Fiorenzo Mazzi

Sheet superconductivity in : crystal structure of the tetragonal matrix

Sheet conductivity was found in oxygen reduced . The sheets are aligned along the twin boundaries of the unreduced starting material . The bulk transforms during the oxygen loss to a phase with tetragonal crystal structure. The space group is with lattice parameters a = 0.739 nm and c = 0.388 nm. The perovskite-like structure contains distorted octahedra with W - O distances between 0.17 nm and 0.218 nm.

The crystal structure of eudialyte

As a result of the present paper, the most probable formula unit of eudialyte is: (Fe2+, Fe3+, Mn, Mg)3Zr3(Zr, Nb) x (Ca, R.E.)6Na12[Si9O27−y (OH) y ]2[Si3O9]2Cl z withx=0.1−0.9;y=1−3;z=0.7−1.4. Space group:R

Bottinoite, Ni(H2O)6[Sb(OH)6]2; crystal structure, twinning, and hydrogen-bond model

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The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15− polyanion

a;a=14.244 A,c=30.080 A,Z=3. The crystal structure was determined from 3-dimensional Weissenberg data by Patterson- and electron density-syntheses, and refined by the least squares method:R=0.075 for 831 obs. reflections. Eudialyte is a cyclosilicate with both threefold and ninefold rings of SiO4-tetrahedra. Si−O distances measure 1.62–1.65 A (for shared oxygens) and 1.58–1.60 A (for unshared oxygens). Zirconium has a practically regular octahedral coordination (Zr−O=2.07–2.08 A), whereas iron is surrounded by only four oxygens in a square arrangement (Fe−O=2.04 A); the coordination polyhedron of calcium is a distorted octahedron (Ca−O=2.33–2.38 A). Two non equivalent sodium atoms have rather irregular coordinations: Na(1) is bound to six oxygens which form a fairly distorted hexagonal ring (Na−O=2.53–2.66 A), whereas Na(2) has four shorter distances to three oxygens (Na−O=2.53–2.57 A) and one chlorine (Na−Cl=2.98 A), and three longer distances to oxygens (Na−O=2.74–2.77 A) in a sort of distorted octahedron with a centered face. Some of the most reliable chemical analyses are reexamined in order to derive a formula unit consistent with the crystal structure and to confirm some of its peculiar features, e.g. the presence of two sets of crystallographic sites only partially occupied by chlorine and zirconium atoms respectively.

The crystal structure of a rare earth bearing leucophanite: (Ca,RE)CaNa2Be2Si4O12(F,O)2

Abstract Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [REE1-x-y(Th,U)xCay](Al3O)2/3[(B4-zSiz)]O10 with x - y + z = 1/3 (Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) Å, V = 172.6 Å3, Z = 1, space group P6̅ 2m. The crystal structure was solved by Patterson methods and refined to Robs = 1.8% (Rall = 2.2%) for 706 unique reflections in the 2q-range 6-136°. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) Å, V = 170.3 Å3, and the structure was refined to Robs = 2.9% and Rall = 3.0% for 271 unique reflections in the 2θ-range 4-80°. The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite, Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is formed by pairs of apical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming Al3O groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a′ = a √ 3 and a ^ a′ = 30°) of three ordered layers randomly translated by ± a.

The structure type of Joaquinite

Abstract The crystal structure of bottinoite, Ni(H2O)6[Sb(OH)6]2, was determined from a twinned crystal. This study revealed that the strong 3̅1m pseudosymmetry shown by all the natural and synthetic crystals examined results from {101̅0} twinning. The structure is trigonal (space group P3) with a = 16.045(4), c = 9.784(2) Å (natural bottinoite) and a = 16.060(3), c = 9.792(1) Å (synthetic analog). The structure consists of a sequence of pairs of layers parallel to (0001) and stacked along the c axis. Each layer consists of isolated octahedra, linked together by hydrogen bonds, which also connect adjacent layers. Ni2+ cations are ordered in two of the ten independent octahedral sites in one layer, whereas the second layer of the pair is made up of only Sb5+ octahedra. The average octahedral bond lengths are 2.06 and 1.98 Å for NiO6 and SbO6, respectively. The ten independent octahedra show four orientations mutually related by pseudosymmetry operations. This particular feature is taken into account in the formulation of a hydrogen-bonding model and a twinning mechanism. Structural and geometrical relationships with the brucite-like M2+(OH)2 structures are also discussed.

The crystal structure of sarcolite

The crystal structures of the two polytypes of the compound CaMgB 2 O 5 , kurchatovite and clinokurchatovite, were determined in the space groups Pc2 1 b and P2 1 /c respectively by Yakubovich et al. (1976) and Simonov et al. (1980). The crystal structures of both minerals have been re-examined and refined until R obs = 3.00 and 2.82%. The lattice parameters are for kurchatovite a = 36.34,b= 11.135,c = 5.499 A, Z = 24, space group Pbca, and for clinokurchatovite a = 12.329,b = 11.146,c = 5.519 A, s = 101.62°, Z = 8, space group P2 1 /c. Common features of both structures are the B 2 O 5 clusters formed by two B-triangles with a common vertex, the Mg-octahedra and Ca-polyhedra with seven corners. The two crystal structures show very close similiarities, both being based on the repeat of a monoclinic module, whose volume is one fourth of that of the unit cell of clinokurchatovite (a 0 = a/4). The possibility of further structures based on the same module is discussed.

Calciobetafite (new mineral of the pyrochlore group) and related minerals from Campi Flegrei, Italy; crystal structures of polymignyte and zirkelite; comparison with pyrochlore and zirconolite

Abstract The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1-xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O1815- polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O33- and NaF32- polyhedra, the occurrence of which is mutually exclusive and statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.