J O U R N A L O F C H E M I S T R Y Materials Influence of pressure on the crystal structure of Nd2CuO4 Heribert Wilhelm,*a C. Cros,b E. Reny,b G. Demazeaub and M. Hanflandc aDe�partement de Physique de la Matie`re Condense�e, Universite� de Gene`ve, 24, Quai Ernest- Ansermet, CH-1211 Geneva 4, Switzerland. E-mail: Heribert.Wilhelm@physics.unige.ch; Tel. (+41)22 702 62 61. Fax. (+41)22 702 68 69.bInstitut de Chimie de la Matie`re Condense�e de Bordeaux, UPR-CNRS 9048, 87, Avenue Dr. Albert Schweitzer, F-33608 Pessac, France cEuropean Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble, France Received 28th July 1998, Accepted 7th October 1998 The pressure evolution of the crystal structure of Nd2CuO4 (tetragonal T¾-type) was studied at room temperature up to 36 GPa using synchrotron radiation.At PT=21.5 GPa a structural transformation into the T-structure (tetragonal K2NiF4-type) occurred. Upon releasing pressure a gradual distortion of the T-structure into the orthorhombic O-phase (Cmca) was found in the pressure range 10<P<18 GPa, and at lower pressure the starting T¾-phase was restored. The pressure evolution of the unit-cell parameters and the interatomic distances were determined by Rietveld refinement of the diVraction pattern.pressure O-phase [distorted K2NiF4-type, Fig. 1(b)] or T- 1 Introduction phase [K2NiF4-type, Fig. 1(c)] the Ln3+ and the Cu2+ ions In a recent study1 the structural evolution of the solid-solution have now a nine- and a six-fold (elongated octahedron) oxygen La2-xNdxCuO4 (T¾-structure, I4/mmm) under pressure was coordination, respectively.Owing to the poor resolution of investigated for 0.6x1.5. At a pressure PT new lines in the the observed lines in the energy-dispersive X-ray study of energy-dispersive diVraction pattern appeared, indicating a La2-xNdxCuO41 it was not possible to determine whether the structural phase transition. The transition pressure PT increases high pressure phase was distorted or not.It was assumed that with the Nd content x. The PT(x) dependence was related to the same phase sequence (T¾AOAT) is followed as was found compressive stress in the Ln–O2 linkages of the fluorite-type for the tolerance factor.4,5 In the O-phase the CuO6 octahedra LnO2-layers in the T¾-structure. The compressive stress rotate cooperatively about the [110] axis and the crystal decreases when the average lanthanide ion size is reduced, i.e.structure is orthorhombic. This structure is found for La2CuO4 xA2. The pressure eVects were also described in terms of a at low pressure (P<3.4 GPa) and ambient temperature6 or at pressure dependent tolerance factor t, defined by Goldschmidt2 low temperature (T<573 K) and ambient pressure.7 to describe the stability of the related perovskite structure.For For Nd2CuO4 itself first indications of a pressure induced the solid solution La2-xNdxCuO4 it was found that the structural change were found around 20 GPa1 which was at transition pressure PT increases as t decreases.1 the limit of the pressure device used. The experiments presented In the T¾-structure of Nd2CuO4 the Cu-ions at the origin here were performed at the European Synchrotron Radiation (2a-site) are surrounded by four oxygen ions, labeled O(1) in Facility (ESRF) in Grenoble.The accessible pressure range Fig. 1(a), which occupy the 4c-site.3 The lanthanides at the was much higher and the high flux of the synchrotron radiation 4e-site have eight nearest oxygen-ion neighbours [O(2) at 4d- should give diVraction patterns which reveal a better insight site] in a pseudo-cubic symmetry.As pressure is applied to into the structural changes and should allow one to determine this structure, the O(2) ions are forced to the 4e-position the symmetry of the high pressure phase. whereas the other ions remain at their sites. In the high 2 Experimental The polycrystalline sample of Nd2CuO4 was synthesised by high temperature reaction of stoichiometric amounts of the oxides CuO and Nd2O3 in air at 950 °C during 24 h, followed by a second treatment at the same conditions after intermediate grinding.The