A new ternary silicide GdFe_1−xSi₂ (x=0.32): preparation, crystal and electronic structure

1 Introduction

Several ternary compounds with the approximate composition RT_1−xSi₂ (R=rare earth, T=transition element) have been reported during the last decades. Their crystal structures are characterized by a stacking of BaAl₄ and AlB₂ related slabs. Most compounds with T=Co–Cu crystallize in the CeNiSi₂-type structure [1]. Different structures were found for the RT_1−xSi₂ compositions. The early rare earth manganese RMn_1−xSi₂ (R=La–Sm) and iron RFe_1−xSi₂ (R=La–Tb) silicides crystallize in the LaMnSi₂-type structure [2], [3], [4], a structure closely related to the CeNiSi₂-type structure, and they are characterized by a site exchange between the transition-metal and the main-group elements within the BaAl₄ block. The atomic positions in the LaMnSi₂-type structure correspond to those in the TbFeSi₂-type structure earlier discovered by Yarovets and Gorelenko [5]. We use the earlier discovered type of structure for our denomination in the text. Investigations in quaternary systems R-T-T′-X led to the characterization of new isotypic representatives of the TbFeSi₂-type structure with the composition RTT′_0.5X_1.5 (R=La–Sm; T=Mn, Co, Rh, Re; T′=Cu, Ni, Pd, Pt; X=Si, Ge) [6], [7], [8], [9], [10], [11]. All compounds show the T′ substitution in the AlB₂ slabs.

Paccard et al. [9] re-investigated TbFeSi₂ first discovered by Yarovets and Gorelenko [5]. From single-crystal X-ray diffraction data of the RFeSi₂ (R=Ho, Tb, Dy) compounds, they concluded that these phases are non-stoichiometric and should rather be named as RFe_0.5Si₂ because an iron non-stoichiometry is evident. Due to the very small difference between the X-ray scattering factors for a fully occupied silicon site or a half-occupied iron site, this study does not clearly distinguish between the two site exchange variants, i.e. the TbFeSi₂ and CeNiSi₂ structure types. Norlidah et al. [10] using neutron diffraction and ⁵⁷Fe Mössbauer spectroscopy for HoFe_0.5Si₂ have suggested that this compound crystallizes in the CeNiSi₂-type structure and partially removed the uncertainties outlined by Paccard et al. [9] concerning the localization of the transition metal in the heavy rare earth RFe_1−xSi₂ compounds (R=Tb, Dy, Ho, Er, Lu). The TbFeSi₂ structure type also has quaternary representatives in the R-Mn-T-Si systems: RMnPd_0.5Si_1.5 (R=La, Ce), RMnPt_0.5Si_1.5 (R=La, Ce, Pr), and RMnCu_0.5Si_1.5 (R=La, Ce–Nd) [11].

For the RT_1−xX₂ compounds with the CeNiSi₂-type structure, the non-stoichiometry increases with the period number of the tetrele element with the same rare earth and transition metal element i.e.: HoFe_0.5Si₂, HoFe_0.38Ge₂, and HoFe_0.14Sn₂ [12]. By fixing the rare earth and tetrele elements, the transition metal content increases with the group number of the transition metal i.e.: LuFe_0.3Si₂ [13], LuCo_0.85Si₂ [14], and LuNiSi₂ [1].

Finally, the RRuSi₂-type compounds (R=La, Ce, Nd, Sm, Gd, Tb) and the stoichiometric RFeSi₂-type compounds (R=Nd, Sm, Gd, Tb) adopt the NdRuSi₂-type structure, a monoclinic variant of the CeNiSi₂ type [3], [15], [16]. The NdRuSi₂-type structure family includes the non-stoichiometric CeNi_1−xSi₂ and CeNi_l−xSi_2+x structures with vacancies or mixed occupations on the transition metal sites [14]. The compounds CeRh_1−xGe_2+x (x=0.325) crystallize in a primitive orthorhombic cell with disordered substitution on half of the transition-metal sites [17], while the TmLi_1−xGe₂ (x=0.5) structure is another monoclinically distorted variant [18].

In the course of systematic phase-analytical studies of the Gd–Fe–Si system we have obtained new phases. A closer inspection of the new compound GeFe_1−xSi₂ became the subject of the present work.

