Unveiling the Potential of Quaternary Heusler Alloys for Spintronic and Thermoelectric Applications: A Valence Electron Count Perspective

crystal architecture provides exceptional compositional flexibility, enabling systematic control of electronic band filling, magnetic exchange interactions, and spin polarization. This tunability gives rise to a wide range of functional states, including half-metallicity, spin-gapless semiconductivity, bipolar magnetic semiconductivity, spin semimetallicity, compensated ferrimagnetism, and topological electronic phases. Among the descriptors governing these properties, the valence electron count (VEC) has proven particularly powerful, providing a predictive framework through the Slater-Pauling rule for correlating composition with magnetic and electronic behavior. In this Review, we comprehensively survey experimentally reported quaternary Heusler alloys and systematically classify them according to their VEC. We first discuss their crystal chemistry, atomic ordering, and disorder mechanisms, highlighting their influence on physical properties. We then establish clear correlations between VEC and magnetic ground states, electronic structure, magnetotransport, and thermoelectric performance, identifying overarching trends and unifying structure-property relationships across diverse compositional families. By integrating insights from structural, magnetic, electronic, and transport studies, this Review provides a coherent framework for understanding quaternary Heusler alloys and offers design principles for discovering next-generation materials for spintronic, thermoelectric, and energy-efficient electronic technologies.