Shift of indirect to direct bandgap in going from K to Cs in MCaF_3 (M = K, Rb, Cs)

摘要

Bonding nature as well as structural, electronic, chemical bonding and optical properties of KCaF_3, RbCaF_3, and CsCaF_3 are studied for first time using the highly accurate full potential linearized augmented plane method within the generalized gradient approximations. It is found that lattice constant and bulk modulus increases with the change of cation (M) from K to Cs in MCaF_3 in accordance with the experimental results, which was previously contradicted by other theoretical results. KCaF_3 and RbCaF_3 have wide and indirect bandgaps, while the bandgap of CsCaF_3 is direct. It is further found that p states of M cation shifts towards the Fermi level while F-2p away from it. The p states of M cations, F-2p, and Ca-4d states control the electronic properties of the compounds. In optical properties real and imaginary parts of dielectric function, refractive index, reflectivity, energy loss function, optical conductivity, and sum rules for the compounds are calculated. Wide direct bandgap and different optical parameters of the materials predict their usefulness in the optoelectronic and optics technology.