The formation energy, phase transition, and negative thermal expansion of Fe2−xScxW3O12

Abstract

Tungstates with a molecular formula A₂W₃O₁₂ exhibits a wider negative thermal expansion (NTE) temperature range than molybdates but are challenging to synthesize, especially when A = Fe or Cr with metastable structures. To enhance the structural stability of Fe₂W₃O₁₂, Sc with lower electronegativity is adopted to substitute Fe according to Fe_2−xSc_xW₃O₁₂, considering the thermodynamic stability of Sc₂W₃O₁₂. It is shown that the solid solutions can be easily synthesized and the phase transition temperature (PTT) can be tuned to well below room temperature (RT). Theoretical calculations and experimental results show that the formation energy decreases and the W–O bond in Fe–O–W gradually strengthens as the substitution of Sc in Fe_2−xSc_xW₃O₁₂ increases, indicating an increase in structural stability. NTE is enhanced after phase transition with an increase in the Sc content. The reduction in PTT and the enhancement in NTE properties of Fe₂W₃O₁₂ could result in a decrease in the effective electronegativity of the Fe-site elements, resulting in a low formation energy and strengthened W–O bond in Fe–O–W, which corresponds to a more stable structure.