The Tl₂S–PbS–SiS₂ system and the crystal and electronic structure of quaternary chalcogenide Tl₂PbSiS₄

Chalcogenides of the quasi-ternary system Tl₂S–PbS–SiS₂ were synthesized in the evacuated silica ampoules by the melting and annealing technique. Phase equilibria in the system was investigated by XRD method. Isothermal section of the system was studied, and two intermediate quaternary phases were discovered to exist (Tl₂PbSiS₄, ∼Tl₂PbSi₃S₈). The quasi-binary section Tl₂SiS₃–PbS was investigated by DTA. Its phase diagram was constructed, and it was established that the equimolar compound melts incongruently at 818 K. The crystal structure of the quaternary compound Tl₂PbSiS₄ was determined by X-ray powder diffraction. It crystallizes in the monoclinic space group P2₁/a with the unit-cell parameters a = 8.8141(4), b = 9.0150(5), c = 10.4383(5) Å, and β = 94.490(4)° (Tl₂PbGeS₄ structure type). Reliability factors calculated in the isotropic approximation were found to be R_I = 0.0564 and R_P = 0.1070. The Tl₂PbSiS₄ single crystal was tested with X-ray photoelectron spectroscopy. In particular, the XPS core-level and valence-band spectra were recorded for pristine and Ar⁺-ion bombarded surfaces of Tl₂PbSiS₄. The Tl₂PbSiS₄ single crystal was found to be rather stable with respect to Ar⁺-ion irradiation. We have also measured the X-ray emission band depicting the energy distribution of mainly the S 3p states and compared it on a common energy scale with the XPS valence-band spectrum of the Tl₂PbSiS₄ crystal. The above comparison indicates that the S 3p states contribute substantively in the upper portion of the valence band of Tl₂PbSiS₄, with their significant contributions in other portions of the valence-band region. - Highlights: • Chalcogenides of the quasi-ternary system Tl₂S–PbS–SiS₂ were synthesized. • Two intermediate quaternary phases were discovered to exist (Tl₂PbSiS₄, ∼Tl₂PbSi₃S₈). • Tl₂PbSiS₄ crystallizes in the monoclinic space group P2₁/a. • Unit cell parameters and atomic positions were determined for Tl₂PbSiS₄. • Electronic structure of Tl₂PbSiS₄ single crystal was studied by XPS and XES.