Single-crystal investigation of the compound SmNi_5.2Mn_6.8

1 Introduction

The structure type ThMn₁₂ [1] is rather widespread among intermetallic compounds. The representatives of this type exist both in the binary systems R-Mn (R=Y, Gd, Tb, Dy, Ho, Er, Tm) [2], [3] and in the ternary systems R-Mn-Т (R=Ce, Pr, Nd, Sm; Т=Fe, Co, Ni) [4], [5]. The transition metals appear to stabilize the compounds with this structure type including the systems with light rare earth metals. The largest number of isotypic compounds have been found in R-T-Al (T=Cr, Mn, Fe, Cu, Re) [6] and R-T-In (T=Cu, Ag) systems [7], while fewer compounds have been reported for related systems with gallium and silicon [6]. The ThMn₁₂ type is characterized by a low content of rare earth metal and high coordination numbers (CN) of all atoms (CN=20 for the Th atoms and CN=12–14 for the Mn atoms). This structure type is closely related to the CаCu₅ structure type [8] and can be derived from it by substitution of one R atom by a pair of T component atoms in the unit cell following the scheme: 2RТ₅=R₂Т₁₀→RТ′₂Т₁₀→RТ₁₂. In cases of strong differences in nature and size of the T component atoms, the atoms can be ordered with the formation of the superstructure type CeMn₄Al₈ [9]. In our earlier work we found the existence of compounds with the ThMn₁₂ type in R-Mn-{Fe, Co, Ni} systems, but only the values of the cell parameters have been determined for them using powder X-ray diffraction [4], [5]. These compounds can be considered as promising magnetic materials.

Therefore, in this paper we report on the synthesis of high-quality SmNi_5.2Mn_6.8 single crystals and the determination of the crystal structure of this compound.

2 Experimental details

A sample of nominal composition Sm_7.7Ni₄₀Mn_52.3 was synthesized from the high-purity elements (Sm≥99.9 wt.%, Ni≥99.92 wt.%, and Mn≥99.90 wt.%) by arc-melting under a purified argon atmosphere, using Ti as a getter and a tungsten electrode. To achieve high efficiency of the interaction between the components, the sample was melted twice. The ingot was annealed at 600°C in an evacuated quartz ampoule for 720 h and subsequently quenched in cold water. The weight loss during the preparation of the sample was less than 1% of the total mass of 2 g.

Laue and rotation diffraction patterns of selected single crystals showed tetragonal symmetry with lattice parameters a~8.6 and c~4.7 Å. Integrated intensities measured at room temperature with graphite-monochromatized MоKα radiation (λ=0.71073 Å) on an Oxford Diffraction Xcalibur four-circle diffractometer equipped with a CCD camera confirmed the tetragonal lattice. Data collection and reduction were made using CrysAlis CCD and CrysAlis Red programs [10] taking into account a numerical absorption correction. The structure was solved by Direct Methods,and refined using the Shelx-2014 program package [11].

The chemical composition of the selected crystals was checked with a field-emission scanning electron microscope (FEINovaNanoSEM 230) equipped with an EDS analyzer (EDAX GenesisXM4, accelerating voltage 20 kV, exposure time 120 s).

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-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-1958794.

3 Results and discussion

Analysis of the XRD data collected for SmNi_5.2Mn_6.8 indicated that the compound crystallizes with a centrosymmetric structure, space group I4/mmm. The structure type ThMn₁₂ was assigned and the structure was refined with anisotropic displacement parameters for all atoms. A final electron density difference map was flat and did not reveal any significant residual peaks. The crystal data and details of the data collection are given in Table 1, while the atomic coordinates and the displacement parameters are listed in Table 2. All crystallographic positions are fully occupied, but the atomic positions 8f and 8j are occupied by a mixture of manganese and nickel atoms with ratios of Mn to Ni of 0.24(3):0.76(3) and 0.46(3):0.54(3), respectively. The refined atomic positions for SmNi_5.2Mn_6.8 are similar to the respective positions of atoms in the ThMn₁₂ type [1]. The refined composition is in good agreement with the results of the EDX analyses (8 at% Sm: 52 at% Mn: 40 at% Ni).