2 Experimental

2.2 Microprobe analysis

For metallographic inspection and for complementary qualitative phase analysis, energy dispersive X-ray spectroscopy (EDX) was employed. The samples were embedded in Wood’s metal (melting point of 75°C, Fluka Chemie, Switzerland). The embedded samples were polished on a nylon cloth using chromium oxide (Bühler Isomet) with grain sizes 1–5 μm. Quantitative and qualitative composition analyses of the polished samples were performed by energy-dispersive X-ray spectroscopy (EDX) on a scanning electron microscope TESCAN 5130 MM with an Oxford Si-detector. From the EDX analysis of the arc-melted sample GdFe_0.4Si₂ the composition Gd_26.2(3)Fe_18.0(3) Si_55.6(3) was deduced. For the chemical microprobe, the polishing procedure had to be performed or repeated just before the measurements. The surface of the phases in the Gd–Fe–Si system appears to be quite stable in air. Metallographic investigation, X-ray powder diffraction and EDX analyses revealed the presence of the phases Gd₂Fe₃Si₅ [19] and Gd_1.2Fe₄Si₉ [20] besides the new compound GdFe_1−xSi₂ (x=0.32) (Fig. 1). After the annealing procedure, only Gd₂Fe₃Si₅ and GdSi₂ [21] were found in the GdFe_0.4Si₂ sample.

Fig. 1:

Backscattered electron image of the arc-cast bulk sample with nominal atomic composition Gd:Fe:Si=1:0.4:2. (1) GdFe_0.68Si₂, (2) Gd₂Fe₃Si₅, (3) Gd_1.2Fe₄Si₉.

3 Results and discussion

The crystal structure of GdFe_1−xSi₂ (x=0.32) is shown in a projection on the (001) plane in Fig. 2. Typical coordination polyhedra (CP) of the various atoms are drawn in Fig. 2a. The gadolinium atoms [GdGd₆Fe₄Si₁₁] present a peculiar environment in the form of a hexagonal prism with additional atoms outside the prism faces. Their coordination number (CN) is therefore 21. The gadolinium atoms have close Si neighbors with Gd–Si distances between 3.011(3) and 3.127(4) Å and one Gd–Si distance of 3.464(8) Å (Table 3). The Gd–Fe distances cover the range between 3.108(4) and 3.137(4) Å for the four Fe neighbors of the Gd atom. The orthorhombic prisms with four centered lateral faces [FeGd₄Fe₄Si₄] with CN=12 are the CPs of the iron atoms. They have four close Si atoms with Fe–Si1 distances between 2.245(5) and 2.290(5) Å and four Fe atoms with Fe–Fe distances of 2.8294(4) Å. The silicon atoms have nine metal atom neighbors, forming a trigonal prism with three additional atoms outside the rectangular faces of the prisms: [Si¹Gd₄Fe₄Si₁] and [Si²Gd₆Si₃]. Si–Si distances are in the range from 2.361(3) to 2.395(8) Å.

Fig. 2:

Structural relationship between (a) GdFe_0.68Si₂, (b) CeNiSi₂ and (c) NdRuSi₂. The T₂X₂ layers and coordination polyhedra of related atoms are emphasized. Black and white spheres within the layers correspond to the transition metal and the silicon atoms, respectively.

GdFe_1−xSi₂ (x=0.32) crystallizes in the TbFeSi₂-type structure that is built up of an intergrowth of BaAl₄ and AlB₂ slabs. This structure type can be considered as a site exchange variant of the CeNiSi₂-type structure by interchanging the silicon and the transition metal in the BaAl₄ slab (Fig. 2). Both structures types TbFeSi₂ and CeNiSi₂ are geometrically similar with all atoms in 4c Wyckoff positions (space group Cmcm). Earlier presented results [2], [13] based on powder X-ray diffraction data for the RFeSi₂ compounds (R=Y, La–Nd, Tb, Ho, Er, Tm, Lu) were not sufficient for the consideration of structural details. From X-ray diffraction studies on single crystals of the RFeSi₂ compounds (R=Ho, Tb, Dy), the authors in reference [9] concluded that these phases should rather be named RFe_0.5Si₂ since an important iron non-stoichiometry is evidenced. The half-occupation of the 4c site by an iron atom does not allow to clearly distinguishing between the two site exchange variants. Norlidah et al. [10] on the basis of ⁵⁷Fe Mössbauer spectroscopy and neutron diffraction data suggested that the RFe_xSi₂ (R=Tb, Dy, Ho, Er, Lu) compounds crystallize in the non-stoichiometric CeNiSi₂-type structure, although the results of the crystal structure refinement were not presented.

Several of the ternary compounds with the approximate compositions RT_1−xSi₂ crystallize in the CeNiSi₂, NdRuSi₂ and TbFeSi₂ types, characterized by different stackings of elementary blocks characteristic of the AlB₂ and BaAl₄ crystal structures. The RT_1−xSi₂ structures are related to one of the well-known ThCr₂Si₂ or CaBe₂Ge₂ structure types [32], [33]. As the atomic composition is different from 1:2:2, only one part of the structures can be compared. In the CeNiSi₂, NdRuSi₂ and TbFeSi₂ structures the BaAl₄ blocks display two different arrangements as a function of the relative positions (sequence) of the T and X elements (Fig. 2). While the spatial distribution within the sheets is always a XTTX elemental block in the ThCr₂Si₂ type, alternating XTTX and TXXT sequences occur in the CaBe₂Ge₂ type. This atomic distribution generates Si–Si pairs in ThCr₂Si₂, whereas no Ge–Ge bonding occurs in one of the sheets in CaBe₂Ge₂.

