Single crystals of Na1.75Yb0.08SO4, Na1.82Y0.06SO4 and Na1.70(La,Yb)0.10SO4 were grown at 1300K by using the Czochralski method. The Yb- or Y-substituted crystals transformed, upon cooling, into a monoclinic phase which was intergrown in a twin relationship. However, the high-temperature defect structure of Na2SO4 could be supercooled by annealing at 1000K and then quenching. The (La,Yb)-doped crystals maintained, even after moderate cooling, the hexagonal symmetry of Na2SO4 but exhibited very weak and diffuse superstructure reflections; thus indicating a tripling of the c-axis. After annealing (1000K) and quenching, the superstructure reflections disappeared and the (La,Yb)-substituted crystals exhibited slightly contracted cell dimensions (a = 0.5331, c = 0.7188nm) as compared to those (a = 0.53472, c = 0.72102nm) of slowly-cooled crystals having the same composition. The crystal structures of the synthesized hexagonal crystals were deduced from room-temperature single-crystal X-ray data, and were refined for the space-group, P63/mmc. The refined structures were strongly related to the high-temperature phase of pure Na2SO4, but exhibited additional SO4 orientations which were related to the amount of substituted Y3+ and Ln3+ on Na1, and also to the numbers of cation vacancies on Na2 and Na3. The structures revealed a disordered arrangement of 5 different SO4 orientations. If a (La,Yb)-substituted crystal was rapidly quenched from 1000K, the lanthanide was concentrated, together with Na, at the Na1 site. During slow cooling, some of the lanthanide diffused to Na2.

Defect-Structure of Ytterbium-Doped Na2SO4 Phase. T.Armbruster, R.Basler, P.Mikhail, J.Hulliger: Journal of Solid State Chemistry, 1999, 145[1], 309-16