Structural basis for high-pressure polymorphism in CuGeO₃

Abstract

Two unusual pressure-induced structural transformations in the inorganic spin-Peierls compound CuGeO₃ have been studied using single-crystal x-ray diffraction and diamond-anvil cell techniques. CuGeO₃-III was formed after nonhydrostatic compression of the ambient pressure phase CuGeO₃-I and the structure determined after quenching back to ambient pressure from 7.0 GPa. CuGeO₃-III is orthorhombic, with space group Pbam. The mechanism of the I-III transformation, which occurs exclusively under nonhydrostatic conditions, involves a shift of half of the Ge atoms to tetrahedral sites adjacent to those occupied in the ambient pressure modification. Hydrostatic compression of CuGeO₃-III from ambient conditions to near 7.0 GPa results in the formation of monoclinic CuGeO₃-IV, with space group P2₁/c. Upon compression, the Ge atoms in CuGeO₃-IV adopt an approximately trigonal bipyramidal coordination environment, formed by the condensation of two tetrahedral sites. Our findings provide a basis for the formulation of a general mechanism explaining the pressure-induced transformations in CuGeO₃ and their sensitivity to deviatoric stress.