TheP–V–Tequation 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

Abstract

Neutron powder diffraction data have been collected from deuterated ice VI (space groupP42/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). ThisP–V–TEoS 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 givenPandT) than H2O ice VI, in agreement with unpublishedcij/ρ 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 theaaxis than for the protonated isotopologue; the incompressibilities of thecaxis for both H2O and D2O ice VI are very similar. TheP–V–TEoS 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.