Structure-Property-Application Correlations of Early Transition Metal Chalcogenides: A Dichalcogenide-Centered Perspective.

Early transition metal (ETM)-based chalcogenides constitute a diverse family of layered materials with tunable structural, electronic, and chemical properties. While this materials class includes dichalcogenides, sesquichalcogenides, and polychalcogenides, research efforts and technological applications have been predominantly concentrated on layered transition metal dichalcogenides. This review provides a dichalcogenide-centered perspective on early transition metal chalcogenides, linking their crystal chemistry and structural polymorphism to functional performance. This review provides a detailed look at various types of ETM-based chalcogenides, including disulfides, sesquichalcogenides, trichalcogenides, and polychalcogenides, along with their crystal structures and coordination geometries. The review also explains how properties can be modified through doping, intercalation, and strain engineering, and how phase transitions and defects influence their performance. Special attention is given to their use in 2D materials, phase-change memory devices, and energy-related applications. By summarizing key experimental findings and structural features, this review offers insight into how ETM-based chalcogenides can be engineered for better functionality. The combination of their rich chemistry and practical tunability makes them promising materials for next-generation electronic, catalytic, and energy technologies. Finally, key challenges related to scalability, phase control, interfacial engineering, and environmental impact are critically discussed, and future research is outlined to guide the rational development of next-generation dichalcogenide-based technologies.