Thermal behavior and crystal structure at 435 K of the mixed rubidium ammonium sulfate tellurate Rb-1.12(NH4)(0.88)SO4 center dot Te(OH)(6)

Crystals of Rb-1.12(NH4)(0.88)SO4. Te(OH)(6) (RNST) have been prepared in a n aqueous solution of H6TeO6 and the corresponding sulfates. The old parame ters and space group found at room temperature are rectified. Indeed, at hi gh temperature, the structure crystallizes in the monoclinic system, space group P2. The lattice constants are determined as follows: a = 7.541(4)Angs trom b = 6.772(4) Angstrom, c = 10.142(6) Angstrom, beta = 98.03(2)degrees, V = 512.8(5) Angstrom (3), Z = 2, D-x = 2.91 g cm(-3), and F(000) = 412. T he residuals are R-1 = 0.037 and wR(2) = 0.082, for 663 observed reflection s refined with 82 parameters. The refined structure, at 435 K, shows that i t is different from that obtained at room temperature, and the bond lengths differ from those obtained at room temperature. The structure is character ized by planes of mixed octahedra and tetrahedra, parallel to be plane, and the cations Rb+/NH4+ are intercalated between them. The structure is stabi lized by two types of hydrogen bonds (0-H . . .0 and N-H . . .O). The phase transitions of the mixed solution RNST are studied by differential scannin g calorimetry and dielectric measurements. It is found that the transition occurs at 458 K to be ferro-paraelectric, whereas the peak at 483 K is attr ibuted to the superprotonic conduction. This transition is confirmed by the conductivity measurements. (C) 2001 Academic Press.