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

The objectives of this study were to refine the phase diagram of the GdF3-Gd2S3 system and to calculate their liquidus, and to synthesize GdSF and to study their properties. The GdSF compound (ST PbFCl, P4/nmm, a (Å) 3.83006(17), c (Å) 6.8529(3), has an optical band gap for a direct interband transi...

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Bibliographic Details
Published in:	Journal of solid state chemistry Vol. 322; p. 123991
Main Authors:	Abulkhaev, M.U., Molokeev, M.S., Oreshonkov, A.S., Aleksandrovsky, A.S., Kertman, A.V., Kamaev, D.N., Trofimova, O.V., Elyshev, A.V., Andreev, O.V.
Format:	Journal Article
Language:	English
Published:	Elsevier Inc 01-06-2023
Subjects:	Direct and indirect bandgaps Electronic structure Gadolinium fluorosulfide Optical properties System phase diagram Tauc plot Thermal properties Thermal properties Tauc plot Electronic structure Gadolinium fluorosulfide Optical properties Direct and indirect bandgaps System phase diagram
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Summary:	The objectives of this study were to refine the phase diagram of the GdF3-Gd2S3 system and to calculate their liquidus, and to synthesize GdSF and to study their properties. The GdSF compound (ST PbFCl, P4/nmm, a (Å) 3.83006(17), c (Å) 6.8529(3), 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 equal to 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 the experimental absorption spectrum. To explain this complicated 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°С, 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. In the system GdF3 - Gd2S3 the balance equations for five phase transformations recorded by the DSC method were compiled. Convergence was achieved in the liquidus of the system constructed according to DSC data and calculated with the use of the Redlich-Kister equation. [Display omitted] •Synthesized single-phase GdSF sample.•The direct optical band gap of GdSF was measured and two indirect band gaps were found.•Comparison of calculated DOS below 0 eV with XPS experimental data.•The phase diagram of the Gd2S3-GdF3 system has been constructed.•The balance equations for five phase transformations in the system, recorded by the DSC method, are compiled.
ISSN:	0022-4596 1095-726X
DOI:	10.1016/j.jssc.2023.123991