SYNTHESIS, CRYSTAL-STRUCTURE, AND THERMAL-BEHAVIOR OF THE RARE-EARTH SULFATES (H5O2)M(SO4)(2) (M = HO, ER, Y)

The compounds (H5O2)M(SO4)(2) (M = Ho, Er Y) were obtained from the me tal oxides M2O3 (M Ho, Er, Y) using diluted sulfuric acid (80%). The c rystal structure of the isotypic compounds has been determined from si ngle-crystal data by direct and Fourier methods [tetragonal, I4(1)/amd (No. 141); (H5O2)Ho(SO4)(2), a = 6.9140(5) Angstrom, c = 17.246(2) An gstrom, R = 0.013; (H5O2)Er(SO4)(2,) a = 6.8967(7) Angstrom, c = 17.16 6(2) Angstrom, R = 0.012; (H5O2)Y(SO4)(2), a = 6.8875(7) Angstrom, c = 17.159(2) Angstrom, R = 0.016]. The characteristic feature of the cry stal structure is a network of edge-sharing [MO8] trigon dodecahedra a nd [SO4] tetrahedra providing channels along [111] which are occupied by disordered H5O2+ ions. In situ X-ray powder investigations exhibit that the compounds are also formed as intermediate phases during the r eaction of the hydrogen sulfates M(HSO4)(3) (M = Ho, Er, Y) with water . According to DSC measurements and temperature-dependent powder diffr action studies, the thermal decomposition of the title compounds follo ws a two-step mechanism. Around 150 degrees C, two molecules of water are driven off, yielding M(HSO4)(SO4) (M = Bo, Er, Y), and finally, at 320 degrees C, H2SO4 (H2O + SO3) is released to give the anhydrous su lfates M-2(SO4)(3).