Neutron structural, X-ray powder and vibrational studies of the mixed solid solution rubidium ammonium sulfate tellurate

At room temperature the rubidium ammonium sulfate tellurate salt Rb-1.12(NH4)(0.88)SO4 center dot Te(OH)(6) (RbNST) has been prepared using the reaction of the rubidium sulfate, ammonium sulfate and telluric acid. its structure has been determined from single crystal using neutron diffraction data. The RbNST crystal is monoclinic structure with P2(1)/a space group. The parameters are: a = 11.440(9) angstrom, b = 6.640(7) angstrom, c = 13.700(1) angstrom, beta = 106.90(7)degrees and Z = 4. The main result is the presence of two different and independent anionic groups (TeO66- and SO42-) in the same crystal. The structure is like built by planes of pure SO4 tetrahedra and pure TeO6 octahedra altering with Rb+ and NH4+ cations statistically disordered on one atomic position. Differential scanning calorimetry traces show one anomaly at 418 K, two epaulments at 483 and 510 K corresponding to three phase transitions and one strong peak at 517 K dealing with the decomposition of the salt. Raman scattering and X-ray powder diffraction measurements on RbNST material, taken between 300 and 620 K are reported in this paper. The spectrums confirm and characterize clearly these phase transitions detected by DSC. (C) 2008 Elsevier B.V. All rights reserved.