ABSTRACT
The ternary germanide Mg5.57Ni16Ge7.43 (cubic, space group Fm-3m, cF116) belongs to the structural family based on the Th6Mn23-type. The Ge1 and Ge2 atoms fully occupy the 4a (m-3m symmetry) and 24d (m.mm) sites, respectively. The Ni1 and Ni2 atoms both fully occupy two 32f sites (.3m symmetry). The Mg/Ge statistical mixture occupies the 24e site with 4m.m symmetry. The structure of the title compound contains a three-core-shell cluster. At (0,0,0), there is a Ge1 atom which is surrounded by eight Ni atoms at the vertices of a cube and consequently six Mg atoms at the vertices of an octahedron. These surrounded eight Ni and six Mg atoms form a [Ge1Ni8(Mg/Ge)6] rhombic dodecahedron with a coordination number of 14. The [GeNi8(Mg/Ge)6] rhombic dodecahedron is encapsulated within the [Ni24] rhombicuboctahedron, which is again encapsulated within an [Ni32(Mg/Ge)24] pentacontatetrahedron; thus, the three-core-shell cluster [GeNi8(Mg/Ge)6@Ni24@Ni32(Mg/Ge)24] results. The pentacontatetrahedron is a new representative of Pavlyuk's polyhedra group based on pentagonal, tetragonal and trigonal faces. The dominance of the metallic type of bonding between atoms in the Mg5.57Ni16Ge7.43 structure is confirmed by the results of the electronic structure calculations. The hydrogen sorption capacity of this intermetallic at 570â K reaches 0.70â wt% H2.
ABSTRACT
The ternary magnesium/lithium boride, MgxLi3 - xB48 - y (x = 1.11, y = 0.40, idealized formula MgLi2B48), crystallizes as its own structure type in P43212, which is closely related to the structural family comprising α-AlB12, Be0.7Al1.1B22 and tetra-gonal ß-boron. The asymmetric unit of title structure contains two statistical mixtures Mg/Li in Wyckoff sites 8b with relative occupancies Mg:Li = 0.495â (9):0.505â (9) and 4a with Mg:Li = 0.097â (8):0.903â (8). The boron atoms occupy 23 8b sites and two 4a sites. One of the latter sites has a partial occupancy factor of 0.61â (2). Both unique Mg/Li atoms adopt a twelvefold coordination environment in the form of truncated tetra-hedra (Laves polyhedra). These polyhedra are connected by triangular faces to four [B12] icosa-hedra. The boron atoms exhibit four kinds of polyhedra, namely penta-gonal pyramid (coordination number CN = 6), distorted tetra-gonal pyramid (CN = 5), bicapped hexa-gon (CN = 8) and gyrobifastigium (CN = 8). At the gas hydrogenation of MgLi2B48 alloy, formation of the eutectic composite hydride LiBH4+Mg(BH4)2 and amorphous boron is observed. In the temperature range 543-623â K, the hydride eutectics decompose, forming MgH2, LiH, MgB4, B and H2.
ABSTRACT
Crystal structures of MgCoGa, Mg0.74CoGa0.52 and Mg0.49CoGa0.15 phases from the Mg-Co-Ga system were investigated using single-crystal diffraction. These structures belong to the family of so-called Laves phases. Hexagonal MgCoGa crystallizes as a disordered phase within the MgZn2 structure type. The orthorhombic structure of Mg0.74CoGa0.52 is a distortion variant of MgZn2 and URe2 structure type, and the structural relation is demonstrated in terms of a Bärnighausen formalism group-subgroup transformation scheme. The structure of trigonal phase Mg0.49CoGa0.15 is strongly disordered, as is shown by the presence of adjacent atomic sites which cannot be occupied simultaneously. In Mg0.49CoGa0.15, two subcells (A and B) were obtained in a ratio of 9:1. Subcell A is closely related to MgZn2-type.
ABSTRACT
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.
