Study of the structural phase transition in Gd2−xCexCuO4

PHYSICA ELSEVIER

Physica C 273 (1997) 239-297

Study of the structuralphase transition in Gd2_xCexCuO4 P. Vigoureux a, M. Braden * ,a,b, A. Gukasov a, W. Paulus a,c, p. Bourges a, A. Cousson a, D. Petitgrand a, j.p. Lauriat d, M. Meven c, S.N. Barilo e, D.I. Zhigunov e, p. Adelmann b, G. Heger c a Laboratoire L~on Brillouin (CEA-CNRS), CE-Saclay, 91191 Gifsur Yvette Cedex, France b Forschungszentrum Karlsruhe, INFP, 76021 Karlsruhe, Germany c lnstitutffir Kristallographie, RWTH-Aachen, Jfigerstr. 17-19, 52056 Aachen, Germany d LURE, Centre Universitaire Paris Sud, 91405 Orsay Cedex, France e Institute of Physics of Solids and Semiconductors, 220726 Minsk, Belarus

Received 7 April 1996; revised manuscript received 22 October 1996

Abstract

The structural phase transition in Gd2_xCe,CuO 4 (x = 0, 0.12) characterized by the rotation of the CuO4-squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd2CuO 4 and below 707.2(7) K for Gdl.88Ce.12CuO 4 with an unusual temperature dependence. The rotation of the CuO4-squares is coupled to a spontaneous strain along the c-axis. Structure analysis of the undistorted high temperature T'-phase in Gd2CuO 4 indicates persisting local distortions.

1. Introduction

Structural phase transitions are frequently observed in high temperature superconductors possessing a perovskite related structure. Most of these transitions are associated with rotations of the oxygen coordination polyhedra. Especially the different distortions in the La2_ x MxCUO 4 system (with M = Sr, Ba) which are due to tilts of the CuO 6 octahedra were widely studied [1]. Furthermore, it is now well established that these tilt transitions are related to the superconductivity in the T-phase [2]. Although an

* Corresponding author. Fax: + 33 1 6908 8261; e-mail: [email protected].

understanding of the coupling mechanism is still missing the interplay between structural phase transitions and superconductivity represents a severe test for proposed models of HTSC. Until recently the structure of the T'-phase of RE2_xCexCuO4_ ~ was considered as less complicated as no long range superstructure was known. However, different experiments indicated that at least a local structural distortion must be present in the compounds with the smaller RE's. MtSssbauer spectroscopy revealed an orthorhombic site symmetry for G d in G d z C u O 4 which is incompatible with the tetragonal T'-structure of space group I 4 / m m m [3]. Additional lines in Raman spectra found in Eu2CuO 4 and Gd2CuO 4 pointed towards a lower symmetry too [4]. Different groups reported the observation of

0921-4534/96/$17.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0921-4534(96)00638-7

240

P. Vigoureux et al./Physica C 273 (1997) 239-247

weak ferromagnetism (WFM) [5] which cannot be understood within space group I 4 / m m m . All these observations indicate a structural distortion sensitively depending on the ionic radius of the RE in RE2CuO 4. Conventional X-ray diffraction was unable to prove a structural distortion associated with the appearance of superstructure reflections even for compounds in which extremely large atomic displacement parameters suggest static contributions [6]. More recently neutron diffraction on an isotope enriched single crystal of GdzCuO 4 allowed to measure long range superstructure reflections and to establish a model for the distorted phase [7]. The distortion in Gd2CuO 4 is characterized by a rotation of the C u Q squares around the c-axis by 5.2 ° at room temperature, (note that in the case of the La2_xMxCuO 4 system the tilt is always around an axis perpendicular to c), and the lattice is enlarged to f}-a × Vr}-a × c (space group Acam). The structural distortion explains the discrepancies described above; it further reflects the softening tendency of the corresponding phonon mode observed in Nd2CuO 4 [8]. The same type of superstructure was also found in (Nd/Tb)2_,CexCuO 4 confirming that the rotation distortion is a general property of the T'-phase with small RE's [9,10]. The absence of superconductivity in Gd2_xCe x CuO4 [11] and an interplay between superconductivity and the WFM observed by different groups [t2] suggest that the rotation transition in the T' phase might have similar effects as the tilt transitions in the T-phase [2]. This interpretation is further supported by the phase diagram of (Nd/Tb)LssCeo.jsCuO 4 where superconducting and structurally distorted compounds are separated [I0]. Hence, a better knowledge of the rotation transition appears desirable. In order to characterize the rotation transition of the T'-phase, different diffraction experiments on Gd2_xCexCuO4 samples were undertaken.

