Pressure-Induced Volume Collapse and Metallization in Inverse Spinel Co$_2$TiO$_4$

Abstract

The structural, vibrational, electronic, and magnetic properties of inverse spinel $Co_2TiO_4$ (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-$I4_1/amd$ 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.