Inverse Nephelauxetic Effect in the Pressure Dependence of R-Line Energy of Ruby
In order to clarify the origin of this discrepancy, we have performed first-principles calculations of multiplet energies of ruby under various pressures. The structural optimization around the impurity Cr3+ ion in α-Al2O3 under pressure was performed using the CASTEP code. The model clusters consisting of 63 atoms were constructed based on the optimized structure and multiplet calculations were performed using the DVME code. The underestimation of electron correlation effects were corrected by the configuration dependent correction (CDC) and the correlation correction (CC). In order to clarify the factors which determine the behavior of 2E energy, the effect of covalency and the effect of correlation correction were investigated quantitatively based on the Coulomb integrals.
The results show that the pressure dependence of 2E energy was well reproduced by the first-principles calculations. The detailed analysis indicated that the Coulomb integral of t2g orbital increases for higher pressure as expected from the contraction of the molecular orbitals. However, due to the strong electron correlation effect, the effective Coulomb integral of t2g orbital significantly decreases, resulting in the decrease of 2E energy. This behavior is opposite to the prediction from the ordinary Nephelauxetic effect and can be called an “Inverse Nephelauxetic effect.”