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Antiferromagnetism in Li substituted YBa2Cu3Oy studied by neutron powder diffraction measurements
Physica C 353 (2001) 93±102
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Antiferromagnetism in Li substituted YBa2Cu3Oy studied by neutron powder diraction measurements F. Maury a,*, M. Nicolas-Francillon b, I. Mirebeau c, F. Bouree c b c
a Laboratoire des Solides Irradi es, Ecole Polytechnique, F-91128 Palaiseau C edex, France Laboratoire de Physique du Solide, ESPCI, 10 rue Vauquelin, F-75231 Paris C edex 05, France Laboratoire L eon Brillouin (CEA-CNRS), CEA/Saclay, F-91191 Gif sur Yvette C edex, France
Received 21 July 2000; received in revised form 2 November 2000; accepted 3 November 2000
Abstract The magnetic structure of two well characterised YBa2 Cu3 x Lix Oy samples, with similar Li concentrations, x 0:08±0:09, and slightly dierent oxygen concentrations, y 6:06 and y 6:18, has been studied by neutron powder diraction measurements. The lithium substitution is found to induce a strong decrease of the Neel temperature and of the mean ordered magnetic moment on the Cu2 sites. TN is 245 K for y 6:06 instead of 415±400 K in unsubstituted YBa2 Cu3 Oy with y 6:0±6.2. The mean ordered moment at low temperature, l0 =lB , is 0:33 0:03 for y 6:06, and 0:17 0:08 for y 6:18, instead of 0.64 in unsubstitued YBCO. These measured values are compared with other data from the literature and the various possible mechanisms of the strong lithium eect are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Bk Keywords: Neutron diraction; High-Tc compounds; Antiferromagnetism
1. Introduction This work belongs to an extensive study of the conduction and magnetic properties of lithium substituted YBCO. This study was initiated with the object of providing a set of data which would allow a better understanding of the link between superconductivity and magnetism in these materials. It has been shown by NMR measurements [1] that the Li substitution induces in the metallic * Corresponding author. Tel.: +33-1-6933-4502; fax: +33-16933-3022. E-mail address: [email protected] (F. Maury).
phase, localised magnetic moments on the Cu ions in the vicinity of the Li. The static magnetisation of superconducting as well as insulating samples [2±4] has been measured as a function of the temperature and of the magnetic ®eld for various Li concentrations up to x 0:4. We have shown [5] that Li substitutes mainly for the Cu of the CuO2 planes (Cu2) when the samples are synthesised under oxygen at T 900±920°C, while about 20% of the Li substitutes for the Cu in the Cu±O± chains (Cu1) when the samples are synthesised at the same temperature in air. The present experiment was aimed at measuring the Li eect on the antiferromagnetic (AF) structure of the insulating samples; we also determined
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 7 3 7 - 8
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the residual impurities present in our samples which may participate in the magnetisation measured at low temperature. We brie¯y recall in Section 2 our experimental conditions. We present and discuss in Section 3 the sample characterisation (residual impurities, lithium and oxygen concentration), in Section 4 the sample magnetisation measured at low temperature, in Section 5 the crystallographic structure of the samples (lithium and oxygen location) and in Section 6 the AF order determined by neutron powder diraction measurements. 2. Experimental Our samples (samples 1 and 3 of Ref. [5]) were prepared by the usual solid state reaction from Y2 O3 , BaCO3 , CuO and Li2 CO3 at 920°C. Sample 1 was synthesised in oxygen and annealed in oxygen at 480°C. Just before the present experiment, it was annealed in argon at 750°C and is thereafter called sample 1b. Sample 3 was synthesised in air and directly annealed in argon at 750°C. Both the Li and the Cu content were determined by emission plasma spectrometry analysis. The Li molar concentration so determined is called ``experimental'' value of x and is xexp 0:115 0:02 for sample 1/1b and xexp 0:09 0:02 for sample 3. The hole content of the samples was determined by iodometric titration and the oxygen content deduced from electrical neutrality. The oxygen molar concentration so determined is called ``experimental'' value of y and is yexp 6:18 0:04 for sample 1b, 6:04 0:04 for sample 3, ®rst measurement, and 6:07 0:04 for the same sample, one year later. The magnetic measurements were performed with the superconducting quantum interference device (SQUID) of the Laboratoire de Physique du Solide de lÕEcole Normale Superieure (Paris). The detailed crystallographic structure of the samples was determined by neutron powder diraction measurements carried out at room temperature on the high resolution powder diractometer 3T2 of the Laboratoire Leon Brillouin with The data were analysed using the k 1:2251 A. Rietveld method. Lattice parameters, atom coordinates, oxygen and Cu/Li site occupancies, iso-
tropic thermal factors, peak shape and width, preferred orientation, scale factor and background were re®ned. The magnetic structure of the samples was determined by neutron powder diraction measurements carried out at various temperatures between 10 and 300 K on the high ¯ux diractometer G6-1 of the Laboratoire Leon Brillouin with k 4:738 The data were analysed using the Rietveld A. method. Only lattice parameters, atom coordinates, peak shape and width, scale factor and mean ordered magnetic moment on the Cu2 sites were re®ned. Site occupancies and thermal factors were taken as determined by the 3T2 experiment. 3. Sample characterisation 3.1. Residual impurities Fig. 1 shows the G6-1 neutron diraction spectra measured at 10 6 T 6 100 K for sample 3 (Fig. 1a) and for sample 1b (Fig. 1b). The scale is much enlarged so as to allow to see distinctly all the impurity peaks that emerge from the background in the domain 50° < 2h < 75°. These peaks are of the same width at half maximum than the YBCO peaks and of much smaller height: the YBCO (0 0 1) peak reaches a maximum intensity of 2250 for sample 1b and 2600 for sample 3 with the units of Fig. 1. All the peaks which are distinguishable from the background have been indexed. They all belong to the three following residual impurities: Ba44 Cu48 (CO3 )6 O81x (hereafter called ``BaCuO2 '') [6], Li2 CuO2 [7] and LiCu2 O2 [8]. The ``BaCuO2 '' unit cell is large and many small diffraction peaks are observed; on the contrary the Li2 CuO2 unit cell is small so that but a few larger peaks are observed. For each of these three impurities, we have calculated, with a gaussian ®t of the diraction peaks, the ratio of the total integrated intensity of the impurity peaks to that of the YBCO peaks. This ratio, fimp , should not be much dierent from the volumic fraction of the impurity present in the sample. Its values are listed in Table 1. We ®nd that sample 3 contains about 0.4% of ``BaCuO2 '' and 0.3% of LiCu2 O2 and that sample 1b contains
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Fig. 1. Neutron diraction data measured with the G6-1 spectrometer. The triangles indicate the position of the (1/2 1/2 l) YBCO peaks with l 0, 1, 2. (a) Sample 3: solid line: 10 6 T 6 100 K; dotted line: T 300 K. (b) Sample 1b: solid line: 10 6 T 6 100 K; dotted line: 150 6 T 6 170 K.
