Properties of GdSF and phase diagram of the GdF3 - Gd2S3 system

http://dx.doi.org/10.2139/ssrn.4330358

The objectives of the article were to synthesize and study the properties of GdSF, to construct a phase diagram of the GdF3-Gd2S3 system, and to calculate the liquids. The GdSF compound (ST PbFCl, P4/nmm, a = 0.3832(7) nm, c = 0.6853(9) nm) has an optical band gap for a direct interband transition of 2.56 eV and is characterized by a pronounced increase in the Kubelka-Munk function in the region of this transition. Direct optical bandgap of GdSF is measured to be 2.77 eV. Two indirect bandgaps are detected to be 1.54 and 2.4 eV. Meta-GGA simulations of band structure predicting 1.481 eV direct bandgap of GdSF are in good agreement with these features of experimental absorption spectrum. To explain this complicate case, we argue that formally direct optical transitions to highly dispersive subbands contribute not to direct but to indirect bandgaps measured by Tauc analysis. The GdSF compound melts incongruently with the formation of a melt and γ-Gd2S3 compound at t = 1280 ± 2 ºС, ΔН = 40.6 ± 2.8 KJ/mol, ΔS 26.1 ± 1.8 J/mol*K. The eutectic has a composition of 13 mol.% Gd2S3 (0.74 GdF3 + 0.26 GdSF), the melting characteristics of the eutectic are 1182 ± 2°С, ΔН = 36.2 ± 2.5 KJ/mol, ΔS = 24.9 ± 1.7 J/mol*K. The phase diagram of the GdF3 - Gd2S3 system has been constructed. The balance equations for five phase transformations in the system, recorded by the DSC method, are compiled. Convergence was achieved in the construction of the liquidus of the system according to DSC data and the calculation according to the Redlich-Kister equation.