The P-V-T equation of state of D₂O ice VI determined by neutron powder diffraction in the range 0 < P < 2.6 GPa and 120 < T < 330 K, and the isothermal equation of state of D₂O ice VII from 2 to 7 GPa at room temperature

Neutron powder diffraction data have been collected from deuterated ice VI (space group P42/nmc, Z = 10) at 76 state points in its field of thermodynamic stability above 150 K (approximately 0.6 < P < 2.1 GPa) and in a region of metastable persistence below 150 K (0 < P < 2.6 GPa). The refined unit-cell parameters and unit-cell volume have been fitted with six-parameter functions based upon a Murnaghan equation of state (EoS), with simple polynomial expressions to describe the temperature dependence of the volume (or axis length) and the bulk modulus (or axial incompressibility). This P-V-T EoS is compared with earlier empirically derived and computationally obtained equations of state, and with the elastic properties determined by ultrasonic or Brillouin scattering methods. It is found that D₂O ice VI is 5% stiffer (i.e. it has a larger bulk modulus, K, at a given P and T) than H₂O ice VI, in agreement with unpublished c ij/data for D₂O ice VI provided by the Shimizu Laboratory, Gifu University, Japan (S. Sasaki, personal communication). This difference is due entirely to a greater incompressibility parallel to the a axis than for the protonated isotopologue; the incompressibilities of the c axis for both H₂O and D₂O ice VI are very similar. The P-V-T EoS is used to estimate the volume change and enthalpy of melting. Additional data on the isothermal compression of ices II and V at 240 K (0.2-0.4 and 0.4-0.7 GPa, respectively), and of ice VII at 295-300 K (2-7 GPa), are also reported and compared with earlier literature data.