High temperature x-ray powder diffractometric studies of the superconducting compound YBa 2Cu 3O 7-x from room temperature to 1300 K in air

Abstract

At room temperature, the compound YBa 2Cu 3O 7-x is orthorhombic, distorted perovskite, deficient in oxygen. X-ray powder diffractograms obtained for this compound over a range of temperatures between room temperature (298 K) and 1300 K in air reveal no change in X-ray diffraction pattern up to 1200 K except for slight shift in peak positions and variations in peak intensities because of thermal expansion and stoichiometry changes in oxygen or metal ion sub-lattices. At temperatures above ∼ 1250 K, the compund undergoes an irreversible transition to a new phase analogous to Y 2BaCuO 5 green phase which is an insulator. Temperature variation of lattice parameters shows an anisotropic expansion. Expansion along c-axis being maximum. Coefficients of linear thermal expansion along a and b axes are almost the same ( δ a ≈ δ b ≈7×10 −6. K −1) while along c-axis it is double ( δ c ≈15×10 -6. K −1) than that of a or b. Coefficient of volume thermal expansion of the unit cell ( δ v ≈29×10 −6. K −1) obeys the relation δ v ≈ δ a + δ b + δ c for an orthorhombic symmetry in the range 298–1200K in air.