Vacancy ordering as a driving factor for structural changes in ternary germanides: the new R2Zn1-xGe6 series of polar intermetallics (R = Rare-Earth Metal)

Synthesis and structural characterization of the new compounds R<inf>2</inf>Zn<inf>1-x</inf>Ge<inf>6</inf> (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-Ce<inf>2</inf>(Ga<inf>0.1</inf>Ge<inf>0.9</inf>)<inf>7</inf> model was established; instead, the Dy compound represents a new structure type (P2<inf>1</inf>/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-La<inf>2</inf>AlGe<inf>6</inf> 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 ∼R<inf>2</inf>MGe<inf>6</inf> 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-SmNiGe<inf>3</inf>) 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 R<inf>2</inf>Zn<inf>3</inf>Ge<inf>6</inf> series, enriched with new members (R = Sm, Gd-Ho) during this work. Electronic structure calculations on La<inf>2</inf>ZnGe<inf>6</inf> 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.