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 novel proton conducting solids, resulted in the new compounds CsNa2(HSO4)3 and Cs2Na(HSO4)3. Single-crystal X-ray diffraction (performed at room temperature) revealed CsNa2(HSO4)3 to crystallize in the cubic space group P213 with lattice parameters a = 10.568(2) Å and Z = 4, whereas Cs2Na(HSO4)3, studied by both single-crystal neutron and X-ray methods, crystallizes in the hexagonal space group P63/m. The latter compound has lattice parameters a = 8.5712(17) and c = 9.980(2) Å, and Z = 2. The unit cell volumes are 1180.4(4) and 634.9(2) Å3, respectively, giving calculated densities of 2.645 and 3.304 mg m-3. Refinement using all observed 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-ray 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 rings. The geometric match between the NaO6 octahedra and the rings suggests the sodium polyhedra may serve to template the (HSO4)3 unit. In CsNa2(HSO4)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, within the “tetrahedral” sites. The rings in Cs2Na(HSO4)3 are linked together by NaO6 octahedra to form infinite Na(HSO4)3 chains that extend along [001]. The hexagonal compound exhibits disorder about the sulfate tetrahedron that suggests a P63/m → P6 phase transition may occur upon cooling.