Vibronic model leading to orthorhombic Jahn-Teller distortions

The eigenvalue problem for the dynamic Jahn-Teller system of an orbital triplet in cubic symmetry coupled to $e$ and $t_2$ vibrational modes has been solved numerically with the simultaneous inclusion of both the linear and quadratic coupling terms. The possibility of orthorhombic Jahn-Teller distortion has been found from the appearance of a nearly-sixfold-degenerate ($T_1+T_2$) ground state in the case when an $E$-type quadratic coupling term or a bilinear coupling term has been considered. The computed result of Ham's reduction factors in the ground state shows the orthorhombic property.