The XTTX elemental block is characterized by a transition metal occupying the center of the BaAl₄ block, in tetrahedral X coordination, while the TXXT layer is characterized by a site inversion between the T and X elements. The XTTX elemental block is mainly encountered in the ThCr₂Si₂- and TbFeSi₂-type structures while the TXXT layer is encountered in the CeNiSi₂ and NdRuSi₂-type structures (Fig. 2). The relative stability of the XTTX and TXXT sequences, is one of the most intriguing problems encountered in the crystal chemistry of these layered compounds. According to Zheng and Hoffmann [34], the elemental RT₂X₂ block is characterized by different band dispersivity of the T and X sites, the more lattice dispersive site being the one that engenders more overlaps between equivalent sites, i.e. the separation between the equivalent sites is smaller than that of other sites. As shown in Fig. 2, in the XTTX block, the T metal occupies the more dispersive site while this site is occupied by the X element in the TXXT elemental block. This leads to important consequences for the relative stability of each block. Depending on the degree of band filling, the more electronegative atom will enter or avoid the more dispersive sites. Hence, for a T metal less electronegative than the X element and for a large band filling, the XTTX block will be more stable than the TXXT elemental block since the latter will have a higher Fermi energy. The numerous compounds crystallizing in the ThCr₂Si₂-type structure account for the great stability of the XTTX block. The stability of the CaBe₂Ge₂-type structure has been investigated by electronic structure calculations by Zheng and Hoffmann [34]. According to this study, the occurrence of compounds with the CaBe₂Ge₂-type structure should be related to the T–X interlayer interactions between the XTTX and TXXT elemental blocks, favoring their simultaneous presence.

For the calculation of the electronic structure of GdFe_xSi₂ based on the LMTO method we have used an ordered and refined structural variant with the composition GdFe_0.5Si₂ in space group Amm2, a subgroup of Cmcm, to avoid the statistical occupation of the Fe site (Table 4). The structure contains zig-zag chains of Si(1) with Si–Si distances of 2.359 Å with terminally bonded Si(2) atoms, as shown in Fig. 3. Based on the Zintl concept formal oxidation states can be assigned resulting in a reasonable formula Gd³⁺(Fe²⁺)_0.5Si(1)¹⁻Si(2)³⁻. The DOS (density of states) curve indicates that GdFe_0.5Si₂ is a metal. The occupied Si states are distributed from –11 eV to the Fermi level, whereas the occupied Fe 3d states are located between –5 eV and 0 eV. The calculated COHP curves for the important interactions in GdFe_0.5Si₂ are shown in Fig. 3. There is a strong bonding interaction between the Fe and Si(2) atoms. The Fe–Si(1) interactions are very weak because of the large Fe–Si(1) distances and are therefore not shown in Fig. 4. Strong Si(1)–Si(1) and Si(2)–Si(1) interactions are found as expected from the analyses based on the Zintl concept (above) whereas the Si(2)–Si(2) interactions are negligibly small. The top valence electrons are all in bonding orbitals and any further filling with electrons, e.g. by additional Fe atoms, will lead to a significant occupation of antibonding states. As the d electron count increases across the first transition series, the electronegativity rises as well (1.55, 1.83, 1.88, 1.91 and 1.90 for Mn, Fe, Co, Ni and Cu, versus 1.9 for Si in Pauling scale). Not only the dispersivity favors the CeNiSi₂ structure (less electronegative atoms occupy the less dispersive sites), but also the d electrons start to fill the antibonding states above the Fermi level indicated in Fig. 3, thus any higher d electron count further favors the CeNiSi₂ structure.

Fig. 3:

Projection of the ordered variant of GdFe_0.5Si₂ in space group Amm2, see Table 4. Large gray circles correspond to Gd atoms, black ones to Fe atoms, medium and light gray spheres to Si(1) and Si(2) atoms, respectively.

Fig. 4:

Calculated total and projected Density of States (left) and cumulative COHP curves (right) for GdFe_0.5Si₂ in space group Amm2, see Table 4.

4 Conclusion

In summary, a new ternary silicide GdFe_1−xSi₂ (x=0.32) has been synthesized from the elements by arc-melting. It crystallizes in space group Cmmm and represents a TbFeSi₂-type structure. The Fe position is not fully occupied and the structure refinement results in a composition GdFe_0.68Si₂ in agreement with a chemical analysis. The structure consists of zig-zag chains of Si(1) atoms which are terminally bound to additional Si(2) atoms. For an ordered variant of GdFe_0.5Si₂, the Zintl concept can be applied which results in formal oxidation states Gd³⁺(Fe²⁺)_0.5Si(1)¹⁻Si(2)³⁻. This assignment is confirmed by results of calculations based on the LMTO method, which also indicate that GdFe_0.5Si₂ is a metal. A more detailed computational analysis will be presented in a separate publication.