- Email: [email protected]
Local structural changes in lithium-doped YBa2Cu3Oy
Physica C 333 Ž2000. 121–132 www.elsevier.nlrlocaterphysc
Local structural changes in lithium-doped YBa 2 Cu 3 O y F. Maury a,) , M. Nicolas-Francillon b, F. Bouree ´ c , R. Ollitrault-Fichet d, M. Nanot d a Laboratoire des Solides Irradies, ´ Ecole Polytechnique, Palaiseau Cedex ´ F-91128, France Laboratoire de Physique du Solide, ESPCI, 10 rue Vauquelin, Paris Cedex ´ 05 F-75231, France c Laboratoire Leon ´ Brillouin, Centre d’Etudes de Saclay, Gif sur YÕette F-91191, France Laboratoire des Ceramiques et Materiaux Mineraux, ESPCI, 10 rue Vauquelin, Paris Cedex ´ ´ ´ ´ 05 F-75231, France b
d
Received 30 November 1999; accepted 1 February 2000
Abstract The structure of orthorhombic and tetragonal samples of lithium ŽLi.-doped YBa 2 Cu 3 O y has been determined by neutron powder diffraction measurements. The experiment shows that when the samples are synthesized in oxygen, Li substitutes for the copper of the CuO 2 planes. When the samples are synthesized in air, a small fraction Žf 20%. of the Li substitutes for the copper of the Cu–O chains. Moreover, the best fits to the diffraction spectra, as well as the best overall understanding of a number of experimental observations, are obtained by assuming that each substitution of a Li for a Cu entails the loss of the neighbouring apical oxygen. This feature seems to be retained during the annealing in argon at 7508C, which transforms the crystallographic structure from orthorhombic to tetragonal. q 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Bk Keywords: Neutron diffraction; High-Tc compounds
1. Introduction This work belongs to an extensive study of the magnetic properties of lithium ŽLi.-doped YBaCuOs and their relation with transport properties. Among all doping elements which can substitute for copper in the high-Tc superconducting cuprates, 7 Li is a good nuclear magnetic resonance ŽNMR. probe. It has been used to evince the appearance of magnetic moments localized around substitutional Li
) Corresponding author. Tel.: q33-1-6933-4502; fax: q33-16933-3022. E-mail address: [email protected] ŽF. Maury..
ions in YBa 2 Cu 3 O 7 w1,2x. Induced magnetization measurements w3x have enabled the determination, in YBa 2 Cu 3yx Li xO 7y1.2 x , of an effective magnetic moment meff s Ž1.1 " 0.15. m B per Li ion, constant up to x s 0.25. Such a magnetic effect is not peculiar to Li but has also been observed in YBa 2 Cu 3 O 7y d doped with nonmagnetic Zn w4–7x and in a number of other doped high-Tc cuprates such as La 1.85 Sr0.15 CuO4 w8,9x or La 2 CuO4q d w10x. Magnetization measurements have also been carried out in insulating YBa 2 Cu 3yx Li xO6q ´ samples w11x, leading to an effective magnetic moment of Ž1.1 " 0.1. m B per Li, surprisingly equal to that obtained in the superconducting YBa 2 Cu 3y x Li xO 7y1.2 x . It has been suggested, on the basis of
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 8 3 - 6
122
F. Maury et al.r Physica C 333 (2000) 121–132
cell parameters and Tc measurements, that contrary to Zn, substitutional Li may occupy the chain Cu1 sites rather than the plane Cu2 sites. The similarity between the magnetization of superconducting and insulating samples could then be attributed to similar local structures around the substitutional Li ions in both types of materials w12x. From neutron powder diffraction measurements, Kwei et al. w13x concluded indeed that, at low doping level, the Li shows a strong preference for the chain Cu1 sites. But both NMR results and a first neutron diffraction experiment on samples prepared like the NMR ones w1x showed that in these samples and at low Li concentration, Li substitutes mainly for Cu in Cu2 sites. Since strongly different Tc values are reported w3,13x for the same Li level, it was suggested that the Li site preference could depend on the synthesis conditions w14x. The present experiment was aimed at ascertaining the Li position and checking the above hypotheses.
2. Experimental The ceramics were prepared by the usual solid state reaction with Y2 O 3 , BaCO 3 , CuO and Li 2 CO 3 as starting materials. The materials were synthesized at T f 9208C either in air or in oxygen. The temperature was optimized through DT and TG analysis. The different atmospheres Žair or oxygen. were used to check the hypothesis according to which the Li site could depend on the synthesis conditions. The ceramics were subsequently annealed in oxygen at 4808C for the superconducting samples. Or they were annealed in argon at f 7508C for the insulating tetragonal ones. The samples prepared in air and annealed in oxygen are labelled A-samples, those prepared in oxygen and annealed in oxygen are labelled O-samples, and the tetragonal ones annealed in argon are labelled T-samples. Both the Li and Cu contents of the samples were determined by emission plasma spectrometry analysis. The uncertainty on the numbers of Cu and Li per mole is "0.02. The total number of Li and Cu per mole is always found to be equal to three within the experimental uncertainty Ž"0.04.. The hole content of the samples was determined by iodometric titration. The oxygen content is then
deduced from electrical neutrality. It is found that the substitution gives rise to an oxygen loss in the doped samples, compared with undoped YBa 2 Cu 3 O 7 , which is proportional to the Li concentration and depends on the sample elaboration: for the A-batch, y, number of O per mole, decreases with x, number of Li per mole, as y s 7 y 1.2 x w3x, while for the O-batch, y s 7 y x. In this latter case, one oxygen exactly is lost per substitutional Li w12x. The crystalline structure of the samples was determined by X-ray diffraction on a Philips PW 1729 diffractometer with exposure times of 11 h. The cell parameters were refined by the U-fit method w15x. The neutron powder diffraction measurements were carried out at room temperature on the highresolution powder spectrometer 3T2 of the Labora˚ The Li toire Leon ´ Brillouin with l s 1.2251 A. scattering length Žy1.90 fm. is very different from that of Cu Ž7.72 fm. or Y Ž7.75 fm., which enables the location of the substitutional Li ions even in dilute samples. We measured the neutron diffraction spectra of four samples: one O-sample, two A-samples and one T-sample. The data were analysed using the Rietveld method. Lattice parameters, atom coordinates, oxygen and CurLi site occupancies, isotropic thermal factors, peak shape and width, preferred orientation, scale factor and background were refined.
3. Cell parameters Fig. 1 shows the cell parameters for the various samples studied either in previous experiments w3,11,16x or in the present one, deduced from X-ray and also, for the present experiment, from neutron diffraction measurements. The samples that have been annealed in oxygen are orthorhombic and superconducting; those that have been annealed in argon are tetragonal and insulating. We see ŽFig. 1a. that the b parameter is independent of the Li concentration in the studied concentration range, 0 - x - 0.4. The a parameter is constant for the O-samples and increases regularly with x for the A-samples. What is expected in the orthorhombic samples due to the mere oxygen defect is indeed an increase of the a parameter w17–19x in this concen-
F. Maury et al.r Physica C 333 (2000) 121–132
123
tration range. But if we now turn to Fig. 1b, we see that for these orthorhombic ŽO- and A-. samples, c is a very slightly decreasing function of x. Now, if but the oxygen defect were to play a role, a significant increase of c Žf q0.3% to q0.5%. should be observed in that concentration range. A tentative explanation for the observed decrease Žf y0.05 to y0.35%. could then be that the Liq ions substitute for the Cuq ions in the chains. The Goldschmidt ionic radius is significantly smaller for Liq than that for Cuq Ž0.68 instead of 0.98 for a coordination number equal to 6.. Yet, if we consider the ionic radii deduced from electron density distribution, then the corresponding values for Cuq, Cu2q and Liq are very near Ž0.74, 0.71 and 0.73, respectively, for a coordination number equal to 4. w20x. Finally, we note ŽFig. 1b. that the dispersion of the data points is larger for the parameter c of the tetragonal samples than for all the orthorhombic ones and much larger than the experimental uncertainty on the measurement. This dispersion could be related to the variable oxygen content of the samples although it is not a clear function of this content.
4. Li location in sample 1
Fig. 1. Cell parameters and cell volume as a function of the lithium concentration. Circles: O-samples. Triangles: A-samples. Squares: T-samples. The full symbols correspond to X-ray measurements, the open symbols to neutron measurements. The lines are linear fits to the data.
Let us consider first the O-sample, YBa 2Cu 2.89 " 0.02 Li 0.11 " 0.02 O6.85 " 0.04 . The measured diffraction pattern for this sample is shown in Fig. 2a. We started the refinement using the Pmmm space group, with the values of the different parameters as already determined for the unsubstituted material w21,22x. The Cu–O chain oxygen ŽO4: x s 0, y s 1r2, z s 0. was assumed to be slightly displaced in the w100x direction perpendicular to the chain axis. All the results discussed in the following are unchanged by assuming unsplit chain oxygens, except for the refined values of the thermal factor B ŽO4., which becomes significantly larger, and for the fit quality which is systematically worsened. To begin with, we supposed that all the substitutional Li ions are on Cu2 sites ŽLi2: x s 1r2, y s 1r2, z Li s z Cu .. All sites were assumed to be fully occupied, except for the two oxygen sites in the Cu1
124
F. Maury et al.r Physica C 333 (2000) 121–132
Ž001. planes: O4 Ž x, 1r2, 0. and O5 Ž1r2, x, 0.. A good fit was obtained Žfit 1 of Table 1. with a refined Li concentration equal to 0.09 " 0.01, a value consistent with the spectrometrically determined one, 0.11 " 0.02 Žtermed ‘‘experimental’’ value in the following.. In a second step, we assumed that all the Li ions were on Cu1 sites ŽLi1: x s y s z s 0.. The fit was worse Ž R Bragg s 4.1% instead of 3.8%. and the refined Li concentration equal to 0.03 " 0.01, much too small compared to the experimental value. Furthermore, if we fix the Li concentration at the experimental value, the fit still worsens and the thermal coefficient of the Cu1rLi1 sites gets negative. If we now allow the Li to occupy the two sites, Cu1 and Cu2 Žfit 2 of Table 1., we get 20% of the Li ions on the Cu1 sites with a Li concentration, x s 0.11 " 0.02, a fit quality which is not significantly improved compared to that of fit 1 and a thermal factor for the ˚ 2 . smaller than Cu1rLi1 site Ž B ŽCu1rLi1. s 0.35 A 2 ˚ ., which is an indicathat in pure YBCO Ž0.5–0.6 A tion of a too low Cu occupancy of this site Ži.e. too much Li1.. Finally, we allowed a fraction of the Liq to substitute for the Y 3q, a possibility considered by Gao et al. w23x, which would also account for the loss of one oxygen per Li. The refinement yielded a value of Ž4 " 9.% for this fraction without improving the fit quality. We can thus conclude that in this O-sample, the Liq ions are mainly on the Cu2 sites and not on the Cu1 sites, contrary to what we had expected on the basis of the cell parameters measurements and of the rather slow Tc decrease in the concentration range 0 - x Li - 0.3. On the other hand, this result is consistent with the NMR results w1,2x, which evidence a unique 7 Li line at low Li concentrations, arising from Liq ions in CuO 2 planes.
5. Lost oxygen location in sample 1 Fig. 2. Neutron diffraction data measured at room temperature with the 3T2 diffractometer. The points are the experimental data, the lines are the calculated profiles. The lower curve represents the difference between the two. Ža. Sample 1, fit 4; Žb. sample 2, fit 5; Žc. sample 3, fit 5.
Let us turn again to the O-sample and consider the refined oxygen concentration of fits 1 or 2. This concentration is y s 6.96 " 0.02, a value which is larger than the ‘‘experimental’’ one: 6.85 " 0.04.
