Correction to the Breakdown of the Long‐Wavelength Approximation and Characteristic Temperatures in Impurity Ion Paramagnetic Resonance

In the temperature dependence of the zero field splitting of PbWO 4 :Mn 2+ , the long‐wavelength approximation breaks down near T b ≈ 150 K. A correction factor is obtained using Bessel functions which explains the observation. In the temperature dependence of the zero field splitting of V 2+ in NaCl, KCl, and RbCl a minimum is found and a zero at characteristic temperatures, T c2 and T 02 , which are related by T 02 = √2 T c2 . This theoretical relation between the two temper‐atures is varified experimentally.