PRESSURE-DEPENDENCE OF HYDROGEN-BONDING IN METAL DEUTEROXIDES - A NEUTRON POWDER DIFFRACTION STUDY OF MN(OD)(2) AND BETA-CO(OD)(2.)

The structures of deuterated pyrochroite, Mn(OD)(2) and beta-Co(OD)(2) have been refined using the Rietveld method and neutron powder diffra ction data collected in an opposed-anvil high pressure (Paris-Edinburg h) 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 consi stent with previous studies of the isostructural brucite, Mg(OD)(2). T he com pressibility of beta-Co(OD)(2) on the other hand is apparently anomalous, The c-axis initially decreases at 3 times the rate of decre ase 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, dete rmined indirectly from the intermolecular D...O distances, decrease st eadily for both materials as pressure is increased.