First-principles study of the structural, electronic, and elastic properties of R Rh3 Bx C1-x (R=Sc and Y)

We study the variation in the structural and electronic properties as well as bulk modulus of perovskite-type R Rh3 Bx C1-x with R=Sc and Y as a function of the boron concentration. These compounds are realized in the whole range of 0≤x≤1 in cubic structure. We use first-principles projected augmented wave method and the generalized gradient approximation for the exchange-correlation functional within the density-functional theory. Different configurations of boron and carbon atoms for a given x have been studied by considering a 2x2x2 supercell. The atomic structures are fully optimized. The most favorable distribution is found to be the one where like atoms (B or C) are nearest neighbors to each other on the B C sublattice. However, the energy difference between different configurations is small and at room temperature, B and C atoms are likely to be randomly distributed. The calculated lattice constants are found to be in very good agreement with the experimental results. Our calculations show that the bulk modulus decreases monotonically with increasing boron concentration. We find strong covalent bonding between boron and carbon 2p and Rh 4d orbitals. There is charge transfer to B and C atoms and it is more significant on the B sites. For R=Sc, the cohesive energy is maximum at x=0 and it decreases monotonically with increasing B concentration. However, for R=Y, the highest cohesive energy is obtained for x∼0.25. For both Sc and Y, the Fermi energy lies in a pseudogap for x=0. Boron doping shifts the Fermi energy in a manner similar to a rigid-band model.