Crystals of binary rare-earth metal germanides grown in molten indium. Synthesis and structural characterization of Nd3Ge5 and Lu3Ge4

Abstract

Reported are the synthesis and the crystal structure determination of a new polymorphic form of the rare-earth metal germanide Nd3Ge5. As established by single-crystal X-ray diffraction methods, Nd3Ge5 crystallizes in a hexagonal crystal system with the space group P2c (No. 190, Z = 2). The unit cell parameters are a = 6.9964(3) Å and c = 8.5820(8) Å. The structure is a derivative of the common structure type AlB2 (space group P6/mmm) where the atoms are stacked in a hexagonal close-packed manner. Digermanide NdGe2 is not known, rather the phase exists as NdGe2–x (x < 0) with a sizeable number of germanium vacancies. When the defect concentration reaches x = 1/3, the vacant Ge sites can undergo long-range ordering, resulting in the formation of Nd3Ge5 (=NdGe1.67). Therefore, the atomic arrangement can be described as a 6-fold superstructure of the AlB2 type structure (a' = a  31/2 and c' = c  2) with flat Ge layers, where every 6th Ge atom is missing; the Ge layers are stacked along the crystallographic c-axis, and are separated by hexagonal layers of rare-earth metal atoms. This is a new polymorph of Nd3Ge5, with the other form being orthorhombic and structurally related to another commonly observed structure, that of ThSi2. Besides the structure of Nd3Ge5, we also report the structure of the known phase Lu3Ge4, which has been previously identified from its powder diffraction pattern but never refined. Lu3Ge4 is isotypic with the rest of the REGe4 germanides (RE = Gd–Tm) and crystallizes in an orthorhombic crystal system with the space group Cmcm (No. 63, Z = 4) with unit-cell parameters a = 3.9591(8) Å; b = 10.429(2) Å; c = 14.019(3) Å.