Table 1 Sample characteristics Sample
Synthesis
Anneal
fBaCuO2 /%
fLi2 CuO2 /%
fLiCu2 O2 /%
1b 3
Oxygen 920°C Air 920°C
Oxygen 480°C argon 750°C Argon 750°C
1:5 0:7 0:4 0:2
1:0 0:4 ±
0:12 0:04 0:3 0:1
about 1.5% of ``BaCuO2 '', 1.0% of Li2 CuO2 and 0.1% of LiCu2 O2 . Besides, we observe on the left side of the (0 0 1) YBCO peak of sample 3, a large shoulder which can be split into two broad peaks, centred at 1 (2h 20:0°) and qmax 0:516 qmax 0:460 A 1 (2h 22:4°). Their total integrated intensity A amounts to 4 % of that of the YBCO peaks. This structure is similar to what has been observed in YBa2 Cu3 Oy annealed in air or in a mixture of Ar and H2 O at 125°C [9]. It results from the diusion and absorption of H2 O on the vacant oxygen chain sites. The ®rst peak evinces the local phase transformation induced by this absorption: the c parameter is increased. The second one most likely stems from very small crystallites of Y2 BaCuO5 . That this structure is observed in sample 3 and not in sample 1b, is easily explainable: sample 3 was elaborated for the 3T2 experiment, that is about eight months before the G6-1 experiment, while
sample 1b was annealed in argon at 750°C, just before the G6-1 experiment. 3.2. Lithium concentration The presence of lithium±copper oxides in our samples results in an overestimate of the Li concentration. Assuming a volumic fraction of Li2 CuO2 equal to fLi2 CuO2 , i.e. equal to 1:0 0:4% in sample 1/1b, we then derive from the spectrometry analysis xexp 0:04 0:035 instead of 0.115. This value is to be compared to the re®ned value, xcalc 0:09 0:01, obtained from the neutron diraction analysis of sample 1 [5]. Besides, this analysis has shown that in sample 1, all the Li is on Cu2 sites. According to the NMR analysis of Bobro et al. [10], we can deduce the Li concentration in the CuO2 planes from the measured superconductivity critical temperature. We measured, for sample 1, Tc 67 K and deduced
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x 0:07±0.08. The Li concentration in sample 1/1b can thus be taken equal to x 0:08 0:01. The Li concentration in sample 3 is changed only slightly due to the presence of a small fraction of LiCu2 O2 . Assuming a volumic fraction of LiCu2 O2 equal to fLiCu2 O2 (0.3%), it becomes xexp 0:08 0:02 instead of 0:09 0:02. We conclude that the Li concentration is about equal, within the experimental uncertainties, in these two samples, 1/1b and 3. 3.3. Oxygen concentration The oxygen content of sample 3, yexp 6:04 0:04, was measured just before the 3T2 experiment [5]. It was remeasured one year later and was yexp 6:07 0:04. The G6-1 experiment was realised in the interval between: we can thus consider that the oxygen content of sample 3 is yexp 6:06 0:04 for the G6-1 experiment. The oxygen content of sample 1b, yexp 6:18 0:04, was measured just before the G6-1 experiment shortly followed by the 3T2 experiment for this sample. We checked that the presence of residual impurities could not change signi®cantly these values. The iodometric titration yields a measure of the excess of charge on the Cu ions compared to Cu . This excess is 4 10 3 F =g for ``BaCuO2 '' and 9:1 10 3 F =g for Li2 CuO2 instead of 3:1
10 3 F =g for YBa2 Cu3 O6 or 6:0 10 3 F =g for YBa2 Cu3 O7 , where F is the Faraday constant. The calculation of the corresponding correction shows that it is negligible compared to the experimental uncertainty on yexp . If we consider, for example, Li2 CuO2 in sample 1b, the overestimate of xexp due to presence of the impurity, leads to an underestimate of yexp which exactly balances the overestimate resulting from the large charge excess per gram of Li2 CuO2 as compared to YBa2 Cu3 O6 . The formula of sample 1b is then: YBa2 Cu2:890:02 Li0:080:01 O6:180:04 , and that of sample 3: YBa2 Cu2:890:02 Li0:090:02 Oy , with y 6:04 0:04 for the 3T2 experiment and y 6:06 0:04 for the G6-1 experiment. 4. Sample magnetisation 4.1. Experimental results Fig. 2 shows the sample magnetisation measured for sample 1b, as a function of the magnetic ®eld at various temperatures between 2 and 30 K. Similar curves are obtained for sample 3. These M B curves are similar to those already measured for similar samples, with xexp ranging from 0.0 to 0.39 [4]. The solid lines are ®ts calculated with
Fig. 2. Magnetisation of sample 1b measured at T 2, 4.5, 8, 15 and 30 K. The lines are best ®ts to the data calculated according to Eq. (1).
F. Maury et al. / Physica C 353 (2001) 93±102
M vlin B Nm1= tanh mB=kT
1= mB=kT 1
where vlin , N and m are three adjustable parameters, the signi®cation of which is discussed in Ref. [4]. The value of vlin is the same for both samples; it decreases linearly with the temperature (see Fig. 4). The value of the magnetic moment m is also equal for both samples: m 13 2lB , where lB is the Bohr magneton. N, the number of those moments in a Langevin model, is larger for sample 1b than for sample 3. Yet the ratio N=NLiO , where NLiO is the number of Li ions extra oxygen ions, is about the same for both samples, as already observed [4]: it is 4:5 10 3 for sample 1b and 4±5:5 10 3 for sample 3. All these values are comparable to those deduced in Ref. [4]. Fig. 3 shows the magnetisation measured for sample 1b, at B 1 T, as a function of temperature. The two components of M according to Eq. (1), Mlin vlin B and ML Nm1= tanh mB=kT 1= mB=kT are indicated separately. 4.2. Discussion Considering that the magnetic moments m stem from superparamagnetic particles made of small
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regions of ferromagnetically ordered moments on Cu2 sites, would lead to a volumic fraction of these regions of a few 10 3 . This is a very small fraction suggesting that ML could be due to residual impurities. The three compounds ``BaCuO2 '', Li2 CuO2 and LiCu2 O2 contain one Cu2 ion per unit formula. In Li2 CuO2 and LiCu2 O2 as in the green phase Y2 BaCuO5 , the spin interaction between the Cu2 ions is antiferromagnetic with an exchange energy, J =k 43 K for Li2 CuO2 [7] and 35 K for LiCu2 O2 [8]. One can check that these two impurities contribute but to a negligible extent to the magnetisation measured below 50 K. ``BaCuO2 '' has been reported as being paramagnetic down to a few Kelvin [11,12]. It can contribute but to a minor part of Mlin since Mlin remains nearly constant from 2 to 30 K. On the contrary, ML can be ®tted quite well, as shown in Fig. 3, with a volumic fraction of ``BaCuO2 '' equal to 2.3% for sample 1b (1.0% if the data for ``BaCuO2 '' are taken from Ref. [12] ). This volumic fraction is 1.0% (0.4%) for sample 3. These values are quite comparable to the values of fBaCuO2 determined in Section 3.1. Moreover if ML stems from residual ``BaCuO2 '', then the magnetisation of ``BaCuO2 '' at low temperature must be the same Langevin function of the magnetic ®eld as ML . This is eectively what is observed [13]. It is thus very likely that the intrinsic magnetisation of our samples is Mlin only. If now we subtract the impurity contributions from the M T data, the eective magnetic moment per Li O, deduced from a Curie ®t in the range 100 < T < 300 K, is 0.9lB for both samples, a value not much smaller than that (1.1lB ) deduced previously [3] .
5. Crystallographic structure 5.1. Oxygen location Fig. 3. Magnetisation of sample 1b measured at B 1 T, either in the M B experiment or in a M T experiment. The solid line is a Curie±Weiss ®t to the saturating part of M, ML , corresponding to 2.3 vol% of ``BaCuO2 '' according to the data of Ref. [11].
After a ®rst neutron diraction measurement on the 3T2 spectrometer, sample 1 was annealed, as a powder, at 750°C in argon. After the G6-1 experiment, its neutron diraction spectrum was
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slightly smaller for sample 1b than for sample 3: xcalc 0:085 0:01 for sample 1b instead of 0:10 0:01 for sample 3. Yet the dierence remains within the experimental uncertainties. The only signi®cant dierence between the re®ned parameters of these samples (see Table 2) is observed for the oxygen concentration. Its re®ned value is ycalc 6:17 0:03 for sample 1b and 6:05 0:03 for sample 3. These values are remarkably equal to the ``experimental'' ones: yexp 6:18 0:04 and 6:04 0:04, respectively. The apical oxygen site occupancy is n O1 1:90 0:02 and 1:91 001, for samples 1b and 3, so that about one apical oxygen is missing per substituted Li. If we now ®x n O1 2, so that the O1 sites be fully occupied, then (see Table 2) ycalc is 6:25 0:02 and 6:12 0:02, for samples 1b and 3, respectively, larger than the experimental values, and the ®t quality is slightly worsened. If we ®x both n O1 2 and n O4 yexp 6, so that the oxygen concentration be equal to the experimental value, then the thermal factor of the oxygen chain sites is much too small: B O4 0:1 0:4 for sample 1b, and even negative: B O4 1:4 0:08 for sample 3, which is an artefact of the program due to a too small constrained value of n(O4). We see that the neutron data seem to indicate that in tetragonal as in orthorhombic samples, one
Fig. 4. Neutron diraction data measured at room temperature with the 3T2 spectrometer for sample 1b. The points are the experimental data, the lines are the calculated pro®les. The lower curve represents the dierence between the two.
