The crystal structure of wycheproofite, NaAlZr(PO (sub 4) ) (sub 2) (OH) (sub 2) . H (sub 2) O

The previously unknown crystal structure of wycheproofite, NaAlZr(PO (sub 4) ) (sub 2) (OH) (sub 2) .H (sub 2) O (space group P1, a = 5.263(1), b = 9.251(2), c = 9.480(2) Aa, alpha = 109.49(3), beta = 98.57(3), gamma = 90.09(3) degrees , V = 429.60(15) Aa (super 3) , Z = 2) has been solved using single-crystal X-ray diffractometer data (CCD area detector, Mo-Kalpha ) and refined to R1 = 4.18% for 1731 "observed" reflections with F (sub o) > 4sigma (F (sub o) ). 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 (sub 6) octahedra and PO (sub 4) tetrahedra. The second layer is built from zigzag chains of edge-sharing AlO (sub 2) (OH) (sub 4) octahedra along [100], and of Na (sub 1-x) O (sub 3) (OH) (sub 2) (H (sub 2) O) (sub 2-y) (x approximately 0.1, y approximately 0.6) polyhedra. Adjacent layers are connected by oxygen ligands shared between PO (sub 4) tetrahedra and AlO (sub 2) (OH) (sub 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 Aa, respectively. The unique Na site is only partially occupied ( approximately 88%) and slightly disordered; it is surrounded by three O atoms, two OH groups and two H (sub 2) O molecules with Na-O distances ranging from 2.247(14) to 2.725(4) Aa. 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(PO (sub 4) ) (sub 2) (OH) (sub 2) .H (sub 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 (sub 2) (PO (sub 4) ) (sub 3) ; selwynite - NaK(Be,Al)Zr (sub 2) (PO (sub 4) ) (sub 4) .2H (sub 2) O; mahlmoodite - FeZr(PO (sub 4) ) (sub 2) .4H (sub 2) O; synthetic alpha -Zr(HPO (sub 4) ) (sub 2) .H (sub 2) O, ZrKH(PO (sub 4) ) (sub 2) and Zr (sub 2) (NaPO (sub 4) ) (sub 4) .6H (sub 2) O) demonstrates that a heteropolyhedral layer composed of corner-linked PO (sub 4) tetrahedra and ZrO (sub 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(XO (sub 4) ) (sub 2) compounds.