TETRAHEDRAL-FRAMEWORK LITHIUM ZINC PHOSPHATE PHASES - LOCATION OF LIGHT-ATOM POSITIONS IN LIZNPO4-CENTER-DOT-H2O BY POWDER NEUTRON DIFFRACTION AND STRUCTURE DETERMINATION OF LIZNPO4 BY AB INITIO METHODS

摘要

The syntheses, crystal structures, and certain properties of two lithium zinc phosphate phases, LiZnPO4.H2O and LiZnPO4, are reported. LiZnPO4.H2O is an isostructure of the Li-A-type zeolite LiAlSiO4.H2O and consists of a fully ordered three-dimensional network of vertex-sharing ZnO4 and PO4 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 LiZnPO4 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. LiZnPO4 may be prepared from LiZnPO4.H2O 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: LiZnPO4.H2O: M(r) = 185.31, orthorhombic, space group Pna2(1) (No. 33), a = 10.575(2) Angstrom, b = 8.0759(9) Angstrom, c = 4.9937(6) Angstrom, V = 426.5(2) Angstrom(3), Z = 4, T = 15(2) K, R(p) = 3.95%, R(wp) = 5.02%, chi(2) = 2.33. LiZnPO4: M(r) = 167.29, orthorhombic, Pn2(1)a (No. 33), a = 10.0207(2) Angstrom, b = 6.6731(2) Angstrom, and c = 4.96548(8) Angstrom, V = 332.04(2) Angstrom(3), Z = 4, T = 298(1) K, R(p) = 11.63%, R(wp) = 15.22%, chi(2) = 2.82. (C) 1995 Academic Press, Inc. [References: 49]