2. Experimental Single crystals of Gd2_xCexCuO4 (x = 0 and x = 0.12(2)) were grown by a flux technique. In order to reduce the absorption for thermal neutrons highly enriched 158Gd was used. Both crystals possess a plate-like shape with the c-axis being along the

shortest dimension. The volumes were about 20 mm 3 and 50 mm 3 for x = 0 and 0.12, respectively. The Ce content was determined by micro-probe analysis at several points of the sample to x = 0.12(2). A piece of the undoped crystal was prepared for X-ray diffraction experiments by grinding it to a sphere of 0.12 mm diameter. A polycrystalline sample of Gd2CuO 4 was obtained by the standard solid state reaction including a reduction step at 900°C. Single crystal X-ray diffraction measurements were performed on an Enraf-Nonius CAD4 fourcircle diffractometer and using synchrotron radiation on the four-circle diffractometer WDIF4C at the LURE in Orsay. The higher resolution of the synchrotron diffractometer was used to analyze peak shapes of fundamental reflections. Due to its higher luminosity it was further possible to observe superstructure reflection intensities. These experiments were restricted to room-temperature. Precise lattice parameters as a function of temperature were determined on a Siemens D500 powder diffractometer using Cu K a 1 radiation obtained by a focusing Ge-(111) monochromator. The sample was heated in air; the temperature in the furnace could be controlled within 1 K. The temperature dependence of the superstructure reflection intensities was studied by single crystal neutron diffraction on the two axis diffractometer 3T.I (A = 1.09 A, Ge-(l 11) monochromator) installed at the ORPHEE reactor. The crystals were heated in vacuum, a temperature stability of + 0.3 K could be achieved. The sample crystals were mounted in the (410)/(001) diffraction plane which yields small absorption (here and in the following all Bragg indices refer to the larger orthorhombic unit cell). The intensities of the (412) superstructure and the (824) fundamental reflections were recorded as a function of temperature. As the (824) reflection is rather weak the A/2 contamination of the superstructure reflection intensity is negligible. A set of 257 Bragg reflection intensities (sin 0/A < 0.77 k - l ) was measured on the four-circle diffractometer 5C.2 (A = 0.831 .~, Cu-(220) monochromator) with the Gd2CuO 4 crystal in the undistorted T'-phase at 673 K (400°C). The furnace allowed a temperature stability of 0.5 K. The intensities were measured in w-scan mode with a resolution adapted variable scan width. The data were corrected

241

P. Vigoureux et al./Physica C 273 (1997) 239-247

1500

for absorption (/z = 8.2 c m - ] ) and averaged to a set of 134 independent reflection intensities of which 125 are larger than 3o-(1). For seven reflections the extinction correction was higher than 60%; they were excluded from the final refinements.

1000

'~ 500 3. Results and discussion

,.a

3.1. Characterization of the distorted structure 0

. . . .

