Abstract
An extended three-body force shell model (ETSM) for alkali cyanides has been developed by including the effects of coupling between the translational and rotational motions of cyanide (CN -) molecules in the framework of original TSM devised by Singh and Sanyal [Physica 132B (1985) 201]. This ETSM has been applied to describe the static, dynamic, anharmonic elastic and photoelastic properties of the alkali cyanide crystals. This model has satisfactorily reproduced their experimental phonon dispersion curves and cohesive energy values. However, the agreements between the experimental and our theoretical values on the third order elastic constants are not so good. These agreements have been found to improve significantly with the exclusion of three-body interaction (TBI) effects, which play a vital role in the prediction of harmonic and anharmonic elastic properties of alkali halides. This might be so due to the overestimated effects of TBI from the Cauchy violations ( C 12– C 44) which are much larger for alkali cyanides than those for alkali halides. It has been concluded that the ETSM, which has reproduced the lattice statics and dynamics of alkali cyanides so well, is incapable to explain their anharmonic elastic and thermal properties. However, this situation is expected to improve by using a more appropriate method for estimating TBI contributions and incorporating the effects of covalent and anharmonic interactions.
Published Version
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