Phase stability of tetragonal SrCuO 2-type compounds in the (Sr,Ca)CuO 2 system

Abstract

Formation regions of orthorhombic and tetragonal SrCuO 2-type compounds in the Sr 1− x Ca x CuO 2 system were studied at 1000°C under both ambient and high pressure conditions. Under ambient pressure, single phase samples of the orthorhombic phase solid solution were formed in the 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 tetragonal phase at x = 0.9. The tetragonal phase solid solution was obtained 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 GPa for the samples with x = 0.0–0.1. No tetragonal phase was obtained in 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 orthorhombic and tetragonal phase were determined to be 0.386 nm. Solid solution limit for both tetragonal and orthorhombic phases was explained by the concept of minimum CuO distance (= 0.193 nm) to stabilize [CuO 4] coordination.