Structural and electronic properties of matlockite MFX (M=Sr, Ba, Pb; X=Cl, Br, I) compounds

First-principles calculations have been used to study the structural and electronic properties of technologically important matlockite compounds MFX (M = Ba, Sr, Pb; X = Cl, Br, I) using a full potential linearized augmented plane-wave method within density functional theory. We used the local density approximation and the generalized gradient approximation, as well as the Engel-Vosko's GGA formalism to find the band gap and the partial density of states at equilibrium volume. We also optimized internal parameters by relaxing the atomic positions in the force directions. The calculated total energy allowed us to investigate several structural properties in particular the equilibrium lattice constants a and c, c/a ratio, bulk modulus, pressure derivative of the bulk modulus, cohesive energy, interatomic distances, interlayer distances along c axis and the angles between different atomic bonds. We calculated the valence charge density at the equilibrium volume for BaFCl and PbFCl and concluded that the bonding nature in these compounds is mainly ionic. Results are discussed and compared with experimental and other theoretical data. (C) 2004 Elsevier Ltd. All rights reserved.