Pressure-Induced Volume Collapse and Metallization in Inverse Spinel Co2TiO4

The structural, vibrational, electronic, and magnetic properties of inverse spinel Co₂TiO₄(CTO-Sp) under high-pressure (HP) conditions are systematically investigated using X-ray diffraction, Raman spectroscopy, in situ optical microscopy, and first-principles density functional theory (DFT) calculations. At ambient conditions, CTO-Sp exhibits a cubic phase with a space group $Fd\bar{3}m$, and it undergoes two notable structural phase transitions at HP. The first transition, occurring at approximately 7.3 GPa, leads to the tetragonal-I4₁/amd phase with minimal alteration in unit cell volume. The second transition takes place near 17.3 GPa, where two orthorhombic phases emerge and coexist above this pressure. This second structural transition corresponds to a first-order phase transition involving a significant reduction in unit cell volume of approximately 17.5%. The bulk compressibility of CTO-Sp and its HP post-spinel phases is almost equal to the average polyhedral compressibility within each phase. DFT calculations reveal a high-spin to low-spin transition, accompanied by the collapse of local magnetic moments in the Cmcm orthorhombic phase, leading to the sample's pressure-induced metallization. .