MgMn4Ga18: a novel three-shell gallium cluster structure.

The new ternary gallide MgMn4Ga18 (magnesium tetramanganese octadecagallium) was synthesized and its crystal structure determined by means of single-crystal X-ray diffraction. The MgMn4Ga18 structure can be described as that of a three core-shell cluster compound. The Mg atoms are surrounded by 16 adjacent Ga atoms, [MgGa16], and the respective coordination polyhedron is an octadecahedron. This [MgGa16] octadecahedron is encapsulated inside a [Ga32] icohexahedron, which is in turn encapsulated inside a [Ga40] pentacontaoctahedron. As a result, a three core-shell cluster, [MgGa16@Ga32@Ga40], is identified. Electronic structure calculations were performed by means of the TB-LMTO-ASA program and additionally confirm the existence of the core-shell packing of the clusters.

New cubic cluster phases in the Mg-Ni-Ga system.

The crystal structure of new Mg9Ni6Ga14 and Mg3Ni2Ga compounds were investigated by single-crystal diffraction. Both structures can be described as three-core-shell cluster compounds. In the Mg6Ni9Ga14 structure, the [Ni6Ga6] icosahedron is encapsulated within the [Mg20] dodecahedron, which is again encapsulated within a [Ni18Ga42] fullerene-like truncated icosahedron, thus the three core-shell cluster [Ni6Ga6@Mg20@Ni18Ga42] results. In the Mg3Ni2Ga structure, the [Mg6] octahedron is encapsulated within the [Ni12Ga6] flattened icosahedron in vertices of which there are 12 nickel atoms, and six lateral edges are centered by gallium atoms, which in turn is encapsulated within a [Mg36] pseudo-rhombicuboctahedron with 12 additional atoms centering the lateral faces; thus for Mg3Ni2Ga the three-shell cluster is [Mg6@Ni12Ga6@Mg36].

A new monoclinic structure type for ternary gallide MgCoGa2.

The crystal structure of MgCoGa2 (magnesium cobalt digallide) was solved by direct methods and refined in two space groups as P21/c (standard choice) and P21/n (non-standard choice). The refined lattice parameters for the standard choice are a = 5.1505 (2), b = 7.2571 (2), c = 8.0264 (3) Šand ß = 125.571 (3)°, and for the non-standard choice are a = 5.1505 (2), b = 7.2571 (2), c = 6.5464 (2) Šand ß = 94.217 (3)°. All parameters for MgCoGa2 refined to R1 = 0.027 and wR2 = 0.042 using 594 reflections. The crystal structure peculiarities of this compound are discussed. Particular attention has been given to relationships with other similar structures, such as YPd2Si and Fe3C. Crystallographic analysis, together with linear muffin-tin orbital electronic structure calculations, reveals the presence of three-dimensional polyatomic nets with partial covalent bonding between the Ga atoms.

LiZn(4†-†x) (x = 0.825) as a (3†+†1)-dimensional modulated derivative of hexagonal close packing.

The (3+1)-dimensional modulated structure of the LiZn(4 - x) (x = 0.825) binary compound has been determined in the superspace. The compound crystallizes in the orthorhombic superspace group Cmcm(α00)0s0 with a = 2.7680 (6), b = 4.7942 (6), c = 4.3864 (9) Å, modulation wavevector: q ≃ 4/7a*. The structure is a derivative from the hexagonal close packing. The cubo-octahedron as a coordination polyhedron (c.n. = 12) is typical for all atoms. Bonding between atoms is explored by means of the TB-LMTO-ASA program package. The absence of strong interatomic interactions in LiZn(4 - x) is the main reason for the possible structure transformations.

Polymorphism of Li2Zn3.

Crystal structures of low- and high-temperature modifications of the binary phase Li(2)Zn(3) were determined by single-crystal X-ray diffraction techniques. The low-temperature modification is a disordered variant of Li(5)Sn(2), space group R\bar 3m (No. 166). The high-temperature modification crystallizes as an anti-type to Li(5)Ga(4), space group P\bar 3m1 (No. 164). Two polymorphs can be described as derivative structures to binary Li(5)Ga(4), Li(5)Sn(2), Li(13)Sn(5), Li(8)Pb(3), CeCd(2) and CdI(2) phases which belong to class 2 with the parent W-type in Krypyakevich's classification. All atoms in both polymorphs are coordinated by rhombic dodecahedra (coordination number CN = 14) like atoms in related structures. The Li(2)Zn(2.76) (for the low-temperature phase) and Li(2)Zn(2.82) (for the high-temperature phase) compositions were obtained after structure refinements. According to electronic structure calculations using the tight-binding-linear muffin-tin orbital-atomic spheres approximations (TB-LMTO-ASA) method, strong covalent Sn-Sn and Ga-Ga interactions were established in Li(5)Sn(2) and Li(5)Ga(4), but no similar Zn-Zn interactions were observed in Li(2)Zn(3).

Li(12)Cu(12.60)Al(14.37): a new ternary derivative of the binary Laves phases.

New ternary dodecalithium dodecacopper tetradecaaluminium, Li(12)Cu(12.60)Al(14.37) (trigonal, R ̅3m, hR39), crystallizes as a new structure type and belongs to the structural family that derives from binary Laves phases. The Li atoms are enclosed in 15- and 16-vertex and the Al3 atom in 14-vertex pseudo-Frank-Kasper polyhedra. The polyhedra around the statistical mixtures of (Cu,Al)1 and (Al,Cu)2 are distorted icosahedra. The electronic structure was calculated by the TB-LMTO-ASA (tight-binding linear muffin-tin orbital atomic spheres approximation) method. The electron localization function, which indicates bond formation, is mostly located at the Al atoms. Thus, Al-Al bonding is much stronger than Li-Al or Cu-Al bonding. This indicates that, besides metallic bonding which is dominant in this compound, weak covalent Al-Al interactions also exist.

A tetragonal form of dysprosium orthomolybdate at room temperature.

In the present tetragonal modification of dysprosium orthomolybdate, Dy(2)(MoO(4))(3), the Dy, one Mo and one O atom are located on a mirror plane with Wyckoff symbol 4e, while another Mo atom is located on a fourfold inverse axis, Wyckoff symbol 2a. A single crystal was selected from a polycrystalline mixture of the Dy(2)O(3)-ZrO(2)-MoO(3) system and was stable at room temperature for at least three months. The structure refinement does not indicate the presence of Zr on the Dy sites (to within 1% accuracy). Thus, the stabilization of the tetragonal form is due to disordered positions for a second O atom and split positions for a third O atom that also maintain the DyO(7) coordination, which is not expected for short Dy-O distances [2.243 (6)-2.393 (5) Å].

Redetermination of iron dialuminide, FeAl(2).

The crystal structure of iron dialuminide [Corby & Black (1973). Acta Cryst. B29, 2669-2677] has been redetermined on a single crystal synthesized from the elements by arc melting. The compound crystallizes in the triclinic space group P-1 with 19 atoms per unit cell, one Fe site being on an inversion centre. The crystal structure can be described as an inclusion-plus-deformation derivative of the orthorhombic YPd(2)Si structure type.