Thermochemistry of A₂M₃O₁₂ negative thermal expansion materials

The enthalpies of the monoclinic to orthorhombic transition for a series of A₂M₃O₁₂ (A = Al, Cr, Fe, In, and Sc; M = Mo or W) compounds were measured by differential scanning calorimetry, and entropies of transition were estimated. The enthalpies of formation from the binary oxides at 25 °C for several A₂M₃O₁₂ samples were obtained from drop solution calorimetry in molten 3Na₂O· 4MoO₃ at 702 °C. The monoclinic and orthorhombic phases of Sc₂Mo₃ O₁₂ and Sc₂W₃O₁₂ are the only phases that are enthalpically stable under ambient conditions. The enthalpies of formation from the oxides (ΔH_f,ox) for orthorhombic Sc₂Mo₃O₁₂ and Sc₂ W₃O₁₂ are -47.2 ± 2.1 kJ/mol and -8.5 ± 2.7 kJ/mol, respectively. For Fe₂Mo₃ O₁₂, In₂Mo₃O₁₂, and In₂W₃O₁₂, ΔH_f,ox values are 51.5 ± 4.5, 7.4 ± 2.9, and 44.5 ± 2.3 kJ/mol, respectively. These phases are entropically stabilized and/or metastable. Enthalpies of formation for phases that could not be measured by calorimetry have been estimated from the enthalpies of transition or trends in the enthalpies of formation. In general, the monoclinic phase is slightly enthalpically stabilized over the orthorhombic phase, while transition to the orthorhombic phase is entropically favored. This confirms that the orthorhombic phase is stable at high temperatures, the monoclinic is stable at low temperatures, and the monoclinic to orthorhombic transition is reversible.