Defect-Structure of Ytterbium(III) Doped Na2SO4 Phase I

Single crystals of Na1.75Yb0.08SO4 Na1.82Y0.06SO4 and Na1.70Ln0.10SO4 (Ln3+=La Yb) were grown at 1300 K by the Czochralski method. The Yb3+ and Y3+ substituted crystals transform upon cooling to a monoclinic phase intergrown in a twin relationship. However the hightemperature defect structure of Na2SO4 type I can be supercooled by annealing at 1000 K and subsequent quenching. (LaYb)-bearing crystals preserve even after moderate cooling hexagonal symmetry of Na2SO4 type I but display very weak and diffuse superstructure reflections indicating tripling of the c-axis. After annealing at 1000 K and subsequent quenching the superstructure reflections disappear and the (La Yb)-substituted crystals exhibit slightly contracted cell dimensions (a=5.331(1) c=7.188(1) ?) compared to the slowly cooled crystals with the same composition (a=5.3472(5) c=7.2102(6) ?). The crystal structures of all synthesized hexagonal crystals were determined from room temperature single-crystal X-ray data and refined in space group P63/mmc. The corresponding refined structures are strongly related to the high temperature phase I of pure Na2SO4 but display additional SO4 orientations correlated with the amount of substituted Y3+ and Ln3+ on Na1 and also with the number of cation vacancies on Na2 and Na3. The structures reveal a disordered arrangement of five different SO4 orientations. If a (LaYb)-substituted crystal is rapidly quenched from 1000 K Ln3+ is concentrated together with Na at the Na1 site whereas upon slow cooling some Ln3+ diffuses to Na2.