Zero Thermal Expansion and Local Structure in K_xMn_xFe_{2- x}Mo₃O₁₂-Based Materials

Zero thermal expansion (ZTE) material is a very interesting research topic for applications. However, achieving ZTE of single-phase materials over a wide range remains a challenge. In this study, K_xMn_xFe_{2- x}Mo₃O₁₂ were designed by adjusting the doping ratio of (KMn)³⁺. Local structure can be controlled to regulate thermal expansion. ZTE over a wide temperature range was achieved in the KMnFeMo₃O₁₂ sample (α_l = 0.51 × 10^-6 K^-1, 100-800 K). A joint study of synchrotron X-ray diffraction, neutron pair distribution function, density functional theory calculations, and Raman spectroscopy has been conducted to investigate the thermal expansion mechanism. Even though the macroscopic crystallographic structure of K_xMn_xFe_{2- x}Mo₃O₁₂ (x = 0.4-1.0) adheres to the hexagonal system (R-3c) according to the results of synchrotron X-ray diffraction, the local structure exhibits a monoclinic distortion in K_0.4Mn_0.4Fe_1.6Mo₃O₁₂. In KMnFeMo₃O₁₂, the local structure still exhibits a hexagonal system. This local distortion affects the coupled rotation of polyhedra and leads to different thermal expansion behaviors. Notably, the KMnFeMo₃O₁₂ exhibits low emissivity in the 1-2.5 µm near-infrared band. This dual functionality, which includes tunable thermal expansion and near-infrared (NIR) low emissivity, endows the material with considerable potential for advanced engineering applications.