-0.3 With the standard X-ray four-circle diffractometer we were unable to observe the superstructure reflections in Gd2CuO 4 in accordance to previous work [6]. These superstructure reflections, however, have intensities up to 5% of the strongest fundamental reflections in the case of neutron diffraction. As the superstructure is caused by oxygen displacements, the related structure factors are essentially smaller in the X-ray case. Calculations based on the Acam structure show that the ratio of superstructure intensities to the strongest fundamental intensity are reduced by about one order of magnitude in the case of X-ray diffraction. Hence, although a long range superstructure has not been detected in several published X-ray studies on RE2CuO 4 with small RE's, it seems reasonable to assume that the same superstructure also exists in the crystals investigated in those studies but could not be observed due to the weak structure factors. Note that neutron diffraction is able to identify the superstructure reflections even for polycrystalline samples [9,10]. Due to the higher luminosity of the synchrotron radiation the superstructure reflections in Gd2CuO 4 can be observed unambiguously with the four-circle diffractometer installed at LURE, see Fig. 1. No difference in the observed full widths at half maximum was obtained for fundamental and superstructure reflections. So, there is no indication that the structural distortion is limited to a restrained space scale in Gd2CuO 4. The proposed symmetry reduction from tetragonal I 4 / m m m to orthorhombic Acam might lead to a twinning of the single crystal. Some of the lost symmetry elements are recovered macroscopically as a twin law, leading to splittings in the peak profiles. The distribution of the differently oriented orthorhombic domains in Gd2CuO 4 should resemble

i

. . . .

-0.2

,

. . . .

-0.1

J

. . . .

b

. . . .

0 0.1 co (degree)

~

,

0.2

,

,

0.3

Fig. 1. Profile of the (410) superstructurereflection observedwith synchrotron radiation at LURE (A= 0.689 ~,) on a G d 2 C u O 4 crystal.

the situation in the orthorhombic phase of the La2CuO 4 compounds, as the reduction of the translation symmetry is identical. The twinning in the latter compounds was investigated in detail [13]: (h00) reflections are split into four and (hhO) reflections into three components. The study of comparable effects in GdzCuO 4 was tempting in order to obtain a confirmation of the orthorhombic symmetry, since the orthorhombic splitting could not be observed even with the best resolution of the synchrotron diffraction experiment. The w-scan profile of the (440)-reflection was measured as a function of the aP-angle, i.e. on rotating the crystal around the [440] direction. Fig. 2a shows the observed peak heights and widths. We find a sinusoidal modulation of both height and width. The maximum height is observed when the c-axis is parallel to the diffraction plane, q t = 90, and the minimum when the c-axis is perpendicular to it, ~ = 0; the width presents just the opposite behavior. The twinning of orthorhombic domains should split the (440) reflection due to the misalignment of the single-crystalline parts [13]. The misorientation of these parts consists of a rotation around the c-axis thereby increasing the profile width and decreasing the height only when the c-axis is perpendicular to the diffraction plane, ~ = 0. So, the behavior in Fig. 2b confirms the orthorhombic twinning qualitatively; the splitting might be beyond the experimental resolution. A more quantitative analysis can be carried out with the following assumptions. When the four

P. Vigoureuxet al./Physica C 273 (1997)239-247

242 ( a ) 0.048

30000

0.046

'~

,

0'044 I " ~ - - < L ~ 0.042 i~// "II ~ 0.040 4 [', "

~

~-, 0.034

~

"

'\ 1

24000 g-

~

0.030 0

26000 ~ = 25000 ;

"

0.032 -50

3.2. Temperature dependence of the order parameter

29000 28000 ~" '~" 27000

'4

0.036

~

;,t'"

~

"g 0.038

: __

50

23000 150

100

~I/ (degree)

(b) 3oooo ~ "~ 25000

[?'1

20000 '~

~' ~

15000 ~

\i

/i

5000 -0.2

.~ I

,r~ /i, "

I 10000 ~

.7' -0.15

\

.

