A Genuine Jahn-Teller System with Compressed Geometry and Quantum Effects Originating from Zero-Point Motion

First-principle calculations together with analysis of the experimental data found for 3d(9) and 3d(7) ions in cubic oxides proved that the center found in irradiated CaO:Ni2+ corresponds to Ni+ under a static Jahn-Teller effect displaying a compressed equilibrium geometry. It was also shown that the anomalous positive g(k) shift (g(parallel to) - g(0)= 0.065) measured at T = 20K obeys the superposition of the vertical bar 3z(2)-r(2)> and vertical bar x(2)-y(2)> states driven by quantum effects associated with the zero-point motion, a mechanism first put forward by O'Brien for static Jahn-Teller systems and later extended by Ham to the dynamic Jahn-Teller case. To our knowledge, this is the first genuine JahnTeller system (i.e. in which exact degeneracy exists at the high-symmetry configuration) exhibiting a compressed equilibrium geometry for which large quantum effects allow experimental observation of the effect predicted by O'Brien. Analysis of the calculated energy barriers for different Jahn-Teller systems allowed us to explain the origin of the compressed geometry observed for CaO:Ni+.