Phase transition, thermal expansion and hygroscopicity of Fe2-2x(HfMg)xW3O12

Abstract

Solid solutions Fe2-2x(HfMg)xW3O12 are synthesized to reduce the monoclinic-to-orthorhombic phase transition temperature of Fe2W3O12 and realize near zero thermal expansion over wide temperature range. The solid solutions Fe2-2x(HfMg)xW3O12 (0.3 ​≤ ​x ​≤ ​1.0) are easier to synthesize than Fe2W3O12. The X-ray diffraction and Raman analyses show that Fe2-2x(HfMg)xW3O12 adopt either a monoclinic (0.3 ​≤ ​x ​≤ ​0.5) or an orthorhombic (0.7 ​≤ ​x ​≤ ​1.0) structure at room temperature. Dilatometry measurements demonstrate that they display either low positive (x ​= ​0.3) or negative (x ​≥ ​0.5) thermal expansion above the temperature of monoclinic-to-orthorhombic phase transition or dehydration. The orthorhombic samples (0.7 ​≤ ​x ​≤ ​1.0) have intrinsically negative thermal expansion covering large temperature range involving room temperature, but are hygroscopic. Crystal water formation and releasing modify their thermal expansion property significantly. The influence of crystal water releasing process on the phonon behaviors is also observed by temperature dependent Raman spectroscopy.