Triclinic phase: The link behind the structural transition in V1−xMgxO2

Metal to insulator (MIT) phase transition accompanied by a structural phase transition (SPT) makes ${\mathrm{VO}}₂$ a potential material for investigations into a strongly correlated transition metal oxide system. It undergoes a high-temperature metallic to a low-temperature insulating phase. The MIT is associated with the SPT from the rutile tetragonal $(R)$ to the monoclinic ($M1$) phase. The structural transition occurs between $M1$ and $R$ via two other insulating metastable phases, namely, monoclinic ($M2$) and triclinic $(T)$. It has gained tremendous attention because of the half century old ``chicken and egg'' debate over the roles played by lattice distortion and electron-electron correlation. Despite several reports on the MIT and SPT between the $R$ and $M1$ phases, a combined and detailed investigation of the relation among the various stable and metastable structural phases is still missing. We have studied the temperature- and pressure-induced structural phase transitions in the ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Mg}}_x{\mathrm{O}}₂$ system by synchrotron x-ray diffraction and Raman spectroscopic measurements. We observe $M2\ensuremath{\rightarrow}T\ensuremath{\rightarrow}M1$ phase transition upon compression, which is completely reversible upon decompression. The transition pressures for $M2\ensuremath{\rightarrow}T$ and $T\ensuremath{\rightarrow}M1$ are observed to increase with the increase in doping concentration. The structural transitions from $M2$ to $T$ to $M1$ in ${\mathrm{VO}}₂$ are found to be second-order continuous phase transition. However, the temperature-driven $M2\ensuremath{\rightarrow}R$ phase transition is found to be first order. We argue that Mott-type first-order metal to insulator transition prompts the MIT from $R$ to $M2$, whereas a second-order structural phase transition/relaxation leads to the observation of $M2$ to $M1$ via the $T$ phase. We further investigated the isothermal and isobaric Gr\"uneisen parameters for individual phonon modes and relaxations of the samples related to their thermal expansion.