F. Maury et al.r Physica C 333 (2000) 121–132
125
Table 1 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for orthorhombic YBa 2 Cu 3 O y ŽFranc¸ois et al. w21x. and YBa 2 Cu 3yx Li x O y Žthis work. Atom positions are Cu1 Ž0, 0, 0., O4 Ž x, 1r2, 0., O5 Ž1r2, x, 0., O1 Ž0, 0, z ., Ba Ž1r2, 1r2, z ., Cu2 Ž0, 0, z ., O2 Ž1r2, 0, z ., O3 Ž0, 1r2, z . and Y Ž1r2, 1r2, 1r2.. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit. YBa 2 Cu 3yx Li x O y Ref. w21x Sample 1, R exp s 5.8% Temperature 270 K 320 K
Sample 1 ŽO-sample., Sample 2, x s 0.11 " 0.02, y s 6.85 " 0.04, R exp s 2.8% R exp s 3.2% 300 K 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
w0x – 6.91 w2x
w0x – 6.91 w2x
w0x – 6.86 w2x
1 0.09 Ž1. w0%x 6.96 Ž2. w2x
2 0.11 Ž2. 20% Ž9. 6.95 Ž2. w2x
3 0.09 Ž1. w0%x 6.89 Ž3. 2 y n Li2
4 0.105 Ž1. w0%x 6.88 Ž3. 2 y n Li2
˚x a wA ˚x w b A
3.8158 Ž1. 3.8822 Ž1.
3.8177 Ž1. 3.8836 Ž1.
3.9180 Ž1. 3.8835 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
˚x c wA
11.6737 Ž3.
11.6827 Ž3. 11.6726 Ž3.
11.6734 Ž3.
11.6734 Ž3. 11.6734 Ž3. 11.6733 Ž3.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽO3.rc z ŽLi2.rc
0.1588 Ž2. 0.1847 Ž2. 0.3552 Ž1. 0.3778 Ž2. 0.3778 Ž2. –
0.1588 Ž2. 0.1844 Ž2. 0.3552 Ž1. 0.3779 Ž2. 0.3780 Ž2. –
0.1592 Ž2. 0.1857 Ž2. 0.3560 Ž1. 0.3782 Ž2. 0.3777 Ž2. –
0.1583 Ž2. 0.1845 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3778 Ž2. z Cu 2 rc
0.1583 Ž2. 0.1846 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3779 Ž2. z Cu2 rc
0.1582 Ž2. 0.1845 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3778 Ž2. z Cu2 rc
0.1583 Ž2. 0.1845 Ž2. 0.3561 Ž1. 0.3783 Ž2. 0.3778 Ž2. 0.30 Ž1.
˚2 x B ŽY. wA ˚2 x Ž . w B Ba A
0.45 Ž2. 0.59 Ž2.
0.49 Ž2. 0.64 Ž2.
0.54 Ž2. 0.65 Ž2.
0.49 Ž3. 0.41 Ž3.
0.50 Ž3. 0.43 Ž3.
0.52 Ž3. 0.42 Ž3.
0.55 Ž3. 0.45 Ž3.
˚2 x B ŽCu1. wA ˚2 x Ž . w B Cu2 A
0.46 Ž2. 0.45 Ž2.
0.51 Ž2. 0.52 Ž2.
0.59 Ž2. 0.50 Ž2.
0.45 Ž3. 0.38 Ž3.
0.35 Ž4. 0.40 Ž3.
0.47 Ž3. 0.39 Ž3.
0.45 Ž3. 0.42 Ž3.
˚2 x B ŽO1. wA ˚2 x Ž . w B O2 A
0.68 Ž2. 0.59 Ž2.
0.75 Ž2. 0.62 Ž2.
0.80 Ž2. 0.65 Ž2.
0.83 Ž3. 0.62 Ž3.
0.85 Ž3. 0.63 Ž3.
0.66 Ž4. 0.62 Ž3.
0.65 Ž4. 0.64 Ž3.
˚2 x B ŽO3. wA ˚2 x Ž . w B O4 A x ŽO4.ra n O5 rŽ n O4 q n O5 .
0.55 Ž2. 0.62 Ž5. 0.034 Ž2. w0x
0.64 Ž2. 0.50 Ž5. 0.036 Ž2. w0x
0.62 Ž3. 0.58 Ž6. 0.038 Ž2. w0x
0.48 Ž3. 0.74 Ž9. 0.032 Ž2. 2% Ž1.
0.49 Ž3. 0.72 Ž9. 0.032 Ž2. 2% Ž1.
0.48 Ž3. 0.87 Ž9. 0.032 Ž2. 2% Ž1.
0.50 Ž3. 0.85 Ž9. 0.034 Ž2. 2% Ž1.
R Bragg w%x R wp w%x R f w%x
7.2 3.2
7.3 3.4
7.5 3.5
3.8 4.5 2.7
3.75 4.5 2.7
3.8 4.6 2.7
3.7 4.5 2.6
This experimental value is indeed an upper limit of y since, if some metallic sites happened to be vacant, considering them as fully occupied when writing the electrical neutrality of the crystal would lead to overestimation of the oxygen concentration. If, in the refinement, we fix this concentration at the experimental value by constraining nŽO4. q
nŽO5. to be equal to 0.85 Žstill assuming that all missing oxygens are chain oxygens., then the fit quality is significantly worse and the refined thermal factor of the chain oxygens, B ŽO4. s 0.32, is significantly smaller than that of the plane oxygens, B ŽO2. s 0.62 and B ŽO3. s 0.51, or that of the apical oxygen, B ŽO1. s 0.84. With x ŽO4. s 0, the fit qual-
126
F. Maury et al.r Physica C 333 (2000) 121–132
ity is worsened further and the refined value of B ŽO4., 0.72, is still smaller than that of B ŽO1., 0.86, and much smaller than the values found in the literature Žsee Refs. w17,18,21x and the more recent works of Villeneuve w24x and Casalta w25x.. The neutron data are thus hardly compatible with the assumption that the hole loss following the Li substitution is due to the loss of a chain oxygen. Since we know that in the O-samples, exactly one oxygen is lost per substitutional Li, we then assumed that the oxygen defect arises from the loss of the apical ŽO1. oxygen, which neighbours the substitutional Li and not from a missing chain oxygen. Fit 3 was calculated under this hypothesis Žwith all the Li ions on Cu2 sites.. It gives acceptable concentrations for both Li and oxygen, reasonable values for B ŽO1. and B ŽO4., and the same R Bragg value of 3.8% as fit 1. We see that the neutron data support the O1 vacancy hypothesis. They do not allow, however, to quite exclude that the oxygen vacancies be shared between the various oxygen sites. Yet the O1 vacancy hypothesis, besides yielding acceptable fits of the neutron data Žcorrect oxygen concentration and reasonable thermal factors., also explains the following experimental observations. Ži. Exactly one oxygen is lost per substitutional Li Žwhich leads to the loss and not to the a priori expected gain of one hole per Li.. Žii. We have observed in a previous work w3x that, within the experimental uncertainties, the oxygen defect does not depend on the atmosphere, either air or oxygen, of the 4808C anneal following the synthesis Žthis observation was made with A-samples, but we will see in Section 6 that A-samples behave as O-samples with regard to the oxygen defect.. Žiii. The c parameter does not increase as it should if oxygen defects were created in the chains. A number of studies of the YBa 2 Cu 3 O 7y d structure as a function of the oxygen deficiency have evidenced this increase. These studies yield the refined z values of the different ions as a function of d . If we look at these values, we see Žcf. for example, Ref. w18x or Ref. w25x. that as d increases, z ŽO1. decreases slightly while z ŽBa. increases significantly. This increase is progressively reduced for the upper Ž001. atomic planes, Cu2, O2 and Y, but is still positive for z ŽY. s cr2, leading to the observed
c increase. These changes are easily explainable by the removal of chain oxygens, if one neglects the variations of the a and b parameters which are much smaller than those of c for small values of the oxygen defect Žand which are zero for our O-samples.. Removing a chain oxygen Ž z s 0. suppresses its repulsive interaction with its two O1 neighbours ˚ neighbouring Ž001. atomic in the z s z ŽO1. f 1.85 A plane, and its attractive interaction with its two Ba ˚ plane Žnext neighbours in the z s z ŽBa. f 2.15 A Ž001. atomic plane.; the result, if one neglects the interactions between more distant neighbours, is a decrease of z ŽO1. and an increase of z ŽBa.. The z ŽCu2. increase is smaller than that of z ŽBa. due to the decrease of z ŽO1. and to the decrease of the mean charge on the Cu2 ions. If we now look at the z values obtained for sample 1, which do not depend on the fitting assumption ŽO1 or O4 loss; see Table 1., we see that they do not follow the above pattern. z ŽO1. and z ŽCu2. are as expected in the case of a mere chain oxygen loss, but z ŽBa., z ŽO2. Žand consequently z ŽY.. are smaller Žsee Fig. 3.. A simple explanation of this difference can be given if we assume that the lost oxygen is an apical oxygen ŽO1. and not a chain oxygen ŽO4.. Removing an apical oxygen suppresses its repulsive interaction with its two O4 neighbours as before: z ŽO1. is about the same in both cases. That also suppresses its attractive interaction with its
Fig. 3. Variation of the refined z coordinate of O1 Žlozenges., Ba Žcircles., Cu2 Žsquares. and O2 Žtriangles. as a function of the oxygen deficiency variation, in orthorhombic YBa 2 Cu 3 O 7y d . Open symbols: data taken from Refs. w18,25x. Full symbols: sample 1.
F. Maury et al.r Physica C 333 (2000) 121–132
four Ba neighbours, but this last interaction has a much smaller component in the c direction than the Ba–O4 interaction: the z ŽBa. increase is smaller for an O1 than for an O4 vacancy. That z ŽCu2. is about the same in YBa 2 Cu 3yx Li xO 7yx as in YBa 2Cu 3 O 7yx is due to the opposite effects of decreasing z ŽBa. and removing O1 Žthe mere substitution of a Liq and a hole for a Cu2 2q does not change the mean charge on the Cu2rLi2 ions and cannot counterpoise the effect of a reduced z ŽBa... z ŽO2., on the contrary, is decreased as a consequence of the suppression of the repulsive interaction between the missing O1 ion and its two O2 close neighbours. Again, this decrease is difficult to understand without the O1 removal. We see that the c decreases as well as the z variations of the different ions are understandable if we assume O1 vacancies, while they are not if we assume O4 vacancies. Although the above explanation neglects to consider the detailed YBaCuO electronic structure, we think that the observed c decrease is a strong argument in favour of missing apical oxygens rather than missing chain oxygens, from the moment when the Li is known to substitute for Cu2 and not Cu1. Now if the O1 site next to each substitutional Li is vacant, the Li may well be displaced from the Cu2 site. We checked this possibility by allowing the coordinate z Li2 to depart from z Cu2 . The refinement gave z Li2rc s 0.30 " 0.01 Žfit 4 of Table 1.. Yet the two fits, fit 3 with z Li2 s z Cu2 and fit 4 with z Li2 / z Cu2 , are almost equivalent: the neutron data do not really allow determination of the Li position. Finally, we checked that assuming one vacant O1 site per Li on a Cu2 site did not change the conclusion reached in Section 4 with regard to the Li location. We allowed the Li to occupy the two sites, Cu1 and Cu2. Neither the Li1 fraction, equal to 20%, ˚ 2, nor the Cu1rLi1 thermal factor, equal to 0.33 A was changed, compared to fit 2. The refined Li concentration was 0.14 " 0.02, at the upper limit of the acceptable values and, as already pointed out, the value of B ŽCu1., smaller than B ŽCu2., indicates that the fraction of Li1 was overestimated. We can thus conclude, from our neutron results together with previous results ŽNMR and cell parameter measurements. that, for ceramics synthesized in oxygen, Li substitutes for Cu in the CuO 2 planes and
127
that, most likely, each substitution entails the loss of the neighbouring apical oxygen.
