The thermal expansion of alkali halides at low temperatures - II. Sodium, rubidium and caesium halides

Abstract

Linear thermal expansions of eleven alkali halides have been determined at room temperature, from 90 to 54 K, and from 30 to 1.5 K. Values are given for the Grüneisen parameter γ = 3 α V / Cx , where α is the linear expansion coefficient, C / V the heat capacity per unit volume, and X is the compressibility. For the four sodium halides γ → γ 0 ≈ 1.0 at the lowest temperature; in NaCl there is evidence of a shallow minimum in γ(T) at about Ɵ D /20. For KCl γ 0 ≈ 0.35, which differs little from our earlier data. Both RbBr and Rbl show a negative expansion below ca . 10 K, having γ 0 ≈ -0.03 and -0.11 respectively, and for RbCl, γ 0 ≈ 0. The limiting values γ 0 agree with γ el 0 , calculated from pressure derivatives of elastic moduli, within limits of experimental uncertainty . CsF which also has NaCl structure was measured but the very limited information about C and X makes γ -values uncertain: approximately γ 0 = 0.3 and γ 283 = 1.7. For CsBr and Csl (each b. c. c.) γ ≈ 2.0 an d is nearly constant.