Structure and Properties of Gd₃Ge₄:  The Orthorhombic RE₃Ge₄ Structures Revisited (RE = Y, Tb−Tm)

The new binary compound Gd₃Ge₄ has been synthesized and its structure has been determined from single-crystal X-ray diffraction. Gd₃Ge₄ crystallizes in the orthorhombic space group Cmcm (No. 63) with unit cell parameters a = 4.0953(11) Å, b = 10.735(3) Å, c = 14.335(4) Å, and Z = 4. Its structure can be described as corrugated layers of germanium atoms with gadolinium atoms enclosed between them. The bonding arrangement in Gd₃Ge₄ can also be derived from that of the known compound GdGe (CrB type) through cleavage of the zigzag chains in GdGe and a subsequent insertion of an extra germanium atom between the resulting triangular fragments. Formally, these characteristics represent isotypism with the Er₃Ge₄ type (Pearson's oC28). However, re-examination of the crystallography in the whole RE₃Ge₄ series (RE = Y, Tb−Tm) revealed discrepancies and called into question the accuracy of the originally determined structures. This necessitated a new rationalization of the bonding, which is provided in the context of a comparative discussion concerning both the original and revised structure models, along with an analysis of the trends across the series. The temperature dependence of the magnetic susceptibility of Gd₃Ge₄ shows that it is paramagnetic at room temperature and undergoes antiferromagnetic ordering below 29 K. Magnetization, resistivity, and calorimetry data for several other members of the RE₃Ge₄ family are presented as well.