In situ neutron diffraction study of deuterated portlandite Ca(OD)₂ at high pressure and temperature

The structure of deuterated portlandite, Ca(OD)₂, has been investigated using time-of-flight neutron diffraction at pressures up to ∼4.5 GPa and temperatures up to ∼823 K. Rietveld analysis of the data reveals that with increasing pressure, unit-cell parameter c decreases at a rate about 4.5 times larger than that for a, which is largely due to rapid contraction of the interlayer spacing in this pressure range. Fitting of the determined cell volumes to the third-order Birch-Murnaghan equation of state yields a bulk modulus (K₀) of 32.2± 1.0 GPa and its first derivative (K₀′) of 4.4 ± 0.6. Moreover, on compression, hydrogen-mediated interatomic interactions within the interlayer become strengthened, as reflected by decreases in interlayer D⋯O and D⋯D distances with increasing pressure. Correspondingly, D-D, the distance between the three equivalent sites over which D is disordered, increases, suggesting a pressure-induced hydrogen disorder. This behavior is similar to that reported in brucite at elevated pressure. On heating at ∼2.1 GPa, cell parameter c increases more rapidly than a, as expected. However, because of the pressure effect, the thermal expansion coefficients, particularly along c, are much smaller than those at ambient pressure. With increasing temperature, the three partially occupied D sites bec ome further apart, and the D-mediated interactions, mainly the interlayer D⋯D repulsion, become weakened.