The crystal structure of Sc₅Co₂In₄

1 Introduction

Ternary compounds with the Lu₅Ni₂In₄-type structure (space group Pbam, oP22, Z = 2) [1] are known in the systems of rare earths (RE) or Zr and Hf [2], transition metals (T) of the 9^th (T = Rh, Ir) [3] and 10^th (T = Ni, Pd, Pt) [1, 4–7] group of the periodic table, and indium. Co- or Fe-based compounds of this structure type have not been observed within the representatives. The structure could be realized for all rare earths with T = Pt, Rh, although light rare earths do not form it with nickel or palladium, and heavy rare earths with iridium. In the case of scandium indides, representatives with Rh [8], Ir [3], Ni [8] and Pt [7] were found. The RE₅T₂In₄ phases (T = Ni, Pd) are accompanied by representatives of the Nd₁₁Pd₄In₉ type [9–12]. Both structures have similar compositions, RE_45.5T_18.2In_36.3 (for Lu₅Ni₂In₄) and RE_45.8T_16.7In_37.5 (for Nd₁₁Pd₄In₉), and similar structures, which are built from CsCl- and AlB₂-type fragments [9]. They contain two layers along the short unit cell axis, where one (z = 0) is occupied by rare earth atoms while the second one (z = 1/2) is filled with T and In. A superstructure of the Lu₅Ni₂In₄ type is known for Zr₅Ir₂In₄ [13].

In this paper, we report on the results of a single-crystal study of the new Sc₅Co₂In₄ phase and its relation to the family of Sc₅T₂In₄ compounds, where T = Rh and Ni [8], Ir [3] or Pt [7].

2 Experimental

A sample with a total weight of ∼0.5 g was obtained by a standard melting procedure: the amounts of the compact metals with the nominal composition of Sc_45.8Co_16.7In_37.5 were arc melted under a pure argon atmosphere on a water-cooled copper hearth with a tungsten electrode and titanium serving as a getter. Ingots of scandium of a purity not lower than 99.9 wt.%, cobalt 99.9 wt.% and indium 99.99 wt.% were used as the starting materials. The button was sealed in an evacuated silica tube and annealed at 870 K for 2 months.

The powdered polycrystalline as-melted and annealed samples were measured using a Stoe STADI P diffractometer [CuK_α1 radiation, bent Ge (111) monochromator, transmission geometry, measured interval 6 ≤ 2θ≤110°, scan step mode, step size in 2θ = 0.015°] at room temperature. The Wincsd [14] program package was used for X-ray Rietveld analysis of the collected data set.

The annealed sample was polished and investigated using a scanning electron microscope (REMMA-102-02, Ukraine) equipped with sensors for the local chemical analysis (EDX).

Irregularly shaped single crystals were selected by mechanical fragmentation of the as-melted sample. Single-crystal X-ray diffraction was performed on a Bruker Apex-II diffractometer equipped with a graphite-monochromatized MoK_α source (λ = 0.71073 Å) in the whole reciprocal sphere at room temperature.

The diffractometer data set of the selected crystal showed a primitive orthorhombic lattice. An analysis of the systematic extinctions revealed the possible space groups Pbam and Pba2. The centrosymmetric group Pbam was found to be correct during structure refinement. The starting atomic parameters were determined from an automatic interpretation of direct methods using Shelxs-97 [15, 16]. The refinement readily revealed isotypism with Lu₅Ni₂In₄ [1]. The final lattice parameters were calculated from all the reflections observed in the actual data collection. The atomic positions of Sc₅Ni₂In₄ [8] were taken as a starting model. The structure was successfully refined with anisotropic displacement parameters for all atoms with Shelxl-97 [16, 17]. Some details of the data collection and refinement parameters are given in Table 1. All crystallographic positions were fully occupied. The final difference Fourier synthesis revealed no significant residual peaks (see Table 1). The atomic parameters and anisotropic displacement parameters are listed in Tables 2 and 3, respectively.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) by quoting the deposition number CSD-428439.

3 Results and discussion

The phase analyses of the as-melted and annealed samples showed a two-phase mixture of Sc₁₁Co₄In₉ (Nd₁₁Pd₄In₉-type structure) and Sc₅Co₂In₄ (Lu₅Ni₂In₄ type) in both cases. Strong texturing, similar to the one described in refs. [8, 18], prevented the successful Rietveld refinement of the structures. At 870 K the refined cell parameters for the Sc₁₁Co₄In₉ phase were: a = 13.836(5), b = 20.758(10), c = 3.351(2) Å and for the Sc₅Co₂In₄ phase were: a = 17.357(2), b = 7.597(1), c = 3.317 (1) Å. The EDX data confirmed the existence of two phases in the sample with compositions Sc_45.9Co_12.5In_41.6 and Sc_48.1Co_14.2In_37.7 which are close to the ideal formulae Sc₁₁Co₄In₉ and Sc₅Co₂In₄.