ABSTRACT
The maximally disordered (MD) phases with the general formula Y5-xPrxSb3-yMy (M = Sn, Pb) are formed with partial substitution of Y by Pr and Sb by Sn or Pb in the binary Y5Sb3 compound. During the electrochemical lithiation and sodiation, the formation of Y5-xPrxSb3-yMyLiz and Y5-xPrxSb3-yMyNaz maximally disordered-high entropy intermetallic phases (MD-HEIP), as the result of insertion of Li/Na into octahedral voids, were observed. Carbon nanotubes (CNT) are an effective additive to improve the cycle stability of the Y5-xPrxSb3-yMy (M = Sn, Pb) anodes for lithium-ion (LIBs) and sodium-ion batteries (SIBs). Modification of Y5-xPrxSb3-ySny alloys by carbon nanotubes allowed us to significantly increase the discharge capacity of both types of batteries, which reaches 280 mAh · g-1 (for LIBs) and 160 mAh · g-1 (for SIBs), respectively. For Y5-xPrxSb3-yPby alloys in which antimony is replaced by lead, these capacities are slightly smaller and are 270 mAh · g-1 (for LIBs) and 155 mAh · g-1 (for SIBs), respectively. Results show that structure disordering and CNT additives could increase the electrode capacities up to 30% for LIBs and up to 25% for SIBs.
ABSTRACT
The multicomponent alumosilicide Li12Mg3Si3Al (cubic, space group I-43d, cI76) belongs to the structural family based on the Cu15Si4 type. The Li atoms are ordered and occupy the site with symmetry 1 and the Mg atoms occupy the site with -4.. symmetry. The Si/Al statistical mixture occupies the site with .3. symmetry. The coordination polyhedra around the Li atoms are 13-vertex distorted pseudo-Frank-Kasper polyhedra. The environments of the Mg and Si/Al atoms are icosahedral. The hydrogen storage characteristics of Li12Mg3Si3Al were investigated. The reversible hydrogen storage capacity of the title compound is excellent and the gravimetric storage capacity of this new material, corresponding to 9.1â wt% H2, is higher compared to Li12Mg3Si4 (8.8â wt%). The enthalpy of hydrogen desorption is 86â kJâ mol-1 and is lower compared to known lithium-based hydrides.
ABSTRACT
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].
ABSTRACT
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 = 5.1505â (2), b = 7.2571â (2), c = 8.0264â (3)â Å and ß = 125.571â (3)°, and for the non-standard choice are a = 5.1505â (2), b = 7.2571â (2), c = 6.5464â (2)â Å and ß = 94.217â (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.
ABSTRACT
The new quaternary aluminide Li20Mg6Cu13Al42 was synthesized from the elements in a sealed tantalum crucible. The crystal structure was studied by single crystal and confirmed by X-ray powder diffraction. Li20Mg6Cu13Al42 {cI162, Im{\overline 3}, a = 13.8451â (2), R[F2 > 2σ(F2)] = 0.023} crystallizes as an ordered version of Mg32(Al,Zn)49 and Li-Cu-X (X = Al, Ga, Si) periodic crystals containing icosahedral clusters. The Li20Mg6Cu13Al42 structure can also be described as three-shell icosahedral clusters of [CuAl12@Li20Cu12@Al60], enclosed inside a distorted triacontahedron. The electronic structure calculations were performed by means of the TB-LMTO-ASA program and confirm the core-shell packing of these clusters. The isostructural compound of Li20Mg6Cu13Ga42 was found in a Li-Mg-Cu-Ga quaternary system.
ABSTRACT
Magnesium alloys are the basis for the creation of light and ultra-light alloys. They have attracted attention as potential materials for the accumulation and storage of hydrogen, as well as electrode materials in metal-hydride and magnesium-ion batteries. The search for new metal hydrides has involved magnesium alloys with rare-earth transition metals and doped by p- or s-elements. The synthesis and characterization of a new quaternary carbide, namely dimagnesium lithium aluminium carbide, Mg1.52Li0.24Al0.24C0.86, belonging to the family of hexagonal close-packed (hcp) structures, are reported. The title compound crystallizes with hexagonal symmetry (space group P-6m2), where two sites with -6m2 symmetry and one site with 3m. symmetry are occupied by an Mg/Li statistical mixture (in Wyckoff position 1a), an Mg/Al statistical mixture (in position 1d) and C atoms (2i). The cuboctahedral coordination is typical for Mg/Li and Mg/Al, and the C atom is enclosed in an octahedron. Electronic structure calculations were used for elucidation of the ability of lithium or aluminium to substitute magnesium, and evaluation of the nature of the bonding between atoms. The presence of carbon in the carbide phase improves the corrosion resistance of the Mg1.52Li0.24Al0.24C0.86 alloy compared to the ternary Mg1.52Li0.24Al0.24 alloy and Mg.