remeasured on the 3T2 spectrometer. The data (Fig. 4) were analysed with the Rietveld method [14] as follows (for more details see Ref. [5]). The space group of tetragonal YBCO is P4/mmm and c the lattice parameters are a b 3:8590 A, 11:8034 A for sample 1b and a b 3:8595 A, c 11:8137 A for sample 3. We start by assuming that all the Li is on Cu2 sites. Then the Li re®ned concentration is found
Table 2 Structural parameters obtained by Rietveld re®nement on neutron data for tetragonal YBa2 Cu3 x Lix Oy a xcalc
% Li1
ycalc
n(O1)
n(O4)
Sample 1b (xexp 0:08 0:01; yexp 6:18 0:04; 0.08 (1) [0] 6.17 (3) 1.90 (2) 0.27 (2) 0.07 (1) [0] 6.25 (2) [2] 0.25 (2) 0.07 (1) [0] [6.18] [2] [0.18] 0.11 (2) 23 (7) 6.16 (3) 1.90 (2) 0.26 (2) [0.08] [30] 6.18 (3) 1.92 (2) 0.26 (2) Sample 3 0.10 (1) 0.08 (1) 0.08 (1) 0.11 (1) [0.09]
(xexp 0:09 0:02; [0] 6.05 (2) [0] 6.12 (2) [0] [6.04] 12 (4) 6.03 (2) [15] 6.04 (2)
B(Y)
B(Ba)
Rexp 4:26 %) 0.63 (3) 0.51 0.58 (3) 0.47 0.57 (3) 0.48 0.64 (3) 0.54 0.61 (3) 0.51
yexp 6:04 0:04; Rexp 2:58%) 1.91 (1) 0.14 (2) 0.58 (2) 0.45 [2] 0.12 (2) 0.53 (2) 0.41 [2] [0.04] 0.53 (2) 0.40 1.90 (1) 0.13 (2) 0.58 (2) 0.46 1.92 (1) 0.13 (2) 0.56 (2) 0.44
B(Cu1)
B(Cu2)
B(O1)
B(O2)
B(O4)
RB (%)
(3) (3) (3) (3) (3)
0.95 0.91 0.91 0.83 0.81
(3) (3) (3) (4) (3)
0.44 0.45 0.45 0.46 0.51
(3) (3) (3) (3) (2)
0.93 1.11 1.10 0.95 0.95
(5) (3 (3 (5) (5)
0.65 0.61 0.62 0.67 0.64
(2) (2) (2) (2) (2)
1.7 1.5 0.1 1.5 1.4
3.90 4.02 4.09 3.79 3.83
(2) (2) (2) (2) (2)
0.96 0.93 0.92 0.91 0.88
(2) (2) (2) (3) (2)
0.40 0.41 0.41 0.41 0.45
(2) (2) (2) (2) (1)
0.95 1.12 1.12 0.96 0.96
(3) (2) (2) (3) (3)
0.62 0.59 0.59 0.63 0.61
(1) (1) (2) (2) (1)
4 (2) 3 (2) )1.4 (8) 3 (2) 3 (2)
(6) (6) (4) (6) (5)
3.26 3.44 3.49 3.21 3.24
a 2 ). Numbers in brackets corOapex O1 (0,0,z), O2 (0.5,0,z), O3 (0,0.5,z), Ochain O4 (x,0.5,0), Cu1 (0,0,0), Cu2 (0,0,z); (B in A respond to ®xed values. Numbers in parentheses following re®ned parameters represent one standard deviation in the last digit. Figures in bold characters correspond to the ®t we consider the best.
F. Maury et al. / Physica C 353 (2001) 93±102
apical oxygen is lost per substituted Li. Yet we want for an independent argument to con®rm this result which is based but on rather small dierences in the ®t quality. 5.2. Lithium location If we now allow the Li to occupy both Cu1 and Cu2 sites, the only parameters which are changed beyond their uncertainty limits are the number of Li on Cu1 sites, n Li1, and the Cu1 thermal factor, B Cu1. The re®ned Li concentration, xcalc n Li1 n Li2, increases with n Li1. Its re®ned value is xcalc 0:11 0:10 0:02 with 23 28 7% of Li1 for sample 1b and xcalc 0:12 0:10 0:01 with 11 14 4% of Li1 for sample 3, where the values in parentheses are calculated with n O1 2 (see Table 2). These values are at the upper limit of xexp . To check the reliability of this determination, we then ®xed xcalc at 0.08 and the fraction of Li1 at dierent values between 0% and 40%, and we compared the re®ned values of the thermal factors of the Cu1 and Cu2 sites with their values in the literature for unsubstituted tetragonal YBa2 Cu3 Oy [15±17]. B(Cu1) is much larger in the tetragonal structure where the oxygen chain sites are vacant and the c axis cell parameter is larger, than in the orthorhombic one, and it is an increasing function of c. Taking this increase into account and comparing our B(Cu1) and B(Cu2) values with expected values, we deduced the following values for the fraction of Li1: 29 (22)% for sample 1b and 14 (7)% for sample 3. These values are about the same as above. The former 3T2 experiment [5] has shown that in sample 1, orthorhombic, all the Li was on Cu2 sites. The present experiment shows that the annealing in argon at 750°C, has modi®ed the Li distribution on the Cu sites. This distribution is as if sample 1b had been synthesised in air and not in oxygen. Finally, the 3T2 experiment seems to indicate that, beside their dierent oxygen content, samples 1b and 3 also dier by their fraction of lithium on Cu1 sites, which is about twice as large in sample 1b as in sample 3.
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6. Magnetic structure 6.1. Experimental results Fig. 1a shows the neutron diraction spectrum measured for sample 3 (x 0:09, y 6:06 in the G6-1 experiment) in the range 50° < 2h < 75°. It displays, together with the various impurity peaks, two diraction peaks which can be indexed (1/2 1/2 1) and (1/2 1/2 2). These peaks which are clearly visible at low temperature, arise from the doubling of the magnetic unit cell along the a and b axes, due to the AF order in the CuO2 planes. They are no longer observed at T 300 K, which shows that they do stem from magnetism and not from residual impurities and that the Neel temperature is lower than 300 K. For sample 1b (see Fig. 1b), these peaks are hardly visible at low temperature and at 150±170 K, they are hidden in the background. The mean ordered magnetic moment on the Cu2 sites was deduced from a Rietveld ®t of the data for 40° < 2h < 100°. (The ®t to the experimental data obtained for sample 3 at T 10 K is displayed in Fig. 5.) The diraction spectrum measured for the sample holder without sample was subtracted from
Fig. 5. Neutron diraction data measured at T 10 K with the G6-1 spectrometer for sample 3. The points are the experimental data, the line is the pro®le calculated with l=lB 0:33.
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6.2. Discussion
Fig. 6. Square of the mean ordered magnetic moment on Cu2 sites determined either by a Rietveld ®t of the neutron data or by a Gaussian ®t of the (1/2 1/2 1) peak. The solid line is calculated with l0 =lB 0:35, TN 242 K and b 0:25.
that measured with the sample. This increases the dispersion on the data points but is necessary since the sample holder spectrum exhibits two peaks (of maximum height 3.5 with the units of Fig. 1) centred at 2h 71:7° and 73.4°, i.e. on each side of the (1/2 1/2 2) magnetic peak which is centred at 2h 72:5°. Fig. 6 shows the re®ned value of (l=lB )2 as a function of the temperature. It is l0 =lB 0:33 0:03 at low temperature, about constant up to T 100 K. No reliable re®ned value of l could be deduced above 200 K. Fig. 6 also shows the integrated intensity of the (1/2 1/2 1) peak, deduced from a gaussian ®t of both YBCO magnetic (1/2 1/2 1) peak and ``BaCuO2 '' (3 2 1) peak in the range 56° < 2h < 58:5°, and normalised to the value of (l=lB )2 at T 10 K. We see that the Neel temperature for sample 3 is TN 245 15 K. For sample 1b, the re®ned value of l=lB is 0:17 0:08 at 10 6 T 6 100 K and 0:05 0:3 at 150 6 T 6 170 K. It is 0:10 0:11 at 150±170 K, from a gaussian ®t of the (1/2 1/2 1) peak. For an unknown reason, the background increased at the end of the sample 3 experiment and is higher for sample 1b than for sample 3, so that no reliable value of l can be obtained for sample 1b above 100 K. Its Neel temperature is TN 150 30 K.
