The thermoelastic properties of MgSO47D2O (epsomite) from powder neutron diffraction and ab initio calculation

time-of-flight powder neutron diffraction has been used to measure the molar volume of MgSO4.7D(2)O (i) from 1.8 - 300 K at ambient pressure, (ii) from 50 - 290 K at 1.4, 3.0, and 4.5 kbar, (iii) from 0 - 5.5 kbar at 290 K, and (iv) from 0 - 4.5 kbar at 50 K. The data have allowed us to determine the temperature dependence of the incompressibility, (partial derivative K/partial derivative T)(p), (thermodynamically equivalent to the pressure dependence of the thermal expansion, (partial derivative alpha/partial derivative P)(T)) of epsomite throughout its stability field. We observed that the a-axis exhibits negative thermal expansion, alpha(a), from 30 - 250 K at room pressure, turning positive above 250 K and being zero below 30 K. However, each of the crystallographic axes exhibits a sharp change in (partial derivative alpha/partial derivative T) at similar to 125 K, and this appears to correspond to significant changes in the axial incompressibilities with the a- and c-axes softening, and the b-axis stiffening considerably below similar to 125 K. Our thermoelastic results are in agreement with ab initio calculations at zero Kelvin; however the calculations offer no obvious insight into the mechanism responsible for the change in behaviour at low temperature.