The quadratic Jahn-Teller effect of a triply degenerate $T$ electronic term in an $O_h$ complex interacting with ${\alpha }_{1g}$, ${\epsilon }_g$, and ${\tau }_{2g}$ modes has been investigated retaining only terms bilinear in the totally symmetrical $Q_1$ coordinate. The resulting coexistence charts in the parameter space confirm the existence of tetragonal and trigonal distortions, while the orthorhombic points can never become minima. A complete treatment involving all of the quadratic terms has been performed numerically for KCI: T${\mathrm{I}}^{+}$ and KBr: T${\mathrm{I}}^{+}$, evaluating the parameters by the angular-overlap model.

• Received 28 March 1979

DOI:https://doi.org/10.1103/PhysRevB.20.5358