Tailored phase transition temperature and negative thermal expansion of Sc-substituted Al2Mo3O12 synthesized by a co-precipitation method

Abstract

Scandium-substituted Al₂Mo₃O₁₂ ceramics corresponding to the general formula Al_2−xSc_xMo₃O₁₂ (0 ≤ x ≤ 0.5) were fabricated by the co-precipitation reaction. Sc³⁺ is proved to successfully replace Al³⁺ and reduces the average grain size of the Al_2−xSc_xMo₃O₁₂ ceramics from 6 μm to 1 μm. Sc³⁺ substitution induces a crystal structure change from monoclinic to orthorhombic symmetry. When the Sc³⁺ substitution content (x) is <0.4, Al_2−xSc_xMo₃O₁₂ ceramics crystalize in a monoclinic structure and exhibit positive thermal expansion, while when x ≥ 0.4, the Al_2−xSc_xMo₃O₁₂ ceramics crystalize in an orthorhombic structure and display negative thermal expansion (NTE), indicating that the phase transition temperature of Sc³⁺-substituted Al₂Mo₃O₁₂ ceramics can be successfully reduced from 204.8 °C to room temperature with increasing substituted Sc³⁺ content. Orthorhombic Al_1.6Sc_0.4Mo₃O₁₂ shows stable NTE above room temperature. Its coefficient of thermal expansion is −2.02 × 10⁻⁶ °C⁻¹ from 25 °C to 500 °C. The theoretical calculation result also accords well with the above experimental data.