Sm2+ fluorescence and absorption in cubic BaCl2: Strong thermal crossover of fluorescence between 4f6 and 4f55d1 configurations

The optical absorption and fluorescence spectra of polycrystalline cubic-phase barium chloride doped with divalent samarium is reported. X-ray diffraction shows that the cubic phase is stabilised at room temperature by the addition of 12.5% lanthanum trichloride: no evidence for the more common orthorhombic or hexagonal phases of barium chloride is found. Optical absorption measurements are in good agreement with a calculated spectrum for Sm2+ ions at a single site of cubic symmetry, even though the material must contain a substantial fraction of chlorine interstitials and lanthanum substitutional ions. The photoluminescence spectrum shows sharp red line emission from the D-5(1) and D-5(0) levels of the 4f(6) configuration, again consistent with a single site of cubic symmetry, together with a broad emission, also in the red, which arises from the lowest levels of the 4f(5)5d(1) configuration. The relative strengths of these emissions are strongly temperature dependent; at room temperature the broad band from the lowest 4f(5)5d(1) levels peaking at 15,130 cm(-1) is dominant, while at low temperature (similar to 12 K) sharp lines from the D-5(0) level dominate. The most intense line corresponds to the D-5(0)-> F-7(1) transition at 14,300 cm-1, but the nominally forbidden D-5(0)-> F-7(0) transition also has significant intensity. The marked temperature dependence of the intensity of the lines and bands is successfully modelled by rate equations and is explained on the basis of thermally activated crossovers between the 4f(6) (5(5)D(0).(1)) levels and the lowest levels of the 4f(5)5d(1) configuration; it is necessary to include the three lowest levels of the 4f(5)5d(1) configuration sandwiched between the D-5(0) and D-5(1) levels. (C) 2012 Elsevier B.V. All rights reserved.