Vacancy Ordering as a Driving Factor for Structural Changes in Ternary Germanides: The New R₂Zn_1–xGe₆Series of Polar Intermetallics (R = Rare-Earth Metal)

Abstract

Synthesis and structural characterization of the new compounds R2Zn1-xGe6 (R = La-Nd, Sm, Gd-Ho) is reported. A structural change was revealed along this series by careful analysis of single-crystal X-ray diffraction data. For light rare earths up to Tb the orthorhombic oS72-Ce2(Ga0.1Ge0.9)7 model was established; instead, the Dy compound represents a new structure type (P21/m, mP34, Z = 4, a = 7.9613(3) Å, b = 8.2480(4) Å, c = 10.5309(5) Å, β = 100.861(1)°) being a superstructure of the mS36-La2AlGe6 prototype. The established structural models support the increase of Zn deficiency along the series, suggested by microprobe analysis, and its key role in governing structural changes. The vacancy ordering criterion was applied as a successful approach to find a general scheme including the structures of the ∼R2MGe6 compounds known up to now (R = rare-earth metal, M = transition metal, Mg, Al, Ga) and highlighting the subtle structural differences within this family. According to this scheme, these structures are obtained from a common aristotype (oS20-SmNiGe3) via symmetry reduction based on group-subgroup relations accompanied by ordering of vacancies. This approach was optimized with the help of the ToposPro software and extended to the R2Zn3Ge6 series, enriched with new members (R = Sm, Gd-Ho) during this work. Electronic structure calculations on La2ZnGe6 confirm the presence of infinite covalent germanium zigzag chains and three-bonded corrugated layers connected via Zn atoms to form a polyanionic network stabilized by La atoms.

Polar‐Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare‐Earth Germanides

“…In previous studies R ‐ M ‐Ge ( R =rare earth metal; M =other metal) systems were characterized from the point of view of phase equilibria and intermediate ternary compound formation . Among these, the R 2 M Ge 6 constitute a continuously growing family, which has been studied with respect both to crystal structure peculiarities, and physical properties . Recent reports suggest that all these compounds belong to the closely related structure types, oS 72‐Ce 2 (Ga 0.1 Ge 0.9 ) 7 and mS 36‐La 2 AlGe 6 (Figure ).…”

“…The fact that R 2 M Ge 6 compounds exist with various metals M (including s‐, p‐ and late transition elements) showing only two structural models makes them very attractive for chemical bonding studies. Following this idea, the compounds La 2 MgGe 6 , La 2 ZnGe 6 , Y 2 PdGe 6 , La 2 AlGe 6 , La 2 PdGe 6 , and the structurally‐revised La 2 LiGe 6 , La 2 CuGe 6 and La 2 AgGe 6 (see Experimental Section), were selected for a chemical bonding analysis in position space. Even though on the basis of interatomic distances it is reasonable to suggest the presence of two‐bonded (2b) Ge zigzag chains and three‐bonded (3b) Ge corrugated layers for all compounds, the Zintl–Klemm concept can be successfully applied only when M is a divalent metal, like Mg and Zn.…”

“…If the sum of the E species' formal charges is consistent with the sum of acceptable appropriate charges of the active metals,t he classical picture pops up with the covalent partial structureso ft he E species,a nd the metals'c ations located inbetween. However,i th as becomec lear on the basis of density of states (DOS) and crystal orbitalH amilton population (COHP) analysis, [1][2][3][4][5] that the easy approximation of complete charge transfer from the metalst ot he E elements should be abandoned in numerous cases. This yields bonding interactions between cationic and anionic partial structures,w hich should be polar-covalent, instead of ionic, and are suspected to feature multicentric character.…”

“…With the aid of this new tool, ac onsistent picture of La/YÀM interactions for the title compounds was extracted.[a] R. coveringt he detailed bondings cenarios and their gradual changes with different M and R.In previouss tudies R-M-Ge (R = rare earth metal; M = other metal)s ystems were characterized from the point of view of phase equilibria and intermediate ternary compound formation. [13][14][15][16][17][18] Among these, the R 2 MGe 6 constitute ac ontinuously growingf amily, [19] which has been studied withr espectb oth to crystal structure peculiarities, [5] and physicalp roperties. [20,21] Recent reports [21][22][23] suggest that all these compounds belong to the closely relateds tructure types, oS72-Ce 2 (Ga 0.1 Ge 0.9 ) 7 and mS36-La 2 AlGe 6 (Figure 1).…”

“…Hereaftert he structure types are notated by Pearson symbol-prototype.The fact that R 2 MGe 6 compounds exist with variousm etals M (including s-, p-andl ate transition elements) showing only two structuralm odels makes them very attractive for chemical bondings tudies. Following this idea, the compounds La 2 MgGe 6 , [14] La 2 ZnGe 6 , [5] Y 2 PdGe 6 , [23] La 2 AlGe 6 , [24] La 2 PdGe 6 , [23] and the structurally-revised La 2 LiGe 6 ,L a 2 CuGe 6 and La 2 AgGe 6 (see Experimental Section), were selected for ac hemical bonding analysis in positions pace.E vent hougho nt he basis of interatomic distances it is reasonable to suggest the presenceo f two-bonded (2b) Ge zigzag chains andt hree-bonded (3b) Ge corrugated layers for all compounds, the Zintl-Klemm concept [25,26] can be successfully applied only when M is ad ivalent metal, like Mg and Zn. In these cases the simplified ionic formula (La 3 + ) 2 (M 2 + )[(2b)Ge 2À ] 2 [(3b)Ge À ] 4 leads to electro-neutrality.A lthough the Zintl-Klemm approach is not fulfilledw hen M is not divalent,i td oes not preventt he studied compounds from having closely relateds tructures, with similar Ge covalently bondedn etworks.…”