RESUMO
The structure of the high-temperature monoclinic variety α-SrHfF6 (strontium hafnium hexafluoride) [and of isostructural α-SrZrF6 (strontium zirconium hexafluoride)] associates Hf2F12 bipolyhedra and SrF8 snub disphenoids, forming zigzag twisted [SrF6]n layers. The distribution of the Hf and Sr polyhedra forms a three-dimensional framework which can be related to the family of anion-excess ReO3-related superstructures. α-SrHfF6 corresponds to a new ABX6 type and is compared to the other main families already described. A partial amorphization of this structure is observed in samples quenched from the melt.
RESUMO
From high-precision Brillouin spectroscopy measurements, six elastic constants (C11, C33, C44, C66, C12, and C14) of a flux-grown GeO2 single crystal with the α-quartz-like structure are obtained in the 298-1273 K temperature range. High-temperature powder X-ray diffraction data is collected to determine the temperature dependence of the lattice parameters and the volume thermal expansion coefficients. The temperature dependence of the mass density, ρ, is evaluated and used to estimate the thermal dependence of its refractive indices (ordinary and extraordinary), according to the Lorentz-Lorenz equation. The extraction of the ambient piezoelectric stress contribution, e11, from the C'11-C11 difference gives, for the piezoelectric strain coefficient d11 , a value of 5.7(2) pC N(-1), which is more than twice that of α-quartz. As the quartz structure of α-GeO2 remains stable until melting, piezoelectric activity is observed until 1273 K.
