Bonding and physical properties of Hume-Rothery compounds with thePtHg4structure

We present a detailed experimental and theoretical study concerning bonding and structural stability of intermetallic electron compounds with the ${\mathrm{PtHg}}_4$ structure. Due to the simplicity of the structure these compounds represent an excellent prototype system for a more general insight into bonding and stability of the large family of $d-sp$ bonded Hume-Rothery compounds. In particular, the representatives ${\mathrm{CrGa}}_4$ and ${\mathrm{MnGa}}_4$ were synthesised and their resistivity, magnetic susceptibility, and bulk modulus measured. We find that both compounds are metallic conductors but show a remarkable large difference in their temperature independent magnetic susceptibilities. The value of the Pauli paramagnetic susceptibility of ${\mathrm{MnGa}}_4$ is about $5\times {10}^{-9}$ ${\mathrm{m}}^3/\mathrm{mol}$ higher than that of ${\mathrm{CrGa}}_4.$ The ${\mathrm{PtHg}}_4$ structure of ${\mathrm{CrGa}}_4$ and ${\mathrm{MnGa}}_4$ is stable up to pressures of about 100 kbar. Full-potential linearized augmented plane wave calculations reproduced very well the experimental structural properties of ${\mathrm{CrGa}}_4$ and ${\mathrm{MnGa}}_4$ and showed strong directional (covalent) bonding between transition metal atoms and Ga atoms in both compounds. The directional bonding is due to a large hybridization of the narrow d bands with the Ga $\mathrm{sp}$ bands. As a consequence a large pseudogap at the Fermi level for ${\mathrm{CrGa}}_4$ and slightly above the Fermi level for ${\mathrm{MnGa}}_4$ is produced. This pseudogap is characteristic and decisive for structural stability of electron compounds with the ${\mathrm{PtHg}}_4$ structure. We find that structural stability appears as a competition between optimizing the pseudogap and minimizing the compound equilibrium volume. Therefore, stable electron compounds are confined to systems $T{\mathrm{Ga}}_4$ with T being a transition metal from group 6 or 7. A complete substitution of Ga for isovalent Al or In is not possible.