Structure and Negative Thermal Expansion in Zr_0.3Sc_1.7Mo_2.7V_0.3O₁₂

A₂M₃O₁₂-based materials have received considerable attention owing to their wide range of negative thermal expansion (NTE) and chemical flexibility toward novel materials design. However, the structure and NTE mechanism remain challenging. Here, Zr⁴⁺ and V⁵⁺ are used as a unit to compensatorily replace Sc³⁺ and Mo⁶⁺ in Sc₂Mo₃O₁₂ to tune its thermal expansion. Its crystal structure, phase transition, NTE property, and corresponding mechanisms are studied by high-resolution synchrotron X-ray diffraction, powder X-ray diffraction, ultralow-frequency Raman spectroscopy, and density functional theory calculations. The results show that Zr_0.3Sc_1.7Mo_2.7V_0.3O₁₂ adopts an orthorhombic (Pbcn) structure at room temperature, with V atoms occupying the position of Mo1 atoms and Zr atoms occupying the position of Sc atoms, and transforms to monoclinic (P2₁/a) structure at ∼133 K (45 K lower than that of Sc₂Mo₃O₁₂). It exhibits excellent NTE in a broader range. Most of the phonon modes below 350 cm^-1 have negative Grüneisen parameters, of which the lowest and next-lowest frequency (38.5 and 45.8 cm^-1) optical phonon modes arising from the translational vibrations of the Sc/Zr and Mo/V atoms in the plane of the nonlinear linkage Sc/Zr-O-Mo/V have the largest and next-largest negative Grüneisen parameters and positive total anharmonicity, and contribute most to the NTE.