High-pressure x-ray diffraction and Raman spectroscopic studies of the tetragonal spinel CoFe₂O₄

In situ x-ray diffraction and Raman spectroscopy have been carried out to pressures of 93.6 and 63.2 GPa, respectively, to explore the pressure-induced phase transformation of CoFe₂O₄ spinel. CoFe ₂O₄ adopts a distorted tetragonal spinel structure at one atmosphere. At a pressure of ∼32.5 GPa, both x-ray diffraction and Raman spectroscopy indicate that CoFe₂O₄ transforms to the orthorhombic CaFe₂O₄ structure, which remains stable to at least 93.6 GPa. The bulk modulus (K₀) of the tetragonal and the high-pressure polymorphs were calculated to be 94(12) and 145(16) GPa, respectively, with K′ ≡ 4. Upon release of pressure the orthorhombic phase persists and appears to be structurally metastable. At zero pressure, laser induced heating leads to a significant transformation back to the tetragonal phase. The high-pressure orthorhombic phase at one atmosphere is 14.7% denser than the tetragonal phase.