The low temperature thermal expansion of thallous chloride and thallous bromide

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Abstract
Measurements of the linear coefficients of thermal expansion of TlCl and TlBr are reported between 25 and 260 K. The results are believed to be accurate in an absolute sense to within 2% down to 50K, falling to 6% at 25K. The Gruneisen parameters gamma have been calculated using earlier data on the specific heats and elastic constants, between 260 and 80K. Within this range it is concluded: (i) Any variation of gamma with temperature is within the limits of experimental uncertainty. (ii) Any variation of gamma with ionic mass ratio is within the limits of experimental uncertainty. (iii) The mean room temperature value of gamma for TlCl and TlBr, 2.45+or-0.18, exceeds the corresponding mean value for CsCl, CsBr and CsI, 2.04+or-0.12. Estimates of gamma0based upon the pressure derivatives of the elastic constants at room temperature suggest that for TlBr statement (i) may be extended to T=0, but for TlCl( gamma0- gammainfinity) may exceed unity. Moments of the vibrational frequency spectra of the two crystals have been calculated and varying degrees of consistency with different theoretical models have been observed.

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