If we compare our results with those in the literature for unsubstituted YBa2 Cu3 Oy [18,19], we see that the Li substitution induces a strong reduction of both l0 and TN , since in unsubstituted YBa2 Cu3 Oy , l0 0:64 0:03 and TN 415 5 K as long as y 6 6:15. The Li eect is much stronger than that of Zn which yields but a small reduction of l and TN , compatible with a mere dilution effect: l=lB 0:62 and TN 350 K for YBa2 Cu2:92 Zn0:08 O6:2 , i.e. for the same x and y concentrations as sample 1b [20]. Moreover, if we compare Fig. 6 with the results of Rossat±Mignot et al. for unsubstituted YBa2 Cu3 Oy with various oxygen dopings (Fig. 3 in Ref. [19]), we see that not only the TN value but also the l value as well as the shape of the l2 T curve are similar for sample 3 and for unsubstituted YBa2 Cu3 Oy with 6:33 < y < 6:36. The observed decrease of the 3D AF order in YBa2 Cu3 Oy with increasing y, stems from hole doping. We know that in unsubstituted YBa2 Cu3 Oy with 6 < y < 6:2, the doped holes are localised on the Cu1 neighbours of the chain oxygens [21] and are of nearly no eect on the AF order [18,19]. The destruction of the 3D AF order appears around y 6:2, when a few mobile holes begin to be created in the CuO2 planes. Muon spin rotation (lSR) measurements on Y1 x Cax Ba2 Cu3 O6 and La2 x Srx CuO4 [22] have indeed shown that the strength of the AF correlations is determined by the planar hole content and does not depend on the concentration of dopant atoms. In Y0:94 Ca0:06 Ba2 Cu3 O6:02 , a concentration of in-plane holes of 3% is sucient to reduce TN to 170 K or less. The similarity between our results for sample 3 and those for YBa2 Cu3 O6:35 [19] seems to indicate that the eect of the Li substitution is a mere hole doping eect. One can thus simply assume that these holes are introduced in the CuO2 planes by the substitution of Li for plane Cu22 while the substitution of Li for chain Cu1 does not modify the hole concentration. Neither does the substitution of Zn2 for Cu22 . This readily explains the observed dierence between Li and Zn. Are these in-plane holes brought in by the Li2 ions, mobile? If we look up in the literature if they
F. Maury et al. / Physica C 353 (2001) 93±102
have to be mobile to aect the 3D AF order, we get various answers. lSR experiments on Y1 x Cax Ba2 Cu3 O6 and La2 x Srx CuO4 [22±24] show that the doped holes become frozen (at the time scale of the lSR technique, sc < 10 6 s) around a transition temperature which increases with x (and is 15 K for Y0:94 Ca0:06 Ba2 Cu3 O6:02 ), and that small concentrations of frozen holes do not aect signi®cantly the mean ordered magnetic moment. Yet, in YBa2 Cu3 Oy (unsubstituted as well as Zn substituted), neutron scattering measurements [19,25] evince, below 15 K, a re-entrant behaviour of the mean ordered magnetic moment, which is attributed to the decrease of the 3D AF order by the static magnetic disorder due to the localisation of the in-plane holes. Finally, neutron scattering measurements on Sr and Li substituted La2 CuO4 [26] show that 5% of Li on the Cu sites eliminate the 3D AF transition completely. The doped holes created by the substitution of Li for Cu2 are inplane holes, like for the Sr doping; but, as the Li substitution, contrary to the Sr one, suppresses the superconductivity, they are not mobile but rather localised in-plane holes. To conclude, we see that mobile holes and possibly localised holes, hinder the 3D AF order, provided they belong to CuO2 planes. Our results for sample 3 can thus be simply interpreted by assuming that the substitution of Li for Cu2 create in-plane holes (``Li2-holes'') and that these holes, in number of n(Li2), are responsible for the suppression the 3D AF order, without the experiment showing if they are mobile or not. This simple model (model 1) implies that, notwithstanding the 3T2 results, we consider that the Li substitution does not entail any loss of apical oxygen. Then it gives a straightforward explanation of the 3D AF order destruction in sample 3. Yet we note that a concentration of 4% of ``Li2holes'' in the CuO2 planes, in sample 3, is not suf®cient to completely suppress the 3D AF order but yields TN 245 K, while 3% of in-plane holes in Y0:94 Ca0:06 Ba2 Cu3 O6:02 , reduce TN down to 6 170 K [22]. If on the contrary, we rely on the 3T2 results, we have to admit that the Li substitution goes with an apical oxygen removal. In our samples where
101
y > 6, the missing apical oxygens are replaced by extra chain oxygens and the holes brought in by the substitution of Li for Cu22 are likely to be localised not in the CuO2 plane but on the near-by Cu1s (that will become Cu12 ). Such holes, as in YBa2 Cu3 Oy with y 6 6:15, should not aect much the 3D AF order. Then, if the strong decrease of TN is due to in-plane holes, these holes have to stem from extra chain oxygens coming near a Cu12 or a Li1 , rather than be ``Li2-holes''. In this hypothesis (model 2), TN should be much more sensitive to a small concentration of extra oxygen than in unsubstituted YBa2 Cu3 O6 . This brings us to compare sample 1b with sample 3 and ask why l and TN are much smaller in sample 1b, with n Li2 0:06 than in sample 3 with n Li2 0:08, given that y is less than 0.2 in both samples. The dierence between the two samples can only come from their dierent oxygen concentration. In model 1, there are, in sample 1b, 2n Li1 0:04 oxygen chain sites where an extra oxygen (sitting near a Li1) can free a mobile hole, and n O4 0:18 extra oxygens per unit formula, while these numbers are 2n Li1 0:02 and n O4 0:06 in sample 3. This dierence can be sucient to explain the dierent TN s. Yet, if we compare sample 1b with Y0:94 Ca0:06 Ba2 Cu3 O6:02 [22], we expect the eect of the chain oxygens in sample 1b to be small, since both samples have similar TN s and the same in-plane hole concentration. In model 2, the in-plane holes responsible for the strong decrease of the 3D AF order are all created by extra oxygens (in concentration y)6) sitting on chain sites near a Cu12 or a Li1 . The concentration of such sites is small, 6 0.10 per unit formula in both samples. Chain oxygen could be retained preferentially in such sites. In this case, the mean ordered magnetic moment, l T , should depend on y even for small values of y, contrary to the case of unsubstituted YBa2 Cu3 Oy . 7. Summary The present experiment has evidenced a strong eect of substituting lithium for copper in YBa2 Cu3 x Lix Oy . For x 0:09 and y 6:06, we
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measured a mean ordered magnetic moment on the Cu2 sites of 0:33 0:03lB at low temperature, with a Neel temperature of 245 15 K. This moment is 0:17 0:08lB for about the same x and y 6:18. These results can be simply interpreted by considering that the Li ions substituted for plane Cu2 ions, create holes in the CuO2 planes and that these holes are responsible for the destruction of the in-plane AF order. Yet a precise characterisation of our samples questions this simple interpretation and suggests the following alternative one: in our samples, most of the apical oxygens neighbouring the Li ions seem to be shifted to chain sites; as a consequence, the holes brought in by the substitution may be localised on chain coppers (Cu12 ) and do not much hinder the AF order in the CuO2 planes. The strong suppression that is observed would then result from chain oxygens sitting near these Cu12 or near Li ions substituted for chain Cu1 ions. More experiments are planned to con®rm or in®rm this latter interpretation.
References [1] K. Sauv, J. Conard, M. Nicolas-Francillon, F. Bouree, Physica C 273 (1996) 49. [2] M. Nicolas-Francillon, F. Maury, R. Ollitrault-Fichet, M. Nanot, P. Legeay, J. Appl. Phys. 84 (2) (1998) 925. [3] M. Nicolas-Francillon, F. Maury, M. Nanot, R. OllitrautFichet, Solid State Commun. 109 (1999) 531. [4] F. Maury, M. Nicolas-Francillon, M. Nanot, R. OllitraultFichet, J. Appl. Phys. 85 (2) (1999) 1002. [5] F. Maury, M. Nicolas-Francillon, F. Bouree, R. OllitraultFichet, M. Nanot, Physica C 333 (2000) 121. [6] M.A.G. Aranda, J.P. Att®eld, Angew. Chem. Int. Ed. Engl. 32 (10) (1993) 1454. [7] R. Homann, R. Hoppe, W. Schafer, Z. anorg. allg. Chem. 578 (1989) 18. [8] F.C. Fritschij, H.B. Brom, R. Berger, Solid State Commun. 107 (12) (1998) 719.
