Spin States of Trivalent Chromium with Tetragonal Symmetry in MgO

Spin transitions between nonadjacent energy levels of Cr³⁺ in tetragonal symmetry in MgO become allowed by the mixing of states which occurs when the applied magnetic field is not along the symmetry axis. The tetragonal electric field arises from association with a next‐nearest‐neighbor (nnn) cation vacancy. When the defect axis is perpendicular to the applied field, energy level and matrix element expressions for the $-\frac{3}{2}\to \frac{3}{2}$ transition are closely related to those for the normally allowed transitions. Two other transitions between alternate levels ($-\frac{3}{2}\to \frac{1}{2}$ and $-\frac{1}{2}\to \frac{3}{2}$ ) become allowed for parallel microwave and static fields. The ESR lines corresponding to these transitions have been observed.