Synthesis, structure, and properties of compounds in the NaHSO4-CsHSO4 system. 1. Crystal structures of CS2Na(HSO4)(3) and CsNa2(HSO4)(3)

Exploratory synthesis in the NaHSO4-CsHSO4 system, aimed at discovering nov el proton conducting solids, resulted in the new compounds CsNa2(HSO4)(3) a nd Cs2Na(HSO4)(3). Single-crystal X-ray diffraction (performed at room temp erature) revealed CsNa2(HSO4)(3) to crystallize in the cubic space group P2 (1)3 with lattice parameters a = 10.568(2) Angstrom and Z = 4, whereas CS2N a(HSO4)(3), studied by both single-crystal neutron and X-ray methods, cryst allizes in the hexagonal space group P6(3)/m. The latter compound has latti ce parameters a = 8.5712(17) and c = 9.980(2) Angstrom, and Z = 2. The unit cell volumes are 1180.4(4) and 634.9(2) Angstrom (3), respectively, giving calculated densities of 2.645 and 3.304 mg m(-3). Refinement using all obs erved reflections yielded a weighted residual, R-w(F-2), of 0.0515 based on F-2 X-ray values for CsNa2(HSO4)(3). For Cs2Na(HSO4)(3) the analogous X-ra y and neutron values were 0.0483 and 0.1715, respectively. Both structures contain a single, crystallographically distinct, asymmetric hydrogen bond ( as confirmed by NMR investigations) and unique, three-membered (HSO4)(3) ri ngs. The geometric match between the NaO6 octahedra and the rings suggests the sodium polyhedra may serve to template the (HSO4)(3) unit. In CsNa2(HSO 4)(3) the rings form a distorted cubic close-packed array. The Cs atoms are located within the "octahedral" sites of this array, and the Na atoms, wit hin the "tetrahedral" sites. The rings in CS2Na(HSO4)(3) are linked togethe r by NaO6 octahedra to form infinite Na(HSO4)(3) chains that extend along [ 001]. The hexagonal compound exhibits disorder about the sulfate tetrahedro n that suggests a P6(3)/m --> P6 phase transition may occur upon cooling.