PHASE-STABILITY OF TETRAGONAL SRCUO2-TYPE COMPOUNDS IN THE (SR,CA)CUO2 SYSTEM

Formation regions of orthorhombic and tetragonal SrCuO2-type compounds in the Sr1-xCaxCuO2 system were studied at 1000 degrees C under both ambient and high pressure conditions. Under ambient pressure, single p hase samples of the orthorhombic phase solid solution were formed in t he region between x = 0.0 and 0.45, mixed phase samples containing the orthorhombic phase in the region between x = 0.5 and 0.65, and the te tragonal phase at x = 0.9. The tetragonal phase solid solution was obt ained under 2-5 GPa in the compositional region between x = 0.2 and 0. 8. The tetragonal phase was stabilized under pressure higher than 4 GP a for the samples with x = 0.0-0.1. No tetragonal phase was obtained i n the samples of Ca content x = 1.0. The tetragonal phase with x = 0.9 obtained under ambient pressure decomposed at above 4 GPa to a mixed phase. The minimum lattice constants for the a-axis in both orthorhomb ic and tetragonal phase were determined to be 0.386 nm. Solid Solution limit for both tetragonal and orthorhombic phases was explained by th e concept of minimum Cu-O distance (= 0.193 nm) to stabilize [CuO4] co ordination. (C) 1997 Elsevier Science B.V.