New rubidium zinc hydrogen phosphate, Rb2Zn2(HPO4)3: synthesis, crystal structure, and 31P single-crystal NMR.

A new rubidium zinc hydrogen phosphate, Rb2Zn2(HPO4)3, is prepared by an unusual method utilizing long nucleation times. This material is crystallized from a gel with an initial composition of 1.0 ZnO/0.94 P2O5/0.96 Rb2O/0.04 Li2O/41 H2O, while the phosphate concentration equals 1.6 M and pH = 3.5. The gel is placed in a sealed Pyrex flask at 52 degrees C, and after 4.5 months crystallization of Rb2Zn2(HPO4)3 is noticed. This new crystalline compound has a three-dimensional framework structure built from spiral chains of alternating PO4 and ZnO4 tetrahedra connected pairwise and assembled by other PO4 tetrahedra, rubidium ions, and hydrogen bonds. The two rubidium ions, Rb(1) and Rb(2), have an exceptionally low number of oxygen contacts in the first coordination sphere, five and seven, respectively. Crystal data: monoclinic, P2(1)/c (no. 14), a = 12.5880(4), b = 12.7170(8), c = 7.5827(8) A, beta = 96.100(1) degrees, Z = 4. A single-crystal 31P NMR investigation of Rb2Zn2(HPO4)3 was performed employing a two-axis goniometer probe and reveals the presence of three chemically and six magnetically nonequivalent phosphorus sites, in accordance with the crystal structure. 31P chemical shielding anisotropies and isotropic chemical shifts (-3.3(3), -2.6(3), and 2.0(3) ppm) have been determined for the three phosphorus sites.