The recently discovered superconductor MgCNi3 crystallizes in the classical cubic perovskite structure. The electronic structures of the MgCNi3-xMx (M = Co, Cu; x = 0, 0.5, 1.0) system have been calculated using the self-consistent tight-binding linear muffin-tin orbital method. The calculations showed that the Fermi level for MgCNi3 is located in the slope descending from a sharp peak originating from d states of Ni atoms. Electron (Cu) and hole (Co) doping of MgCNi3 reconstructs its band structure but does not lead to magnetic order.

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The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi(2) and Ce(2)Rh(3)Si(5) (orthorhombic CeNiSi(2)- and U(2)Co(3)Si(5)-type structures, respectively) were studied over wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce L(III)-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confronted with the results of ab initio calculations of the electronic band structures in both compounds.