Negative and Zero Thermal Expansion in High-Entropy NASICON Structure Oxide Material

Zero thermal expansion (ZTE) materials can significantly enhance the structural stability and service life of precision components, which is of great significance to industrial technology. In this work, the high-entropy strategy is adopted to design anomalous thermal expansion material KMg_1/3Mn_1/3Co_1/3Sc_1/2In_1/2Mo₃O₁₂ with a NASICON structure. It shows volumetric negative thermal expansion (NTE) behavior below 300 K (α_V = -9.97×10^-6 K^-1) and achieves a ZTE (α_V = 1.56×10^-6 K^-1, 300-700 K). The highly anisotropic thermal vibration of O atoms causes the rigid rotation of the polyhedron within the structural unit of the lantern, further manifested as NTE. From the perspective of crystal structure, anomalous thermal expansion behavior originates from the mutual compensation between the negative thermal expansion induced by the bending vibrations of the polyhedron within the ab plane and the positive thermal expansion arising from the thermal vibrations of K⁺ ions along the c-axis. Temperature-dependent Raman spectroscopy further confirms the role of polyhedral coupled rotation in the anomalous thermal expansion behavior of the system. This work provides a design strategy for the development of NTE and ZTE materials.