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

The adiabatic elastic constants C 11 , C 12 and C 44 of 7 LiH and 7 LiD have been obtained from measurements of ultrasonic wave transit times through single crystals over the range 4.2-300K. The temperature variation follows quite closely the approximation C ij (T)=C ij (0)(1-D epsilon ). The anisotropy factor A=C 44 /(C 11 -C 12 ) and the deviation ( Delta =C 12 -C 44 ) from the Cauchy relation have been used to make comparisons with the alkali halides. It has been found that the Gruneisen parameter gamma is approximately constant at 1.2 for both 7 LiH and 7 LiD from 50 to 270K. There are signs of anomalous behaviour of the elastic constants in the region of 11K, where a lambda -type heat capacity anomaly has been reported.