resulting product was identified by X-ray diVraction, using a conventional powder diVractometer (Cu-Ka and h<70°).The high pressure experiments were performed at ambient temperature using a membrane-type diamond anvil cell (DAC). A well powdered specimen was filled into a 0.125 mm bore, which was drilled in a stainless steel gasket. The gasket was placed between the two diamonds of the DAC. Nitrogen served as pressure transmitting medium. This ensured quasi-hydrostatic pressure conditions up to the highest pressure.The pressure was determined with the ruby luminescence technique8 using the non-linear ruby pressure scale.9,10 Under these circumstances the experimental error in Fig. 1 Schematic view of the structure of (a) the T¾-phase (tetragonal, determining the pressure was <0.2 GPa. The X-ray powder Nd2CuO4-type), (b) the O-phase (orthorhombic, distorted K2NiF4- diVraction spectra were recorded using synchrotron radiation type) and (c) the T-phase (tetragonal, K2NiF4-type) of the Ln2CuO4 at the beamline ID09 at the ESRF.The high X-ray flux of the oxides (Ln=lanthanide). The oxygen atoms O(2) are distinguished from the O(1) ones by a central point. synchrotron combined with the image plate (size A3) provides J. Mater. Chem., 1998, 8, 2729–2732 2729a much better resolution than the technique used in the recent investigation.1 The diVraction images were collected at a wavelength of l=0.4558 A° (E#25 keV) during 60 s exposure time.The images were integrated with the program fit2d.11 The structural parameters and interatomic distances were obtained by Rietveld refinement12 of the diVraction pattern in the range (3°<2h<23°). Isotropic temperature factors were used for all atoms.In each pattern the temperature factors of the Nd and Cu atoms and those of the oxygen atoms were kept the same and these two parameters were refined independently. The N2 diVraction lines, originating from the pressure transmitting medium, were also refined. 3 Results and discussion Fig. 2 shows diVraction patterns of Nd2CuO4 at ambient pressure and P=30.5 GPa.At pressures up to P=20 GPa the tetragonal T¾-phase is stable. In the pressure range 21.5<P<31 GPa a structural phase transformation into the tetragonal T-phase takes place gradually, which is clearly seen from the relative positions of some lines in the pattern recorded at 30.5 GPa in comparison to those in the pattern at ambient Fig. 3 Relative unit-cell volume V/V0 of Nd2CuO4 versus pressure.At pressure [for example, (103) and (110), (114) and (200), (213) PT=21.5 GPa the T¾-phase (bold squares) starts to transform into the high pressure T-structure (bold circles). The T-phase fraction increases and (107)] [see Fig. 2(b)]. The pressure where first signs of with pressure as is depicted in the inset.Upon releasing pressure (open the high pressure phase were observed is chosen as transition symbols) the orthorhombic O-phase (diamonds) appears and at low pressure, and therefore PT=21.5 GPa. Upon releasing pressure the T¾-phase (squares) is restored again. pressure, the T-phase exists down to #18 GPa. The gradual splitting of some characteristic lines [(110), (114), (213), ...] in the pressure range 18>P>10 GPa, indicates a distortion The refinement of the diVraction pattern gave Rwp values of the tetragonal T-phase into the orthorhombic (O) one.This between 3 and 6% and x2<5 (Table 1). is shown explicitly in the inset of Fig. 2 with a pattern obtained The V(P) diagram of Nd2CuO4 is shown in Fig. 3. For the at 12.0 GPa and indexed according to the space group Cmca.T¾-phase the Murnaghan equation of state (EOS)13 Below 8 GPa the low pressure T¾-phase was recovered again. V(P)=V0 AB0¾ B0 P+1B-1/B0¾ (1) was adjusted to the data and a bulk modulus B0=145(1) GPa and its pressure derivative B0¾=4.1(1), with V0= 189.252(1) A° 3 (solid line) and for the high pressure T-phase B0=69(1) GPa and B0¾=8.7(1) were obtained,ively.Using the Birch EOS14 P(V)= 3 2 B0 {x7/3-x5/3} C1- 3 4 (4-B0¾) (x2/3-1)D (2) with x=V0/V(P), gives B0=146(1) GPa and B0¾=4.0(2) for the T¾-phase. Rather diVerent values were