On Chemical Bonding in ht-Ga₃Rh and Its Effect on Structural Organization and Thermoelectric Behavior

In the course of systematic studies of intermetallic compounds Ga₃TM (TM─transition metal), the compound Ga₃Rh is synthesized by direct reaction of the elements at 700 °C. The material obtained is characterized as a high-temperature modification of Ga₃Rh. Powder and single-crystal X-ray diffraction analyses reveal tetragonal symmetry (space group P4₂/mnm, No. 146) with a = 6.4808(2) Å and c = 6.5267(2) Å. Large values and strong anisotropy of the atomic displacement parameters of Ga atoms indicate essential disorder in the crystal structure. A split-position technique is applied to describe the real crystal structure of ht-Ga₃Rh. Bonding analysis in ht-Ga₃Rh performed on ordered models with the space groups P1̅, P4₂nm, and P4₂2₁2 shows, besides the omnipresent heteroatomic Ga-Rh bonds in the rhombic prisms _∞³[Ga_8/2Rh₂], the formation of homoatomic Ga-Ga bonds bridging the Rh-Rh contacts and the absence of significant Rh-Rh bonding. These features are essential reasons for the experimentally observed disorder in the lattice. In agreement with the calculated electronic density of states, ht-Ga₃Rh shows temperature-dependent electrical resistivity of a "bad metal". The very low lattice thermal conductivity of less than 0.5 W m^-1 K^-1 at 300 K, being lower than those for most other Ga₃TM compounds, correlates with the enhanced bonding complexity.