Thermal Behavior and Crystal Structure at 435 K of the Mixed Rubidium Ammonium Sulfate Tellurate Rb_1.12(NH_4)_0.88SO_4 centre dot Te(OH)_6

摘要

cRYSTALS OF rB_1.12(NH_4)_0.88SO_4 centre dot Te(OH)_6 (RNST) have been prepared in an aqueous solution f H_6TeO_6 and the corresponding sulfates. The old parameters and space group found at room temperature are rectified. Indeed, at high temperature, the structure crystalizes in the monoclinic system, space group P2. The lattice constants are determined as follows: a = 7.541(4) A, b = 6.772(4) A, c = 10.142(6) A, #beta# = 98.03(2)~deg, V = 512.8(5) A~3, Z = 2, D_x = 2.91 g cm~(-3), and F(000) = 412. The residuals are R_1 = 0.037 and wR_2 = 0.082, for 663 observed reflections refined with 82 parameters. The refined structure, at 435 K, shows that it is different from that obtained at room temperature, and the bond leg5hs differ from those obtained at room temperature. The structure is characterized by planes of mixed octahedra and tetrahedra, parallel to bc plane, and the cations Rb~+/NH_4~+ are intercalated between them. The structure is stabilitzed by two types of hydrogen bonds (O-H … and N-H …O). The phase transitions of the mixed solution RNST are studied by differential scanning 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 attributed to the superprotonic conduction. This transition is confirmed by the conductivity measurements.