The low temperature thermal expansion and related thermodynamic properties of alkali halides with a caesium chloride structure

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Abstract
Interferometric measurements of the linear thermal expansion coefficients x of CsCl and CsBr between 20 and 273°k are reported. The results are believed to be accurate to within 2% down to 30°K, falling to 3% at 25°k. The Debye characteristic temperatures θ of CsCl and CsI are shown to rise with increasing temperature at intermediate temperatures, in contrast to the results for alkali halides possessing a rocksalt structure. Attributing this to the influence of anharmonic effects, the anharmonic contribution to the heat capacity of Csl is shown to be proportional to temperature above T∼2θ D. Any temperature variation of the Grüneisen parameters γ=β/CVXT, calculated from the volume expansion coefficients β and earlier heat capacity and elasticity measurements, and in the case of CsBr from the pressure derivatives of the elastic constants, is smaller than the uncertainty of the values derived, i.e. approximately 5% at 270°k, rising to approximately 20% at 25°K. In the cases of CsCl and CsI the quasi-harmonic approximation has been applied to calculate the maximum frequencies VD{n) of the Debye distributions having the same nth moments as the actual crystals for −3≤n≤6. As in the case of alkali halides with a rocksalt structure. vD(n) has been found to fall with increasing mass of the halide ion and the variations of vD(n) with n display characteristic minima.

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