6. Neutron results for sample 2 Let us now turn to the A-sample, YBa 2Cu 2.85 " 0.02 Li 0.18 " 0.02 O6.79 " 0.04 . The measured diffraction pattern for this sample is shown in Fig. 2b. The sample quality is not as good as that of sample 1. The measured diffraction pattern exhibits a background level that is twice as high as that of sample 1 for similar experimental conditions and signal level. Traces of green phase ŽY2 BaCuO5 . were detectable on the X-ray diagram, yet the quantity of green phase is small enough not to give rise to any clearly distinguishable peak in the diffraction spectrum of Fig. 2b. We started the data analysis as for sample 1 by assuming that all the Li were on Cu2 sites and found again that the refined oxygen concentration was too high compared to the experimental one. Indeed, one does not see the reason why, if this oxygen is lost in the O-samples sintered in oxygen, it should not be lost in the A-samples sintered in air. As for sample 1, the fit is worsened and the refined Li concentration much too small, without anything changed concerning the oxygen concentration and the oxygen thermal factors, when all the Li are assumed to occupy Cu1 sites. We thus assumed that for A-samples as for Osamples, the Li substitutes mainly for the Cu2 and that one apical oxygen is lost for each Li on a Cu2 site. Fit 3 of Table 2 was calculated with such a hypothesis and, as a first step, all the Li on Cu2 sites. When considering the corresponding values of the refined parameters, one notes two things: the ˚ 2 , is yttrium thermal factor, B ŽY. s Ž0.69 " 0.05. A larger than expected and the Cu2 thermal factor, ˚ 2 , smaller than expected, B ŽCu2. s Ž0.31 " 0.05. A since all the fits of Table 1 Žsample 1 as well as Ref. w21x. yield: 0.45 F B ŽY. F 0.55 and 0.4 F B ŽCu2. F 0.5. The high value of B ŽY. is likely to come from an overestimate of the yttrium site occupancy, nŽY., assumed equal to 1. We remember that traces of green phase were detectable by X-rays in this sample. If Y2 BaCuO5 is formed at the expense of
F. Maury et al.r Physica C 333 (2000) 121–132
128
Table 2 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for orthorhombic YBa 2 Cu 3yx Li x O y ŽA-samples. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit. YBa 2 Cu 3yx Li x O y Temperature
Sample 2 ŽA-sample. x s 0.18 " 0.02, y s 6.79 " 0.04, R exp s 2.5% 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
3 0.15 Ž2. w0%x 6.79 Ž5. 2 y n Li2
3b 0.17 Ž2. w0%x 6.77 Ž6. 2 y n Li2
4 0.19 Ž2. w0%x 6.75 Ž6. 2 y n Li2
5 0.18 Ž2. w25%x 6.77 Ž4. 2 y n Li
3 0.65 Ž1. w0%x 6.90 Ž3. 2 y n Li
5 w0.06x w20x% 6.91 Ž2. 2 y n Li
˚x a wA ˚x b wA
3.8281 Ž1. 3.8871 Ž1.
3.8282 Ž1. 3.8871 Ž1.
3.8281 Ž1. 3.8871 Ž1.
3.8281 Ž1. 3.8871 Ž1.
3.8226 Ž1. 3.8860 Ž1.
3.8226 Ž1. 3.8860 Ž1.
˚x c wA
11.6646 Ž5.
11.6647 Ž5.
11.6647 Ž5.
11.6646 Ž5.
11.6738 Ž3.
11.6737 Ž3.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽO3.rc z ŽLi2.rc
0.1574 Ž3. 0.1862 Ž3. 0.3570 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu 2 rc
0.1574 Ž3. 0.1861 Ž3. 0.3571 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu2rc
0.1574 Ž3. 0.1860 Ž3. 0.3569 Ž2. 0.3772 Ž3. 0.3780 Ž3. 0.29 Ž1.
0.1574 Ž3. 0.1863 Ž3. 0.3567 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu2rc
0.1578 Ž2. 0.1849 Ž2. 0.3559 Ž1. 0.3787 Ž2. 0.3778 Ž2. z Cu2rc
0.1578 Ž2. 0.1849 Ž2. 0.3559 Ž1. 0.3787 Ž2. 0.3778 Ž2. z Cu2rc
nŽY. ˚2 x B ŽY. wA
w1x
0.97 Ž2. 0.59 Ž7.
0.97 Ž2. 0.60 Ž7.
0.98 Ž1. 0.64 Ž7.
w1x
w1x
0.56 Ž5.
0.61 Ž5.
0.57 Ž5.
0.58 Ž3. 0.43 Ž3.
0.57 Ž3. 0.42 Ž3.
0.66 Ž5. 0.31 Ž5.
0.70 Ž5. 0.30 Ž5.
0.66 Ž5. 0.36 Ž4.
0.46 Ž4. 0.40 Ž4.
0.51 Ž3. 0.36 Ž3.
0.44 Ž3. 0.40 Ž2.
0.72 Ž5. 0.55 Ž5.
0.72 Ž5. 0.56 Ž5.
0.69 Ž5. 0.60 Ž5.
0.79 Ž5. 0.57 Ž5.
0.77 Ž4. 0.66 Ž3.
0.80 Ž3. 0.65 Ž3.
˚2 x B ŽO4. wA x ŽO4.ra n O5 rŽ n O4 q n O5 .
0.51 Ž5. 1.1 Ž2. 0.040 Ž3. 5% Ž2.
0.55 Ž5. 1.1 Ž2. 0.040 Ž3. 5% Ž2.
0.57 Ž5. 1.1 Ž2. 0.041 Ž2. 5% Ž2.
0.54 Ž5. 1.0 Ž2. 0.040 Ž2. 4% Ž2.
0.50 Ž3. 0.84 Ž10. 0.035 Ž2. 4% Ž1.
0.49 Ž3. 0.80 Ž10. 0.035 Ž2. 4% Ž1.
R Bragg w%x R wp w%x R f w%x
6.2 4.4 4.25
6.15 4.3 4.3
6.1 4.3 4.2
6.1 4.3 4.2
4.05 4.5 2.8
4.05 4.5 2.8
˚2 x B ŽBa. wA ˚2 x Ž . B Cu1 wA ˚2 x B ŽCu2. wA ˚ Ž . w B O1 A2 x ˚2 x B ŽO2. wA ˚2 x Ž . w B O3 A
0.69 Ž5. 0.54 Ž5.
YBa 2 Cu 3 O 7 , this could result in a lack of yttrium in YBa 2 Cu 3 O 7 . Thus, we refined the Y site occupancy. ˚ 2 for nŽY. s B ŽY. was found equal to 0.59 " 0.07 A 0.97 " 0.02 Žfit 3b of Table 2.. The refined values of the other parameters were not much changed. In particular, B ŽCu2. was still low. It is slightly increased if one allows the Li2 ions to be displaced from the Cu2 sites Žfit 4.: B ŽCu2. s 0.36 Žinstead of 0.30. with z Li2rc s 0.29. Yet it remains low, indicating that the Cu2 site occupancy by Cu can be
Sample 4 ŽA-sample., x s 0.06 " 0.02, R exp s 2.9% 300 K X
X
underestimated and, consequently, the Li2 concentration overestimated. In the meantime, B ŽCu1. is high, whereas in undoped YBa 2 Cu 3 O 7 w21,24,25x and in sample 1, B ŽCu1. and B ŽCu2. are about equal. We are thus led to assume that a fraction of the Li may occupy Cu1 sites. Fit 5 of Table 2 was then calculated by assuming 25% of the Li on Cu1 sites. This fit is of the same quality as fit 3b with a Li concentration equal to the experimental one and reasonable values of the ther-
F. Maury et al.r Physica C 333 (2000) 121–132
mal factors. The refined value of y, 6.77 " 0.04, is now to be compared not to 6.79 but to 6.76 " 0.04, which is the experimental y value when the yttrium defect is taken into account. Similar results are obtained with 20% of the Li on Cu1 sites and z Li2rc s 0.29. If now a fraction of f 20% of the Li substitutes for Cu on Cu1 sites, and since for the A-samples, y s 7 y 1.2 x, this means that f 2 oxygens are lost per Li1. This is indeed sufficient to explain the observed variations of the cell volume if one assumes that these oxygens are chain oxygens. For x s 0.4, about 0.15 chain oxygen will be missing, which should result in an increase of the cell volume ˚ 3 w18x. This is effectively the difference of f 0.5 A observed between our A- and O-samples for this x value Žsee Fig. 1.. Yet the loss of two chain oxygens per Li1 does not explain why c is slightly smaller in A-samples than in O-samples since it should result in an increase of both V and c. That suggests that the corresponding defects might not be simply Liq ions on Cu1 sites between two missing chain oxygens. The concentration of Li1 is too small to enable the determination of their structure, that is, either to locate the missing oxygens or to measure a possible displacement of the substitutional Li1 off the Cu1 sites. We checked that none of the refined parameters is changed beyond its uncertainty limits when two O1 instead of two O4 are supposed to be missing per Li1, or when the Li1s are assumed to be displaced from the Cu1 sites, or when the thermal factor, B ŽLi1., is fixed at a value significantly larger than B ŽCu1.. If we now return to Table 2, we see Žfit 5. that the disorder in the Cu1 chains is greater for sample 2 than for sample 1. x O4 ra s 0.040 " 0.002 instead of 0.032 " 0.002, B ŽO4. s 1.0 " 0.2 instead of 0.87 " 0.09 and n O5 rŽ n O4 q n O5 . s 4% instead of 2%. A question then arises: Is the location of a fraction of the Li on Cu1 sites a mere consequence of the poorer quality of the sample or does it result from the sample elaboration conditions, different for this sample than for sample 1? To answer this question, we measured the diffraction spectra of a second A-sample: sample 4 ŽYBa 2 Cu 2.94 " 0.02 Li 0.06 " 0.02 O y ., which was of as good quality as sample 1 and had a smaller Li concentration. The data analysis was performed as
129
for samples 1 and 2. Fit 3X of Table 2 was calculated by assuming 100% Li2 and n O1 s 2 y n Li2 . We see that the Y and Ba thermal factors are the same as for sample 1, confirming the good quality of the sample, but that B ŽCu1. and B ŽCu2. are, as for sample 2, respectively, too high and too low, compared to sample 1. Thus, in sample 4 as in sample 2, a non-negligible fraction of the Li must occupy the Cu1 sites. Fit 5X of Table 2 was calculated by fixing x at the experimental value and the fraction of Li1 at 20%. The thermal factors are then comparable to those of undoped YBa 2 Cu 3 O 7 or of sample 1. We thus conclude that the presence of a fraction of the Li on Cu1 sites results from the synthesis conditions. It is indeed easy to understand that, if at least two oxygens are to be missing in order that one Li may substitute for one Cu1, such a possibility is more likely to occur when the synthesis is realized in air where the oxygen partial pressure is smaller by a factor 5 than in oxygen. The formation of the parasitic Y2 BaCuO5 phase, rich in oxygen, can also favour the Li1 sites. Small amounts of green phase were indeed observed by Kwei et al. w13x in samples containing large fractions of Li1. The greater disorder in the Cu1 chains, visible in samples 2 and 4, must stem from the presence of Li on Cu1 sites.
7. Neutron results for sample 3 The tetragonal sample, YBa 2 Cu 2.89 " 0.02 Li 0.09 " 0.02 O6.04 " 0.04 , synthesized in air, was of good quality and the background level of its diffraction spectrum ŽFig. 2c. was the same as that of sample 1 or 4. We started the refinement using the P4rmmm space group and, as for sample 1, values of the different parameters as already determined for the undoped material w22x. The chain oxygens were assumed to be slightly displaced from Ž0, 1r2, 0. in the x direction perpendicular to the chain axis. Their ˚ 2 . We thermal factor was fixed at B ŽO4. s 1.0 A checked that none of the refined values of the parameters is changed if one assumes B ŽO4. s 0.6 or 1.5, e.g., instead of 1.0. We first assumed Žfit 1 of Table 3. that all the Li ions are on Cu2 sites and that all sites are fully occupied except for the chain oxygen ones, most of
F. Maury et al.r Physica C 333 (2000) 121–132
130
Table 3 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for tetragonal YBa 2 Cu 3 O y ŽJohnston et al. w17x, Jorgensen et al. w18x. and YBa 2 Cu 3yx Li x O y Žthis work. Atom positions are Cu1 Ž0, 0, 0., O4 Ž x, 1r2, 0., O1 Ž0, 0, z ., Ba Ž1r2, 1r2, z ., Cu2 Ž0, 0, z ., O2 Ž1r2, 0, z . and Y Ž1r2, 1r2, 1r2.. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit.
Temperature
Ref. w17x, y s 6.00, R exp s 2.8% 296 K
Ref. w18x, y s 6.09, R exp s 3.6% 300 K
Sample 3 ŽT-sample., x s 0.09 " 0.02, y s 6.04 " 0.04, R exp s 2.6% 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
w0x – 6.13 Ž5. 2.01 Ž2.
w0x – 6.10 Ž3. 2.00 Ž2.
