FeWO₄ Single Crystals: Structure, Oxidation States, and Magnetic and Transport Properties

Synthetic crystals of the ferberite FeWO4 have been grown by the chemical transport reaction starting from a polycrystalline sample of this phase. Magnetic susceptibility measurements showed an antiferromagnetic Néel temperature of TN = 75 K. The anisotropy in the magnetic susceptibility can be ascribed to the magnetocrystalline anisotropy of the Fe2+ ion, the oxidation state of which was confirmed by X-ray absorption spectroscopy. While X-ray photoemission analysis indicated that all W ions are in the expected 6+ charge state, the dielectric permittivity of the FeWO4 crystals was found to be leaky, hindering changes at TN to be detected. Subsequent thermoelectric power measurements suggested the presence of about 1.5% Fe3+. X-ray diffraction experiments confirmed the basic crystal structure of the wolframite type and revealed some structural disorder in the 1% range. Transmission electron microscopy allowed us to unveil the occurrence of stacking faults attributed to the similarity of the atomic environment of the Fe and W species.