: '~

-0.1 -0.05 0 0.05 co (degree)

0. l

0.15

0.2

Fig. 2. (a) Dependence of the FWHM and peak heights of the co-scan profile of the (,140)-reflection on the ~ angle (i.e. the rotation around [440]) in Gd2CuO4; the scans were performed using synchrotron radiation with a = 0.689 .~. q* = 0 corresponds to the configuration where the c-axis is parallel to the ~-scan rotation axis (maximum separation); (b) observed profile of (440) for gt = 0, the line represents a fit corresponding to ideally distributed twinning of orthorhombic domains.

different domain orientations occupy the same volume in the crystal, the intensity ratio of the contributions should be 1 : 2 : 1 , as the center peak in the (440)-scan results from two different domain orientations. Furthermore, we suppose that for c parallel to the diffraction plane, ~ = 90, the intrinsic width of the un-twined crystal is seen. Within these assumptions the observed profile at ~ = 0 can be described by only two parameters, the height of a single contribution and the splitting A which is related to the orthorhombic distortion by A = 90 ° - 2 arctan(b/a) [13]; the A-value obtained with the fit indicates an orthorhombic splitting (b - a)/a ~ 2 × 10 -4 which is well beyond the resolution of the experiment.

Fig. 3 shows the temperature dependence of the (412) superstructure reflection intensity normalized to the intensity of the (824) reflection (the intrinsic shift of the (824) structure factor due to the Ce substitution was corrected). The data confirm a continuous transition from the tetragonal T'-phase towards the orthorhombic structure. The effect of the Ce doping is found to increase the transition temperature, Tl_ A, by 49(2) K which agrees well with the shift found in (Nd/Th)2_xCe,CuO 4 [10]. The sign of the shift in T~_ A can be understood by the fact that Ce 4+ is smaller than Gd 3+ and should render the T'-phase even more unstable. The doping induced transfer of electrons into the CuO2-plane where they occupy anti-bonding orbitals, further enhances the instability. The intensity of the superstructure reflection is in first approximation proportional to the square of the order parameter characterized by the rotation angle of the CuO4-squares. The intensity data can be satisfactorily described with a power law I = 1 0 ( 1 T/T1_A) 2t~ thereby determining the critical exponent /3, see Fig. 3b. When considering only one CuO 2 plane the rotation transition possesses a one-component order parameter in contrast to the T-phase tilt transitions. The single layer distortion should, hence, be described by a 2d-Ising model. However, the stacking of the distorted planes together with the concomitant displacement of the O2-sites leads to two configurations corresponding to the low temperature A- and B-centered phases. The three-dimensional structural transition is, therefore, described by the 3d-XY model with a two component order parameter as in the case of the T-phase tilt transitions. There is only little difference in the expected r-values for the one and two component order parameter transitions, 0.325 and 0.346 respectively [14]; the observed /3-exponents, 0.34(2) and 0.31(2) for x = 0 and x = 0.12, respectively, are in fair agreement with these predictions. The power law could be fitted to the data for a large temperature range. Furthermore, the saturation of the rotation angle seems to start at quite elevated temperatures. Both effects are at variance with observations in typical displacive phase transitions in perovskite compounds, where a linear temperature de-

P, Vigoureux et al. / Physica C 273 (1997) 239-247

(a)

i

.

©

.

.

t

¢q

=

.

0.8

.

.

.

.

.

.

.

'

'

'

'

Gd 1 88Ce0 12CUO4

'

]_~

ttttitlr~Tt % ~ .

/

'5

tJtz Iti

0.6

!!

tiiti{t?

o

0.4 t

.s

i

GdzCUO4

0.2

5 1f

© :

t I t

0

I

i

i

,

300

I

r

t

i

I

,

,

i

,

,

' ii~---r

400 500 600 700 Temperature (K)

(b) 1000

Gdl,ssCe0,nCuO4 ~,_~i c'q

5

loo

I

o -~

l!