1 0.083 Ž6. w0%x 6.10 Ž1. w2x
3.8577 Ž2. 11.8274 Ž8.
3.8600 Ž1.
3.8595 Ž1.
3.8593 Ž1.
3.8595 Ž1.
˚x c wA
11.8168 Ž2.
11.8136 Ž2.
11.8137 Ž2.
11.8137 Ž2.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽLi2.rc
0.1528 Ž3. 0.1954 Ž3. 0.3608 Ž2. 0.3795 Ž2. –
0.1524 Ž2. 0.1946 Ž2. 0.3611 Ž1. 0.3795 Ž1. –
0.1522 Ž1. 0.1954 Ž1. 0.3622 Ž1. 0.3789 Ž1. z Cu 2 rc
0.1522 Ž1. 0.1954 Ž1. 0.3622 Ž1. 0.3789 Ž1. z Cu2rc
0.1522 Ž1. 0.1954 Ž1. 0.3621 Ž1. 0.3789 Ž1. z Cu2rc
˚2 x B ŽY. wA ˚2 x Ž . w B Ba A
0.54 Ž6. 0.55 Ž7.
0.42 Ž3. 0.50 Ž3.
0.53 Ž2. 0.40 Ž2.
0.58 Ž2. 0.44 Ž2.
0.58 Ž2. 0.46 Ž2.
˚2 x B ŽCu1. wA ˚2 x Ž . w B Cu2 A
0.97 Ž7. 0.39 Ž4.
0.91 Ž4. 0.37 Ž2.
0.93 Ž2. 0.41 Ž2.
0.97 Ž2. 0.40 Ž1.
0.89 Ž3. 0.41 Ž2.
˚2 x B ŽO1. wA ˚2 x Ž . w B O2 A
1.16 Ž9. 0.51 Ž5.
1.12 Ž2. 0.59 Ž1.
0.95 Ž3. 0.62 Ž1.
0.96 Ž3. 0.63 Ž1.
˚2 x B ŽO4. wA x ŽO4.ra
4 Ž3. w0x
w1.0x 0.086 Ž7.
w1.0x 0.084 Ž7.
w1.0x 0.085 Ž7.
R Bragg w%x R wp w%x R p w%x
5.2 4.0
3.4 4.25 3.4
3.3 4.2 3.35
3.2 4.2 3.35
YBa 2 Cu 3yx Li x O y
˚x a wA
0.62 Ž2. w2x w0x
8.0
which should be vacant. The refined Li concentration was x s 0.083 " 0.006, consistent with the experimental value. We then assumed that all the Li are on Cu1 sites. The refined Li concentration was x s 0.007 " 0.005, much too low, and the fit quality slightly worse. We can then conclude that, in this sample as in the orthorhombic ones, the Li is mainly on the Cu2 sites. If we now turn to the oxygen concentration, we see ŽTable 3. that the refined value, y s 6.10 " 0.01, is slightly too high compared to the experimental one. It is not evident whether, in this sample, one apical oxygen should be missing per Li on a Cu2 site, since the crystal structure is tetragonal and
3 w0.09x w0%x 6.01 Ž2. 1.91 Ž1.
5 0.12 Ž1. 17% Ž6. 6.01 Ž2. 1.92 Ž1.
since, in particular, its c-axis cell parameter is larger than in the orthorhombic samples. We thus fixed the Li concentration at the experimental value and refined the O1 site occupancy. The refined value is n O1 s 1.91 " 0.01 Žwhich corresponds exactly to one missing O1 per Li., when all the Li are assumed to occupy the Cu2 sites, with an oxygen concentration of 6.01 " 0.02, a value consistent with the experimental one. The neutron data thus favour the hypothesis of one missing apical oxygen for each substituted Li2, in tetragonal as well as in orthorhombic samples. That apical oxygen vacancies may be observed in tetragonal YBa 2 Cu 3 O y is supported by the work of Siddique w26x who reports measured values
F. Maury et al.r Physica C 333 (2000) 121–132
of y as low as 5.83, in vacuum annealed YBa 2 Cu 3 O y , and attributes them to the loss of apical oxygens. Finally, we assumed that in this tetragonal sample as in the orthorhombic ones, one apical oxygen is lost per Li2, and we allowed the Li to occupy both Cu1 and Cu2 sites. The best fit was obtained for 17% of Li1 Žfit 5 of Table 3. and x Li s 0.12 " 0.02. None of the refined parameters is changed beyond its uncertainty limits if the Li concentration is fixed at the experimental value, except for a slight increase of B ŽCu2.. To determine whether the Li1 fraction was fixed by the synthesis conditions or was liable to be modified by the anneal at 7508C, which transforms the structure from orthorhombic to tetragonal, we recently annealed sample 1 at 7508C in argon. ŽThis sample was annealed as a powder and not, like all other samples, as ceramics.. Preliminary measurements seem to indicate a non-zero fraction of Li1 in this sample, thus showing that the 7508C anneal in argon does modify the Li location. Before concluding, let us turn again to the cell parameters. As already mentioned, in the tetragonal samples, c is not found to vary regularly with the Li content Ž x . of the sample Žsee Fig. 1b. or with the chain oxygen content of the sample Žequal to ´ or G x q ´ , depending on which oxygens we assume to be missing. or with a combination of both. We interpret this as an indication that in these samples with large c values, c is strongly sensitive to all causes of decrease such as the presence of oxygens in the chains, of Li on Cu1 sites, etc., which can depend on the exact annealing conditions Žargon pressure, sample state: powder or ceramics, etc... 8. Conclusion The present neutron powder diffraction experiment has evidenced two things. Ž1. In Li-doped YBaCuO samples, synthesized in oxygen at f 9208C, the Li substitutes for the copper of the CuO 2 planes. When the samples are synthesized in air, about 20% of the Li substitutes for the copper of the Cu–O chains. In tetragonal samples subsequently annealed in argon at 7508C, this fraction seems to depend not only on the synthesis conditions but also on the annealing conditions.
131
Ž2. One apical oxygen is lost per Li on a Cu2 site and about two oxygens per Li on a Cu1 site. This feature does not seem to be modified by the transformation from the orthorhombic to the tetragonal structure. It allows us to explain fairly well the cell parameter variations with the Li content of the sample. In the tetragonal samples, it can also explain why the induced magnetic moment is found constant when calculated per Li ion q extra oxygen Ž x q ´ . and not per Li w11,16x. The sample magnetization would stem from the presence of oxygen ions in the Cu1 chains and not from the mere presence of Li in the CuO 2 planes. Up to f 415 K, YBa 2 Cu 3 O6q ´ is known to be antiferromagnetic w27x. At low doping level and at room temperature or below, the random substitution of Cu2 ions by nonmagnetic Li ions suppresses magnetic moments which are, at random, parallel or antiparallel to the magnetic field; it may also modify the magnetic moments of their Cu2 near neighbours without changing their antiferromagnetic state. Both effects will result in no variation of the measured sample magnetization, due to the defects in the Cu1 chains only. We can now revert to our starting question: Is the similarity between the magnetization of superconducting and insulating samples to be attributed to similar local structures around the substitutional Li ions in both types of materials or is it fortuitous? If, in the superconducting samples, the magnetic moments stem from Cu2 ions, near neighbours of substitutional Li2, and if, in the insulating samples, they stem from chain defects, the answer is that the coincidence is fortuitous. Finally, we remark that the loss of the apical oxygen may be one of the reasons why it is difficult to make samples with high Li concentrations. Never could we reach values of x larger than f 0.5, whereas values of 0.7 w22x or 1 w28x are reported in the literature for Fe or Co. Is this loss particular to Li or does the same happen with other dopants? The question is worthy to be asked. References w1x K. Sauv, J. Conard, M. Nicolas-Francillon, F. Bouree, ´ Physica C 273 Ž1996. 49. w2x J. Bobroff, Thesis, Universite´ Paris-Sud, Orsay, France, 1997.
132
F. Maury et al.r Physica C 333 (2000) 121–132
w3x M. Nicolas-Francillon, F. Maury, R. Ollitrault-Fichet, M. Nanot, P. Legeay, J. Appl. Phys. 84 Ž2. Ž1998. 925. w4x G. Xiao, F.H. Streiftz, A. Gavrin, Y.W. Du, C.L. Chien, Phys. Rev. B 35 Ž16. Ž1987. 8782. w5x S. Zagoulaev, P. Monod, J. Jegoudez, Phys. Rev. B 52 Ž14. ´ Ž1995. 10474. w6x P. Mendels, H. Alloul, G. Collin, N. Blanchard, J.F. Marucco, J. Bobroff, Physica C 235–240 Ž1994. 1595. w7x H. Alloul, P. Mendels, H. Casalta, J.F. Marucco, J. Arabski, Phys. Rev. Lett. 67 Ž22. Ž1991. 3140. w8x G. Xiao, M.Z. Cieplak, J.Q. Xiao, C.L. Chien, Phys. Rev. B 42 Ž13. Ž1990. 8752. w9x K. Ishida, Y. Kitaoka, K. Yamazoe, K. Asayama, Y. Yamada, Phys. Rev. Lett. 76 Ž3. Ž1996. 531. w10x G.D. Liu, Z.X. Zhao, G.C. Che, Solid State Commun. 109 Ž1999. 495. w11x M. Nicolas-Francillon, F. Maury, M. Nanot, R. OllitrautFichet, Solid State Commun. 109 Ž1999. 531. w12x M. Nicolas-Francillon, F. Maury, R. Ollitrault-Fichet, M. Nanot, Physica C 317–318 Ž1999. 579. w13x G.H. Kwei, K.C. Ott, E.J. Peterson, Physica C 194 Ž1992. 307. w14x K. Sauv, Thesis, University Paris VI, Paris, France, 1996. w15x M. Evain, U-Fit: A Cell Parameter Refinement Program, Laboratoire des Materiaux de Nantes, France, 1992. ´ w16x F. Maury, M. Nicolas-Francillon, M. Nanot, R. OllitraultFichet, J. Appl. Phys. 85 Ž2. Ž1999. 1002. w17x D.C. Johnston, A.J. Jacobson, J.M. Newsam, J.T. Lewandowski, D.P. Goshorn, D. Xie, W.B. Yelon, in: Chem-
w18x
w19x
w20x w21x
w22x w23x w24x w25x
w26x w27x
w28x
istry of High-Temperature Superconductors, AMS, Washington, DC, 1987, p. 136, Chapter 14. J.D. Jorgensen, B.W. Veal, A.P. Paulikas, L.J. Nowicki, G.W. Crabtree, H. Claus, W.K. Kwok, Phys. Rev. B 41 Ž4. Ž1990. 1863. R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck, L.W. Rupp Jr., Physica C Ž1990. 419. B.K. Vainshtein, V.M. Fridkin, V.L. Indenbom, Structure of Crystals, Springer-Verlag, Berlin, 1995. M. Franc¸ois, A. Junod, K. Yvon, A.W. Hewat, J.J. Capponi, P. Strobel, M. Marezio, P. Fischer, Solid State Commun. 66 Ž10. Ž1988. 1117. A.M. Balagurov, F. Bouree, ´ I.S. Lyubutin, I. Mirebeau, Physica C 228 Ž1994. 299. Q. Gao, Y.-S. Zhou, L.-Y. Zhang, Physica C 199 Ž1992. 121. R. Villeneuve, Thesis, University Paris XI, Orsay, France, 1996. H. Casalta, P. Schleger, P. Harris, B. Lebech, N.H. Andersen, R. Liang, P. Dosanjh, W.N. Hardy, Physica C 258 Ž1996. 321. R.K. Siddique, Physica C 228 Ž1994. 365. J. Rossat-Mignot, P. Burlet, M.J. Jugens, C. Vettier, L.P. Regnault, J.Y. Henry, C. Ayache, L. Forro, H. Noel, M. Potel, P. Gougeon, J.C. Levet, J. Phys. ŽFrance. 49 Ž12. Ž1988. 2119. B. Fisher, J. Genossar, L. Patlagan, G.M. Reisner, A. Knizhnik, J. Appl. Phys. 80 Ž2. Ž1996. 89816.
