Investigations of the Pressure Dependence of Optical Spectra for NiO Crystal

Abstract

The optical spectra and their pressure dependence for NiO crystal have been calculated by use of a modified crystal‐field theory, where the normalization parameters Nt and Ne describe the covalency reduction of electronic interactions for t2 and e orbitals and the pressure coefficients dNt/dP and dNe/dP are obtained from a cluster approach. The calculated results are in agreement with the observed values. The difficulty of the discrepancy in the change in Racah parameters B under pressure for the 3T1a and 3T1b transitions occurring in the conventional crystal‐field theory is removed.

Evolution of Chemical Bonding and Spin-Pairing Energy in Ferropericlase across Its Spin Transition

“…The observed behavior with increasing pressure in the HS state shows a decrease in the Racah B parameter, which is in contradiction with molecular orbital calculations of Sherman who argued that there is no significant increase in covalency with pressure. However, it has been experimentally confirmed for several transition metal-bearing complexes that the shortening of interatomic distances at high pressure leads to an increase in covalency. − Furthermore, the decrease in the Racah B parameter from 653 cm –1 (corrected for the d 7 configuration) at 1 atm to 612 cm –1 at 48 GPa (6.3% with (∂ B /∂ P ) T = −0.85 cm –1 /GPa) is broadly consistent with the previous estimations that reported a minor reduction of ∼2–6% over a 20 GPa. ,, …”