Thermal expansion of FeWO₄ (ferberite) and FeWO₄:Fe₂WO₆ (7:1): a comparative X-ray and neutron diffraction study

The thermal expansion of natural FeWO₄ (ferberite) and synthetic FeWO₄:Fe₂WO₆ (7:1) was investigated over the 2-1123 K temperature range, combining single-crystal and powder X-ray diffraction together with neutron powder diffraction. High-precision lattice parameters were obtained for both samples. The temperature dependence of the unit-cell volume was analyzed using physically based thermodynamic models, including the Kroll and Berman approaches as implemented in EoSFit7-GUI. All datasets are well reproduced within their respective temperature intervals. However, significant differences are observed between the behavior of ferberite and FeWO₄:Fe₂WO₆, which has a ∼40% smaller thermal expansion coefficient and a reduced reference volume. The possible origins of these differences, including microstructural and phase-coexistence effects, are discussed. The results provide a comprehensive description of the thermal expansion behavior of FeWO₄ across a wide temperature range.