Local structural changes in lithium-doped YBa 2 Cu 3 O y F. Maury a,) , M. Nicolas-Francillon b, F. Bouree ´ c , R. Ollitrault-Fichet d, M. Nanot d a Laboratoire des Solides Irradies, ´ Ecole Polytechnique, Palaiseau Cedex ´ F-91128, France Laboratoire de Physique du Solide, ESPCI, 10 rue Vauquelin, Paris Cedex ´ 05 F-75231, France c Laboratoire Leon ´ Brillouin, Centre d’Etudes de Saclay, Gif sur YÕette F-91191, France Laboratoire des Ceramiques et Materiaux Mineraux, ESPCI, 10 rue Vauquelin, Paris Cedex ´ ´ ´ ´ 05 F-75231, France b
d
Received 30 November 1999; accepted 1 February 2000
Abstract The structure of orthorhombic and tetragonal samples of lithium ŽLi.-doped YBa 2 Cu 3 O y has been determined by neutron powder diffraction measurements. The experiment shows that when the samples are synthesized in oxygen, Li substitutes for the copper of the CuO 2 planes. When the samples are synthesized in air, a small fraction Žf 20%. of the Li substitutes for the copper of the Cu–O chains. Moreover, the best fits to the diffraction spectra, as well as the best overall understanding of a number of experimental observations, are obtained by assuming that each substitution of a Li for a Cu entails the loss of the neighbouring apical oxygen. This feature seems to be retained during the annealing in argon at 7508C, which transforms the crystallographic structure from orthorhombic to tetragonal. q 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Bk Keywords: Neutron diffraction; High-Tc compounds
1. Introduction This work belongs to an extensive study of the magnetic properties of lithium ŽLi.-doped YBaCuOs and their relation with transport properties. Among all doping elements which can substitute for copper in the high-Tc superconducting cuprates, 7 Li is a good nuclear magnetic resonance ŽNMR. probe. It has been used to evince the appearance of magnetic moments localized around substitutional Li
) Corresponding author. Tel.: q33-1-6933-4502; fax: q33-16933-3022. E-mail address: [email protected] ŽF. Maury..
ions in YBa 2 Cu 3 O 7 w1,2x. Induced magnetization measurements w3x have enabled the determination, in YBa 2 Cu 3yx Li xO 7y1.2 x , of an effective magnetic moment meff s Ž1.1 " 0.15. m B per Li ion, constant up to x s 0.25. Such a magnetic effect is not peculiar to Li but has also been observed in YBa 2 Cu 3 O 7y d doped with nonmagnetic Zn w4–7x and in a number of other doped high-Tc cuprates such as La 1.85 Sr0.15 CuO4 w8,9x or La 2 CuO4q d w10x. Magnetization measurements have also been carried out in insulating YBa 2 Cu 3yx Li xO6q ´ samples w11x, leading to an effective magnetic moment of Ž1.1 " 0.1. m B per Li, surprisingly equal to that obtained in the superconducting YBa 2 Cu 3y x Li xO 7y1.2 x . It has been suggested, on the basis of
0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 0 8 3 - 6
122
F. Maury et al.r Physica C 333 (2000) 121–132
cell parameters and Tc measurements, that contrary to Zn, substitutional Li may occupy the chain Cu1 sites rather than the plane Cu2 sites. The similarity between the magnetization of superconducting and insulating samples could then be attributed to similar local structures around the substitutional Li ions in both types of materials w12x. From neutron powder diffraction measurements, Kwei et al. w13x concluded indeed that, at low doping level, the Li shows a strong preference for the chain Cu1 sites. But both NMR results and a first neutron diffraction experiment on samples prepared like the NMR ones w1x showed that in these samples and at low Li concentration, Li substitutes mainly for Cu in Cu2 sites. Since strongly different Tc values are reported w3,13x for the same Li level, it was suggested that the Li site preference could depend on the synthesis conditions w14x. The present experiment was aimed at ascertaining the Li position and checking the above hypotheses.
2. Experimental The ceramics were prepared by the usual solid state reaction with Y2 O 3 , BaCO 3 , CuO and Li 2 CO 3 as starting materials. The materials were synthesized at T f 9208C either in air or in oxygen. The temperature was optimized through DT and TG analysis. The different atmospheres Žair or oxygen. were used to check the hypothesis according to which the Li site could depend on the synthesis conditions. The ceramics were subsequently annealed in oxygen at 4808C for the superconducting samples. Or they were annealed in argon at f 7508C for the insulating tetragonal ones. The samples prepared in air and annealed in oxygen are labelled A-samples, those prepared in oxygen and annealed in oxygen are labelled O-samples, and the tetragonal ones annealed in argon are labelled T-samples. Both the Li and Cu contents of the samples were determined by emission plasma spectrometry analysis. The uncertainty on the numbers of Cu and Li per mole is "0.02. The total number of Li and Cu per mole is always found to be equal to three within the experimental uncertainty Ž"0.04.. The hole content of the samples was determined by iodometric titration. The oxygen content is then
deduced from electrical neutrality. It is found that the substitution gives rise to an oxygen loss in the doped samples, compared with undoped YBa 2 Cu 3 O 7 , which is proportional to the Li concentration and depends on the sample elaboration: for the A-batch, y, number of O per mole, decreases with x, number of Li per mole, as y s 7 y 1.2 x w3x, while for the O-batch, y s 7 y x. In this latter case, one oxygen exactly is lost per substitutional Li w12x. The crystalline structure of the samples was determined by X-ray diffraction on a Philips PW 1729 diffractometer with exposure times of 11 h. The cell parameters were refined by the U-fit method w15x. The neutron powder diffraction measurements were carried out at room temperature on the highresolution powder spectrometer 3T2 of the Labora˚ The Li toire Leon ´ Brillouin with l s 1.2251 A. scattering length Žy1.90 fm. is very different from that of Cu Ž7.72 fm. or Y Ž7.75 fm., which enables the location of the substitutional Li ions even in dilute samples. We measured the neutron diffraction spectra of four samples: one O-sample, two A-samples and one T-sample. The data were analysed using the Rietveld method. Lattice parameters, atom coordinates, oxygen and CurLi site occupancies, isotropic thermal factors, peak shape and width, preferred orientation, scale factor and background were refined.
3. Cell parameters Fig. 1 shows the cell parameters for the various samples studied either in previous experiments w3,11,16x or in the present one, deduced from X-ray and also, for the present experiment, from neutron diffraction measurements. The samples that have been annealed in oxygen are orthorhombic and superconducting; those that have been annealed in argon are tetragonal and insulating. We see ŽFig. 1a. that the b parameter is independent of the Li concentration in the studied concentration range, 0 - x - 0.4. The a parameter is constant for the O-samples and increases regularly with x for the A-samples. What is expected in the orthorhombic samples due to the mere oxygen defect is indeed an increase of the a parameter w17–19x in this concen-
F. Maury et al.r Physica C 333 (2000) 121–132
123
tration range. But if we now turn to Fig. 1b, we see that for these orthorhombic ŽO- and A-. samples, c is a very slightly decreasing function of x. Now, if but the oxygen defect were to play a role, a significant increase of c Žf q0.3% to q0.5%. should be observed in that concentration range. A tentative explanation for the observed decrease Žf y0.05 to y0.35%. could then be that the Liq ions substitute for the Cuq ions in the chains. The Goldschmidt ionic radius is significantly smaller for Liq than that for Cuq Ž0.68 instead of 0.98 for a coordination number equal to 6.. Yet, if we consider the ionic radii deduced from electron density distribution, then the corresponding values for Cuq, Cu2q and Liq are very near Ž0.74, 0.71 and 0.73, respectively, for a coordination number equal to 4. w20x. Finally, we note ŽFig. 1b. that the dispersion of the data points is larger for the parameter c of the tetragonal samples than for all the orthorhombic ones and much larger than the experimental uncertainty on the measurement. This dispersion could be related to the variable oxygen content of the samples although it is not a clear function of this content.
4. Li location in sample 1
Fig. 1. Cell parameters and cell volume as a function of the lithium concentration. Circles: O-samples. Triangles: A-samples. Squares: T-samples. The full symbols correspond to X-ray measurements, the open symbols to neutron measurements. The lines are linear fits to the data.
Let us consider first the O-sample, YBa 2Cu 2.89 " 0.02 Li 0.11 " 0.02 O6.85 " 0.04 . The measured diffraction pattern for this sample is shown in Fig. 2a. We started the refinement using the Pmmm space group, with the values of the different parameters as already determined for the unsubstituted material w21,22x. The Cu–O chain oxygen ŽO4: x s 0, y s 1r2, z s 0. was assumed to be slightly displaced in the w100x direction perpendicular to the chain axis. All the results discussed in the following are unchanged by assuming unsplit chain oxygens, except for the refined values of the thermal factor B ŽO4., which becomes significantly larger, and for the fit quality which is systematically worsened. To begin with, we supposed that all the substitutional Li ions are on Cu2 sites ŽLi2: x s 1r2, y s 1r2, z Li s z Cu .. All sites were assumed to be fully occupied, except for the two oxygen sites in the Cu1
124
F. Maury et al.r Physica C 333 (2000) 121–132
Ž001. planes: O4 Ž x, 1r2, 0. and O5 Ž1r2, x, 0.. A good fit was obtained Žfit 1 of Table 1. with a refined Li concentration equal to 0.09 " 0.01, a value consistent with the spectrometrically determined one, 0.11 " 0.02 Žtermed ‘‘experimental’’ value in the following.. In a second step, we assumed that all the Li ions were on Cu1 sites ŽLi1: x s y s z s 0.. The fit was worse Ž R Bragg s 4.1% instead of 3.8%. and the refined Li concentration equal to 0.03 " 0.01, much too small compared to the experimental value. Furthermore, if we fix the Li concentration at the experimental value, the fit still worsens and the thermal coefficient of the Cu1rLi1 sites gets negative. If we now allow the Li to occupy the two sites, Cu1 and Cu2 Žfit 2 of Table 1., we get 20% of the Li ions on the Cu1 sites with a Li concentration, x s 0.11 " 0.02, a fit quality which is not significantly improved compared to that of fit 1 and a thermal factor for the ˚ 2 . smaller than Cu1rLi1 site Ž B ŽCu1rLi1. s 0.35 A 2 ˚ ., which is an indicathat in pure YBCO Ž0.5–0.6 A tion of a too low Cu occupancy of this site Ži.e. too much Li1.. Finally, we allowed a fraction of the Liq to substitute for the Y 3q, a possibility considered by Gao et al. w23x, which would also account for the loss of one oxygen per Li. The refinement yielded a value of Ž4 " 9.% for this fraction without improving the fit quality. We can thus conclude that in this O-sample, the Liq ions are mainly on the Cu2 sites and not on the Cu1 sites, contrary to what we had expected on the basis of the cell parameters measurements and of the rather slow Tc decrease in the concentration range 0 - x Li - 0.3. On the other hand, this result is consistent with the NMR results w1,2x, which evidence a unique 7 Li line at low Li concentrations, arising from Liq ions in CuO 2 planes.
5. Lost oxygen location in sample 1 Fig. 2. Neutron diffraction data measured at room temperature with the 3T2 diffractometer. The points are the experimental data, the lines are the calculated profiles. The lower curve represents the difference between the two. Ža. Sample 1, fit 4; Žb. sample 2, fit 5; Žc. sample 3, fit 5.
Let us turn again to the O-sample and consider the refined oxygen concentration of fits 1 or 2. This concentration is y s 6.96 " 0.02, a value which is larger than the ‘‘experimental’’ one: 6.85 " 0.04.
F. Maury et al.r Physica C 333 (2000) 121–132
125
Table 1 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for orthorhombic YBa 2 Cu 3 O y ŽFranc¸ois et al. w21x. and YBa 2 Cu 3yx Li x O y Žthis work. Atom positions are Cu1 Ž0, 0, 0., O4 Ž x, 1r2, 0., O5 Ž1r2, x, 0., O1 Ž0, 0, z ., Ba Ž1r2, 1r2, z ., Cu2 Ž0, 0, z ., O2 Ž1r2, 0, z ., O3 Ž0, 1r2, z . and Y Ž1r2, 1r2, 1r2.. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit. YBa 2 Cu 3yx Li x O y Ref. w21x Sample 1, R exp s 5.8% Temperature 270 K 320 K
Sample 1 ŽO-sample., Sample 2, x s 0.11 " 0.02, y s 6.85 " 0.04, R exp s 2.8% R exp s 3.2% 300 K 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
w0x – 6.91 w2x
w0x – 6.91 w2x
w0x – 6.86 w2x
1 0.09 Ž1. w0%x 6.96 Ž2. w2x
2 0.11 Ž2. 20% Ž9. 6.95 Ž2. w2x
3 0.09 Ž1. w0%x 6.89 Ž3. 2 y n Li2
4 0.105 Ž1. w0%x 6.88 Ž3. 2 y n Li2
˚x a wA ˚x w b A
3.8158 Ž1. 3.8822 Ž1.
