The new ternary phases of La₃(Zn_0.874Mg_0.126)₁₁ and Ce₃(Zn_0.863Mg_0.137)₁₁

Recently, intermetallic compounds containing rare earths, transition metals and magnesium have been of particular interest to researchers in relation to their useful properties as modern lightweight alloys and hydrogen-storage materials. The crystal structures, physical properties and hydrogenation behaviour of these materials have been reviewed by Rodewald et al. (2007). Until now, the most heavily studied intermetallic compounds in this class have been those with transition metals such as Ni and Cu. Only one ternary compound, i.e. La₂Mg₃Zn₃ (cubic, a = 7.145 Å), was investigated from the La–Mg–Zn ternary system (Melnik, Kinzhibalo et al., 1978). The Ce–Mg–Zn ternary system was first reported by Melnik, Kostina et al. (1978), and the isothermal section of the phase diagram was constructed partially up to 60 at.% of Zn and 50 at.% of Ce at 573 K. Four new ternary compounds with preliminary compositions CeMg₇Zn₁₂, Ce(Mg_0.5–0.85Zn_0.5–0.15)₉, CeMg₃Zn₅ and Ce₂Mg₃Zn₃ were reported in this region. The last compound was found to crystallize with a cubic unit cell (a = 7.064 Å), whereas the crystal structures of the first three compounds remain unknown. We decided to explore the rest of the phase diagram starting from the Zn-rich region. During the investigation of the Ce–Mg–Zn phase diagram in the Zn-rich concentration range, several ternary phases were found. In our previous papers, the crystal structures of CeMgZn₂ (Pavlyuk et al., 2007) and Ce₂₀Mg₁₉Zn₈₁ (Pavlyuk et al., 2008) were reported, and it was found that the CeMgZn₂ ternary phase [structure type of MnCu₂Al, cubic, cF16, a = 7.0358 (4) Å] belongs to a numerous family of Heusler-type structures, which are an ordered variant of the BiF₃ cubic structure type, while the ternary compound Ce₂₀Mg₁₉Zn₈₁ crystallizes with a large cubic unit cell [space group F43 m, a = 21.1979 (8) Å] and represents a new type of structure. The results of crystallographic studies of two further intermetallic compounds, i.e. La₃(Zn_0.874Mg_0.126)₁₁, (I), and Ce₃(Zn_0.863Mg_0.137)₁₁, (II), are presented here.

The title compounds crystallize with the orthorhombic La₃Al₁₁ structure type (space group Immm) with 28 atoms per unit cell. The formation of compounds with this structure type is typical for the R₃Al₁₁ (Buschow & Van Vucht, 1967) and R₃Zn₁₁ (Bruzzone et al., 1970) binary intermetallics (R = rare earth), and for the ternary compounds in the following systems: R–Ag–Al, R–Cu–Al (Stel'makhovych et al., 2000), R–T–Ga (T = Cu, Ag, Au, Pd, Pt, Rh, Ir) (Grin et al., 1993) and Yb–Zn–Al (Fornasini et al., 2005). The shortest interatomic distances in (I) and (II) are in the ranges typical for intermetallic compounds containing La (or Ce), Mg and Zn, and indicate metallic-type bonding. The projection of the unit cell and coordination polyhedra of the atoms are shown in Fig. 1. The number of neighbour atoms correlates well with the dimensions of the central atoms. The largest La or Ce atoms are enclosed in 17- and 18-vertex polyhedra that can be treated as distorted pseudo Frank–Kasper polyhedra. The statistical mixture of (Zn/Mg)6 is characterized by the monocapped cuboctahedron polyhedra having the coordination number (CN) of 13. The statistical mixture of (Zn/Mg)5 and the Zn3 atom are surrounded by 12 neighbours in the form of distorted cuboctahedra (CN =12), while the atomic environment of the Zn4 atom is a bicapped tetragonal antiprism (CN 8 + 2).

Although the isostructural compounds (I) and (II) are very similar to the La₃Al₁₁ structure type in terms of having the same space group, the same Wyckoff positions and similar lattice parameters, these two compounds cannot be treated as isostructural with it. Comparing the structure of (I) with that of La₃Al₁₁ (Gomes de Mesquita & Buschow, 1967) we observe a significant decrease in the unit-cell dimension along the b axis of 11.9% [b = 10.132 (7) Å for La₃Al₁₁ and b = 9.0514 (8) Å for (I)], while the dimensions in the two other directions increase by 3.6% [a = 4.431 (5) Å for La₃Al₁₁ and a = 4.5992 (4) Å for (I)] and 5.2% [c = 13.142 (10) Å for La₃Al₁₁ and c = 13.8635 (11) Å for (I)]. This atypical deformation of the unit cell at the transition from the structure type La₃Al₁₁ to (I) cannot be associated only with the geometric factor of the difference in atomic radius of Al (r = 1.43 Å) and Zn (r = 1.38 Å) (Slater, 1964). The relative reduction of the radius in this case is only 3.5%. Rather, the reason for this atypical deformation is closer packing of atoms in the unit cell of (I) caused by shifts in atomic positions resulting from the strengthening of metallic-type bonding. The last assertion is based on the fact that Zn and Mg are still common metal atoms (d- and s-block elements, respectively), while Al has electronic nature as a p-block element, though with metal properties. Thus, bonding between the d electrons of the La atom and d electrons of the Zn atom (or bonding between the d electrons of the La atom and s electrons of the Mg atom) in structure (I) is stronger than bonding between the d electrons of the La atom and p electrons of the Al atom in the La₃Al₁₁ structure type.

