time-of-flight powder neutron diffraction has been used to measure the molar volume of MgSO4.7D2O (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, (∂K/∂T)p, (thermodynamically equivalent to the pressure dependence of the thermal expansion, (∂α/∂P)T) of epsomite throughout its stability field. We observed that the a-axis exhibits negative thermal expansion, αa, 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 (∂α/∂T) at ∼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 ∼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.

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