Abstract: First-principle calculations together with analysis of the experimental data found for 3d9 and 3d7 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 gk shift (gk¢g0=0.065) measured at T=20 K obeys the superposition of the j 3z 2¢r 2 i and j x 2¢y 2 i 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 Jahn-Teller 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+ .

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