Electrostrictive Constants of Insulating Crystals

The electrostrictive constants of insulating crystals have been related to the microscopic coupling parameters of lattice theory. This is done within the framework of the Born-Huang method of long waves. It is shown that the electrostrictive constants, as commonly defined, include the effects of Maxwell stresses and local stresses in the crystal. The connection between the theory and the properties of displacive ferroelectrics in the unpolarized phase is discussed. The relation between the observed temperature dependence of the electrostrictive coefficients and the temperature dependent "ferroelectric mode" is exhibited. Order-of-magnitude agreement is obtained between the theory and experiment for SrTi${\mathrm{O}}_3$.