A detailed crystal chemical analysis shows that in the case of the La₃Al₁₁ structure, the Al2 atom has the coordination polyhedron of a trigonal prism with three additional capping atoms (CN = 9), while in the La₃(Zn_0.874Mg_0.126)₁₁ and Ce₃(Zn_0.863Mg_0.137)₁₁ structures the Zn4 atom (which occupies the same 8l Wyckoff position as Al2 in La₃Al₁₁) has the coordination polyhedron of a tetragonal antiprism with two added atoms (CN = 10) (Fig. 2). Significant deformation in the direction of the b-cell axis and shifts of atoms in the same direction cause the observed changes in the coordination polyhedra. In fact, there is a close relationship between the tetragonal antiprism (CN = 8), in the ideal case, and trigonal prism with two additional atoms (CN = 6 + 2) (Fig. 2a). Transformation of the tetragonal antiprism to trigonal prism with two additional capping atoms is due only to the shifts of two atoms of a square face, which is bent forming two triangular faces. The same method of transformation of the 8l coordination polyhedron occurs at the transition between La₃(Zn_0.874Mg_0.126)₁₁ (Fig. 2b) and La₃Al₁₁ (Fig. 2c). The Al2—La2 distance in the La₃Al₁₁ structure is large (3.7382 Å), and thus the La2 atom does not belong to the Al2 polyhedron. By contrast, in the La₃(Zn_0.874Mg_0.126)₁₁ structure (and also in the Ce analogue), the distance between the corresponding atoms, i.e. betwen Zn4 and La2, is much smaller [3.3198 (10) Å] causing atom La2 to belong to the Zn4 polyhedron. As a result of these differences the title compounds and La₃Al₁₁ belong to different structural classes in the classification scheme of Krypyakevich (1977). La₃(Zn_0.874Mg_0.126)₁₁ and Ce₃(Zn_0.863Mg_0.137)₁₁ belong to class 9 (tetragonal antiprism as a coordination polyhedron), while La₃Al₁₁ belongs to class 10 (trigonal prism as a coordination polyhedron).

If we compare the known Al-containing compounds from systems R–Ag–Al, R–Cu–Al (Stel'makhovych et al., 2000) with the structure of La₃Al₁₁, we do not observe this atypical deformation. For these ternary compounds it is observed that the decrease in the unit-cell dimension along the b axis is in the range 1.19–3.21% for ₃(CuAl)₁₁ and 0.51–1.58% for R₃(AgAl)₁₁ depending on the rare earth element (R). By contrast, in the binary compounds R₃Zn₁₁ (Bruzzone et al., 1970), the decrease in the unit-cell dimension along the b axis is in the range 11.61–12.90% depending on R. Solid evidence that this anomaly is characteristic only for compounds containing Zn is obtained by comparing our results with data for compounds that contain Ga instead of Al (i.e. from systems R–T–Ga where T = Cu, Ag, Au, Pd, Pt, Rh, Ir; Grin et al., 1993). The atomic radius of Ga (r = 1.35 Å) (Slater, 1964) is very close to the radius of Zn (r = 1.38 Å), but the b axis in the Ga-containing compounds is larger than that in the Zn-containing compounds by an average of 5%. This is entirely inconsistent with the geometric factor (radius) and supports our assumption that the structural deformation is driven by closer packing in the Zn-containing structure caused by strengthening of metallic-type bonding in consequence of changes in the electronic nature of the element.

The crystal structures of the title compounds are also closely related to the tetragonal BaAl₄ (space group I4/mmm; Andress & Alberti, 1935) and orthorhombic LaAl₄ (space group Imm2; Zalutskii & Krypyakevych, 1967) structural types, i.e. types adopted by the binary rare earth compounds RAl₄ (R = La, Ce, Pr, Nd). In these structures the rare earth atoms are embedded in the three-dimensional networks, which are formed by the smaller metal atoms (Fig. 3). The primary fragment of such networks has the composition [RM₁₈]. For example, in the BaAl₄ structure each Ba atom is surrounded by 18 Al atoms. Further, the structural [RM₁₈] fragment is connected with the six other identical fragments. The structures of La₃(Zn_0.874Mg_0.126)₁₁ and Ce₃(Zn_0.863Mg_0.137)₁₁ also contain the analogous [RM₁₈] fragments and each of them is connected with six fragments containing 16 smaller atoms [RM₁₆]. The [RM₁₆] fragment type can be derived from the [RM₁₈] fragment by internal deformation and multiple substitutions (Fig. 4).