3.8177 Ž1. 3.8836 Ž1.
3.9180 Ž1. 3.8835 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
3.8198 Ž1. 3.8847 Ž1.
˚x c wA
11.6737 Ž3.
11.6827 Ž3. 11.6726 Ž3.
11.6734 Ž3.
11.6734 Ž3. 11.6734 Ž3. 11.6733 Ž3.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽO3.rc z ŽLi2.rc
0.1588 Ž2. 0.1847 Ž2. 0.3552 Ž1. 0.3778 Ž2. 0.3778 Ž2. –
0.1588 Ž2. 0.1844 Ž2. 0.3552 Ž1. 0.3779 Ž2. 0.3780 Ž2. –
0.1592 Ž2. 0.1857 Ž2. 0.3560 Ž1. 0.3782 Ž2. 0.3777 Ž2. –
0.1583 Ž2. 0.1845 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3778 Ž2. z Cu 2 rc
0.1583 Ž2. 0.1846 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3779 Ž2. z Cu2 rc
0.1582 Ž2. 0.1845 Ž2. 0.3562 Ž1. 0.3782 Ž2. 0.3778 Ž2. z Cu2 rc
0.1583 Ž2. 0.1845 Ž2. 0.3561 Ž1. 0.3783 Ž2. 0.3778 Ž2. 0.30 Ž1.
˚2 x B ŽY. wA ˚2 x Ž . w B Ba A
0.45 Ž2. 0.59 Ž2.
0.49 Ž2. 0.64 Ž2.
0.54 Ž2. 0.65 Ž2.
0.49 Ž3. 0.41 Ž3.
0.50 Ž3. 0.43 Ž3.
0.52 Ž3. 0.42 Ž3.
0.55 Ž3. 0.45 Ž3.
˚2 x B ŽCu1. wA ˚2 x Ž . w B Cu2 A
0.46 Ž2. 0.45 Ž2.
0.51 Ž2. 0.52 Ž2.
0.59 Ž2. 0.50 Ž2.
0.45 Ž3. 0.38 Ž3.
0.35 Ž4. 0.40 Ž3.
0.47 Ž3. 0.39 Ž3.
0.45 Ž3. 0.42 Ž3.
˚2 x B ŽO1. wA ˚2 x Ž . w B O2 A
0.68 Ž2. 0.59 Ž2.
0.75 Ž2. 0.62 Ž2.
0.80 Ž2. 0.65 Ž2.
0.83 Ž3. 0.62 Ž3.
0.85 Ž3. 0.63 Ž3.
0.66 Ž4. 0.62 Ž3.
0.65 Ž4. 0.64 Ž3.
˚2 x B ŽO3. wA ˚2 x Ž . w B O4 A x ŽO4.ra n O5 rŽ n O4 q n O5 .
0.55 Ž2. 0.62 Ž5. 0.034 Ž2. w0x
0.64 Ž2. 0.50 Ž5. 0.036 Ž2. w0x
0.62 Ž3. 0.58 Ž6. 0.038 Ž2. w0x
0.48 Ž3. 0.74 Ž9. 0.032 Ž2. 2% Ž1.
0.49 Ž3. 0.72 Ž9. 0.032 Ž2. 2% Ž1.
0.48 Ž3. 0.87 Ž9. 0.032 Ž2. 2% Ž1.
0.50 Ž3. 0.85 Ž9. 0.034 Ž2. 2% Ž1.
R Bragg w%x R wp w%x R f w%x
7.2 3.2
7.3 3.4
7.5 3.5
3.8 4.5 2.7
3.75 4.5 2.7
3.8 4.6 2.7
3.7 4.5 2.6
This experimental value is indeed an upper limit of y since, if some metallic sites happened to be vacant, considering them as fully occupied when writing the electrical neutrality of the crystal would lead to overestimation of the oxygen concentration. If, in the refinement, we fix this concentration at the experimental value by constraining nŽO4. q
nŽO5. to be equal to 0.85 Žstill assuming that all missing oxygens are chain oxygens., then the fit quality is significantly worse and the refined thermal factor of the chain oxygens, B ŽO4. s 0.32, is significantly smaller than that of the plane oxygens, B ŽO2. s 0.62 and B ŽO3. s 0.51, or that of the apical oxygen, B ŽO1. s 0.84. With x ŽO4. s 0, the fit qual-
126
F. Maury et al.r Physica C 333 (2000) 121–132
ity is worsened further and the refined value of B ŽO4., 0.72, is still smaller than that of B ŽO1., 0.86, and much smaller than the values found in the literature Žsee Refs. w17,18,21x and the more recent works of Villeneuve w24x and Casalta w25x.. The neutron data are thus hardly compatible with the assumption that the hole loss following the Li substitution is due to the loss of a chain oxygen. Since we know that in the O-samples, exactly one oxygen is lost per substitutional Li, we then assumed that the oxygen defect arises from the loss of the apical ŽO1. oxygen, which neighbours the substitutional Li and not from a missing chain oxygen. Fit 3 was calculated under this hypothesis Žwith all the Li ions on Cu2 sites.. It gives acceptable concentrations for both Li and oxygen, reasonable values for B ŽO1. and B ŽO4., and the same R Bragg value of 3.8% as fit 1. We see that the neutron data support the O1 vacancy hypothesis. They do not allow, however, to quite exclude that the oxygen vacancies be shared between the various oxygen sites. Yet the O1 vacancy hypothesis, besides yielding acceptable fits of the neutron data Žcorrect oxygen concentration and reasonable thermal factors., also explains the following experimental observations. Ži. Exactly one oxygen is lost per substitutional Li Žwhich leads to the loss and not to the a priori expected gain of one hole per Li.. Žii. We have observed in a previous work w3x that, within the experimental uncertainties, the oxygen defect does not depend on the atmosphere, either air or oxygen, of the 4808C anneal following the synthesis Žthis observation was made with A-samples, but we will see in Section 6 that A-samples behave as O-samples with regard to the oxygen defect.. Žiii. The c parameter does not increase as it should if oxygen defects were created in the chains. A number of studies of the YBa 2 Cu 3 O 7y d structure as a function of the oxygen deficiency have evidenced this increase. These studies yield the refined z values of the different ions as a function of d . If we look at these values, we see Žcf. for example, Ref. w18x or Ref. w25x. that as d increases, z ŽO1. decreases slightly while z ŽBa. increases significantly. This increase is progressively reduced for the upper Ž001. atomic planes, Cu2, O2 and Y, but is still positive for z ŽY. s cr2, leading to the observed
c increase. These changes are easily explainable by the removal of chain oxygens, if one neglects the variations of the a and b parameters which are much smaller than those of c for small values of the oxygen defect Žand which are zero for our O-samples.. Removing a chain oxygen Ž z s 0. suppresses its repulsive interaction with its two O1 neighbours ˚ neighbouring Ž001. atomic in the z s z ŽO1. f 1.85 A plane, and its attractive interaction with its two Ba ˚ plane Žnext neighbours in the z s z ŽBa. f 2.15 A Ž001. atomic plane.; the result, if one neglects the interactions between more distant neighbours, is a decrease of z ŽO1. and an increase of z ŽBa.. The z ŽCu2. increase is smaller than that of z ŽBa. due to the decrease of z ŽO1. and to the decrease of the mean charge on the Cu2 ions. If we now look at the z values obtained for sample 1, which do not depend on the fitting assumption ŽO1 or O4 loss; see Table 1., we see that they do not follow the above pattern. z ŽO1. and z ŽCu2. are as expected in the case of a mere chain oxygen loss, but z ŽBa., z ŽO2. Žand consequently z ŽY.. are smaller Žsee Fig. 3.. A simple explanation of this difference can be given if we assume that the lost oxygen is an apical oxygen ŽO1. and not a chain oxygen ŽO4.. Removing an apical oxygen suppresses its repulsive interaction with its two O4 neighbours as before: z ŽO1. is about the same in both cases. That also suppresses its attractive interaction with its
Fig. 3. Variation of the refined z coordinate of O1 Žlozenges., Ba Žcircles., Cu2 Žsquares. and O2 Žtriangles. as a function of the oxygen deficiency variation, in orthorhombic YBa 2 Cu 3 O 7y d . Open symbols: data taken from Refs. w18,25x. Full symbols: sample 1.
F. Maury et al.r Physica C 333 (2000) 121–132
four Ba neighbours, but this last interaction has a much smaller component in the c direction than the Ba–O4 interaction: the z ŽBa. increase is smaller for an O1 than for an O4 vacancy. That z ŽCu2. is about the same in YBa 2 Cu 3yx Li xO 7yx as in YBa 2Cu 3 O 7yx is due to the opposite effects of decreasing z ŽBa. and removing O1 Žthe mere substitution of a Liq and a hole for a Cu2 2q does not change the mean charge on the Cu2rLi2 ions and cannot counterpoise the effect of a reduced z ŽBa... z ŽO2., on the contrary, is decreased as a consequence of the suppression of the repulsive interaction between the missing O1 ion and its two O2 close neighbours. Again, this decrease is difficult to understand without the O1 removal. We see that the c decreases as well as the z variations of the different ions are understandable if we assume O1 vacancies, while they are not if we assume O4 vacancies. Although the above explanation neglects to consider the detailed YBaCuO electronic structure, we think that the observed c decrease is a strong argument in favour of missing apical oxygens rather than missing chain oxygens, from the moment when the Li is known to substitute for Cu2 and not Cu1. Now if the O1 site next to each substitutional Li is vacant, the Li may well be displaced from the Cu2 site. We checked this possibility by allowing the coordinate z Li2 to depart from z Cu2 . The refinement gave z Li2rc s 0.30 " 0.01 Žfit 4 of Table 1.. Yet the two fits, fit 3 with z Li2 s z Cu2 and fit 4 with z Li2 / z Cu2 , are almost equivalent: the neutron data do not really allow determination of the Li position. Finally, we checked that assuming one vacant O1 site per Li on a Cu2 site did not change the conclusion reached in Section 4 with regard to the Li location. We allowed the Li to occupy the two sites, Cu1 and Cu2. Neither the Li1 fraction, equal to 20%, ˚ 2, nor the Cu1rLi1 thermal factor, equal to 0.33 A was changed, compared to fit 2. The refined Li concentration was 0.14 " 0.02, at the upper limit of the acceptable values and, as already pointed out, the value of B ŽCu1., smaller than B ŽCu2., indicates that the fraction of Li1 was overestimated. We can thus conclude, from our neutron results together with previous results ŽNMR and cell parameter measurements. that, for ceramics synthesized in oxygen, Li substitutes for Cu in the CuO 2 planes and
127
that, most likely, each substitution entails the loss of the neighbouring apical oxygen.
