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 M3+AsO4·2H2O (M3+ = 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.