20

,

0.001

;d uO4

i

,

EIIIII

0.01

r

,11

, I

0.1 (Tt-T) / T t

243

pendence of the order parameter square corresponding to mean field theory is fulfilled over a large temperature interval [15]. Deviations from this linearity are usually observed only close to the transition temperature where they are due to fluctuations, and at low temperatures due to order parameter saturation [16]. The behavior observed in Gd2_,CexCuO 4 may point to some first order character of the the continuous transition. The temperature dependences observed in the two crystals differ in the behavior above Tt_ A. In the doped compound there persists some intensity in the T'-phase, which may be explained by precursors induced by the intrinsic disorder due to the Ce s u b s t i t u t i o n similar to o b s e r v a t i o n s in La2_xSGCuO4. Phase separation is frequently discussed for T and T' type cuprate superconductors [17]; however, on the length scale of coherent neutron diffraction (several hundreds of A) the Ce doped single crystal shows no indication for such effects as there is a clear doping induced shift of the transition temperature, Fig. 3a, which still is well defined in the Ce containing sample. A long range separation in doped and undoped regions should become evident in a split transition. Of course, a phase separation on a local scale cannot be observed via a single Bragg reflection. The lattice parameters of GdzCuO 4 as a function of temperature are shown in Fig. 4. There is a clear kink in the c parameter at TI_ A = 658 K whereas no anomaly can be detected for the a-axis. The rotation transition increases the c parameter. The same conclusion was deduced from the dependence of the c lattice parameter as function of the Tb concentration in (Nd/Tb)l.85Ce0.15CuO 4 [10]. Also the dependence of the c-axis in RE2CuO 4 on the RE ionic radius exhibits the same kink [18,19]. For the REzCuO4's with RE's smaller than Eu the c parameter is essentially larger than the extrapolation from the larger RE's. So, there is a coupling of the order parameter to the spontaneous strain parallel to the c-axis.

Fig. 3. (a) Dependence of the intensity of the (412) superstructure reflection normalized to that of the (824) fundamental reflection as function of temperature; the measurements were performed on the neutron diffractometer 3T.I; (b) as (a) in double logarithmic scale, the lines represent least squares fits with a power law.

244

P. Vigoureux et al. / Physica C 273 (1997) 239-247

In the flame of the phenomenological Landau theory such a coupling is usually taken into consideration by adding a coupling term (linear in the strain, E, and quadratic in the order parameter, Q), ot EQ 2, and a term describing the elastic energy of the deformation, ot E2, to the expansion of the free energy, G. The condition that the system has to be stress-free, aG/Oe = 0, results into a proportionality between the square of the order parameter and the spontaneous strain [15]. This relation should remain valid for the entire system. In the case of La2CuO 4 these proportionalities were confirmed by varying stoichiometry, temperature and pressure [20]. In the case of the T'-phase one may compare the strain observed in Gd2CuO 4 as a function of temperature, with that observed as a function of the RE ionic radius [18,19]. Gd2CuO 4 presents at room temperature a c parameter which is 0.022 ~, larger than the

3,910 . . . . . . . . . . . . . . . . . . 3,908

11,94 ~

11,93

3,906 11,92

,~ 3,904 3,902

/ / ~

"~

11,91

~. 3,900 , / /

11,90

3,898 11,89

3,896 1,0176 -' . . . . . . . . . . . . . . . 1,0174 1,o172

, ,

11,88

t --

1,0170

1,0168 . . . . . . . . . . . . . . . . . . . 300 400 500 600

700

Temperature (K) Fig. 4. Lattice parametersof Gd2CuO4 obtainedby X-ray powder diffraction as function of temperature.

extrapolation from higher temperatures, however it is even 0.10 A larger than the extrapolation from larger ionic radii. This discrepancy might indicate persisting local distortions in the high temperature phase of Gd2CuO 4. In this sense the character of the phase transition would shift towards an order-disorder transition, which might further explain the unusual temperature dependences of the superstructure reflection intensities.

