Anisotropic thermal expansion and hydrogen bonding behavior of portlandite: A high-temperature neutron diffraction study

The structure of deuterated portlandite, Ca(OD)₂, was investigated using time-of-flight neutron diffraction in the temperature range 308-643 K. Rietveld analysis reveals that with increasing temperature, the c dimension expands at a rate ∼4.5 times larger than that for a. This anisotropy of thermal expansion is due to rapid increase in the interlayer thickness along c with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α=α₀+α₁T, where α₀=5.966×10^-5 K^-1 and α₁=3.333×10^-8 K^-2. On heating, hydrogen-mediated interatomic interactions within the interlayer become weakened, as reflected by increases in the interlayer D⋯O and D⋯D distances with increasing temperature. Correspondingly, the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at high temperatures.