6. Neutron results for sample 2 Let us now turn to the A-sample, YBa 2Cu 2.85 " 0.02 Li 0.18 " 0.02 O6.79 " 0.04 . The measured diffraction pattern for this sample is shown in Fig. 2b. The sample quality is not as good as that of sample 1. The measured diffraction pattern exhibits a background level that is twice as high as that of sample 1 for similar experimental conditions and signal level. Traces of green phase ŽY2 BaCuO5 . were detectable on the X-ray diagram, yet the quantity of green phase is small enough not to give rise to any clearly distinguishable peak in the diffraction spectrum of Fig. 2b. We started the data analysis as for sample 1 by assuming that all the Li were on Cu2 sites and found again that the refined oxygen concentration was too high compared to the experimental one. Indeed, one does not see the reason why, if this oxygen is lost in the O-samples sintered in oxygen, it should not be lost in the A-samples sintered in air. As for sample 1, the fit is worsened and the refined Li concentration much too small, without anything changed concerning the oxygen concentration and the oxygen thermal factors, when all the Li are assumed to occupy Cu1 sites. We thus assumed that for A-samples as for Osamples, the Li substitutes mainly for the Cu2 and that one apical oxygen is lost for each Li on a Cu2 site. Fit 3 of Table 2 was calculated with such a hypothesis and, as a first step, all the Li on Cu2 sites. When considering the corresponding values of the refined parameters, one notes two things: the ˚ 2 , is yttrium thermal factor, B ŽY. s Ž0.69 " 0.05. A larger than expected and the Cu2 thermal factor, ˚ 2 , smaller than expected, B ŽCu2. s Ž0.31 " 0.05. A since all the fits of Table 1 Žsample 1 as well as Ref. w21x. yield: 0.45 F B ŽY. F 0.55 and 0.4 F B ŽCu2. F 0.5. The high value of B ŽY. is likely to come from an overestimate of the yttrium site occupancy, nŽY., assumed equal to 1. We remember that traces of green phase were detectable by X-rays in this sample. If Y2 BaCuO5 is formed at the expense of
F. Maury et al.r Physica C 333 (2000) 121–132
128
Table 2 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for orthorhombic YBa 2 Cu 3yx Li x O y ŽA-samples. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit. YBa 2 Cu 3yx Li x O y Temperature
Sample 2 ŽA-sample. x s 0.18 " 0.02, y s 6.79 " 0.04, R exp s 2.5% 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
3 0.15 Ž2. w0%x 6.79 Ž5. 2 y n Li2
3b 0.17 Ž2. w0%x 6.77 Ž6. 2 y n Li2
4 0.19 Ž2. w0%x 6.75 Ž6. 2 y n Li2
5 0.18 Ž2. w25%x 6.77 Ž4. 2 y n Li
3 0.65 Ž1. w0%x 6.90 Ž3. 2 y n Li
5 w0.06x w20x% 6.91 Ž2. 2 y n Li
˚x a wA ˚x b wA
3.8281 Ž1. 3.8871 Ž1.
3.8282 Ž1. 3.8871 Ž1.
3.8281 Ž1. 3.8871 Ž1.
3.8281 Ž1. 3.8871 Ž1.
3.8226 Ž1. 3.8860 Ž1.
3.8226 Ž1. 3.8860 Ž1.
˚x c wA
11.6646 Ž5.
11.6647 Ž5.
11.6647 Ž5.
11.6646 Ž5.
11.6738 Ž3.
11.6737 Ž3.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽO3.rc z ŽLi2.rc
0.1574 Ž3. 0.1862 Ž3. 0.3570 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu 2 rc
0.1574 Ž3. 0.1861 Ž3. 0.3571 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu2rc
0.1574 Ž3. 0.1860 Ž3. 0.3569 Ž2. 0.3772 Ž3. 0.3780 Ž3. 0.29 Ž1.
0.1574 Ž3. 0.1863 Ž3. 0.3567 Ž2. 0.3772 Ž3. 0.3780 Ž3. z Cu2rc
0.1578 Ž2. 0.1849 Ž2. 0.3559 Ž1. 0.3787 Ž2. 0.3778 Ž2. z Cu2rc
0.1578 Ž2. 0.1849 Ž2. 0.3559 Ž1. 0.3787 Ž2. 0.3778 Ž2. z Cu2rc
nŽY. ˚2 x B ŽY. wA
w1x
0.97 Ž2. 0.59 Ž7.
0.97 Ž2. 0.60 Ž7.
0.98 Ž1. 0.64 Ž7.
w1x
w1x
0.56 Ž5.
0.61 Ž5.
0.57 Ž5.
0.58 Ž3. 0.43 Ž3.
0.57 Ž3. 0.42 Ž3.
0.66 Ž5. 0.31 Ž5.
0.70 Ž5. 0.30 Ž5.
0.66 Ž5. 0.36 Ž4.
0.46 Ž4. 0.40 Ž4.
0.51 Ž3. 0.36 Ž3.
0.44 Ž3. 0.40 Ž2.
0.72 Ž5. 0.55 Ž5.
0.72 Ž5. 0.56 Ž5.
0.69 Ž5. 0.60 Ž5.
0.79 Ž5. 0.57 Ž5.
0.77 Ž4. 0.66 Ž3.
0.80 Ž3. 0.65 Ž3.
˚2 x B ŽO4. wA x ŽO4.ra n O5 rŽ n O4 q n O5 .
0.51 Ž5. 1.1 Ž2. 0.040 Ž3. 5% Ž2.
0.55 Ž5. 1.1 Ž2. 0.040 Ž3. 5% Ž2.
0.57 Ž5. 1.1 Ž2. 0.041 Ž2. 5% Ž2.
0.54 Ž5. 1.0 Ž2. 0.040 Ž2. 4% Ž2.
0.50 Ž3. 0.84 Ž10. 0.035 Ž2. 4% Ž1.
0.49 Ž3. 0.80 Ž10. 0.035 Ž2. 4% Ž1.
R Bragg w%x R wp w%x R f w%x
6.2 4.4 4.25
6.15 4.3 4.3
6.1 4.3 4.2
6.1 4.3 4.2
4.05 4.5 2.8
4.05 4.5 2.8
˚2 x B ŽBa. wA ˚2 x Ž . B Cu1 wA ˚2 x B ŽCu2. wA ˚ Ž . w B O1 A2 x ˚2 x B ŽO2. wA ˚2 x Ž . w B O3 A
0.69 Ž5. 0.54 Ž5.
YBa 2 Cu 3 O 7 , this could result in a lack of yttrium in YBa 2 Cu 3 O 7 . Thus, we refined the Y site occupancy. ˚ 2 for nŽY. s B ŽY. was found equal to 0.59 " 0.07 A 0.97 " 0.02 Žfit 3b of Table 2.. The refined values of the other parameters were not much changed. In particular, B ŽCu2. was still low. It is slightly increased if one allows the Li2 ions to be displaced from the Cu2 sites Žfit 4.: B ŽCu2. s 0.36 Žinstead of 0.30. with z Li2rc s 0.29. Yet it remains low, indicating that the Cu2 site occupancy by Cu can be
Sample 4 ŽA-sample., x s 0.06 " 0.02, R exp s 2.9% 300 K X
X
underestimated and, consequently, the Li2 concentration overestimated. In the meantime, B ŽCu1. is high, whereas in undoped YBa 2 Cu 3 O 7 w21,24,25x and in sample 1, B ŽCu1. and B ŽCu2. are about equal. We are thus led to assume that a fraction of the Li may occupy Cu1 sites. Fit 5 of Table 2 was then calculated by assuming 25% of the Li on Cu1 sites. This fit is of the same quality as fit 3b with a Li concentration equal to the experimental one and reasonable values of the ther-
F. Maury et al.r Physica C 333 (2000) 121–132
mal factors. The refined value of y, 6.77 " 0.04, is now to be compared not to 6.79 but to 6.76 " 0.04, which is the experimental y value when the yttrium defect is taken into account. Similar results are obtained with 20% of the Li on Cu1 sites and z Li2rc s 0.29. If now a fraction of f 20% of the Li substitutes for Cu on Cu1 sites, and since for the A-samples, y s 7 y 1.2 x, this means that f 2 oxygens are lost per Li1. This is indeed sufficient to explain the observed variations of the cell volume if one assumes that these oxygens are chain oxygens. For x s 0.4, about 0.15 chain oxygen will be missing, which should result in an increase of the cell volume ˚ 3 w18x. This is effectively the difference of f 0.5 A observed between our A- and O-samples for this x value Žsee Fig. 1.. Yet the loss of two chain oxygens per Li1 does not explain why c is slightly smaller in A-samples than in O-samples since it should result in an increase of both V and c. That suggests that the corresponding defects might not be simply Liq ions on Cu1 sites between two missing chain oxygens. The concentration of Li1 is too small to enable the determination of their structure, that is, either to locate the missing oxygens or to measure a possible displacement of the substitutional Li1 off the Cu1 sites. We checked that none of the refined parameters is changed beyond its uncertainty limits when two O1 instead of two O4 are supposed to be missing per Li1, or when the Li1s are assumed to be displaced from the Cu1 sites, or when the thermal factor, B ŽLi1., is fixed at a value significantly larger than B ŽCu1.. If we now return to Table 2, we see Žfit 5. that the disorder in the Cu1 chains is greater for sample 2 than for sample 1. x O4 ra s 0.040 " 0.002 instead of 0.032 " 0.002, B ŽO4. s 1.0 " 0.2 instead of 0.87 " 0.09 and n O5 rŽ n O4 q n O5 . s 4% instead of 2%. A question then arises: Is the location of a fraction of the Li on Cu1 sites a mere consequence of the poorer quality of the sample or does it result from the sample elaboration conditions, different for this sample than for sample 1? To answer this question, we measured the diffraction spectra of a second A-sample: sample 4 ŽYBa 2 Cu 2.94 " 0.02 Li 0.06 " 0.02 O y ., which was of as good quality as sample 1 and had a smaller Li concentration. The data analysis was performed as
129
for samples 1 and 2. Fit 3X of Table 2 was calculated by assuming 100% Li2 and n O1 s 2 y n Li2 . We see that the Y and Ba thermal factors are the same as for sample 1, confirming the good quality of the sample, but that B ŽCu1. and B ŽCu2. are, as for sample 2, respectively, too high and too low, compared to sample 1. Thus, in sample 4 as in sample 2, a non-negligible fraction of the Li must occupy the Cu1 sites. Fit 5X of Table 2 was calculated by fixing x at the experimental value and the fraction of Li1 at 20%. The thermal factors are then comparable to those of undoped YBa 2 Cu 3 O 7 or of sample 1. We thus conclude that the presence of a fraction of the Li on Cu1 sites results from the synthesis conditions. It is indeed easy to understand that, if at least two oxygens are to be missing in order that one Li may substitute for one Cu1, such a possibility is more likely to occur when the synthesis is realized in air where the oxygen partial pressure is smaller by a factor 5 than in oxygen. The formation of the parasitic Y2 BaCuO5 phase, rich in oxygen, can also favour the Li1 sites. Small amounts of green phase were indeed observed by Kwei et al. w13x in samples containing large fractions of Li1. The greater disorder in the Cu1 chains, visible in samples 2 and 4, must stem from the presence of Li on Cu1 sites.
7. Neutron results for sample 3 The tetragonal sample, YBa 2 Cu 2.89 " 0.02 Li 0.09 " 0.02 O6.04 " 0.04 , synthesized in air, was of good quality and the background level of its diffraction spectrum ŽFig. 2c. was the same as that of sample 1 or 4. We started the refinement using the P4rmmm space group and, as for sample 1, values of the different parameters as already determined for the undoped material w22x. The chain oxygens were assumed to be slightly displaced from Ž0, 1r2, 0. in the x direction perpendicular to the chain axis. Their ˚ 2 . We thermal factor was fixed at B ŽO4. s 1.0 A checked that none of the refined values of the parameters is changed if one assumes B ŽO4. s 0.6 or 1.5, e.g., instead of 1.0. We first assumed Žfit 1 of Table 3. that all the Li ions are on Cu2 sites and that all sites are fully occupied except for the chain oxygen ones, most of
F. Maury et al.r Physica C 333 (2000) 121–132
130
Table 3 Structural parameters obtained by Rietveld refinement on neutron powder diffraction data for tetragonal YBa 2 Cu 3 O y ŽJohnston et al. w17x, Jorgensen et al. w18x. and YBa 2 Cu 3yx Li x O y Žthis work. Atom positions are Cu1 Ž0, 0, 0., O4 Ž x, 1r2, 0., O1 Ž0, 0, z ., Ba Ž1r2, 1r2, z ., Cu2 Ž0, 0, z ., O2 Ž1r2, 0, z . and Y Ž1r2, 1r2, 1r2.. Numbers in brackets correspond to fixed values. Numbers in parentheses following refined parameters represent one standard deviation in the last digit.
Temperature
Ref. w17x, y s 6.00, R exp s 2.8% 296 K
Ref. w18x, y s 6.09, R exp s 3.6% 300 K
Sample 3 ŽT-sample., x s 0.09 " 0.02, y s 6.04 " 0.04, R exp s 2.6% 300 K
Fit number x n Li1 rŽ n Li1 q n Li2 . y n O1
w0x – 6.13 Ž5. 2.01 Ž2.
w0x – 6.10 Ž3. 2.00 Ž2.