[9] S. Edo, T. Takama, Jpn. J. Appl. Phys. Part 1 37 (7) (1998) 3956. [10] J. Bobro, W.A. MacFarlane, H. Alloul, P. Mendels, N. Blanchard, G. Collin, J.F. Marucco, Phys. Rev. Lett. 83 (1999) 4381. [11] R. Troc, Z. Bukowski, R. Horyn, J. Klamut, Phys. Lett. A 124 (4) (1987) 222. [12] D.C. Vier, S.B. Osero, C.T. Salling, J.F. Smyth, S. Schultz, Y. Dalichaouch, B.W. Lee, M.B. Maple, Phys. Rev. B 36 (16) (1987) 8888. [13] Z.R. Wang, D.C. Johnston, L.L. Miller, D. Vaknin, Phys. Rev. B 52 (10) (1995) 7384. [14] J. Rodriguez-Carjaval, Abstract Sat. Meeting on Powder Diraction, IUCr. Conf., Toulouse, 1990, p. 127. [15] D.C. Johnston, A.J. Jacobson, J.M. Newsam, J.T. Lewandowski, D.P. Goshorn, D. Xie, W.B. Yelon, Chemistry of High Temperature Superconductors, AMS, Washington DC, 1987, p. 136 (Chapter 14). [16] J.D. Jorgensen, B.W. Veal, A.P. Paulikas, L.J. Nowicki, G.W. Crabtree, H. Claus, W.K. Kwok, Phys. Rev. B 41 (4) (1990) 1863. [17] H. Casalta, P. Schleger, P. Harris, B. Lebech, N.H. Andersen, R. Liang, P. Dosanjh, W.N. Hardy, Physica C 258 (1996) 321. [18] J.M. Tranquada, A.H. Moudden, A.I. Goldman, P. Zolliker, D.E. Cox, G. Shirane, S.K. Sinha, D. Vaknin, D.C. Johnston, M.S. Alvarez, A.J. Jacobson, Phys. Rev. B 38 (4) (1988) 2477. [19] J. Rossat-Mignot, et al., J. de Physique 49 (12) (1988) 2119. [20] Y. Sidis, P. Bourges, B. Hennion, R. Villeneuve, G. Collin, J.F. Marucco, Physica C 235±240 (1994) 1591. [21] H. Tolentino, F. Baudelet, A. Fontaine, T. Gourieux, G. Krill, J.Y. Henry, J. Rossat-Mignod, Physica C 192 (1992) 115. [22] C. Niedermayer, C. Bernhard, T. Blasius, A. Golnik, A. Moodenbaugh, J.I. Budnick, Phys. Rev. Lett. 80 (17) (1998) 3843. [23] C. Bucci, R. De Renzi, G. Guidi, G. Allodi, F. Licci, Hyperf. Interact. 105 (1997) 71. [24] F. Borsa, P. Carretta, J.H. Cho, F.C. Chou, Q. Hu, D.C. Johnston, A. Lascialfari, D.R. Torgeson, Phys. Rev. B 52 (10) (1995) 7334. [25] Y. Sidis, P. Bourges, B. Hennion, R. Villeneuve, G. Collin, J.F. Marucco, unpublished results. [26] Y. Endoh, K. Yamada, R.J. Birgeneau, D.R. Gabbe, H.P. Jenssen, M.A. Kastner, C.J. Peters, P.J. Picone, T.R. Thurston, J.M. Tranquada, G. Shirane, Y. Hidaka, M. Oda, Y. Enomoto, M. Suzuki, T. Murakami, Phys. Rev. B 37 (13) (1988) 7443.
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Antiferromagnetism in Li substituted YBa2Cu3Oy studied by neutron powder diraction measurements F. Maury a,*, M. Nicolas-Francillon b, I. Mirebeau c, F. Bouree c b c
a Laboratoire des Solides Irradi es, Ecole Polytechnique, F-91128 Palaiseau C edex, France Laboratoire de Physique du Solide, ESPCI, 10 rue Vauquelin, F-75231 Paris C edex 05, France Laboratoire L eon Brillouin (CEA-CNRS), CEA/Saclay, F-91191 Gif sur Yvette C edex, France
Received 21 July 2000; received in revised form 2 November 2000; accepted 3 November 2000
Abstract The magnetic structure of two well characterised YBa2 Cu3 x Lix Oy samples, with similar Li concentrations, x 0:08±0:09, and slightly dierent oxygen concentrations, y 6:06 and y 6:18, has been studied by neutron powder diraction measurements. The lithium substitution is found to induce a strong decrease of the Neel temperature and of the mean ordered magnetic moment on the Cu2 sites. TN is 245 K for y 6:06 instead of 415±400 K in unsubstituted YBa2 Cu3 Oy with y 6:0±6.2. The mean ordered moment at low temperature, l0 =lB , is 0:33 0:03 for y 6:06, and 0:17 0:08 for y 6:18, instead of 0.64 in unsubstitued YBCO. These measured values are compared with other data from the literature and the various possible mechanisms of the strong lithium eect are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ld; 74.72.Bk Keywords: Neutron diraction; High-Tc compounds; Antiferromagnetism
1. Introduction This work belongs to an extensive study of the conduction and magnetic properties of lithium substituted YBCO. This study was initiated with the object of providing a set of data which would allow a better understanding of the link between superconductivity and magnetism in these materials. It has been shown by NMR measurements [1] that the Li substitution induces in the metallic * Corresponding author. Tel.: +33-1-6933-4502; fax: +33-16933-3022. E-mail address: [email protected] (F. Maury).
phase, localised magnetic moments on the Cu ions in the vicinity of the Li. The static magnetisation of superconducting as well as insulating samples [2±4] has been measured as a function of the temperature and of the magnetic ®eld for various Li concentrations up to x 0:4. We have shown [5] that Li substitutes mainly for the Cu of the CuO2 planes (Cu2) when the samples are synthesised under oxygen at T 900±920°C, while about 20% of the Li substitutes for the Cu in the Cu±O± chains (Cu1) when the samples are synthesised at the same temperature in air. The present experiment was aimed at measuring the Li eect on the antiferromagnetic (AF) structure of the insulating samples; we also determined
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 7 3 7 - 8
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the residual impurities present in our samples which may participate in the magnetisation measured at low temperature. We brie¯y recall in Section 2 our experimental conditions. We present and discuss in Section 3 the sample characterisation (residual impurities, lithium and oxygen concentration), in Section 4 the sample magnetisation measured at low temperature, in Section 5 the crystallographic structure of the samples (lithium and oxygen location) and in Section 6 the AF order determined by neutron powder diraction measurements. 2. Experimental Our samples (samples 1 and 3 of Ref. [5]) were prepared by the usual solid state reaction from Y2 O3 , BaCO3 , CuO and Li2 CO3 at 920°C. Sample 1 was synthesised in oxygen and annealed in oxygen at 480°C. Just before the present experiment, it was annealed in argon at 750°C and is thereafter called sample 1b. Sample 3 was synthesised in air and directly annealed in argon at 750°C. Both the Li and the Cu content were determined by emission plasma spectrometry analysis. The Li molar concentration so determined is called ``experimental'' value of x and is xexp 0:115 0:02 for sample 1/1b and xexp 0:09 0:02 for sample 3. The hole content of the samples was determined by iodometric titration and the oxygen content deduced from electrical neutrality. The oxygen molar concentration so determined is called ``experimental'' value of y and is yexp 6:18 0:04 for sample 1b, 6:04 0:04 for sample 3, ®rst measurement, and 6:07 0:04 for the same sample, one year later. The magnetic measurements were performed with the superconducting quantum interference device (SQUID) of the Laboratoire de Physique du Solide de lÕEcole Normale Superieure (Paris). The detailed crystallographic structure of the samples was determined by neutron powder diraction measurements carried out at room temperature on the high resolution powder diractometer 3T2 of the Laboratoire Leon Brillouin with The data were analysed using the k 1:2251 A. Rietveld method. Lattice parameters, atom coordinates, oxygen and Cu/Li site occupancies, iso-
tropic thermal factors, peak shape and width, preferred orientation, scale factor and background were re®ned. The magnetic structure of the samples was determined by neutron powder diraction measurements carried out at various temperatures between 10 and 300 K on the high ¯ux diractometer G6-1 of the Laboratoire Leon Brillouin with k 4:738 The data were analysed using the Rietveld A. method. Only lattice parameters, atom coordinates, peak shape and width, scale factor and mean ordered magnetic moment on the Cu2 sites were re®ned. Site occupancies and thermal factors were taken as determined by the 3T2 experiment. 3. Sample characterisation 3.1. Residual impurities Fig. 1 shows the G6-1 neutron diraction spectra measured at 10 6 T 6 100 K for sample 3 (Fig. 1a) and for sample 1b (Fig. 1b). The scale is much enlarged so as to allow to see distinctly all the impurity peaks that emerge from the background in the domain 50° < 2h < 75°. These peaks are of the same width at half maximum than the YBCO peaks and of much smaller height: the YBCO (0 0 1) peak reaches a maximum intensity of 2250 for sample 1b and 2600 for sample 3 with the units of Fig. 1. All the peaks which are distinguishable from the background have been indexed. They all belong to the three following residual impurities: Ba44 Cu48 (CO3 )6 O81x (hereafter called ``BaCuO2 '') [6], Li2 CuO2 [7] and LiCu2 O2 [8]. The ``BaCuO2 '' unit cell is large and many small diffraction peaks are observed; on the contrary the Li2 CuO2 unit cell is small so that but a few larger peaks are observed. For each of these three impurities, we have calculated, with a gaussian ®t of the diraction peaks, the ratio of the total integrated intensity of the impurity peaks to that of the YBCO peaks. This ratio, fimp , should not be much dierent from the volumic fraction of the impurity present in the sample. Its values are listed in Table 1. We ®nd that sample 3 contains about 0.4% of ``BaCuO2 '' and 0.3% of LiCu2 O2 and that sample 1b contains
F. Maury et al. / Physica C 353 (2001) 93±102
95
Fig. 1. Neutron diraction data measured with the G6-1 spectrometer. The triangles indicate the position of the (1/2 1/2 l) YBCO peaks with l 0, 1, 2. (a) Sample 3: solid line: 10 6 T 6 100 K; dotted line: T 300 K. (b) Sample 1b: solid line: 10 6 T 6 100 K; dotted line: 150 6 T 6 170 K.