A projection of the unit cell of SmNi_5.2Mn_6.8 onto the crystallographic ab plane and the coordination polyhedra of all atoms are shown on Fig. 1. The samarium atom is in the center of a 20-vertex polyhedron [SmMn₄Т₁₆] similar to the environment of the thorium atoms in the ThMn₁₂ structure. The smallest atoms in SmNi_5.2Mn_6.8, i.e. the mixture of Mn/Ni atoms, are located inside the icosahedra [Т1Т₆Mn₄Sm₂] and [Т2Т₆Mn₄Sm₂] with CN=12, while the bigger manganese atoms have 14 neighbors [MnТ₈Mn₅Sm]. Similar situations have been observed for the compounds ErCu_5.1In_6.9 [12] and YbAg_5.4In_6.6 [13], where the bigger In atoms, compared with the mixtures (Cu/In) or (Ag/In), also reside in 14-vertex polyhedra.

Fig. 1:

Projection of the unit cell of SmNi_5.2Mn_6.8 onto the crystallographic ab plane and coordination polyhedra of the atoms.

The main interatomic distances and their values of reductions orientated at the sum of atomic radii (Δ=100(d−Σr)/Σr, where Σr is the sum of the respective atomic radii) along with the coordination numbers of atoms for SmNi_5.2Mn_6.8 are listed in Table 3 (values of the atomic radii are taken from ref. [14]: r(Sm)=1.80, r(Ni)=1.24, r(Mn)=1.27, and calculated r(Т1)=1.24 and r(Т2)=1.25 Å). Most deviations of the interatomic distances from the sums of relevant atomic radii are very small (no more than 8%). Some Mn–Mn and Т1–Т1 interatomic distances are slightly shorter compared to the sum of the respective atomic radii. Such shortening of interatomic distances allows us to consider a pairing of the respective atoms. The manganese atoms form dumb-bells, while the Т1 atoms are combined to chains along the c direction. Two Sm atoms, four Mn atoms and 14 T atoms form the coordination sphere of the Mn₂ dumb-bells in the SmNi_5.2Mn_6.8 structure (Fig. 2). Such clusters [Mn₂(Sm₂Mn₄T₁₄)] are connected through common faces and form grids perpendicular to the c direction, in the holes of which the Sm atoms are located. The coordination polyhedra of the Т1 atoms are connected along the c direction to form tubes [Т1₂(Sm₃Mn₆T₆)] in the middle of which there is the Т1 atom chain (Fig. 3). Further, these tubes are connected through common faces and completely fill the space in the structure of SmNi_5.2Mn_6.8.

Fig. 2:

The arrangement of the clusters [Mn₂(Sm₂Mn₄T₁₄)] in the crystal structure of SmNi_5.2Mn_6.8.

Fig. 3:

The [Т1₂(Sm₃Mn₆T₆)]_∞ tube and the arrangement of these tubes in the crystal structure of SmNi_5.2Mn_6.8.

The SmNi_5.2Mn_6.8 structure can also be considered as a combination of simple fragments: tetragonal motifs of the CeMg₂Si₂ structure (P4/mmm, a=4.250, c=5.765 Å) [8] and slabs of a hypothetical substructure “Mn₄T₄” (Fig. 4).

Fig. 4:

Packing of Mn₄T₄ and CeMg₂Si₂ slabs in the crystal structure of SmNi_5.2Mn_6.8.

The results of our investigation together with all previous data [4], [5] indicate that Sm(Mn,Ni)₁₂ forms a homogeneity range with Mn/Ni substitutions. The formula of the compound can be described as SmNi_8.10−3.94Mn_3.90−8.06. The lattice parameters increase due to substitution of smaller Ni atoms with larger Mn atoms (a=8.465–8.693, c=4.766–4.864 Å).

4 Conclusions

The crystal structure of SmNi_5.2Mn_6.8 has been investigated by using single crystal X-ray data. This compound forms a tetragonal ThMn₁₂-type unit cell (space group I4/mmm, a=8.6528(3), c=4.8635(3) Å), observed before for the binary phases RMn₁₂ with R=Y, Gd, Tb, Dy, Ho, Er, Tm) [2], [3] and for the ternary systems R-Mn-Т with R=Ce, Pr, Nd, Sm and Т=Fe, Co, Ni [4], [5]. Mixtures of Mn/Ni atoms occupy two crystallographic positions in the structure of this compound while the remaining two positions are occupied solely by Sm and Mn atoms.