Theoretical insight into the Jahn–Teller system NaCl:Rh2+

The RhCl64− Jahn–Teller centre in NaCl has been explored through Density Functional calculations on clusters involving up to 39 atoms for different values of the Qθ (≈3z2−r2) coordinate, where Qθ=(12/3)(Rax−Req), and Rax and Req mean the axial and equatorial Rh2+–Cl− distances. The equilibrium situation is found for Qθ=30.3 pm (Rax=270.7; Req=244.5 pm) while another minimum is found at Qθ=−21.7 pm whose energy lies 213 cm−1 above. These facts and the different curvature at the two minima are all shown to arise from the anharmonicity of the Eg mode. As a salient feature both the axial (As(ax)) and equatorial (As(eq)) isotropic superhyperfine constants are found to decrease when Qθ increases, As(ax)/As(eq) being around 5 at equilibrium. This surprising result which is pointed out to be related to the 4d–5s hybridisation concurs with experimental findings. The crystal-field and charge transfer transitions calculated at the equilibrium geometry reasonably account for the experimental optical transitions of NaCl:Rh2+.