Table 1 Sample characteristics Sample
Synthesis
Anneal
fBaCuO2 /%
fLi2 CuO2 /%
fLiCu2 O2 /%
1b 3
Oxygen 920°C Air 920°C
Oxygen 480°C argon 750°C Argon 750°C
1:5 0:7 0:4 0:2
1:0 0:4 ±
0:12 0:04 0:3 0:1
about 1.5% of ``BaCuO2 '', 1.0% of Li2 CuO2 and 0.1% of LiCu2 O2 . Besides, we observe on the left side of the (0 0 1) YBCO peak of sample 3, a large shoulder which can be split into two broad peaks, centred at 1 (2h 20:0°) and qmax 0:516 qmax 0:460 A 1 (2h 22:4°). Their total integrated intensity A amounts to 4 % of that of the YBCO peaks. This structure is similar to what has been observed in YBa2 Cu3 Oy annealed in air or in a mixture of Ar and H2 O at 125°C [9]. It results from the diusion and absorption of H2 O on the vacant oxygen chain sites. The ®rst peak evinces the local phase transformation induced by this absorption: the c parameter is increased. The second one most likely stems from very small crystallites of Y2 BaCuO5 . That this structure is observed in sample 3 and not in sample 1b, is easily explainable: sample 3 was elaborated for the 3T2 experiment, that is about eight months before the G6-1 experiment, while
sample 1b was annealed in argon at 750°C, just before the G6-1 experiment. 3.2. Lithium concentration The presence of lithium±copper oxides in our samples results in an overestimate of the Li concentration. Assuming a volumic fraction of Li2 CuO2 equal to fLi2 CuO2 , i.e. equal to 1:0 0:4% in sample 1/1b, we then derive from the spectrometry analysis xexp 0:04 0:035 instead of 0.115. This value is to be compared to the re®ned value, xcalc 0:09 0:01, obtained from the neutron diraction analysis of sample 1 [5]. Besides, this analysis has shown that in sample 1, all the Li is on Cu2 sites. According to the NMR analysis of Bobro et al. [10], we can deduce the Li concentration in the CuO2 planes from the measured superconductivity critical temperature. We measured, for sample 1, Tc 67 K and deduced
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x 0:07±0.08. The Li concentration in sample 1/1b can thus be taken equal to x 0:08 0:01. The Li concentration in sample 3 is changed only slightly due to the presence of a small fraction of LiCu2 O2 . Assuming a volumic fraction of LiCu2 O2 equal to fLiCu2 O2 (0.3%), it becomes xexp 0:08 0:02 instead of 0:09 0:02. We conclude that the Li concentration is about equal, within the experimental uncertainties, in these two samples, 1/1b and 3. 3.3. Oxygen concentration The oxygen content of sample 3, yexp 6:04 0:04, was measured just before the 3T2 experiment [5]. It was remeasured one year later and was yexp 6:07 0:04. The G6-1 experiment was realised in the interval between: we can thus consider that the oxygen content of sample 3 is yexp 6:06 0:04 for the G6-1 experiment. The oxygen content of sample 1b, yexp 6:18 0:04, was measured just before the G6-1 experiment shortly followed by the 3T2 experiment for this sample. We checked that the presence of residual impurities could not change signi®cantly these values. The iodometric titration yields a measure of the excess of charge on the Cu ions compared to Cu . This excess is 4 10 3 F =g for ``BaCuO2 '' and 9:1 10 3 F =g for Li2 CuO2 instead of 3:1
10 3 F =g for YBa2 Cu3 O6 or 6:0 10 3 F =g for YBa2 Cu3 O7 , where F is the Faraday constant. The calculation of the corresponding correction shows that it is negligible compared to the experimental uncertainty on yexp . If we consider, for example, Li2 CuO2 in sample 1b, the overestimate of xexp due to presence of the impurity, leads to an underestimate of yexp which exactly balances the overestimate resulting from the large charge excess per gram of Li2 CuO2 as compared to YBa2 Cu3 O6 . The formula of sample 1b is then: YBa2 Cu2:890:02 Li0:080:01 O6:180:04 , and that of sample 3: YBa2 Cu2:890:02 Li0:090:02 Oy , with y 6:04 0:04 for the 3T2 experiment and y 6:06 0:04 for the G6-1 experiment. 4. Sample magnetisation 4.1. Experimental results Fig. 2 shows the sample magnetisation measured for sample 1b, as a function of the magnetic ®eld at various temperatures between 2 and 30 K. Similar curves are obtained for sample 3. These M B curves are similar to those already measured for similar samples, with xexp ranging from 0.0 to 0.39 [4]. The solid lines are ®ts calculated with
Fig. 2. Magnetisation of sample 1b measured at T 2, 4.5, 8, 15 and 30 K. The lines are best ®ts to the data calculated according to Eq. (1).
F. Maury et al. / Physica C 353 (2001) 93±102
M vlin B Nm1= tanh mB=kT
1= mB=kT 1
where vlin , N and m are three adjustable parameters, the signi®cation of which is discussed in Ref. [4]. The value of vlin is the same for both samples; it decreases linearly with the temperature (see Fig. 4). The value of the magnetic moment m is also equal for both samples: m 13 2lB , where lB is the Bohr magneton. N, the number of those moments in a Langevin model, is larger for sample 1b than for sample 3. Yet the ratio N=NLiO , where NLiO is the number of Li ions extra oxygen ions, is about the same for both samples, as already observed [4]: it is 4:5 10 3 for sample 1b and 4±5:5 10 3 for sample 3. All these values are comparable to those deduced in Ref. [4]. Fig. 3 shows the magnetisation measured for sample 1b, at B 1 T, as a function of temperature. The two components of M according to Eq. (1), Mlin vlin B and ML Nm1= tanh mB=kT 1= mB=kT are indicated separately. 4.2. Discussion Considering that the magnetic moments m stem from superparamagnetic particles made of small
97
regions of ferromagnetically ordered moments on Cu2 sites, would lead to a volumic fraction of these regions of a few 10 3 . This is a very small fraction suggesting that ML could be due to residual impurities. The three compounds ``BaCuO2 '', Li2 CuO2 and LiCu2 O2 contain one Cu2 ion per unit formula. In Li2 CuO2 and LiCu2 O2 as in the green phase Y2 BaCuO5 , the spin interaction between the Cu2 ions is antiferromagnetic with an exchange energy, J =k 43 K for Li2 CuO2 [7] and 35 K for LiCu2 O2 [8]. One can check that these two impurities contribute but to a negligible extent to the magnetisation measured below 50 K. ``BaCuO2 '' has been reported as being paramagnetic down to a few Kelvin [11,12]. It can contribute but to a minor part of Mlin since Mlin remains nearly constant from 2 to 30 K. On the contrary, ML can be ®tted quite well, as shown in Fig. 3, with a volumic fraction of ``BaCuO2 '' equal to 2.3% for sample 1b (1.0% if the data for ``BaCuO2 '' are taken from Ref. [12] ). This volumic fraction is 1.0% (0.4%) for sample 3. These values are quite comparable to the values of fBaCuO2 determined in Section 3.1. Moreover if ML stems from residual ``BaCuO2 '', then the magnetisation of ``BaCuO2 '' at low temperature must be the same Langevin function of the magnetic ®eld as ML . This is eectively what is observed [13]. It is thus very likely that the intrinsic magnetisation of our samples is Mlin only. If now we subtract the impurity contributions from the M T data, the eective magnetic moment per Li O, deduced from a Curie ®t in the range 100 < T < 300 K, is 0.9lB for both samples, a value not much smaller than that (1.1lB ) deduced previously [3] .
5. Crystallographic structure 5.1. Oxygen location Fig. 3. Magnetisation of sample 1b measured at B 1 T, either in the M B experiment or in a M T experiment. The solid line is a Curie±Weiss ®t to the saturating part of M, ML , corresponding to 2.3 vol% of ``BaCuO2 '' according to the data of Ref. [11].
