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.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.