3.3. High temperature tetragonal T'-phase The T' structure model was refined with the 673 K data set obtained on the Gd2CuO4-crystal leading to the results shown in Table 1 and in Fig. 5a). There is a satisfying description of the data. The atomic displacement parameters (ADP) are roughly doubled in comparison to the room-temperature values [6] except for the plane oxygen, O1, which is the site most sensitive to the structural distortion. One has to take into consideration the different orientation of the lattice in the room-temperature phase with space group Acam. The ADP perpendicular to the CuO bond and parallel to the planes rises from 0.0092 ~2 at 295 K [6] to 0.0358(12) ,&2 at 673 K. So, this parameter is enhanced by more than what would be expected for standard harmonic behavior. For a displacive transition the corresponding phonon mode softens close to the transition temperature which might enhance certain ADP's. However, a structural study on the related structural transition in SrTiO 3 revealed that this enhancement remains rather small [1]. The phonon softening is restricted only to a small part of the Brillouin zone whose volume is too small for significantly influencing the ADP's. Even though the softening in the T'-structure might occur on a line in reciprocal space due to the weak coupling between the adjacent layers, it appears unlikely that this effect could be sufficient in order to explain the observed large ADP. A lattice dynamical model which was developed for the Nd2CuO 4 compound [22] allows further analysis. The calculated phonon contribution to U~~(O1) amounts to 0.020 A2 at 673 K which is twice the room temperature value but significantly lower than the experimental observation. However, the model is based on the Nd dispersion curves and no attempt has been made to describe a compound which is

P. Vigoureux et al. / Physica C 273 (1997) 239-247

245

(a)

(b)

(c)

Fig. 5. (a) Ortep plot of the Gd2CuO4-structure at 673 K; the drawn ellipsoids represent an probability density of 80%. CuO 2 planes described in (b) the ideal T' structure, and (c) with a statistical distribution of the Ol-site.

246

P. Vigoureux et al. / Physica C 273 (1997) 239-247

Table 1 Results of the structural refinements with the data set obtained at 673 K. The left column corresponds to the ideal T-structure and the right column to the split model where the Ol-site is statistically distributed, see also Fig. 5c. The lattice parameters were determined by powder X-ray diffraction, a = 3.9073(3) ~. and c = 11.922( 1) ,~ Gd2CuO 4 14/mmm R~(F 2) [%] R(F 2) [%] G.O.F.

4.03 3.86 2.65

Gd2CuO 4 split model ! 4/mmm 4.02 3.85 2.64

Gd Occ z Gd Gd Ut, [~2]

0.863(10) 0.3494(1) 0.0096(5)

0.864(9) 0.3494(1) 0.0096(5)

Gd U33 [,~2]

0.0086(6)

0.0086(6)

Cu Ull [~2] Cu U33 [~2]

0.0067(5) 0.0134(7)

0.0066(5) 0.0134(7)

x O41) O4 o U~ [~2 ]

0.000 0.0360(11)

0.028(10) 0.023(11)

0 41)U22 [~2 ] 041) U33 [~2]

0.0074(8) 0.0210(9)

0.0074(8) 0.0210(9)

o~2)u,, [~2]

O.OLO0(5)

O.OLOO(5)

O42)U33 [~2]

0.0161(7)

0.0161(7)

giso X 104

1.03(9)

1.03(9)

Gd_Oo) [~2]

2.6534(8)

Gd_O(2) [,~2] Cu.Ot, ) [.~2]

2.2850(6) 1.9536(1)

unstable against the rotation transition like Gd2CuO 4. An investigation of the lattice dynamics in Gd2CuO 4 is under progress and should help to clarify the character of the transition. Furthermore, one may compare U~(O1) to the corresponding parameter in E H 2 C u O 4 at room-temperature, Uj j(O1) = 0.0130 ~2 [23], which is also higher than the lattice dynamical prediction. The Eu compound presents no long range distortion at room-temperature; however, magnetic and Raman measurements indicate that it is close to the transition [4,5] which was indeed recently observed by neutron diffraction [24]. (Extrapolating the room-temperature Eu ADP to the temperature of 673 K results in 0.030 ,~2 which is still below the value observed in Gd2CuO4.)

