Absorption and fluorescence properties of Cr3+ doped nonstoichiometric green spinel

Publisher Website
Google Scholar
Abstract
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 μs lifetime that does not depend on the temperature (at least up to ∼300 K) that we examine in detail.