After a ®rst neutron diraction measurement on the 3T2 spectrometer, sample 1 was annealed, as a powder, at 750°C in argon. After the G6-1 experiment, its neutron diraction spectrum was
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slightly smaller for sample 1b than for sample 3: xcalc 0:085 0:01 for sample 1b instead of 0:10 0:01 for sample 3. Yet the dierence remains within the experimental uncertainties. The only signi®cant dierence between the re®ned parameters of these samples (see Table 2) is observed for the oxygen concentration. Its re®ned value is ycalc 6:17 0:03 for sample 1b and 6:05 0:03 for sample 3. These values are remarkably equal to the ``experimental'' ones: yexp 6:18 0:04 and 6:04 0:04, respectively. The apical oxygen site occupancy is n O1 1:90 0:02 and 1:91 001, for samples 1b and 3, so that about one apical oxygen is missing per substituted Li. If we now ®x n O1 2, so that the O1 sites be fully occupied, then (see Table 2) ycalc is 6:25 0:02 and 6:12 0:02, for samples 1b and 3, respectively, larger than the experimental values, and the ®t quality is slightly worsened. If we ®x both n O1 2 and n O4 yexp 6, so that the oxygen concentration be equal to the experimental value, then the thermal factor of the oxygen chain sites is much too small: B O4 0:1 0:4 for sample 1b, and even negative: B O4 1:4 0:08 for sample 3, which is an artefact of the program due to a too small constrained value of n(O4). We see that the neutron data seem to indicate that in tetragonal as in orthorhombic samples, one
Fig. 4. Neutron diraction data measured at room temperature with the 3T2 spectrometer for sample 1b. The points are the experimental data, the lines are the calculated pro®les. The lower curve represents the dierence between the two.
remeasured on the 3T2 spectrometer. The data (Fig. 4) were analysed with the Rietveld method [14] as follows (for more details see Ref. [5]). The space group of tetragonal YBCO is P4/mmm and c the lattice parameters are a b 3:8590 A, 11:8034 A for sample 1b and a b 3:8595 A, c 11:8137 A for sample 3. We start by assuming that all the Li is on Cu2 sites. Then the Li re®ned concentration is found
Table 2 Structural parameters obtained by Rietveld re®nement on neutron data for tetragonal YBa2 Cu3 x Lix Oy a xcalc
% Li1
ycalc
n(O1)
n(O4)
Sample 1b (xexp 0:08 0:01; yexp 6:18 0:04; 0.08 (1) [0] 6.17 (3) 1.90 (2) 0.27 (2) 0.07 (1) [0] 6.25 (2) [2] 0.25 (2) 0.07 (1) [0] [6.18] [2] [0.18] 0.11 (2) 23 (7) 6.16 (3) 1.90 (2) 0.26 (2) [0.08] [30] 6.18 (3) 1.92 (2) 0.26 (2) Sample 3 0.10 (1) 0.08 (1) 0.08 (1) 0.11 (1) [0.09]
(xexp 0:09 0:02; [0] 6.05 (2) [0] 6.12 (2) [0] [6.04] 12 (4) 6.03 (2) [15] 6.04 (2)
B(Y)
B(Ba)
Rexp 4:26 %) 0.63 (3) 0.51 0.58 (3) 0.47 0.57 (3) 0.48 0.64 (3) 0.54 0.61 (3) 0.51
yexp 6:04 0:04; Rexp 2:58%) 1.91 (1) 0.14 (2) 0.58 (2) 0.45 [2] 0.12 (2) 0.53 (2) 0.41 [2] [0.04] 0.53 (2) 0.40 1.90 (1) 0.13 (2) 0.58 (2) 0.46 1.92 (1) 0.13 (2) 0.56 (2) 0.44
B(Cu1)
B(Cu2)
B(O1)
B(O2)
B(O4)
RB (%)
(3) (3) (3) (3) (3)
0.95 0.91 0.91 0.83 0.81
(3) (3) (3) (4) (3)
0.44 0.45 0.45 0.46 0.51
(3) (3) (3) (3) (2)
0.93 1.11 1.10 0.95 0.95
(5) (3 (3 (5) (5)
0.65 0.61 0.62 0.67 0.64
(2) (2) (2) (2) (2)
1.7 1.5 0.1 1.5 1.4
3.90 4.02 4.09 3.79 3.83
(2) (2) (2) (2) (2)
0.96 0.93 0.92 0.91 0.88
(2) (2) (2) (3) (2)
0.40 0.41 0.41 0.41 0.45
(2) (2) (2) (2) (1)
0.95 1.12 1.12 0.96 0.96
(3) (2) (2) (3) (3)
0.62 0.59 0.59 0.63 0.61
(1) (1) (2) (2) (1)
4 (2) 3 (2) )1.4 (8) 3 (2) 3 (2)
(6) (6) (4) (6) (5)
3.26 3.44 3.49 3.21 3.24
a 2 ). Numbers in brackets corOapex O1 (0,0,z), O2 (0.5,0,z), O3 (0,0.5,z), Ochain O4 (x,0.5,0), Cu1 (0,0,0), Cu2 (0,0,z); (B in A respond to ®xed values. Numbers in parentheses following re®ned parameters represent one standard deviation in the last digit. Figures in bold characters correspond to the ®t we consider the best.
F. Maury et al. / Physica C 353 (2001) 93±102
apical oxygen is lost per substituted Li. Yet we want for an independent argument to con®rm this result which is based but on rather small dierences in the ®t quality. 5.2. Lithium location If we now allow the Li to occupy both Cu1 and Cu2 sites, the only parameters which are changed beyond their uncertainty limits are the number of Li on Cu1 sites, n Li1, and the Cu1 thermal factor, B Cu1. The re®ned Li concentration, xcalc n Li1 n Li2, increases with n Li1. Its re®ned value is xcalc 0:11 0:10 0:02 with 23 28 7% of Li1 for sample 1b and xcalc 0:12 0:10 0:01 with 11 14 4% of Li1 for sample 3, where the values in parentheses are calculated with n O1 2 (see Table 2). These values are at the upper limit of xexp . To check the reliability of this determination, we then ®xed xcalc at 0.08 and the fraction of Li1 at dierent values between 0% and 40%, and we compared the re®ned values of the thermal factors of the Cu1 and Cu2 sites with their values in the literature for unsubstituted tetragonal YBa2 Cu3 Oy [15±17]. B(Cu1) is much larger in the tetragonal structure where the oxygen chain sites are vacant and the c axis cell parameter is larger, than in the orthorhombic one, and it is an increasing function of c. Taking this increase into account and comparing our B(Cu1) and B(Cu2) values with expected values, we deduced the following values for the fraction of Li1: 29 (22)% for sample 1b and 14 (7)% for sample 3. These values are about the same as above. The former 3T2 experiment [5] has shown that in sample 1, orthorhombic, all the Li was on Cu2 sites. The present experiment shows that the annealing in argon at 750°C, has modi®ed the Li distribution on the Cu sites. This distribution is as if sample 1b had been synthesised in air and not in oxygen. Finally, the 3T2 experiment seems to indicate that, beside their dierent oxygen content, samples 1b and 3 also dier by their fraction of lithium on Cu1 sites, which is about twice as large in sample 1b as in sample 3.
99
6. Magnetic structure 6.1. Experimental results Fig. 1a shows the neutron diraction spectrum measured for sample 3 (x 0:09, y 6:06 in the G6-1 experiment) in the range 50° < 2h < 75°. It displays, together with the various impurity peaks, two diraction peaks which can be indexed (1/2 1/2 1) and (1/2 1/2 2). These peaks which are clearly visible at low temperature, arise from the doubling of the magnetic unit cell along the a and b axes, due to the AF order in the CuO2 planes. They are no longer observed at T 300 K, which shows that they do stem from magnetism and not from residual impurities and that the Neel temperature is lower than 300 K. For sample 1b (see Fig. 1b), these peaks are hardly visible at low temperature and at 150±170 K, they are hidden in the background. The mean ordered magnetic moment on the Cu2 sites was deduced from a Rietveld ®t of the data for 40° < 2h < 100°. (The ®t to the experimental data obtained for sample 3 at T 10 K is displayed in Fig. 5.) The diraction spectrum measured for the sample holder without sample was subtracted from
Fig. 5. Neutron diraction data measured at T 10 K with the G6-1 spectrometer for sample 3. The points are the experimental data, the line is the pro®le calculated with l=lB 0:33.