The enhancement of the U,, parameter should be interpreted by a local distortion in the high temperature T'-phase, which was already conjectured from the temperature and composition dependences of the c-parameter. Therefore, a second structural model where O1 is distributed statistically over ( + x , 0.5, 0.0) was refined, see Table 1; (the arrangement in the CuO 2 planes is depicted in figure 5c). This split model converges but gives no significant improvement of the data description. The strong correlation between x and Uj~ avoids a proper determination, but the obtained UI~ value agrees with the lattice dynamical calculations. It appears most probable that also the Eu compound at room temperature presents a local distortion characterized by a smaller rotation angle. The weak inter-layer coupling of the structural distortion may favor an only two-dimensional coupling, before the three-dimensional structural transition takes place. For Ce-doped samples [10] a local deformation of the rotation scheme should, furthermore, follow a possible phase separation, in doped and undoped clusters which was proposed in [25]. The phase-diagram of Ndl.85_xTb0.15CexCuO4 [10] shows an intermediate region where the samples exhibit neither superconductivity nor a long range distortion. It appears most likely, that these samples are locally distorted.

4. Conclusion

We have presented different diffraction experiments in order to characterize the transition from the tetragonal T'-phase towards the orthorhombic distorted structure in Gd2_~CexCuO 4. In spite of the fact that there is no observed orthorhombic splitting, the peak profiles and intensity ratios point towards an orthorhombic twined structure confirming the structure analysis of Gd2CuO 4 at room temperature in space group Acam [7]. The temperature dependence of the order parameter indicates a continuous transition which is shifted to higher temperatures by Ce doping. There is a coupling to a spontaneous strain parallel to the c-axis; the rotation enhances the c parameter. However, the quantitative analysis of the temperature dependences reveals an unusual behavior compared to typical displacive phase transitions. Furthermore, the in-

P. Vigoureux et al. / Physica C 273 (1997) 239-247

crease of c with respect to the temperature dependence is smaller than that with respect to the substitution dependences. These observations are interpreted as an indication for a local distortion persisting in the high temperature phase. Hence, the transition might possess a significant order disorder character. Other explanations would need rather unusual assumptions for the substitution influence on the c parameter. The high temperature structure analysis reveals for most of the thermal parameters values close to the expectations from lattice dynamical calculations. Only the Ull parameter of the plane oxygen which is the most sensitive to the rotation transition is substantially enhanced. It seems unlikely that this enhancement can be accounted for by a harmonic softening of the rotation mode. Again the assumption of disorder in the high temperature phase can explain the experimental observation.

[7]

[8]

[9]

[10] [11]

[12]

[13] [14]

Acknowledgments We are pleased to acknowledge various stimulating discussions with W. Reichardt; we further thank him for providing us some results of his lattice dynamical calculations.

[15] [16] [17]

[18]

References [1] R.J. Birgeneau, G. Shirane, in: Physical Properties of High Temperature Superconductors I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989), [2] J.D. Axe, A.H. Moudden, D. Hohlwein, D.E. Cox, K.M. Mohanty, A.R. Moodenbaugh and Y. Xu, Phys. Rev. Lett. 62 (1989) 2751. [3] P. Adelmann, R. Ahrens, G. Czjzek, G. Roth, H. Schmidt and C. Steinleitner, Phys. Rev. B 46 (1992) 3619. [4] M.A. Laguna, M.L. Sanjuan and M. Kanehisa, Physica C 235-240 (1994) 1193; M. Udagawa, Y. Nagaoka, N. Ogita, M. Masada, J. Akimitsu and K. Ohbayashi, Phys. Rev. B 49 (1994) 585. [5] S.B. Oseroff, D. Rao, F. Wright, D.C. Vier, S. Schultz, J.D. Thompson, Z. Fisk, S.-W. Cheong, M.F. Hundley and M. Tovar, Phys. Rev B 41 (1990) 1934. [6] K.A. Kubat-Martin, Z. Fisk and R. Ryan, Acta Cryst. C 44 (1988) 1518;

