Gallium Arsenate Dihydrate under Pressure: Elastic Properties, Compression Mechanism, and Hydrogen Bonding.

Gallium arsenate dihydrate is a member of a class of isostructural compounds, with the general formula M(3+)AsO4·2H2O (M(3+) = Fe, Al, In, or Ga), which are being considered as potential solid-state storage media for the sequestration of toxic arsenic cations. We report the first high-pressure structural analysis of a metal arsenate dihydrate, namely, GaAsO4·2H2O. This compound crystallizes in the orthorhombic space group Pbca with Z = 8. Accurate unit cell parameters as a function of pressure were obtained by high-pressure single-crystal X-ray diffraction, and a bulk modulus of 51.1(3) GPa for GaAsO4·2H2O was determined from a third-order Birch-Murnaghan equation of state fit to the P-V data. Assessment of the pressure dependencies of the unit cell lengths showed that the compressibility of the structure along the axial directions increases in the order of [010] < [100] < [001]. This order was found to correlate well with the proposed compression mechanism for GaAsO4·2H2O, which involves deformation of the internal channel void spaces of the polyhedral helices that lie parallel to the [010] direction, and increased distortion of the GaO6 octahedra. The findings of the high-pressure diffraction experiment were further supported by the results from variable-pressure Raman analysis of GaAsO4·2H2O. Moreover, we propose a revised and more complex model for the hydrogen-bonding scheme in GaAsO4·2H2O, and on the basis of this revision, we reassigned the peaks in the OH stretching regions of previously published Raman spectra of this compound.