High-pressure crystal chemistry of chromous orthosilicate, Cr_2SiO_4. A single-crystal X-ray diffraction and electronic absorption spectroscopy study

摘要

The high-pressure behaviour of chromous orthosilicate, Cr_2SiO_4, has been studied by means of single-crystal X-ray diffraction and electronic absorption spectroscopy. X-ray diffraction data show that the structure remains orthorhombic to the highest pressure reached of 9.22 GPa. The compressibility of the unit-cell is strongly anisotropic with the c axis approximately six times more compressible than the a and b axes. A third-order Birch-Murnaghan equation of state fitted to the volume-pressure data yields V_o = 610.10(3) A, K = 94.7(4) GPa, K' = 8.32(14). Cr_2SiO_4 is therefore more compressible than the isostructural Cd analogue, even though its molar volume is smaller. This unusual behaviour can be attributed to the fact that the Cr atom is too small for the six-coordinated site that it occupies, and the site is therefore strongly distorted. Structure refinements indicate that under high pressures the Cr atom remains strongly displaced from the central position of the octahedron. Polarized and unpolarized electronic absorption spectra include a strong absorption band occuring at 18.300 cm~(-1) for E//c (which is parallel to the shortest Cr-Cr vector in the structure) which has an unusually large half width (5000 cm~(-1)), indicative of electronic interaction between metal centres. Deconvolution of unpolarized high-pressure spectra show that the relative integrated intensity of this component increases linearly from 40% at 1 bar to 60% at 11.2 GPa. Both the structural changes and the absorption spectra at high pressures suggest that pairs of adjacent Cr atoms in chromous orthosilicate form chromium dimers with a weak metal-metal bond, which is consistent with the diamagnetic response found at ambient pressure.