1 0.083 Ž6. w0%x 6.10 Ž1. w2x
3.8577 Ž2. 11.8274 Ž8.
3.8600 Ž1.
3.8595 Ž1.
3.8593 Ž1.
3.8595 Ž1.
˚x c wA
11.8168 Ž2.
11.8136 Ž2.
11.8137 Ž2.
11.8137 Ž2.
z ŽO1.rc z ŽBa.rc z ŽCu2.rc z ŽO2.rc z ŽLi2.rc
0.1528 Ž3. 0.1954 Ž3. 0.3608 Ž2. 0.3795 Ž2. –
0.1524 Ž2. 0.1946 Ž2. 0.3611 Ž1. 0.3795 Ž1. –
0.1522 Ž1. 0.1954 Ž1. 0.3622 Ž1. 0.3789 Ž1. z Cu 2 rc
0.1522 Ž1. 0.1954 Ž1. 0.3622 Ž1. 0.3789 Ž1. z Cu2rc
0.1522 Ž1. 0.1954 Ž1. 0.3621 Ž1. 0.3789 Ž1. z Cu2rc
˚2 x B ŽY. wA ˚2 x Ž . w B Ba A
0.54 Ž6. 0.55 Ž7.
0.42 Ž3. 0.50 Ž3.
0.53 Ž2. 0.40 Ž2.
0.58 Ž2. 0.44 Ž2.
0.58 Ž2. 0.46 Ž2.
˚2 x B ŽCu1. wA ˚2 x Ž . w B Cu2 A
0.97 Ž7. 0.39 Ž4.
0.91 Ž4. 0.37 Ž2.
0.93 Ž2. 0.41 Ž2.
0.97 Ž2. 0.40 Ž1.
0.89 Ž3. 0.41 Ž2.
˚2 x B ŽO1. wA ˚2 x Ž . w B O2 A
1.16 Ž9. 0.51 Ž5.
1.12 Ž2. 0.59 Ž1.
0.95 Ž3. 0.62 Ž1.
0.96 Ž3. 0.63 Ž1.
˚2 x B ŽO4. wA x ŽO4.ra
4 Ž3. w0x
w1.0x 0.086 Ž7.
w1.0x 0.084 Ž7.
w1.0x 0.085 Ž7.
R Bragg w%x R wp w%x R p w%x
5.2 4.0
3.4 4.25 3.4
3.3 4.2 3.35
3.2 4.2 3.35
YBa 2 Cu 3yx Li x O y
˚x a wA
0.62 Ž2. w2x w0x
8.0
which should be vacant. The refined Li concentration was x s 0.083 " 0.006, consistent with the experimental value. We then assumed that all the Li are on Cu1 sites. The refined Li concentration was x s 0.007 " 0.005, much too low, and the fit quality slightly worse. We can then conclude that, in this sample as in the orthorhombic ones, the Li is mainly on the Cu2 sites. If we now turn to the oxygen concentration, we see ŽTable 3. that the refined value, y s 6.10 " 0.01, is slightly too high compared to the experimental one. It is not evident whether, in this sample, one apical oxygen should be missing per Li on a Cu2 site, since the crystal structure is tetragonal and
3 w0.09x w0%x 6.01 Ž2. 1.91 Ž1.
5 0.12 Ž1. 17% Ž6. 6.01 Ž2. 1.92 Ž1.
since, in particular, its c-axis cell parameter is larger than in the orthorhombic samples. We thus fixed the Li concentration at the experimental value and refined the O1 site occupancy. The refined value is n O1 s 1.91 " 0.01 Žwhich corresponds exactly to one missing O1 per Li., when all the Li are assumed to occupy the Cu2 sites, with an oxygen concentration of 6.01 " 0.02, a value consistent with the experimental one. The neutron data thus favour the hypothesis of one missing apical oxygen for each substituted Li2, in tetragonal as well as in orthorhombic samples. That apical oxygen vacancies may be observed in tetragonal YBa 2 Cu 3 O y is supported by the work of Siddique w26x who reports measured values
F. Maury et al.r Physica C 333 (2000) 121–132
of y as low as 5.83, in vacuum annealed YBa 2 Cu 3 O y , and attributes them to the loss of apical oxygens. Finally, we assumed that in this tetragonal sample as in the orthorhombic ones, one apical oxygen is lost per Li2, and we allowed the Li to occupy both Cu1 and Cu2 sites. The best fit was obtained for 17% of Li1 Žfit 5 of Table 3. and x Li s 0.12 " 0.02. None of the refined parameters is changed beyond its uncertainty limits if the Li concentration is fixed at the experimental value, except for a slight increase of B ŽCu2.. To determine whether the Li1 fraction was fixed by the synthesis conditions or was liable to be modified by the anneal at 7508C, which transforms the structure from orthorhombic to tetragonal, we recently annealed sample 1 at 7508C in argon. ŽThis sample was annealed as a powder and not, like all other samples, as ceramics.. Preliminary measurements seem to indicate a non-zero fraction of Li1 in this sample, thus showing that the 7508C anneal in argon does modify the Li location. Before concluding, let us turn again to the cell parameters. As already mentioned, in the tetragonal samples, c is not found to vary regularly with the Li content Ž x . of the sample Žsee Fig. 1b. or with the chain oxygen content of the sample Žequal to ´ or G x q ´ , depending on which oxygens we assume to be missing. or with a combination of both. We interpret this as an indication that in these samples with large c values, c is strongly sensitive to all causes of decrease such as the presence of oxygens in the chains, of Li on Cu1 sites, etc., which can depend on the exact annealing conditions Žargon pressure, sample state: powder or ceramics, etc... 8. Conclusion The present neutron powder diffraction experiment has evidenced two things. Ž1. In Li-doped YBaCuO samples, synthesized in oxygen at f 9208C, the Li substitutes for the copper of the CuO 2 planes. When the samples are synthesized in air, about 20% of the Li substitutes for the copper of the Cu–O chains. In tetragonal samples subsequently annealed in argon at 7508C, this fraction seems to depend not only on the synthesis conditions but also on the annealing conditions.
131
Ž2. One apical oxygen is lost per Li on a Cu2 site and about two oxygens per Li on a Cu1 site. This feature does not seem to be modified by the transformation from the orthorhombic to the tetragonal structure. It allows us to explain fairly well the cell parameter variations with the Li content of the sample. In the tetragonal samples, it can also explain why the induced magnetic moment is found constant when calculated per Li ion q extra oxygen Ž x q ´ . and not per Li w11,16x. The sample magnetization would stem from the presence of oxygen ions in the Cu1 chains and not from the mere presence of Li in the CuO 2 planes. Up to f 415 K, YBa 2 Cu 3 O6q ´ is known to be antiferromagnetic w27x. At low doping level and at room temperature or below, the random substitution of Cu2 ions by nonmagnetic Li ions suppresses magnetic moments which are, at random, parallel or antiparallel to the magnetic field; it may also modify the magnetic moments of their Cu2 near neighbours without changing their antiferromagnetic state. Both effects will result in no variation of the measured sample magnetization, due to the defects in the Cu1 chains only. We can now revert to our starting question: Is the similarity between the magnetization of superconducting and insulating samples to be attributed to similar local structures around the substitutional Li ions in both types of materials or is it fortuitous? If, in the superconducting samples, the magnetic moments stem from Cu2 ions, near neighbours of substitutional Li2, and if, in the insulating samples, they stem from chain defects, the answer is that the coincidence is fortuitous. Finally, we remark that the loss of the apical oxygen may be one of the reasons why it is difficult to make samples with high Li concentrations. Never could we reach values of x larger than f 0.5, whereas values of 0.7 w22x or 1 w28x are reported in the literature for Fe or Co. Is this loss particular to Li or does the same happen with other dopants? The question is worthy to be asked. References w1x K. Sauv, J. Conard, M. Nicolas-Francillon, F. Bouree, ´ Physica C 273 Ž1996. 49. w2x J. Bobroff, Thesis, Universite´ Paris-Sud, Orsay, France, 1997.
132
F. Maury et al.r Physica C 333 (2000) 121–132
w3x M. Nicolas-Francillon, F. Maury, R. Ollitrault-Fichet, M. Nanot, P. Legeay, J. Appl. Phys. 84 Ž2. Ž1998. 925. w4x G. Xiao, F.H. Streiftz, A. Gavrin, Y.W. Du, C.L. Chien, Phys. Rev. B 35 Ž16. Ž1987. 8782. w5x S. Zagoulaev, P. Monod, J. Jegoudez, Phys. Rev. B 52 Ž14. ´ Ž1995. 10474. w6x P. Mendels, H. Alloul, G. Collin, N. Blanchard, J.F. Marucco, J. Bobroff, Physica C 235–240 Ž1994. 1595. w7x H. Alloul, P. Mendels, H. Casalta, J.F. Marucco, J. Arabski, Phys. Rev. Lett. 67 Ž22. Ž1991. 3140. w8x G. Xiao, M.Z. Cieplak, J.Q. Xiao, C.L. Chien, Phys. Rev. B 42 Ž13. Ž1990. 8752. w9x K. Ishida, Y. Kitaoka, K. Yamazoe, K. Asayama, Y. Yamada, Phys. Rev. Lett. 76 Ž3. Ž1996. 531. w10x G.D. Liu, Z.X. Zhao, G.C. Che, Solid State Commun. 109 Ž1999. 495. w11x M. Nicolas-Francillon, F. Maury, M. Nanot, R. OllitrautFichet, Solid State Commun. 109 Ž1999. 531. w12x M. Nicolas-Francillon, F. Maury, R. Ollitrault-Fichet, M. Nanot, Physica C 317–318 Ž1999. 579. w13x G.H. Kwei, K.C. Ott, E.J. Peterson, Physica C 194 Ž1992. 307. w14x K. Sauv, Thesis, University Paris VI, Paris, France, 1996. w15x M. Evain, U-Fit: A Cell Parameter Refinement Program, Laboratoire des Materiaux de Nantes, France, 1992. ´ w16x F. Maury, M. Nicolas-Francillon, M. Nanot, R. OllitraultFichet, J. Appl. Phys. 85 Ž2. Ž1999. 1002. w17x D.C. Johnston, A.J. Jacobson, J.M. Newsam, J.T. Lewandowski, D.P. Goshorn, D. Xie, W.B. Yelon, in: Chem-
w18x
w19x
w20x w21x
w22x w23x w24x w25x
w26x w27x
w28x
istry of High-Temperature Superconductors, AMS, Washington, DC, 1987, p. 136, Chapter 14. J.D. Jorgensen, B.W. Veal, A.P. Paulikas, L.J. Nowicki, G.W. Crabtree, H. Claus, W.K. Kwok, Phys. Rev. B 41 Ž4. Ž1990. 1863. R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck, L.W. Rupp Jr., Physica C Ž1990. 419. B.K. Vainshtein, V.M. Fridkin, V.L. Indenbom, Structure of Crystals, Springer-Verlag, Berlin, 1995. M. Franc¸ois, A. Junod, K. Yvon, A.W. Hewat, J.J. Capponi, P. Strobel, M. Marezio, P. Fischer, Solid State Commun. 66 Ž10. Ž1988. 1117. A.M. Balagurov, F. Bouree, ´ I.S. Lyubutin, I. Mirebeau, Physica C 228 Ž1994. 299. Q. Gao, Y.-S. Zhou, L.-Y. Zhang, Physica C 199 Ž1992. 121. R. Villeneuve, Thesis, University Paris XI, Orsay, France, 1996. H. Casalta, P. Schleger, P. Harris, B. Lebech, N.H. Andersen, R. Liang, P. Dosanjh, W.N. Hardy, Physica C 258 Ž1996. 321. R.K. Siddique, Physica C 228 Ž1994. 365. J. Rossat-Mignot, P. Burlet, M.J. Jugens, C. Vettier, L.P. Regnault, J.Y. Henry, C. Ayache, L. Forro, H. Noel, M. Potel, P. Gougeon, J.C. Levet, J. Phys. ŽFrance. 49 Ž12. Ž1988. 2119. B. Fisher, J. Genossar, L. Patlagan, G.M. Reisner, A. Knizhnik, J. Appl. Phys. 80 Ž2. Ž1996. 89816.