On Chemical Bonding in -GaRh and Its Effect on Structural Organization and Thermoelectric Behavior.

In the course of systematic studies of intermetallic compounds GaTM (TM─transition metal), the compound GaRh is synthesized by direct reaction of the elements at 700 °C. The material obtained is characterized as a high-temperature modification of GaRh. Powder and single-crystal X-ray diffraction analyses reveal tetragonal symmetry (space group 4/, No. 146) with = 6.4808(2) Å and = 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 -GaRh. Bonding analysis in -GaRh performed on ordered models with the space groups 1̅, 4, and 422 shows, besides the omnipresent heteroatomic Ga-Rh bonds in the rhombic prisms [GaRh], 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, -GaRh shows temperature-dependent electrical resistivity of a "bad metal". The very low lattice thermal conductivity of less than 0.5 W m K at 300 K, being lower than those for most other GaTM compounds, correlates with the enhanced bonding complexity.