Influence of pressure and temperature on the crystal structure of deuterated ammonium copper tutton salt, (ND4)2[Cu(D2O)6](SO4)2

The influence of pressure on the unit cell parameters of deuterated ammonium copper(II) sulfate hexahydrate at various temperatures from 50 to 325 K obtained from pulsed neutron powder diffraction is reported. Application of pressure causes the structure to switch to a packing arrangement like that observed for the corresponding hydrogenous compound over the complete temperature range, with a higher pressure being required at lower temperatures. A pressure hysteresis occurs upon the release of pressure in the temperature range 303275 K and below similar to295 K the compound remains in the high pressure (high density) modification at 1 bar. As the temperature is raised from similar to296 to 298 K, the structure reverts progressively to the low-pressure modification. Using the Clapeyron equation, an enthalpy DeltaH of 259 cm(-1) at 260 K was evaluated for the transition. With zero applied pressure, a negative temperature expansion coefficient is observed above 230 K, apparently due to an increasing thermal population of the high-density packing arrangement at higher temperatures.