obtained for the T-phase [B0=56(5) GPa and B0¾=14(2)]. The deviation of the extrapolated EOS (dotted line in Fig. 3) from the data points above PT is interpreted as a sign of structural changes starting in this pressure range.Up to #30 GPa both the T¾- and T-phase were used to refine the diVraction pattern. As shown in the inset of Fig. 3, the fraction of the T-phase, i.e. the high pressure form, increases in the transition region. It was also possible to use the orthorhombic structure to refine the pattern in this transition region. However, neither the R-values were improved nor the splitting of e.g.the (110) line was obvious. Therefore, the higher symmetry phase is used to describe the diVraction pattern. Above 30 GPa the pattern are well described using the Tphase only. The pattern recorded during pressure release showed a gradual and clear splitting [see inset of Fig. 2(a)] and the orthorhombic O-structure was used to refine the pattern.The corresponding V(P)-data are included in Fig. 3 (open diamonds). Below 10 GPa the same structural parameters were obtained as upon increasing pressure, indicating Fig. 2 Powder diVraction pattern, refined and diVerence pattern as the reversibility of the structural changes. For each phase the well as the peak positions of (a) the low pressure T¾-phase (P=1 bar) lattice parameters, the unit-cell volume, the fractional coordi- and (b) the high pressure T-phase (P=30.5 GPa) of Nd2CuO4.In the nates, and the temperature factors are given in Table 1. The inset the diVraction pattern of the orthorhombic O-phase, clearly entries are chosen for the lowest (highest) pressure at which indicated by the splitting of several lines, obtained on pressure release, is shown. the T-structure (O-structure) were observed. 2730 J. Mater. Chem., 1998, 8, 2729–2732Table 1 Symmetry, structural parameters, site symmetry, fractional coordinates, temperature factors (multiplied by 100), Rwp and x2 values of the ambient (T¾) and high pressure phases (T and O) of Nd2CuO4. The orthorhombic O-phase was obtained upon releasing pressure T¾ (I4/mmm) Z=2 T (I4/mmm) Z=2 O(Cmca) Z=4 P=1 bar P=21.5 GPa P=17.3 GPa a/A° 3.943(1) 3.629(3) 5.1374(4) b/A° 3.943(1) 3.629(3) 12.450(1) c/A° 12.1704(5) 12.40(2) 5.1815(4) V/A° 3 189.252(1) 163.3(3) 331.32(3) atom x/a y/b z/c U/A° 2 x/a y/b z/c U/A° 2 x/a y/b z/c U/A° 2 Nd 4e 0 0 0.352(1) 0.9(1) 4e 0 0 0.387(3) 1.6(1) 8f 0 0.13(1) 0.49(1) 1.6(1) Cu 2a 0 0 0 0.9(1) 2a 0 0 0 1.6(1) 4a 0 0.5 0.5 1.6(1) O(1) 4c 0 0.5 0 1.5(4) 4c 0 0.5 0 2.6(1) 8f 0 0.43(1) 0.51(2) 0.3(2) O(2) 4d 0 0.5 0.25 1.5(4) 4e 0 0 0.16(2) 1.5(4) 8e 0.25 0.48(1) 0.25 0.3(2) Rwp(%) 4.8 5.4 5.7 x2 3.6 4.4 4.4 The transition pressure PT=21.5 GPa found for Nd2CuO4 and B0¾=7.0(5), respectively.The relative lattice parameter change at the transition is #-3% and #+7% for the a- and is in good agreement with the pressure where in a recent work1 first signs of a structural transition were observed.c-axis, respectively. In the crystal structure of the T¾-phase the only free Furthermore, this value gives additional support for the relation between PT and the tolerance factor t to describe the parameter is the z-value of the Nd ion (zNd). It is almost pressure independent up to PT and jumps from z=0.352 to stability of the La2-xNdxCuO4 solid solution.1 For Nd2CuO4 the tolerance factor is t=0.8509 and a transition pressure of zNd=0.387 (Fig. 5). It strongly decreases down to zNd=0.362 at P=27 GPa and is then almost pressure independent up to about 20 GPa can be deduced from the PT–t relation given in Fig. 4 of ref. 1. As the tolerance factor increases (tA1) the the highest pressures.For the T-phase also the position of the oxygen ion O(2) is a free parameter (zO(2)). Its value increases phase sequence T¾AOAT was obtained at normal conditions, 4,5 suggesting that under pressure the O-structure monotonically with pressure from zO(2)#0.155 at PT= 21.5 GPa to #0.195 at P=36.6 GPa (see inset Fig. 5). should occur before the T-phase. However, the refinement of the synchrotron data gave no evidence that the intermediate