MxSe

MxSe refers to a family of metal selenide compounds with non-stoichiometric metal content, where M denotes a metal element and x indicates the metal-to-selenium ratio. The label is used in the literature to describe phases with variable metal occupancy, leading to defect-rich chemistry and tunable properties. MxSe encompasses a range of compositions, including iron selenide, molybdenum selenide, and other transition- or post-transition-metal selenides, with x varying by system and synthesis conditions.

Structure and defects

The crystalline structures of MxSe are often layered and related to transition metal dichalcogenides, though exact symmetry and layering depend on the metal and the degree of non-stoichiometry. Non-stoichiometry arises from metal vacancies, interstitials, or selenium vacancies, and these defects strongly influence electrical, magnetic, and catalytic behavior.

Properties and examples

The properties of MxSe are highly composition-dependent. Iron selenide (FeSe) is a notable member, exhibiting superconductivity with a transition temperature around 8 K under ambient pressure, a value that can be enhanced by pressure or intercalation. Molybdenum selenide (MoSe2) is typically semiconducting and serves in optoelectronics and catalysis; non-stoichiometric variants can modify electronic structure and catalytic activity, including hydrogen evolution reactions. Other MxSe systems similarly show tunable conductivity, magnetism, or catalytic performance, driven by the exact metal, selenium content, and defect landscape.

Applications

MxSe materials find use in energy storage, catalysis, and electronic or optoelectronic devices. Defect engineering and controlled phase composition enable tailoring of performance for specific applications, such as electrode materials, catalysts for hydrogen production, or two-dimensional electronic components.

Synthesis

Common synthesis methods include solid-state reactions, high-temperature annealing, chemical vapor transport, chemical vapor deposition, and solvothermal routes. Handling of selenium compounds requires appropriate safety measures due to toxicity.

Overview

The term MxSe is descriptive rather than naming a single compound, reflecting the variability of stoichiometry and structure across metal selenide systems.