Pressure dependence of the optical properties of alkali halide crystals

An equation of Szigeti relating the compressibility of an ionic lattice to its optical properties is generalized so that it is valid at all pressures. This equation is then used to re-analyse the data of Jones (1967) on (∂ In ε0/∂p)T, and leads to less anomalous estimates of the volume dependence of the effective ionic charge. However, detailed investigation of the most recent data still indicates that the effective charge varies less with volume than has been predicted theoretically. Calculations of (dε0/dp) and the Grüneisen parameters yt and yl ; are carried out for two static deformation dipole models of KC1 with the volume dependences suggested respectively by Hardy (1962) and Mitskevich (1964). Disagreement of (dε0/dp) with a low temperature value of (∂ε0/∂p)T extrapolated from Jones's measurements confirms the existence of an apparent anomaly. The values of yt and yl differ little from those of a rigid ion model with the same interionic potential.