Defect-structure of ytterbium(III) doped Na2SO4 phase I

Single crystals of Na1.75Yb0.08SO4, Na1.82Y0.06SO4, and Na(1.70)Ln(0.10)SO(4) (Ln(3+) = 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 high-temperature defect structure of Na2SO4 type I can be supercooled by annealing at 1000 K and subsequent quenching. (La,Yb)-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 e-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) Angstrom) compared to the slowly cooled crystals with the same composition (a = 5.3472(5), c = 7.2102(6) Angstrom). The crystal structures of all synthesized hexagonal crystals were determined from room temperature single-crystal X-ray data and refined in space group P6(3)/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 Ln(3+) 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 (La,Yb)-substituted crystal is rapidly quenched from 1000 K, Ln(3+) is concentrated together with Na at the Na1 site whereas upon slow cooling some Ln(3+) diffuses to Na2. (C) 1999 Academic Press.