The cesium halides have been studied in further Mössbauer measurements employing the 81-keV transition in ${\mathrm{Cs}}^{133}$ at 4.2°K. The values obtained for the recoilless fraction $f_a$ are $100fa=1.59\pm 0.06$ for CsF, 1.45±0.07 for CsCl, 1.35±0.07 for CsBr, and 0.94±0.08 for CsI. The characteristic temperature ${\Theta }_M$ defined by $f_a=\exp (-\frac{3E_r}{2k{\Theta }_M})$ has the value (in °K) 109±1 for CsF, 106±1 for CsCl, 104±1 for CsBr, and 96±2 for CsI. These results are qualitatively discussed in terms of the dispersion and polarization of the lattice modes. Both for CsF, which has the NaCl structure, and for CsI, which can be regarded as monatomic, values of the Debye temperature are deduced from the Mössbauer results and compared with available data.

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