The crystal structure of a new ternary aluminide of refined composition Sc6Cu24.1(2)Al11.9(2) was solved by direct methods from X-ray powder diffraction data. It belongs to the cubic space group Im-3, Pearson symbol cI176-8, a = 13.5337(5) Å. The structure of Sc6Cu24.1Al11.9 may be described as a packing of 16-vertex coordination polyhedra of the Sc atoms, which form icosahedral clusters around the origin and the center of the body-centered unit cell. The voids at the centers of the clusters are filled by Cu4 tetrahedra, disordered between two possible orientations. The structure is closely related to the structure types Ru3Be17 and Ce6Au27.6Sn6.8.
The crystal structure of a new ternary aluminide, Sc 3 Cu 7.5 Al 7.5 , was solved by direct methods from X-ray powder diffraction data. It crystallizes with a partly disordered occupation variant of the structure type Sc 3 Ni 11 Ge 4 (Pearson symbol hP38, space group P6 3 /mmc, a = 8.4848(1), c = 8.8593(1) Å). The hexagonal unit cell of Sc 3 Cu 7.5 Al 7.5 contains 36 atoms, distributed over seven crystallographic sites (Sc atoms: one site, statistical mixtures of Cu and Al atoms: four sites, Cu atoms alone: two partly occupied sites). The main feature of the structure is the clustering of the Sc atoms into triangles with relatively short interatomic distances (δ Sc-Sc = 3.446(2) Å). The coordination polyhedra of the Sc atoms are pentagonal prisms (with seven capping atoms) and those of the sites occupied by atoms of the statistical mixtures icosahedra, and those of the remaining Cu atoms trigonal prisms (with three capping atoms) or trigonal antiprisms.
The isothermal section at 800°C of the phase diagram of the ternary system Sc–Cu–Al was constructed in the whole concentration range using X-ray powder diffraction data. The existence of eight ternary compounds was confirmed: ScCu4.9‑6.0Al7.1‑6.0 (structure type ThMn12, Pearson symbol tI26, space group I4/mmm), Sc6Cu24.1Al11.9 (own structure type, cI176, Im-3), Sc2Cu6.25Al4.75 (own structure type, oS108, Cmmm), Sc3Cu7.5Al7.5 (Sc3Ni11Ge4, hP38, P63/mmc), ScCu2Al (MnCu2Al, cF16, Fm-3m), ScCu0.8Al0.2 (CsCl, cP2, Pm-3m), ScCu0.6Al1.4 (MgNi2, hP24, P63/mmc), and ScCuAl (MgZn2, hP12, P63/mmc), and the existence of a continuous solid solution ScCu1-xAlx (x = 0-1), based on the binary compounds ScCu and ScAl with CsCl-type structure (cP2, Pm-3m), was established. The investigation of the Ti–Cu–Al system at 800°C confirmed the existence of four ternary compounds: TiCu0.25Al2.75 (ZrAl3, tI16, I4/mmm), TiCu0.3-0.6Al2.7-2.4 (Cu3Au, cP4, Pm-3m), TiCu2-2.7Al1-0.3 (MnCu2Al, cF16, Fm-3m), and TiCu0.54-1.16All.46-0.84 (MgZn2, hP12, P63/mmc). Electrical resistivity measurements were performed for three compounds in the Sc–Cu–Al system and confirmed metal behavior in the temperature range 5-290 K.
One known and nine new quaternary aluminides in the Al-rich region of R–T–Ag–Al (R = Y, Ce, Pr, Nd, Sm, and Gd, T = V, Nb, Ta, Cr, and Mo) systems were synthesized by arc melting and their crystal structures were studied by X-ray powder diffraction. The crystal structures of the compounds R3T(Ag,Al)20Al12 (R = Ce, Pr, Nd, and Sm, T = V, Nb, Ta, Cr, and Mo) and R3TAgx(Ag,Al)20Al12 (R = Y and Gd, T = Ta) belong to new structure types: Ce3Ta (Ag,Al)20Al12 (Pearson symbol сP36, space group Pm-3m, a = 8.6675(2) Å) and Gd3TaAgx(Ag,Al)20Al12 (сP4912.86, Pm-3m, a = 8.6158(5) Å), respectively. Within the rows of isotypic compounds the unit-cell parameter increases with increasing metallic radius of the rare-earth or d-element atom. The structure type Ce3Ta (Ag,Al)20Al12 (Ce3TaAg7.97Al24.03) is a partly ordered quaternary substitution derivative of the structure type BaHg11, whereas Gd3TaAgx(Ag,Al)20Al12 (Gd3TaAg8.89Al23.25) is a distorted fill-in quaternary substitution variant of the same prototype. The latter can also be described as a derivative of the structure type Y3TaNi6+xAl26.
Six rare-earth nickel alumogermanides RNiAl 4 Ge 2 (R = Gd, Tb, Dy, Ho, Tm, Lu) were synthesized by arc melting and their crystal structures were studied by X-ray powder diffraction. They are isotypic to SmNiAl 4 Ge 2 , Pearson symbol hR24, space group R-3m. The structures of the RNiAl 4 Ge 2 compounds can be described as formed by two types of alternating slabs. The first type of slab is built up of cubes of composition Al 8 , every second cube being centered by a Ni atom, and the second one is formed by octahedra of composition Ge 6 , every second octahedron being centered by an R atom.Rare-earth nickel alumogermanide / X-ray powder diffraction / Crystal structure
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