ABSORPTION AND FLUORESCENCE PROPERTIES OF CR-3+ DOPED NONSTOICHIOMETRIC GREEN SPINEL

The ground-state as well as the excited-state absorption and fluorescence properties of single crystals of Cr3+ doped nonstoichiometric green spinels MgO-2.6Al2O3 have been studied at various temperatures and in a broad spectral range (300-1700 nm). Evidence is found of the existence of four distinct classes of Cr3+ sites associated with weak, medium, and strong crystal fields with very specific absorption and fluorescence characteristics. The medium and strong field Cr3+ sites are associated with Cr3+ ions having more or less complex environments of Al3+ and Mg2+ ions and of vacancies due to the site inversions which characterize these spinel crystals and to their nonstoichiometry. They give rise to long-lived fluorescences between about 700 and 780 nm typical of (2E,4T2) --> 4A2 optical transitions. The weak field Cr3+ sites are attributed to Cr3+ ions having a nearest-neighbor cation vacancy in a [011] direction, thus to Cr3+ centers having a well-defined rhombic site symmetry. They give rise to a very interesting broadband infrared fluorescence peaking around 875 nm with a 23-mu-s lifetime that does not depend on the temperature (at least up to approximately 300 K) that we examine in detail.