X-RAY INVESTIGATIONS OF La2CuO4 AND Pr2CuO4 UNDER HIGH PRESSURE

X-ray investigations ofPr2CuO4 andLa2CuO4 under pressurehave beencarriedoutat 300 K and 30 K. Thelatticeparameters decrease monotonically for both structures thus preserving the structure of the materials For La2CuO4the lattice parameter reduction is isotropic with increasingpressure butthe lattice parameters of Pr2CuO4 arefound tobehave anisotropic as the c axis decreases fasterthan the a axis La2CuO4 hasabulk modulus of 1850kbarat 300 K, whereasPr2CuO4 hasabulk modulus of 1600 kbar at 300 K and 1650 kbarat 30 K due to the fasterdecreasing c axis lattice parameter In the a and b direction both materialshave asimilar compressibility Assigningthis behaviourto the commoncopper-oxide layers ofboth structuresgives the possibility togeneralize these results.Forseveral high temperaturesuperconductorswe estimated thelinear compressibilities in the a, b directionbeing in the rangeof 1.5 to 2.&x l0—~kbar~.In thec-direction thecompressibilityapparently depends on the degeneracyof theoxygen octahedraandthe number ofintercalatedlayers or structuralelements.

1. IntroductIon comparison of La2CuO4 with the non-superconducting Pr2CuO4 we decided to investigate the un-La2CuO4 has the K2NiF4 structure. At tempera-doped and thus non-superconducting La2CuO4 rather tures above 420 K it is tetragonal, but at lower tem-than the doped La2CuO4. peratures it becomes orthorhombjc [1]. There are Pr2CuO4 has a Nd2CuO4 structure [7], which is small distortions of the a-and b-lattice parameters closely related to the K2NiF4 structure. The structure and the Cu-0 octahedra are smoothly tilted. The of Pr2CuO4 is shown in fig. lb. The materials crysstructure is sketched in fig. 1a. For simplicity, we tallizing in the Nd2CuO4 structure are tetragonal. The show the tetragonal unit cell, since the orthorhombic strikingdifference between the structuresof La2CuO4 distortions are too small to be visible. If La2CuO4 is and Pr2CuO4 is the location of the oxygen 0(11) site doped with Ba or Sr it is found to be superconduct- [8]. These oxygen atoms are marked in fig. 1. In ing [2], which was the starting point of the new high-La2CuO4 these oxygen atoms complete the octahe-T~superconducting materials. With increasing con-dral coordination of the copper atom, whereas due tent x of the dopant A = Sr, Ba, La2 _~A~Cu04 be-to the change of the location of the oxygen atoms at comes tetragonal [3,4] also at low temperatures. The the 0(11) site, the Cu-atoms have a square planar superconductivity of La2CuO4 may also be induced environment in Pr2CuO4. by doping with excess oxygen, which ends in the for- The structural investigation of these two materials mula La2CuO4÷ [5,  to the higher compressibility of the NaF. Fig. 3 shows the (200), (020), and (004) reflection of La2CuO4. dependence of the lattice parameters of high-Ta materials [9][10][11][12][13]. transmitting n~edium(NaC1, NaF, ...) and filled into a 0.2 mm bor; which was drilled into an inconel gas-25.6 26. 6 20 ket. The gask~twas placed between the two diamoods of a high-pressure cell [14], which permits   reflections shows just a slight broadening due to uni-~I kbar] axial pressure components. Thus, the nearly con- The Pr2CuO~(101) reflection shows no splitting in the whole in~restigated pressure range. The broadening at 255 $cbar is caused by uniaxial pressure To evaluate the dependence of the lattice parameters under pressure, the lattice parameter g(P) is fitted by the expression (with g0=g at ambient pressure). This gives the linear compressibility /i~, which is normalized and thus independent of the size of the lattice parameter g.
3.0 L _.~For orthorhombic materials, the three linear cornpressibilities are Pa, Pb and fl~. The parameter ög indicates the change of the linear compressibility with F increasing pressure. on pressure of a-and b-lattice parametersãre essen- Fig. 9. c/a ratio of Pr2CuO4 as a function of pressure at 300 K tially the same for La2CuO4 and Pr2CuO4. The only (X).
clear change was observed for the c-axis lattice parameter, with a linear compressibility for Pr2CuO4 (110) reflection. At 180 kbar these two reflections being twice the value of La2CuO4. These different form a single peak. Therefore the c-lattice parameter compressibilities in the c-direction are reflected in shows a stronger decrease under pressure than the a-the bulk moduli, which have been ca1cui~tedusing lattice parameter. We extrapolated the decrease of the Birch equation ofstate [18]. La2CuO4 hasa bulk the a-and c-lattice parameter up to pressures of 255 modulus ofB= 1815 ±100 kbar with a slope B' =5.6, kbar by using the values determined up to 180 kbar.
directions. Regarding the structure of the high-Ta materials, the Cu-0 planes are a common feature. If these planes are mainly responsible for the low compress-Acknowledgement ibility of La 2CuO4 and Pr2CuO4 in the a-, b-direction, there should be similar linear compressibilities We like to thank D. Schiferl and S. Johnson for Pa and I 3b for all high-T 0 materials containing such sharing with us the results of their powder X-ray Cu-0 planes. We estimated the linear compressibil-measurements at high pressures on Pr2CuO4, which ities for five hi~h-Tmaterials using experimental re-initialized our own investigations. sults, which were previously published [9][10][11]13,19].
The obtained values are shown in table 2. For YBa2Cu3O7 and EuBa2Cu3O7 the calculation of the References linear compressibilities is complicated by the ambiguity ofthe àxperimental results. The c-axis lattice