Inhomogeneous broadening of ESR lines of rare earth impurities in scheelite CaWO4 induced by internal electric field gradients

The inhomogeneous broadening of ESR line in erbium-doped CaWO(4) crystal is studied both theoretically and experimentally in the special case when external magnetic field is parallel to crystal c axis. Lorenzian shapes of ESR lines are observed and ascribed to the broadening from random electric field gradients produced by charged defects. The defect concentrations are estimated on the grounds of a simple statistical model and crystal-field calculations of impurity-defect coupling constants. Samples with different concentrations of erbium ions are used in order to find out how Er(3+) impurities affect the linewidth.