Pressure Dependence of Elastic Constants and Crystal Stability of Alkali Halides: NaI and KI

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Abstract
The single crystal elastic constants and their first and second pressure derivatives have been measured ultrasonically at 298 °K for NaI and KI. Fair agreement with lattice theoretical data calculated from an interaction potential composed of Coulomb, van der Waals and first and second nearest neighbor repulsive forces is found. On the basis of this model it is possible to explain semi‐quantitatively that the first pressure derivatives of the elastic constants of all alkali halides depend almost exclusively on the cation. The convergence of the Taylor expansion of the elastic constants with respect to pressure is investigated theoretically. On the basis of Born's stability criteria the relations between the pressure dependence of the shear moduli c44 and cS to the B1‐B2 transition and the melting curve are investigated.

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