A projection of the unit cell on the xy plane and the coordination polyhedra of the atoms of Sc₅Co₂In₄ are shown in Fig. 1. The polyhedra were drawn according to the atomic coordination numbers (CN) and the interatomic distances presented in Table 4. In1 is surrounded by a Sc tetragonal prism, which is capped by one cobalt and three indium atoms [In1Sc₈CoIn₃]. The coordination polyhedron [In2Sc₈Co₂In₃] of In2 is similar to the previous one but has one additional cobalt atom and, consequently, its CN is 13. The coordination polyhedra of the Co atoms are trigonal prisms of the Sc atoms with all the lateral faces capped by indium atoms [CoSc₆In₃]. Sc1 is surrounded by a distorted cuboctahedron [Sc1In₈Sc₄]. The nearest neighbors of Sc2 form a base-capped pentagonal prism [Sc2Co₄In₆Sc₂] at distances with maximal enlargement of 3.1 % as compared to the sum of the atomic radii. The prism is additionally equatorially capped by four scandium atoms at distances up to 22 % larger than the sum of the atomic radii. These atoms can be also included in the first coordination sphere (CP [Sc2Co₄In₆Sc₆]) in analogy to the isostructural compounds with the larger rare earth elements [1]. The resulting Sc2 coordination number becomes 16, which is very high for Sc; however, it is known in some other Sc-T-In compounds [20]. A similar situation is observed for the Sc3 polyhedron. Eleven atoms with short distances (deviations from the sum of interatomic distances reached 3.1 %) building a tetragonal prism with two bases and one face capped by Sc atoms [Sc3Co₂In₆Sc₃]. Three more Sc atoms (Δ = 8.7–14.0 %) are located at the equatorial plane of this polyhedron. Consequently, the Sc3 CN is 14 in a coordination polyhedron [Sc3Co₂In₆Sc₆].

Fig. 1:

Projection of the unit cell of Sc₅Co₂In₄ on the xy plane and coordination polyhedra of the atoms.

The structure can be presented as a polyhedral space filling (Fig. 2). Thus, the Co prisms share Sc triangles along the z direction and Sc-In edges along the x direction forming the corrugated walls in the xz plane. The second layer is constituted of Sc1-centered interpenetrating cuboctahedra, which, in turn, build rods along the z direction and share edges and corners with the Co prisms. The combination of both units includes all atoms of the structure. The main features of the crystal chemistry of Lu₅Ni₂In₄-type indides have been discussed in detail in previous papers [1–8]. The most important of these is that the crystal structure is two-layered in the direction of the shorter cell parameter, where the scandium atom nets at z=0 alternate with nets of cobalt and indium atoms at z=1/2; thus the cobalt and indium atoms have trigonal prismatic and distorted square prismatic scandium coordination, respectively. The structure can be considered as an intergrowth of distorted AlB₂ (ScCo₂)- and CsCl (ScIn)-related slabs, belonging to a homological series of compounds with the total formula described as RE_{m + n}T_2nX_m, where the T and X atoms are transition metals and indium (boron), respectively. The numbers n and m correspond to the number of AlB₂ and CsCl blocks. The structures of Nd₁₁Pd₄In₉, Mo₂FeB₂, Mn₂AlB₂ and Cr₃AlB₄-type indides also belong to this series [11].

Fig. 2:

The arrangement of the clusters [CoSc₆In₃] (red) and [Sc1In₈Sc₄] (light green) in the Sc₅Co₂In₄ structure.

Herein we focus only on the peculiarities of the scandium compounds Sc₅T₂In₄ (T = Co, Rh, Ir, Ni, Pt). The refined cell parameters for Sc₅Co₂In₄ are slightly larger than those of Sc₅Ni₂In₄ [8], and the cell volumes of the compounds agree well with the atomic radii of the T elements [19] in the V–r(T) dependence (Fig. 3). As was derived from Fig. 3, the resulting interatomic interactions within the compounds of group 10 T element are stronger than those of group 9 resulting in a lower V–r(T) dependence for T=Ni, Pt (bottom line) as compared to the ones for T=Co, Rh, Ir (upper line). The point for the hypothetical Sc₅Pd₂In₄ compound placed with the expected cell parameters is slightly lower than those for Sc₅Pt₂In₄ (Fig. 3).

Fig. 3:

Cell volumes of Sc₅T₂In₄ (T = Co, Rh, Ir, Ni, Pt) and hypothetical Sc₅Pd₂In₄versus the atomic radii of the T elements [19].

The complete crystal structure investigations of Sc₅Ni₂In₄ and Sc₅Rh₂In₄ allow us to compare them with Sc₅Co₂In₄. An analysis of the interatomic distances of the related Sc₅T₂In₄ compounds shows that the strongest bonding occurs in Sc₅Ni₂In₄, and the weakest bonds are observed in Sc₅Rh₂In₄, while Sc₅Co₂In₄ has an intermediate strength. According to the layered structure, the Sc1–Sc1 distances correspond to the shortest cell parameter c. They are somewhat shorter than average Sc–Sc distances of 3.28 Å in hcp scandium [21]. The shortest bonds have been observed between the transition metals and Sc3, where the relative reduction of interatomic distances reaches more than 10 % for the rhodium compound. Very strong interactions of the atoms occur in the coordination polyhedra of the transition metals in a distorted trigonal prism. In these prisms the T–Sc3 distances have a reduction of 8.8, 9.2 and 10.8 % for T = Ni, Co, Rh, respectively. Other distances in the structure are close to the sum of the respective atomic radii. The T atoms are well separated from each other. The In–In distances are in the range of 3.15 (In1–In2) to 3.30 Å (In1–In1) for all compounds, which is less than the distances in tetragonal indium (4 × 3.25 and 8 × 3.38 Å) [21].