Crystal chemical features of ternary phases in the R-Li-Ge (R = rare earth element) systems

The synthesis and structural characterization of phases in the R–Li–Ge (R = rare earth) systems are reported. The R4LiGe4 compounds with R = La–Nd, Sm complete the series, where the analogues with R = Gd–Tm, Lu were confirmed, all crystallizing with the Tm4LiGe4 type. Nd4LiGe4 and Ho4LiGe4 were structurally studied by single crystal X-ray diffraction (space group Pnma, Z = 4, Pearson code oP36). Nd4LiGe4: a = 7.367 (1) Å, b = 15.116 (5) Å, c = 7.947 (3) Å, wR2 = 0.054; Ho4LiGe4: a = 7.077 (4) Å, b = 14.585 (6) Å, c = 7.670 (3) Å, wR2 = 0.067. The phase Pr7Li8Ge10 crystallizes with a novel orthorhombic structure (space group Cmmm, Z = 2, Pearson code oS50): a = 6.8945 (7) Å, b = 33.531 (6) Å, c = 4.4400 (2) Å, wR2 = 0.060. Isotypic compounds were found also with R = La, Ce, Nd. A third series R3Li4Ge4 with R = La–Nd, Sm crystallizes with the Gd3Cu4Ge4 type. The structure was refined for the Nd analogue with the final composition Nd3Li3.65(2)Ge4.35(2) (space group Immm, Z = 2, Pearson code oI22): a = 4.3935 (5) Å, b = 6.811 (1) Å, c = 14.619 (3) Å, wR2 = 0.068. The Pr7Li8Ge10 structure is closely related to both Ce2Li2Ge3 and Gd3Cu4Ge4 types. They belong to a structural series with general formula R2m+nM4mX2m+2n formed by m and n segments of the MgCuAl2 and AlB2 types, respectively. All the examined structures are characterized by the trigonal prismatic coordination around the germanium atoms, which represents a common feature of the R–Li–Ge compounds.