100
F. Maury et al. / Physica C 353 (2001) 93±102
6.2. Discussion
Fig. 6. Square of the mean ordered magnetic moment on Cu2 sites determined either by a Rietveld ®t of the neutron data or by a Gaussian ®t of the (1/2 1/2 1) peak. The solid line is calculated with l0 =lB 0:35, TN 242 K and b 0:25.
that measured with the sample. This increases the dispersion on the data points but is necessary since the sample holder spectrum exhibits two peaks (of maximum height 3.5 with the units of Fig. 1) centred at 2h 71:7° and 73.4°, i.e. on each side of the (1/2 1/2 2) magnetic peak which is centred at 2h 72:5°. Fig. 6 shows the re®ned value of (l=lB )2 as a function of the temperature. It is l0 =lB 0:33 0:03 at low temperature, about constant up to T 100 K. No reliable re®ned value of l could be deduced above 200 K. Fig. 6 also shows the integrated intensity of the (1/2 1/2 1) peak, deduced from a gaussian ®t of both YBCO magnetic (1/2 1/2 1) peak and ``BaCuO2 '' (3 2 1) peak in the range 56° < 2h < 58:5°, and normalised to the value of (l=lB )2 at T 10 K. We see that the Neel temperature for sample 3 is TN 245 15 K. For sample 1b, the re®ned value of l=lB is 0:17 0:08 at 10 6 T 6 100 K and 0:05 0:3 at 150 6 T 6 170 K. It is 0:10 0:11 at 150±170 K, from a gaussian ®t of the (1/2 1/2 1) peak. For an unknown reason, the background increased at the end of the sample 3 experiment and is higher for sample 1b than for sample 3, so that no reliable value of l can be obtained for sample 1b above 100 K. Its Neel temperature is TN 150 30 K.
If we compare our results with those in the literature for unsubstituted YBa2 Cu3 Oy [18,19], we see that the Li substitution induces a strong reduction of both l0 and TN , since in unsubstituted YBa2 Cu3 Oy , l0 0:64 0:03 and TN 415 5 K as long as y 6 6:15. The Li eect is much stronger than that of Zn which yields but a small reduction of l and TN , compatible with a mere dilution effect: l=lB 0:62 and TN 350 K for YBa2 Cu2:92 Zn0:08 O6:2 , i.e. for the same x and y concentrations as sample 1b [20]. Moreover, if we compare Fig. 6 with the results of Rossat±Mignot et al. for unsubstituted YBa2 Cu3 Oy with various oxygen dopings (Fig. 3 in Ref. [19]), we see that not only the TN value but also the l value as well as the shape of the l2 T curve are similar for sample 3 and for unsubstituted YBa2 Cu3 Oy with 6:33 < y < 6:36. The observed decrease of the 3D AF order in YBa2 Cu3 Oy with increasing y, stems from hole doping. We know that in unsubstituted YBa2 Cu3 Oy with 6 < y < 6:2, the doped holes are localised on the Cu1 neighbours of the chain oxygens [21] and are of nearly no eect on the AF order [18,19]. The destruction of the 3D AF order appears around y 6:2, when a few mobile holes begin to be created in the CuO2 planes. Muon spin rotation (lSR) measurements on Y1 x Cax Ba2 Cu3 O6 and La2 x Srx CuO4 [22] have indeed shown that the strength of the AF correlations is determined by the planar hole content and does not depend on the concentration of dopant atoms. In Y0:94 Ca0:06 Ba2 Cu3 O6:02 , a concentration of in-plane holes of 3% is sucient to reduce TN to 170 K or less. The similarity between our results for sample 3 and those for YBa2 Cu3 O6:35 [19] seems to indicate that the eect of the Li substitution is a mere hole doping eect. One can thus simply assume that these holes are introduced in the CuO2 planes by the substitution of Li for plane Cu22 while the substitution of Li for chain Cu1 does not modify the hole concentration. Neither does the substitution of Zn2 for Cu22 . This readily explains the observed dierence between Li and Zn. Are these in-plane holes brought in by the Li2 ions, mobile? If we look up in the literature if they
F. Maury et al. / Physica C 353 (2001) 93±102
have to be mobile to aect the 3D AF order, we get various answers. lSR experiments on Y1 x Cax Ba2 Cu3 O6 and La2 x Srx CuO4 [22±24] show that the doped holes become frozen (at the time scale of the lSR technique, sc < 10 6 s) around a transition temperature which increases with x (and is 15 K for Y0:94 Ca0:06 Ba2 Cu3 O6:02 ), and that small concentrations of frozen holes do not aect signi®cantly the mean ordered magnetic moment. Yet, in YBa2 Cu3 Oy (unsubstituted as well as Zn substituted), neutron scattering measurements [19,25] evince, below 15 K, a re-entrant behaviour of the mean ordered magnetic moment, which is attributed to the decrease of the 3D AF order by the static magnetic disorder due to the localisation of the in-plane holes. Finally, neutron scattering measurements on Sr and Li substituted La2 CuO4 [26] show that 5% of Li on the Cu sites eliminate the 3D AF transition completely. The doped holes created by the substitution of Li for Cu2 are inplane holes, like for the Sr doping; but, as the Li substitution, contrary to the Sr one, suppresses the superconductivity, they are not mobile but rather localised in-plane holes. To conclude, we see that mobile holes and possibly localised holes, hinder the 3D AF order, provided they belong to CuO2 planes. Our results for sample 3 can thus be simply interpreted by assuming that the substitution of Li for Cu2 create in-plane holes (``Li2-holes'') and that these holes, in number of n(Li2), are responsible for the suppression the 3D AF order, without the experiment showing if they are mobile or not. This simple model (model 1) implies that, notwithstanding the 3T2 results, we consider that the Li substitution does not entail any loss of apical oxygen. Then it gives a straightforward explanation of the 3D AF order destruction in sample 3. Yet we note that a concentration of 4% of ``Li2holes'' in the CuO2 planes, in sample 3, is not suf®cient to completely suppress the 3D AF order but yields TN 245 K, while 3% of in-plane holes in Y0:94 Ca0:06 Ba2 Cu3 O6:02 , reduce TN down to 6 170 K [22]. If on the contrary, we rely on the 3T2 results, we have to admit that the Li substitution goes with an apical oxygen removal. In our samples where
101
y > 6, the missing apical oxygens are replaced by extra chain oxygens and the holes brought in by the substitution of Li for Cu22 are likely to be localised not in the CuO2 plane but on the near-by Cu1s (that will become Cu12 ). Such holes, as in YBa2 Cu3 Oy with y 6 6:15, should not aect much the 3D AF order. Then, if the strong decrease of TN is due to in-plane holes, these holes have to stem from extra chain oxygens coming near a Cu12 or a Li1 , rather than be ``Li2-holes''. In this hypothesis (model 2), TN should be much more sensitive to a small concentration of extra oxygen than in unsubstituted YBa2 Cu3 O6 . This brings us to compare sample 1b with sample 3 and ask why l and TN are much smaller in sample 1b, with n Li2 0:06 than in sample 3 with n Li2 0:08, given that y is less than 0.2 in both samples. The dierence between the two samples can only come from their dierent oxygen concentration. In model 1, there are, in sample 1b, 2n Li1 0:04 oxygen chain sites where an extra oxygen (sitting near a Li1) can free a mobile hole, and n O4 0:18 extra oxygens per unit formula, while these numbers are 2n Li1 0:02 and n O4 0:06 in sample 3. This dierence can be sucient to explain the dierent TN s. Yet, if we compare sample 1b with Y0:94 Ca0:06 Ba2 Cu3 O6:02 [22], we expect the eect of the chain oxygens in sample 1b to be small, since both samples have similar TN s and the same in-plane hole concentration. In model 2, the in-plane holes responsible for the strong decrease of the 3D AF order are all created by extra oxygens (in concentration y)6) sitting on chain sites near a Cu12 or a Li1 . The concentration of such sites is small, 6 0.10 per unit formula in both samples. Chain oxygen could be retained preferentially in such sites. In this case, the mean ordered magnetic moment, l T , should depend on y even for small values of y, contrary to the case of unsubstituted YBa2 Cu3 Oy . 7. Summary The present experiment has evidenced a strong eect of substituting lithium for copper in YBa2 Cu3 x Lix Oy . For x 0:09 and y 6:06, we
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F. Maury et al. / Physica C 353 (2001) 93±102
measured a mean ordered magnetic moment on the Cu2 sites of 0:33 0:03lB at low temperature, with a Neel temperature of 245 15 K. This moment is 0:17 0:08lB for about the same x and y 6:18. These results can be simply interpreted by considering that the Li ions substituted for plane Cu2 ions, create holes in the CuO2 planes and that these holes are responsible for the destruction of the in-plane AF order. Yet a precise characterisation of our samples questions this simple interpretation and suggests the following alternative one: in our samples, most of the apical oxygens neighbouring the Li ions seem to be shifted to chain sites; as a consequence, the holes brought in by the substitution may be localised on chain coppers (Cu12 ) and do not much hinder the AF order in the CuO2 planes. The strong suppression that is observed would then result from chain oxygens sitting near these Cu12 or near Li ions substituted for chain Cu1 ions. More experiments are planned to con®rm or in®rm this latter interpretation.
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