The pressure effect on the crystal structure and vibration spectra of the brownmillerite Ca₂FeCoO₅ compound

Pressure-induced phase transitions in complex perovskite materials can significantly alter their physical properties. The discovery of new types of these transitions is important not only for fundamental research, but also for potential practical applications. The structural and vibrational properties of the brownmillerite compound Ca₂FeCoO₅, synthesized using the sol-gel method, have been studied using X-ray diffraction and Raman spectroscopy at pressures up to 24 GPa. A gradual structural transition was observed at pressures above ∼6 GPa, evidenced by changes in lattice parameters, unit cell compressibility, and anomalies in vibration mode pressure behaviors. The lattice parameters, unit cell volumes and vibration modes as functions of pressure for the initial orthorhombic phase and the pressure induced phase of brownmillerite Ca₂FeCoO₅ compound were obtained. It is assumed that the observed phase transition is attributed to the reorientation and rearrangement of Fe(Co)O₄ tetrahedra and the enhanced distortion of oxygen octahedra Fe(Co)O₆. It is also confirmed by the anomalies in the pressure behavior of Raman modes associated with the stretching and bending vibrations of the tetrahedral and octahedral oxygen units.