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

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 Fe2-xScxW3O12, 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 Fe2-xScxW3O12 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.

A linear scaling law for predicting phase transition temperature via averaged effective electronegativity derived from A2M3O12-based compounds.

The chemical flexibility of A2M3O12-based compounds enables the design of materials with versatile functionalities such as ferroelastic switching, ion conduction and negative thermal expansion (NTE) above the ferroelastic transition temperature (Tt), which is promising for a variety of applications. Quantitative prediction of Tt is essential but lacking. Herein we propose a concept of averaged effective electronegativity (AEE) and establish a linear relationship between the Tt and AEE for A2M3O12-based compounds. The linear scaling law is validated using first principles calculations of the effective charge on oxygen and its effectiveness is verified experimentally by designing high entropy compounds Scx1Zrx2Hfx3Fex4Moy1Vy2O12 and a NTE compound Zr2MoVPO12 with expected Tt. Generalization of the linear scaling law to other NTE oxides with displacive phase transition is also demonstrated. The findings can be used as a simple and effective approach to guide the design of novel compounds with desired properties and Tt.

Hydrate formation and its effects on the thermal expansion properties of HfMgW3O12.

HfMgW3O12 is a representative material with negative thermal expansion in the ABM3O12 (A = Zr, Hf; B = Mg, Mn, Zn, M = W, Mo) family. Herein we report a novel feature of hydration in HfMgW3O12 and its effect on the thermal expansion and its structures which have not been determined previously. It is found that hydrate formation in HfMgW3O12 occurs under ambient or moisture conditions and restrain the low energy librational and translational and even high energy bending and stretching motions of the polyhedra. The coefficient of thermal expansion could be tailored from negative to zero and positive depending on the hydration levels. The unhydrated HfMgW3O12 adopts an orthorhombic structure with space group Pna21 (33) without phase transition at least from 80 K to 573 K, but pressure-induced structure transition and amorphization are found to occur at about 0.19 Gpa and above 3.93 GPa, respectively.