Stabilization of the γ form of the rare earth sesquisulfides at lower temperature by doping with Calcium

The preparation of several samples forming a solid solution that can be formulated as Ca(3/2)yR2−y0.25−(1/2)yS3 (R = Ce, Sm, Gd) (0 ≤ y ≤ 0.30) is reported, together with their structural characterization, mainly through transmission electron microscopy. The introduction of Ca2+ into the rare earth metal sesquisulfide matrix stabilizes the γ form phase at 900 °C. This effect can be related to the non-stoichiometric nature of this phase, R3−xxS4, because the introduction of Ca2+ requires the elimination of cation vacancies from the structure: 2R3+ + → 3Ca2+ (R = rare earth metal;  = cation vacancies). However, a NaCl-type solid solution is formed for R = Eu, formulated as Eu1−yCayS. Well-ordered crystals are found in every sample, as it is revealed by transmission electron microscopy images and diffraction patterns. The color properties of the samples have been evaluated with reflectance spectra in the visible range and with L*–a*–b* coordinates.

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► Stabilization of the γ-phase of rare earth sesquisulfides at 900 °C by Ca doping.
► Microstructure characterization with TEM and associated techniques.
► Cation vacancies in crystal structures can be the key for γ-phase stabilization.
► Color properties are evaluated according to Ca content.