Pressure dependence of hydrogen bonding in metal deuteroxides: A neutron powder diffraction study of Mn(OD)₂ and β-Co(OD)₂

The structures of deuterated pyrochroite, Mn(OD)₂ and β-Co(OD)₂ have been refined using the Rietveld method and neutron powder diffraction data collected in an opposed-anvil high pressure (Paris-Edinburgh) cell from room pressure to 9 GPa. The equation of state for Mn(OD)₂ was determined (K=41(3) GPa for fixed K'=4.7) and found to be consistent with previous studies of the isostructural brucite, Mg(OD)2. The compressibility of β-Co(OD)₂ on the other hand is apparently anomalous. The c-axis initially decreases at 3 times the rate of decrease of the a-axis; the ratio decreases to about 1.5 at an estimated 6 GPa before increasing again beyond this pressure. There is no obvious corresponding anomaly in the details of the atomic structure. In both materials there is an increase in the D-site disorder with pressure. A split-site model for the D-positions best fits the data at pressures above 8 GPa. There is no statistically significant increase in the O-D interatomic distance at increased pressure while the hydrogen bonding interaction D...O appears to increase as this distance decreases and the O-D...O angle increases. The intramolecular O-D bond valences, determined indirectly from the intermolecular D...O distances, decrease steadily for both materials as pressure is increased.