For the pressure evolution of the crystal structure of Nd2CuO4 a few interatomic distances are important.In the O-structure is attained upon increasing pressure. The T¾-structure transforms in a relatively wide pressure range into T¾-phase the CuKO(1) distance, i.e. dCu–O(1)=a/2=1.9717(5) A° at ambient pressure decreases to dCu–O(1)=1.9146(6) A°at the T-structure. During pressure release however, the O-structure is found at intermediate pressures before the 20.2 GPa.Its pressure dependence is given by that of the aaxis. The pressure dependence of the NdKO(2) distance as T¾-phase is finally formed at low pressure. The T¾-structure is more compressible along the c-axis than well as that of NdKO(1) are well described by the c-axis compressibility. As far as interatomic distances in the high along the a-axis.This is seen from the c/a-ratio (Fig. 4) which decreases from 3.09 to #3.01 at the transition pressure. The pressure phase are concerned, the NdKO(2) and CuKO(2) distances are of particular interest because they determine the pressure dependence of the lattice parameters (inset in Fig. 4) is described by the Murnaghan EOS and gives for the a- and height of the CuO6-octahedron. As is shown in Fig. 5, the c-axis B0=527(3) GPa and B0=326(4) GPa with B0¾=16.7(4) Fig. 5 The fractional coordinate zNd in Nd2CuO4 as function of Fig. 4 The c/a-ratio of Nd2CuO4 versus pressure up to 36 GPa. In the pressure. Above 25 GPa the Nd-position in the T-phase is pressure independent.The inset shows the fractional coordinate zO(2). It inset the pressure variation of the a and c lattice parameter is shown for the T¾- and T-phase. Open symbols represent data obtained during increases with pressure, i.e. the CuO(2)-octahedron is enlarged along the c-axis. pressure release. J. Mater. Chem., 1998, 8, 2729–2732 2731fractional z-coordinate of Nd is rather pressure independent References above 30 GPa and therefore the NdKO(2) distance is deter- 1 H.Wilhelm, C.Cros, F. Arrouy and G. Demanzeau, J. Solid State mined through the pressure variation of zO(2). Above 30 GPa Chem., 1996, 126, 88. the NdKO(2) distance decreases rather strongly (#.7%) and 2 V.M. Goldschmidt, Akad. Oslo I. Mater. Natur., 1926, 2, 7. as a consequence the CuKO(2) distance increases by the same 3 H.Mu¡§ ller-Buschbaum and W. Wollschla¡§ger, J. Anorg. Allg. amount. This means that the CuO6 octahedron is elongated Chem., 1975, 414, 76. with increasing pressure. 4 A. Manthiram and J. B. Goodenough, J. Solid State Chem., 1990, 87, 402. 5 J. F. Bringley, S. S. Trail and B. A. Scott, J. Solid State Chem., 4 Conclusion 1990, 86, 310. 6 J.Shu, J. Akella, J. Z. Liu, H. K. Mao and L. Finger, Physica C, A structural phase transition from the low pressure T�ú- to the 1991, 176, 503. high pressure T-phase was observed for Nd2CuO4 at PT= 7 B. Grande, H. Mu¡§ ller-Buschbaum and M. Schweitzer, Z. Anorg. 21.5 GPa at room temperature using synchrotron radiation. Allg. Chem., 1977, 428, 120. Above PT the T-phase fraction increases and the transition is 8 G. J. Piermarini, S. Block, J. D. Barnett and R. A. Forman, completed at 30 GPa. The transition pressure is in good J. Appl. Phys., 1975, 46, 2774. agreement with the value predicted from the relation between 9 H. K. Mao, P. M. Bell, J. W. Shanner and D. J. Steinberg, Appl. PT and the tolerance factor t of this system. An orthorhombic Phys., 1978, 49, 3276. distorted O-phase was observed during pressure release in the 10 H. K. Mao, J. Xu and P. M. Bell, J. Geophys. Res., 1986, 91, range 10<P<18 GPa. This phase is not observed in the 4673. 11 A. P. Hammersly, ESRF Internal Report EXP/AH/95-01, 1995. pattern upon increasing pressure due to hysteresis in th12 A. C. Larson, GSAS manual, LAUR 86-748, 1986. transition. At low pressure the initial T�ú-phase was found again. 13 F. D. Murnaghan, Proc. Natl. Acad. Sci. USA, 1944, 430, 244. 14 F. Birch, Phys. Rev., 1947, 47, 809. 5 Acknowledgements We would like to acknowledge helpful discussions with Dr. R. C¢� erny about the Rietveld refinement. Paper 8/05886E 2732 J. Mater. Chem., 1998, 8, 2729.