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The Mn4+ center in mainly octahedral symmetry has been studied in magnesium fluorogermanate phosphors. Emission and absorption spectra and the temperature dependence of various parameters have been evaluated. Absorption takes place from the 4A2 ground state into the 4F2, 4F1, and higher excited states whereas both emission bands, which lie in the red region of the spectrum, correspond to transitions from the split 4F2 state to the ground state. Electrons in 4F1 and higher quartet states decay nonradiatively into the 4F2 state. The crystal field parameters Dq for the absorption and emission are 2400 and 1600 cm—1, respectively. The electronic and vibrational structure, the temperature dependence and kinetics of emission can be explained by a configurational (normal) coordinate diagram. This coordinate is associated with the distortions from octahedral symmetry and is thus different from that associated with the octahedral crystal field parameter Dq.

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We consider the various physical mechanisms that contribute to the ground-state energy (Eg) of the ordered pyrite (OP) and the marcasite (M) phases of Na02. We find that the ionic aisd molecular-crystal contributions to Eg (Madelung potential, 02 -Na+ and 02 -02 repulsion, Van der Waals and quadrupole-quadrupole interaction between 02 molecules) favor the OP structure by an energy of 2204 K per 02 molecule. Contributions to Eg arising from splitting ot the orbital degeneracy of the 02 ion, quadrupoleelectric-field gradient interaction, and antiterromagnetic exchange coupling between 02 spins favor the M structure by 2468 K per 02 molecule. An estimate of the librational zero-point energies in both phases suggests that the energies of the two phases are very close to each other. Within the present accuracies of our calculation we cannot definitely conclude that M structure has lower energy at T =0 as seen experimentally. However the molecular orientations obtained in each phase from energy niinimization agree well with the experiment. The first-order phase transition from OP to disordered pyrite (DOP) has been studied in a molecular-field approximation. The results are in reasonable agreement with experiment. We find that 02 -Na+ repulsion plays an important role in the observed orientational order-disorder transition. The theoretical value of the order-disorder transition temperature is calculated to be 300 K compared to 223 g obtained experimentally. The discontinuity in the entropy at the transition is calculated to be k ln1. 62 whereas the experimental value is k ln2. 35. The possible sources ot discrepancy between theory and experiment are discussed,