Abstract
Metal to insulator (MIT) phase transition accompanied by a structural phase transition (SPT) makes 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 to the monoclinic () phase. The structural transition occurs between and via two other insulating metastable phases, namely, monoclinic () and triclinic . 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 and 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 system by synchrotron x-ray diffraction and Raman spectroscopic measurements. We observe phase transition upon compression, which is completely reversible upon decompression. The transition pressures for and are observed to increase with the increase in doping concentration. The structural transitions from to to in are found to be second-order continuous phase transition. However, the temperature-driven phase transition is found to be first order. We argue that Mott-type first-order metal to insulator transition prompts the MIT from to , whereas a second-order structural phase transition/relaxation leads to the observation of to via the phase. We further investigated the isothermal and isobaric Grüneisen parameters for individual phonon modes and relaxations of the samples related to their thermal expansion.
- Received 1 December 2023
- Revised 16 April 2024
- Accepted 18 April 2024
DOI:https://doi.org/10.1103/PhysRevB.109.184107
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