CsCl (B2, Pm-3m) phase of lead chalcogenides and their ternary alloys: A computational study

We report ab initio density functional theory calculations for PbX (X = S, Se, Te) compounds in the CsCl-type structure (B2, Pm-3m), and their related ternary alloys PbS1-xSex, PbS1-xTex, and PbSe1-xTex. The structural, electronic, vibrational, and optical properties have been investigated under zero and finite pressures. Our calculated lattice constant values for the binary compounds are in a reasonable agreement with previous experimental and theoretical studies. The lattice constants of ternary alloys have a linear dependence on the concentration x, indicating that the bonding character in ternary and binary alloys of lead chalcogenides are identical. We find that PbX (X = S, Se, Te) and their related ternary alloys have metallic character, and the bonding has both ionic and covalent contributions. The features in the electronic structures are closely related to the charge transfer from the Pb atom to the chalcogen, and electronegativity differences of the chalcogen atoms. Frequency-dependent dielectric functions show peculiar features to help identify the CsCl-phases by optical spectroscopy. Calculated phonon spectra indicate that both the binary compounds and the ternary alloys are dynamically stable only under pressure. A binary lead chalcogenide may be substitutionally doped by the other chalcogens up to 12% dopant concentrations.