Exploring Phase Transitions in Double Perovskite Oxides under Extreme Conditions of Pressure

Mukherjee, Bidisha (2025) Exploring Phase Transitions in Double Perovskite Oxides under Extreme Conditions of Pressure. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Bidisha Mukherjee (19RS037)) 19RS037.pdf - Submitted Version Restricted to Repository staff only Download (42MB)

Abstract

This thesis investigates the possible structural, electronic, and magnetic phase transitions of double perovskite oxides at high pressures using synchrotron X-ray diffraction, Raman spectroscopy, and first principle Density Functional Theory calculations. The primary goal is to understand how structural transitions affect the materials’ electronic and magnetic behaviour. Detailed descriptions of experimental tools used to study the structural, vibrational, and magnetic properties of these materials are provided in the thesis. We have investigated the structural response of transition metal double perovskite oxide Nd₂CoFeO₆ under pressure by XRD and Raman spectroscopic measurements. From XRD data, we have observed a pressure-induced structural transition from the orthorhombic phase to the monoclinic phase at about 14.8 GPa. An anomalous increase in compressibility at a much lower pressure (∼1.1 GPa) is seen where no structural transition occurs. At about the same pressure, a sudden drop in the slope of the Raman shift is observed. Further investigation at low temperatures reveals that the B₁g Raman mode is strongly affected by magnetic interactions. Additional high-pressure Raman experiments with the application of a magnetic field have indicated that the mentioned anomaly around 1.1 GPa can be explained by a high-spin to low-spin transition of Co³⁺. A thorough investigation of vibrational, structural, and electronic properties of perovskite-type rhombohedral Ba₂ZnTeO₆ (BZTO) under systematic application of pressure has been carried out.To carry out the analysis of structural and vibrational properties, we have performed pressure-dependent Raman spectroscopic measurements, synchrotron XRD, and for the analysis of electronic properties we have performed density functional theory-based calculations. At ambient conditions, BZTO has R¯3m space group, which under pressure undergoes a structural transition to a monoclinic phase with space group C2/m at around 18 GPa. In-depth Raman analysis reveals softening of a phonon mode Eg (∼ 28cm⁻¹), which leads to the structural phase transition. First principle DFT calculations also indicate that the instability introduced by Eg phonon mode, which corresponds to the TeO₆ octahedral rotation along with Ba and Zn translation, solel ydrives the structure to a lower symmetry phase C2/m. Investigation of the high-pressure behavior of another perovskite-type rhombohedral compound Ba₂NiTeO₆ (BNTO) has been performed. In contrast of BZTO, a magnetic ion is present in BNTO, which paves the way of understanding the relationship among spin- lattice-charge degrees of freedom under high pressure. High pressure synchrotron XRD, Raman spectroscopic measurements and pressure dependent DFT calculations are used to explore the structural, vibrational properties and DFT is used for the study of magnetic and electronic properties. Pressure dependent electronic and structural transitions are found as a function of pressure at two different pressure points (namely at around 1.8GPa and 20 GPa) using DFT and XRD, respectively. The signature of both of these transitions are also present in pressure variation of Raman modes. Increase of pressure suppress the spin-phonon coupling in the system and drives the structural transition. An anomalous behaviour of the pressure dependent frequency and FWHM of Raman modes has been found near 11GPa, which indicates the anharmonicity of the sample is increasing above that pressure and consequently the structural transition takes place. High-pressure Raman spectroscopy of Ho₂CoMnO₆ (HCMO) exhibited slope changes in Raman modes at 6GPa and 16GPa, with the FWHM reaching a minimum at 6GPa and followed by a slope change at 16GPa. However, no structural transition is observed at those pressure points from high pressure XRD data, suggesting a possible electronic, spin- state, or magnetic transition at these pressures. Overall, this thesis advances the understanding of how extreme pressure conditions influence the structural, electronic, and magnetic properties of double perovskite oxides.

Actions (login required)

View Item