Investigations on Rare Earth Manganites, Tantalates, and Vanadates under Extreme Conditions of Temperature and Pressure

Rare earth compounds contain rare earth elements along with other combination of elements, each having applications of strategic importance. The materials that we are investigating are manganites, tantalates, and vanadates. These materials have diverse applications which include thin films, microcircuits, passivation devices, field effect transistors, integrated circuits, capacitors, gate insulators, electrode materials in high-performance super-capacitors, as antioxidants in the biomedical industry, for selective hydrogenation, in sensing devices such as solar cells due to their extraordinary luminescence features, as buffer layers for gamma-ray shielding, in the generation of acoustic resonators, for making photoluminescence based gas sensors, as drug delivery agents etc. The wide-ranging applications of these materials underscore their significance; however, they are often subjected to extreme external influences such as pressure and temperature, which may affect their structure and properties. Understanding the stability and material behaviour in response to extreme conditions is crucial for their optimization and utilization in various technological applications across strategic sectors. This thesis focuses on investigating the stability of a few rare earth compounds (manganites, tantalates & vanadates) under varying external temperature and pressure conditions and their related structural, magnetic and vibrational properties. A comprehensive study on Nd1-xSrxMnO3 (x=0.4, 0.5), La1-xRuxTaO4 (x=0,0.09,0.12,0.2), and GdVO4, has been undertaken, which is expected to lead to a multi-faceted understanding of the behavior of these rare earth compounds under extreme environments and provide valuable insights into the fundamental properties of these materials. The primary investigation tools are temperature-dependent Raman spectroscopy and magnetization studies, which assisted in elucidating their mutual correlation, as well as to gain insights into the material's behaviour under different temperatures. Additionally, high-pressure X-ray diffraction (XRD) studies are also employed to study the pressure stability of structural phases. To begin with, Manganites -Nd0.6Sr0.4MnO3 & Nd0.5Sr0.5MnO3 (NSMO) belonging to the Perovskite family with an orthorhombic structure were synthesized using solid-state reaction method. Magnetic measurements on these samples pointed towards the three magnetic transitions observed at 280 K, 150 K and at 40 K. Magnetic measurements and Raman measurements showed a direct correlation in terms of slope change. Further, pure and doped LTO (9,12 & 20% Ru doped LaTaO4) belonging to the Perovskite family with a polymorphic structure showed phonon softening with an increase in temperature from 80-460 K. Anharmonic constants showed dominancy of three-phonon decay process over four phonon process. Magnetic measurements pointed towards the diamagnetic nature of all the materials. GdVO4 belonging to the Perovskite family with zircon structure showed phonon softening with an increase in temperature. However, one mode interestingly showed phonon hardening. Anharmonic constants again showed dominancy of three phonon process over four phonon process. High pressure XRD investigations on NSMO-0.4 up to 33.6 GPa showed stability of the structure. The bulk modulus and its first derivative were estimated to be 214±1.4 GPa and 9.3± 0.13 respectively. High pressure studies on GdVO4 , upto 27.1 GPa revealed a structural transition from ambient zircon to scheelite phase which was found to be initiated at 0.4 GPa and completed at 12.2 GPa. The bulk modulus of zircon and scheelite phases were found to be 85.1 GPa and 145.2 GPa respectively. High-pressure studies were also done on pure LTO up to 36.1 GPa, which showed a transition from the mixed (orthorhombic+monoclinic) phase to the new monoclinic phase. The orthorhombic phase has bulk modulus of 113 GPa with first derivative of 8, while the monoclinic phase demonstrated a bulk modulus of 145 GPa with first derivative of 6. On the other hand, the high-pressure monoclinic phase exhibited a bulk modulas of 241 GPa accompanied by the first derivative of 12.04. Finally, in Raman metrology investigations, the effect of spectrometer calibration in various wavenumber ranges was investigated. The calibration of the spectrometer in the lower wavenumber was seen to affect the peak positions in the middle and higher wavenumber range. Likewise, middle wavenumber calibration leads to shift in the lower and higher wavenumber etc. However, after the linearity corrections of the spectrometer motors, these deviations were minimized.<p></p>