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

Abstract

The previously unknown crystal structure of wycheproofite, NaAlZr(PO 4 ) 2 (OH) 2 -H 2 O (space group P1, a = 5.263(1), b = 9.251(2), c = 9.480(2) A, α = 109.49(3), β = 98.57(3), γ = 90.09(3)°, V = 429.60(15) A 3 Z = 2) has been solved using single-crystal X-ray diffractometer data (CCD area detector, Mo- K α) and refined to R 1 =4.18% for 1731 ’observed’ reflections with F 0 > 4σ( F 0 ). 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 ZrO 6 octahedra and PO 4 tetrahedra. The second layer is built from zigzag chains of edge-sharing AlO 2 (OH) 4 octahedra along [100], and of Na 1-x O 3 (OH) 2 (H 2 O) 2-y (x ∼ 0.1, y ∼ 0.6) polyhedra. Adjacent layers are connected by oxygen ligands shared between PO 4 tetrahedra and AlO 2 (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 A, 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 H 2 O molecules with Na-O distances ranging from 2.247(14) to 2.725(4) A. Two of its O ligands (Ow 11 and Ow12) are also only partially occupied, and split (Ow 11) or somewhat disordered (Ow 12), in agreement with bond-valence calculations. The originally given chemical formula, NaAlZr(PO 4 ) 2 (OH) 2 -H 2 O, 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. , kosnarite -KZr 2 (PO 4 ) 3 ; selwynite - NaK(Be,Al)Zr 2 (PO 4 ) 4 ·2H 2 O; mahlmoodite - FeZr(PO 4 ) 2 -4H 2 O; synthetic α-Zr(HPO 4 ) 2 -H 2 O, ZrKH(PO 4 ) 2 and Zr 2 (NaPO 4 ) 4 ·6H 2 O) demonstrates that a heteropolyhedral layer composed of corner-linked PO 4 tetrahedra and ZrO 6 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(X O 4 ) 2 compounds.