The low-temperature variation of the elastic constants of lithium hydride and lithium deuteride

The adiabatic elastic constants C11, C12 and C44 of 7LiH and 7LiD have been obtained from measurements of ultrasonic wave transit times through single crystals over the range 4.2-300 K. The temperature variation follows quite closely the approximation Csub(ij)(T) = Csub(ij)(O)(1 - Depsilon-bar). The anisotropy factor A = C44/(C11 - C12) and the deviation (Δ = C12 - C44) from the Cauchy relation have been used to make comparisons with the alkali halides. It has been found that the Grueneisen parameter γ is approximately constant at 1.2 for both 7LiH and 7LiD from 50 to 270 K. There are signs of anomalous behaviour of the elastic constants in the region of 11 K, where a lambda-type heat capacity anomaly has been reported. (author)