X-ray Diffraction Study of the Structure and Defect System of Nominally Pure and Er~(3+)- and Ce~(3+)-Activated (Na_(0.5)La_(0.5))MoO_4 Crystals

摘要

Nominally pure and activated (Er~(3+) and Er~(3+) + Ce~(3+)) sodium lanthanum molybdate single crystals grown by the Czochralski technique from the (Na_(0.5)La_(0.5))MoO_4 melt in different environments and then oxidized in air at 1000℃ are characterized in detail by x-ray diffraction (lattice parameters and structure refinement by the Rietveld method and single-crystal techniques). The results confirm that (Na_(0.5)La_(0.5))MoO_4 crystallizes in a tetragonal scheelite structure (sp. gr. I4_1/a). The crystals (especially unannealed crystals grown in neutral atmosphere) are shown to contain oxygen vacancies. In addition, some of the samples contain Mo vacancies. The Er~(3+) distribution over some of the activated crystals is highly inhomogeneous. As a result, the crystals contain Er-enriched zones with a distorted scheelite structure.