Tetrahedral-Framework Lithium Zinc Phosphate Phases: Location of Light-Atom Positions in LiZnPO 4 · H 2O by Powder Neutron Diffraction and Structure Determination of LiZnPO 4 by ab initio Methods

Abstract

The syntheses, crystal structures, and certain properties of two lithium zinc phosphate phases, LiZnPO 4 · H 2O and LiZnPO 4, are reported. LiZnPO 4 · H 2O is an isostructure of the Li- A-type zeolite LiAlSiO 4 · H 2O and consists of a fully ordered three-dimensional network of vertex-sharing ZnO 4 and PO 4 tetrahedral units surrounding 8-, 6-, and 4-ring windows. The extraframework lithium cation and water molecule are located in this cavity system. These nonframework species (including protons) were unambiguously located by Rietveld refinement against powder neutron data. The structure of LiZnPO 4 was solved ab initio using synchrotron X-ray powder data and consists of a new "semicondensed" tetrahedral-framework structure, incorporating the guest lithium cations in squashed 6-ring channels. LiZnPO 4 may be prepared from LiZnPO 4 · H 2O by a first-order phase transition which involves Zn/P/O bond breaking/making. This transformation is briefly discussed and related to similar transformations in other framework systems. Crystal data: LiZnPO 4 · H 2O: M r = 185.31, orthorhombic, space group Pna 2 1 (No. 33), a = 10.575(2) Å, b = 8.0759(9) Å, c = 4.9937(6) Å, V = 426.5(2) Å 3, Z = 4, T = 15(2) K, R p = 3.95%, R wp = 5.02%, χ 2 = 2.33. LiZnPO 4: M r = 167.29, orthorhombic, Pn 2 1 a (No. 33), a = 10.0207(2) Å, b = 6.6731(2) Å, and c = 4.96548(8) Å, V = 332.04(2) Å 3, Z = 4, T = 298(1) K, R p = 11.63%, R wp = 15.22%, χ 2 = 2.82.