Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO₄): A High-Pressure Study

This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO₄) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO₄ crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO₄ (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO₄, in the pressure range investigated. The bulk modulus (B₀ = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO₄ (131(3) GPa). According to our optical absorption measurements, FeWO₄ has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO₄ and are similar to those previously reported for MgWO₄. Our results on FeWO₄ are also compared to previous results on other wolframite-type compounds.