Original paper

The crystal structure of wycheproofite, NaAlZr(PO4)2(OH)2 H2O

Kolitsch, Uwe

Abstract

The previously unknown crystal structure of wycheproofite, NaAlZr(PO4)2(OH)2·H O (space group P1,a = 5.263(1), b = 9.251(2), c = 9.480(2) Å, α = 109.49(3), β = 98.57(3), γ = 90.09(3)°, V = 429.60(15) Å3, Z = 2) has been solved using singlecrystal X-ray diffractometer data (CCD area detector, Mo-Kα) and refined to R1 = 4.18 % for 1731 'observed' reflections with F0 > 4σ(F0). The structure determination led to a completely revised unit cell. The atomic arrangement of wycheproofite is characterised by two different alternating layers parallel to (001): the first layer is composed of corner-linked ZrO6 octahedra and PO4 tetrahedra. The second layer is built from zigzag chains of edge-sharing AlO2(OH)4 octahedra along [100], and of Na1- O3(OH)2(H2O)2-y (x ~ 0.1, y ~ 0.6) polyhedra. Adjacent layers are connected by oxygen ligands shared between PO4 tetrahedra and AlO2(OH)4 octahedra. Only very weak hydrogen bonding is necessary to reinforce the structural framework. The Al-, Zr- and P-O polyhedra are all fairly regular and average Al-O, Zr-O and P-O bond lengths are 1.898, 2.063 and 1.529 Å, respectively.The unique Na site is only partially occupied (~ 88 %) and slightly disordered; it is surrounded by three O atoms, two OH groups and two H2O molecules with Na-O distances ranging from 2.247(14) to 2.725(4) Å. Two of its O ligands (Ow11 and Ow12) are also only partially occupied, and split (Ow11) or somewhat disordered (Ow12), in agreement with bond-valence calculations. The originally given chemical formula, NaAlZr(PO4)2(OH)2·H2O, is therefore an idealised formula, although it is very close to the presently obtained formula. Comparisons with the structures of the few other known natural and synthetic zirconium phosphates (e.g., kosnariteKZr2(PO4)3; selwynite- NaK(Be,Al)Zr2(PO4)4·2H2O; mahlmoodite- FeZr(PO4)2·4H2O; synthetic α-Zr(HPO4)2·H2O, ZrKH(PO4)2 and Zr2(NaPO4)4·6H2O) demonstrates that a heteropolyhedral layer composed of corner-linked PO4 tetrahedra and ZrO6 octahedra is a common structural feature of several of these phosphates, and that it links them closely to the family of yavapaiite-related layered AM(XO4)2 compounds.

Keywords

wycheproofitecrystal structurecrystal chemistryzirconium phosphatesyavapaiite