Bonding and physical properties of Hume-Rothery compounds with the PtHg4 structure -: art. no. 125118

We present a detailed experimental and theoretical study concerning bonding and structural stability of intermetallic electron compounds with the PtHg4 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 CrGa4 and MnGa4 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 MnGa4 is about 5x10(-9) m(3)/mol higher than that of CrGa4. The PrHg4 structure of CrGa4 and MnGa4 is stable up to pressures of about 100 kbar. Full-potential linearized augmented plane wave calculations reproduced very well the experimental structural properties of CrGa4 and MnGa4 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 sp bands. As a consequence a large pseudogap at the Fermi level for CrGa4 and slightly above the Fermi level for MnGa4 is produced. This pseudogap is characteristic and decisive for structural stability of electron compounds with the PtHg4 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 TGa4 with T being a transition metal from soup 6 or 7. A complete substitution of Ga for isovalent Al or In is not possible.