Lattice Dynamics of CsI Containing Impurity Ions

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Abstract
A new effective force constant in crystals introduced earlier in the framework of a nearest-neighbor rigid-ion model of lattice dynamics has been calculated in a self-consistent manner. The present force constant is general in the sense that it can be evaluated for any frequency between zero and infinity. The effective force constants defined by the earlier workers are seen to be special cases of this force constant. The method is applied to determine force constants both at the positive-ion and at the negative-ion sites in CsI, which possesses a CsCl crystal structure. The eigenfrequencies and the eigenvectors for CsI obtained by Mahler in the breathing shell model have been utilized for evaluating the pertinent Green's functions. The results of the calculation of these Green's functions are also reported.