Elastic Constants of the Alkali Halides at 4.2°K

Adiabatic elastic constants of LiCl, NaF, NaCl, NaBr, KF, RbCl, RbBr, and RbI were measured at 4. 2 and 300^, using the ultrasonic pulse-echo technique. Data are also given for several of these materials, for the temperature interval 4. 2 to 300^. It is observed that the elastic anisotropy factor, A=2c₄₄ ({c₁₁-c₁₂) }, increases with temperature. This results in the Li halides becoming less anisotropic and the Na, K, and Rb halides becoming more anisotropic as the temperature decreases toward T=0^K. The Cauchy relation (c₁₂=c₄₄) is not satisfied at 4. 2^ for any of the materials studied. The degree of failure of the Cauchy relation (=c₁₂-c₄₄) is larger at 4. 2^ than at room temperature for the Li and Na halides but is smaller for the K and Rb halides. Also calculated was the T=0^K value {₀}^el of the Debye characteristic temperature from the 4. 2^ elastic-constant data, using the tables given by de Launay. The values of {₀}^el in ^ are: LiCl (4292. 2), NaF (491. 52. 4), NaCl (321. 21. 6), NaBr (224. 61. 2), KF (335. 91. 7), RbCl (168. 90. 85), RbBr (136. 50. 7), and RbI (108. 00. 55). Comparison with {₀}^c the Debye characteristic temperature derived from low-temperature specific-heat data, shows the two to be equal within experimental error in almost all cases. However, there is some indication of a trend toward {₀}^el being larger than {₀}^c by an amount on the order of 1 to 3%. Whether this trend is real or the result of some systematic error in the experiments or analysis is not known at this time.