Athermochemical Investigation of the La-Sr-Cu-O System
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ATHERMOCHEMICAL INVESTIGATION OF THE La-Sr-Cu-O SYSTEM JOSEPH BULARZIK AND ALEXANDRA NAVROTSKY Dept. of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544 BRUCE SCOTT, JOE BRINGLEY AND STEVE TRAIL IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT The perovskite related La2.xMxCuO4.y oxides substituted with alkaline earth metals were one of the first classes of high temperature superconductors discovered. Determining the thermodynamic properties is important to understand the stability and superconducting mechanism of these structures. High temperature solution calorimetry, using a molten lead borate solvent, has been performed on La2 CuO 4 and the related Sr substituted oxides. Calorimetric measurements on CuO, La2 0 3 and SrCO 3 yield heats of formation. A change in the trend of the heats of formation appears at Sr content 0.1, the reported orthorhombic to tetragonal transition, the onset of supercgnductivity and loss of oxygen. INTRODUCTION The La(2.x)Sr(x)CuO(4.y) series of compounds based on the K2 NiF 4 structure contains perovskite structure alternating with rocksalt-like layers. The compound is single phase with an orthorhombic to tetragonal transition at x =0.10. Superconductivity has been measured above 4K between x = 0.10 and 0.30, with the highest Tc at x =0.15. The substitution of La+3 by Sr+2 is charge-compensated by the conversion of Cu+2 to Cu+3 , at least interms of formal oxidation state. Loss of oxygen compensates for some of the charge change of the substitution. The oxygen content can be changed by different preparation methods, and is reversible. Different oxygen contents effect the superconducting properties. Rather little is known about the thermodynamic properties of these new materials. Important questions are: "Can the energetics of the coupled ionic substitutions be separated from the vacancy forming reduction reaction?' and "Are the energetics of the reduction reaction constant per mole of 02 as in the
case of YBa 2 Cu3 Ox?" EXPERIMENTAL Material Preparation The La2.xSrxCuO4.y samples were prepared by three cycles of grinding, pressing into a pellet and heating the stoichiometric amounts of CuO, La2 0 3 and SrCO 3 . All heatings were done in air between 1173 and 1273 K,with a final step of slowly cooling the pellet from 1273 K. X-ray diffraction patterns matched those of Goodenough [1]. The samples were sealed under argon until needed for calorimetry, then kept ina desiccator. The lattice parameters did not change after heating for several hours at 975 K,which is analogous to calorimetric conditions. Calorimetry Thermochemical measurements were made using a high temperature Calvet-type reaction calorimeter operating at 975 K,with molten 2PbO.B2 0 3 as the solvent [2]. Three experimental procedures were used. Solution calorimetry consists of suspending a 10 - 50 mg sample above the solvent to reach thermal equilibrium, and then stirring the sample into the solvent, measuring a heat of solution. Transposed temperature-drop calorimetry co
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