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Crystals of synthetic Mg-whitlockite, Ca,8Mg2H2(P04)14, have been grown by hydrothermal techniques and their structure refined by full-matrix least-squares methods, using 1626 unique reflections measured with a Syntex PT automatic diffractometer, to a final R value of 0.046. The crystals have space group R3c with Z, = 1, a = 13.765(8) A and a = 44.25(5)" with the equivalent hexagonal parameters a = 10.350(5), c = 37.085(12) A and ZFl = 3. The structure, although similar to that of (3Ca3(P04),, differs significantly. The structure contains interconnected infinite chains of polyhedra paralleling the hexagonal c axis with links in the chains consisting of three CaO, polyhedra separated by two PO, tetrahedra. Six of these chains surround a chain of M g 0 6 octahedra and P 0 4 H groups which lie on the three-fold axes. These chains however contain only half the formula unit density of the calcium containing ones and a proton, presumably disordered, is attached to the oxygen atom on the triad axes.Des cristaux de Mg-whitlockite synthetique Ca18MgZH2(P04)14 ont it6 developpes par des techniques calorimetriques et leur structure raffinee par une matrice complete des methodes de moindres carris, utilisant la reflection unique 1626 mesurke par un diffractornetre automatique

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STRUCTURE CRISTALLINE DE L'HYPOVANADATE CaV409pliquant bien pour celui-ci non pas un environnement octa6drique mais un environnement pyramidal b. base carr6e.(~ 2')~ 3, 06 CusV2Ol0 is monoclinic with a=8"393 (2), b=6-0652 (8), c= 16.156 (3)~, fl= 108"09 (2) °, Z=4, and space group P2~/c. The crystal structure was refined by full-matrix least-squares analysis to a wR = 0.032 using 1629 reflexions measured on a Syntex automatic diffractometer. The structure consists of a network made up of double chains of Cu-O6 octahedra running parallel to b and chains forrned from Cu-O6 octahedra and Cu-O5 trigonal bipyramids running parallel to c. These chains are linked to each other by edge sharing of the octahedra and trigonal bipyramids and corner sharing of the V(2)Oa tetrahedra. Two such networks at x= ¼ and k are linked by the V(1)O4 tetrahedra. Cu(1) and Cu(2) are octahedrally coordinated by oxygen atoms and show typical Jahn-Teller distortion. Cu(4) is also octahedrally coordinated but has an unusual distortion in which one equatorial and one apical, rather than 2 apical Cu-O bonds, are elongated. The mean Cu-O distances differ significantly for the three octahedra. These differences and those in other Cu-containing oxides are related to the degree of distortion of the octahedra. Cu(3) and Cu(5) are each surrounded by five oxygen atoms in the form of distorted trigonal bipyramids. The apical bonds are shorter than the equatorial bonds as in Cu2OSO4, CHaAszOs and Cu3WO 6. The differences in individual V-O distances are related to the bond-strength sums around the oxygen atoms.