Formation of Layered Sulfide–Hydroxide (Valleriite) Materials under Hydrothermal Conditions

This paper examines processes underlying the formation of layered materials, analogs of the natural mineral valleriite, CuFeS2·1.53[(Mg,Al)(OH)2], made up of alternating two-dimensional sulfide and hydroxide layers, under hydrothermal conditions. The synthesized materials have been characterized by X-ray diffraction, scanning and transmission electron microscopies, X-ray photoelectron spectroscopy, and laser diffraction. The results demonstrate that the formation of valleriite phase at 160°C in an autoclave proceeds through the formation and subsequent consumption of reaction intermediates: erdite (NaFeS2·2H2O), haycockite (Cu4Fe5S8), and chalcopyrite (CuFeS2). The formation of phase-pure valleriite has been shown to occur at a hydrothermal treatment time from 25 to 70 h, whereas shorter or longer treatment times lead to contamination of the reaction product with impurity phases. The nature of the anion in the starting materials ( $${\text{SO}}_{4}^{{2 - }}$$ or $${\text{NO}}_{3}^{ - }$$ ) has been shown to have little or no effect on characteristics of the synthesis product. The use of thiourea as a sulfur source instead of sodium sulfide makes it possible to obtain valleriite phase contaminated only slightly with spherical magnesium carbonate particles. Our results demonstrate that, under hydrothermal conditions, equilibrium in the formation of the material can be reached if chalcopyrite phase is used as a precursor of 2D valleriite layers.