Structural basis for high-pressure polymorphism inCuGeO3

Abstract

Two unusual pressure-induced structural transformations in the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_{3}$ have been studied using single-crystal x-ray diffraction and diamond-anvil cell techniques. ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{III}$ was formed after nonhydrostatic compression of the ambient pressure phase ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{I}$ and the structure determined after quenching back to ambient pressure from 7.0 GPa. ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{III}$ is orthorhombic, with space group $\mathrm{Pbam}.$ 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 ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{III}$ from ambient conditions to near 7.0 GPa results in the formation of monoclinic ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{IV},$ with space group ${P2}_{1}/c.$ Upon compression, the Ge atoms in ${\mathrm{CuGeO}}_{3}\ensuremath{-}\mathrm{IV}$ 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 ${\mathrm{CuGeO}}_{3}$ and their sensitivity to deviatoric stress.