High-pressure synthesis and superconductivity of a new binary lanthanum germanide LaGe₃ with triangular Ge₃ cluster units

We prepared a new binary lanthanum germanide, LaGe₃, under high-pressure and high-temperature conditions (3-12 GPa, 500-1200 °C). It crystallizes in the BaPb₃ structure (the space group R3̄m) with lattice constants of a = 6.376(1) Å, c = 22.272(3) Å, and V = 784.1(2) ų. We refined the structure using Rietveld analysis from X-ray powder data. The structure is composed of two types of close-packed atom layers. In one layer, every La atom is surrounded solely by Ge atoms with the same distance of 3.188 Å. The other layer contains Ge₃ regular, triangular cluster units with a Ge-Ge distance of 2.634 Å. The electron localization function and crystal orbital Hamilton population calculations suggest that the triangular cluster is composed of three Ge-Ge covalent bonds and that each Ge atom has a lone pair. The temperature dependence of the magnetic susceptibility and electrical conductivity measurements revealed that LaGe₃ is metallic and shows superconductivity at 7.4 K. This critical temperature is highest for the La-Ge system.