Received 22 April 2011
A new rhombohedral modification of EuNi5In
A rhombohedral modification of europium pentanickel indide, r-EuNi5In, crystallizes in the Rm space group and adopts the UCu5In structure type. The structure is closely related to the hexagonal, h-EuNi5In, form (CeNi5Sn type). Both EuNi5In modifications are composed of CaCu5 (EuNi5)-, MgCu2 (InNi2)- and NiAs (EuNi)-type slabs in a 1:2:1 ratio. The atoms in the structure have high coordination numbers, viz. 20 and 18 for europium, 14 for indium, and 12 and 10 for nickel. The structure features a two-dimensional network of 2[Ni8] tetrahedral clusters arranged in the ab plane.
Extensive investigations focused on the interaction of rare earths (RE) with transition metals and indium have clearly demonstrated that the formation of numerous intermetallic compounds is typical for RE-Ni-In systems (Kalychak et al., 2004; Kalychak, 1997). More than 25 structure types have been reported for these intermetallics (Kalychak et al., 2004). Besides the large structural variety, the RExNiyInz compounds have attracted considerable interest owing to their distinctive magnetic and electrical properties, as well as their hydrogen-storage behavior. In this context, the representatives with cerium, europium and ytterbium are the most interesting. Several of these compounds show valence instabilities or unusual magnetic ordering phenomena. Rare earth intermetallics containing europium exhibit a wide range of interesting and unusual physical properties, which are mostly related to their mixed-valence nature (II/III) (Zaremba et al., 2006; Pöttgen et al., 1996). Unfortunately, these systems remain much less well known, which may be explained, along with other reasons, by the experimental difficulties of alloy synthesis. To date, the existence of seven compounds in the Eu-Ni-In system has been confirmed: EuNi7+xIn6-x (LaNi7In6 structure type; Zaremba et al., 2006), EuNi9In2 (YNi9In2 structure type; Kalychak et al., 1984), EuNi3In6 (LaNi3In6 structure type; Kalychak et al., 1997), EuNi5In (CeNi5Sn structure type; Baranyak et al., 1992), EuNiIn4 (YNiAl4 structure type; Kalychak et al., 1988; Pöttgen et al., 1996), EuNiIn2 (MgCuAl2 structure type; Kalychak et al., 1997) and EuNi0.5In1.5 (AlB2 structure type; Baranyak, Dmytrakh et al., 1988).
During the study of the ternary Eu-Ni-In phase diagram, the novel modification of EuNi5In was found and its crystal structure determined by single-crystal X-ray diffraction. The compound adopts the UCu5In (Stepien-Damm et al., 1999; Hlukhyy, 2003) structure type. An orthorhombic projection of the unit cell and the coordination polyhedra of the atoms are shown in Fig. 1. Two types of polyhedra were observed for Eu1 and Eu2 atoms on Wyckoff sites 3a and 3b, namely eight-capped hexagonal [Eu1Ni18In2] prisms and six equatorially capped and two base-capped pentagonal [Eu2Ni12In6] antiprisms. Distorted [Ni1Ni9Eu3] and [Ni3Ni7In2Eu3] icosahedra are filled by Ni1 and Ni3 atoms, respectively, and Ni2 has ten-vertex [Ni2Ni3In4Eu3] polyhedra. The In atoms are located at the centers of [InNi10Eu4] polyhedra.
EuNi5In is a nickel-rich compound. Three crystallographically different nickel sites, with Ni-Ni distances ranging from 2.4330 (15) to 2.4784 (16) Å, can be found within the rhombohedral EuNi5In (r-EuNi5In) structure. Compared to the Ni-Ni distance of 2.49 Å in the face-centered cubic (f.c.c.) structure of nickel (Donohue, 1974), we can assume a significant degree of Ni-Ni bonding. Here, the central building unit is a distorted tetrahedron formed by three Ni3 atoms in the 18h position and one Ni1 or Ni2 atom, both occupying the 6c sites. The tetrahedra now alternately share vertices and faces along the c axis, thereby forming an [Ni8] unit (Fig. 2a). Each fragment is connected through Ni3 atoms with three other fragments rotated by 180°. Six [Ni8] units are linked together by vertices in the ab plane, building up ring units of 18 tetrahedra (Fig. 2b). The resulting substructure of Ni atoms features a two-dimensional network in the ab plane (Fig. 2c). Eu1 atoms fill holes in the hexagonal rings thus formed; Eu2 and In1 atoms separate different sheets of nickel networks. The vertices of the tetrahedra in the next layer are located under the centers of the hexagonal rings. The orientation of the layers in the structure can be described as an ABC sequence (Fig. 2d). The different nickel clusters have been found for several compounds with a high content of transition element in RE-Ni-In systems (RE is a rare earth): a three-dimensional network of 3[Ni4] corner-sharing tetrahedra is a characteristic of CeNi4In (Koterlin et al., 1998), two-dimensional 2[Ni2] fragments occur in LaNi2In (Kalychak & Zaremba, 1994) and one-dimensional 1[Ni5] and 1[Ni7] cluster chains are present in Ce4Ni7In8 (Baranyak, Kalychak et al., 1988) and EuNi7In6 (Zaremba et al., 2006), respectively.
