Structural Transitions in Cu₁.₈₅S Single Crystals
Abstract
Cu1.85S is of significant current interest due to its complex crystal chemistry, wide homogeneity range, and unique physicochemical properties. These materials belong to the class of digenites and exhibit various structural transformations and reversible phase transitions that are highly sensitive to copper content. The synthesis, growth, and investigation of the structural behavior of Cu1.85S single crystals provide essential insights into phase stability and transformation mechanisms in the Cu2-xS system. Such knowledge is crucial for potential applications in semiconductor devices, catalysts, and energy conversion systems, where the crystal structure and phase composition directly influence material performance. This paper presents the results of the synthesis, growth of single crystals, and X-ray phase analysis of the nonstoichiometric compound Cu1.85S, which belongs to the class of digenites. The single crystals were obtained by combining the Bridgman directional crystallization method with slow cooling. A comprehensive microstructural and X-ray analysis was carried out, including the use of Weissenberg photographs and temperature-dependent diffraction studies in the range from room temperature to 420℃. It was established that at room temperature, the Cu1.85S sample is biphasic and consists of orthorhombic (Pnma) and monoclinic ( ) phases. Upon heating, two structural transitions were observed: first to a tetragonal phase at approximately 93℃, and then to a high-temperature face-centered cubic (FCC) lattice at around 120℃. All transitions are reversible and occur via a single-crystal-to-single-crystal mechanism with well-defined orientational relationships between the lattices. The biphasic nature at room temperature is attributed to the accumulation of copper atoms in twin regions. This work contributes to understanding structural transformations in the Cu1.85S system and confirms the existence of stable interphase transitions that depend on copper content.
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