On the Pressure Derivatives of the Second‐Order Elastic Constants of Ionic Solids

Abstract

Abstract A method of direct differentiation is proposed for obtaining expressions for the first‐ and secondorder pressure derivatives of the second‐order elastic constants (SOEC) from the expressions for the SOEC and the equilibrium condition corresponding to a state of the solid under a hydrostatic pressure. The method is simple and straightforward and can be used effectively as a check on the correctness of the expressions for the third‐ and fourth‐order elastic constants. An application of the corrected expressions for the fourth‐order elastic constants of CsCl structure solids gives the values of the second‐order pressure derivatives of the SOEC in much better agreement with the experimental values than those obtained from the erroneous expressions.