Previously, the hexagonal EuNi5In (h-EuNi5In; Baranyak et al., 1992) intermetallic of the CeNi5Sn type was observed at 670 K. The synthesis of our compound was carried out at 870 K. Therefore, we believe that the new compound with the UCu5In type is probably a high-temperature polymorphic modification of h-EuNi5In with the CeNi5Sn type. Structures of both intermetallics can be considered as an intergrowth of the CaCu5 (Bruzzone, 1971), MgCu2 (Ohba et al., 1984) and NiAs (Brand & Briest, 1965) related slabs (Fig. 3) with the compositions EuNi5, InNi2 and EuNi in the ratio of 1:2:1, viz. 2EuNi5In = EuNi5 + 2InNi2 + EuNi. Hence, the unit cell of each compound in the orthorhombic projection is deduced as follows: for the UCu5In structure type, 3Eu2Ni10 (6CaCu5) + 6In2Ni4 (12MgCu2) + 3Eu2Ni2 (6NiAs) = Eu12Ni60In12 = 12 r-EuNi5In; and for the CeNi5Sn structure type, 2Eu2Ni10 (4CaCu5) + 4In2Ni4 (8MgCu2) + 2Eu2Ni2 (4NiAs) = Eu8Ni40In8 = 8 h-EuNi5In. In both structures, the fragments alternate along the c axis. Consequently, the unit-cell dimension c is proportional to the number of layers: the hexagonal phase has two sets of fragments and c 20 Å, and the rhombohedral phase has three sets of fragments and c 30 Å. It should be noted that the original prototype UCu5In of the novel compound (Stepien-Damm et al., 1999; Hlukhyy, 2003) has three polymorphic modifications, two of which have the same structure as reported herein.
| || Figure 1 |
A projection of the EuNi5In unit cell in an orthorhombic aspect and a view of the coordination polyhedra of the atoms. Generic atom labels without consideration of symmetry operators are used.
| || Figure 2 |
(a) The [Ni8] tetrahedral unit; (b) the hexagonal ring of six [Ni8] units; (c) the 2[Ni8] two-dimensional layer; (d) the ABC sequence of 2[Ni8] layers in the EuNi5In structure.
| || Figure 3 |
The packing of (c) CaCu5-, (d) MgCu2- and (e) NiAs-related slabs in the (a) UCu5In and (b) CeNi5Sn forms of EuNi5In.
A sample of composition EuNi4In and a weight of 2 g was prepared by arc-melting of the pure components (the purity of the ingredients was higher than 99.9 wt%) under a high-purity argon atmosphere. The ingot was remelted twice to ensure homogeneity. The sample was wrapped in tantalum foil and sealed in an evacuated quartz tube. The ampoule was heated to 1070 K, followed by cooling to 870 K for 24 h. The annealing was carried out at this temperature for 75 h. After the thermal treatment, the ampoule with samples was quenched in water. The sample obtained showed a small weight loss which could be explained by evaporation of Eu. The sample is air and moisture sensitive and decomposes outside of the inert atmosphere within a few days. An X-ray diffraction powder pattern was collected using monochromatic Cu K radiation on a DRON-3 diffractometer. X-ray phase analysis revealed the presence of two phases, viz. EuNi5In and a small amount of In, which could be explained by a partial decomposition of the sample. Single crystals of EuNi5In of an irregular form were extracted from the crushed sample. In contrast to the powder, these remain stable in air for a long time.
Analyses of the systematic absences for the single-crystal data led to the possible space groups R (No. 148), R3 (No. 146), R3m (No. 160), R32 (No. 155) and Rm (No. 166). The space group with the highest symmetry, Rm, was found to be correct during the structure refinement. The starting atomic parameters were deduced from an automatic interpretation of direct methods and the structure was successfully refined with anisotropic atomic displacement parameters for all atoms. All crystallographic positions are fully occupied. Final difference Fourier synthesis revealed a slightly elevated, but not significant, residual peak of 2.55 e Å-3. However, it was too close to an Ni-atom position (0.90 Å) to be indicative of an additional atomic site. It is probably due to the irregular shape of the crystal and consequently an incomplete absorption correction.
Data collection: SMART (Bruker, 1996); cell refinement: SAINT (Bruker, 1996); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: KU3048 ). Services for accessing these data are described at the back of the journal.
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