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

The thermal expansion of natural FeWO$_4$ (ferberite) and synthetic FeWO$_4$:Fe$_2$WO$_6$ (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 analysed using physically based thermodynamic models, including the Kroll and Berman approaches as implemented in EoSFit7. All datasets are well reproduced within their respective temperature intervals. However, significant differences are observed between the behavior of ferberite and FeWO$_4$:Fe$_2$WO$_6$, which has a \~40% smaller thermal expansion coefficient and a reduced reference volume. Possible origins, including microstructural and phase-coexistence effects, are discussed. The results provide a comprehensive description of the thermal expansion behavior of FeWO$_4$ across a wide temperature range.