ISOMORPHOUS SUBSTITUTION IN NONLINEAR OPTICAL KTIOPO4 - POWDER DIFFRACTION STUDIES OF K0.5RB0.5SNOPO4,K0.5NA0.5TI0.5SN0.5OPO4,NA0.5RB0.5TI0.5SN0.5OPO4 AND K0.5RB0.5TI0.5SN0.5OPO4

The structures of K0.5Rb0.5SnOPO4, K0.5Na0.5Ti0.5Sn0.5OPO4, Na0.5Rb0.5,Ti0.5Sn0.5OPO4 and K0.5Rb0.5Ti0.5Sn0.5OPO4 have been determined through joint Rietveld analysis of time-of-flight neutron and X-ray powder diffraction data: Pn2(1)a, K0.5Rb0.5SnOPO4 a = 13.2382(4) angstrom, b = 10.7083(3) angstrom, c = 6.5581(2) angstrom, V = 929.66(6) angstrom3; K0.5Na0.5Ti0.5Sn0.5OPO4 a = 12.8826(4) angstrom, b = 10.6384(4) angstrom, c = 6.3898(2) angstrom, V = 875.72(8) angstrom3; Na0.5Rb0.5Ti0.5Sn0.5OPO4 a = 13.1435(5) angstrom, b = 10.6433(3) angstrom, c = 6.5478(3) angstrom, V = 915.97(4) angstrom3; K0.5Rb0.5Ti0.5Sn0.5OPO4 a = 13.0440(5) angstrom, b = 10.6304(4) angstrom, c = 6.4961(3) angstrom, V = 900.8(1) angstrom3 . The ordering of Na in other KTP isomorphs is reduced by Sn substitution into the framework, while the partial cation ordering seen in K0.5Rb0.5TiOPO4 decreases only slightly in K0.5Rb0.5Ti0.5Sn0.5OPO4. The preference of Sn for one of the two Ti sites, Ti(2), is unaltered by substitution for K.