Pressure dependence of the hydrogen-bond geometry in topaz-OD from neutron powder diffraction

The crystal structure of deuterated topaz [topaz-OD; Al₂SiO₄ (OD)₂], synthesized at 10 GPa and 800 °C, has been determined using neutron powder diffraction at pressures up to 7.5 GPa. The linear axial compressibilities obtained from regressions of the lattice constants vs. pressure are β _a = 1.87(1) × 10^-3 GPa^-1, β_b = 1.7t(1) × 10^-3 GPa^-1, and β_c, = 2.73(1) × 10^-3 GPa^-1. The occupancy of the D1 site was found to be greater than that of D2, as shown independently using neutron diffraction and infrared spectra at ambient conditions. A bifurcated hydrogen bond involving the D1 site, O4-D1⋯O2 and O4-D1⋯O3, and a trifurcated hydrogen bond involving D2 site, O4-D2⋯O1, O4-D2⋯O2, and O4-D2⋯O4 are proposed for hydrogen-bond donor and acceptor pairs in addition to those reported previously. The observed pressure dependences of the hydrogen-bonding geometry show that these donor and acceptor pairs are classifiable into two types of interaction: (1) those that strengthen as a function of pressure (O4-D1⋯O3, O4-D2⋯O2, and O4-D2⋯O4) and (2) those that weaken (O4-D1⋯O1 and O4-D2⋯O1). These results also demonstrate that the reason for the contrasting behavior of the v(OH) between F-rich natural topaz and topaz-OH are both the cooperative effect, O4-D2⋯O4-D2⋯O3, and the increasing A1-O4 distance.