[19] [20]

[21] [22]

[23]

[24] [25]

247

P. Galez, P. Schweiss, G. Collin and R. Bellisent, J. LessCommon Met. 164-165 (1990) 784. M. Braden, W. Paulus, A. Cousson, P. Vigoureux, G. Heger, A. Gukasov, P. Bourges and D. Petitgrand, Europhys. Lett. 25 (1994) 625. N. Pyka, N.L. Mitrofanov, P. Bourges, L. Pintschovius, W. Reichardt, A.Y. Rumiantsev and A.S. Ivanov, Europhys. Lett. 18 (1992) 711. M. Braden, P. Adelmann, W. Paulus, P. Vigoureux, G. Andr6, P. Schweiss, A. Cousson, A. Gukasov, S.N. Barilo, D.I. Zhigunov and G. Heger, Physica C 235-240 (1994) 793. M. Braden, P. Adelmann, P. Schweiss and T. Woisczyk, Phys. Rev. B 53 (1996) 2975. M.B. Maple, N.Y. Ayoub, J. Beille, T. Bjornholm, Y. Dalichaouch, E.A. Early, S. Ghamaty, B.W. Lee, J.T. Markert, J.J. Neumeier, G. Nieva, L.M. Paulius, I.K. Schuller, C.L. Seaman and P.K. Tsai, Transport Properties of Superconductors (World Scientific, Singapore, 1990) pp. 536. C. Lin, G. Lu, Z.X. Liu and Y.F. Zhang, Physica C 194 (1994) 66; D. Fuchs, M. Noll, A. Grauel, C. Geibel and F. Steglich, Physica B 194-196 (1994) 2255. M. Braden, G. Heger, P. Schweiss, Z. Fisk, K. Gamayunov, I. Tanaka and H. Kojima, Physica C 191 (1992) 455. J.C. Le Guillou and J. Zinn-Justin, Phys. Rev. Lett. 39 (1977) 95. A.D. Bruce and R.A. Cowley, Structural Phase Transitions (Taylor and Francis, London, 1981). E. Salje, Phase Transitions in Coelastic and Ferroelastic Crystals (Cambridge University Press, Cambridge, 1990). For a review see: K.A. Miiller and G. Benedek, eds., Phase Separation in Cuprate Superconductors (World Scientific, Singapore, 1993). P. Bordet, J.J. Capponi, C. Chaillout, D. Chateigner, J. Chenavas, T. Fournier, J.L. Hodeau, M. Marezio, M. Perroux, G. Thomas and A. Varela, Physica C 193 (1992) 178. H. Okada, M. Takano and Y. Takeda. Physica C 166 (1990) 111. M. Braden, W. Schnelle, W. Schwarz, N. Pyka, G. Heger, Z. Fisk, K. Gamayunov, I. Tanaka and H. Kojima, Z. Phys. B 94 (1994) 29; H. Takahashi, H. Shaked, B.A. Hunter, P.G. Radaelli, R.L. Hittermann, D.G. Hinks and J.D. Jorgensen, Phys. Rev. B 50 (1994) 3221. M. Braden, unpublished data. L. Pintschovius, N. Pyka, W. Reichardt, A.Yu. Rumiantsev, N.L. Mitrofanov, A.S. lvanov, G. Collin, P. Bourges, Physica B 174 (1991) 323; W. Reichardt, private communication. P. Vigoureux, W. Paulus, M. Braden, A. Cousson, G. Heger, J.Y. Henry, V. Kvardakov, A. Ivanov and P. Galez, Physica C 235 (1994) 1263. P. Vigoureux, Ph.D. thesis at the Universit6 Paris Sud (1995). W. Henggeler, G. Guntze, J. Mesot, M. Klausa, G. SaemanIschenko and A. Furrer, Europhys. Lett. 29 (1995) 233.