Crystal structure and optical investigation of Pb2Na1 - xLaxNb5 - xFexO15 (0 ≤ x ≤ 1) phases

The Pb2Na1-xLaxNb5-xFexO15 (0 <= x <= 1) compounds with tungsten bronze structure were refined by the Rietveld method from X-Ray powder diffraction recorded at room temperature and characterized by optical spectroscopy. The results show that the structure depends on x value. For x=0.2, the cell is orthorhombic with non-centrosymmetrical space group Cm2m. The lattice parameters are a=17.6053(39) angstrom, b=17.8245(60) angstrom and c=3.8698(02) angstrom. For x=0.8, the lattice is tetragonal with space group P4/mbm and cell parameters are a=b=12.4911(97) angstrom and c=3.8932(22) angstrom. The luminescence properties of studied materials activated with Eu3+ have been investigated in the visible region at room and low temperatures. The result confirms that the rare earth is located at a single center in the structure. A room temperature Mossbauer study confirmed the +3 valence state of iron and its six-coordination. The site occupancies suggested by optical and Mossbauer data are in good agreement with those obtained from crystal structure refinement. (C) 2012 Elsevier Ltd. All rights reserved.