Structural investigations on rare earth oxides & scandates under extreme conditions of pressure and temperature

Rare Earth elements and their oxides have been of vast interest due to their strategic importance and extensive applicability. However, the successful implementation of these materials can be ensured only when their stability under different experimental conditions is established. This is because, many of the applications of such materials involve their operation under extreme thermodynamic conditions such as temperature and pressure or even under hostile chemical environments, high radiations exposure, high electric and magnetic fields, chemical bonding related wear and abrasion etc. It is well known that changes in pressure or temperature, may result in changes to the microstructure or even structural phase changes, all of which would directly alter their properties. The current work, therefore, focuses on understanding the influence of externally applied pressures or temperatures on the structural stability of rare earth sesquioxides and rare earth scandates. The research presented in this thesis involves the synthesis of rare earth scandates using the solid-state method, their characterization as well as comprehensive study of the structural behavior of rare earth sesquioxides and rare earth scandates under extreme thermodynamic conditions of pressure and temperature. The investigations were carried out on three rare earth sesquioxides namely Eu2O3, Lu2O3 and Tm2O3 and rare earth scandates DyScO3 and GdScO3. The materials were characterized using a suite of material characterization tools including Raman spectroscopy, X-ray diffraction, Scanning electron microscopy and Energy dispersive X-ray spectroscopy. The high-pressure effects were investigated using high energy synchrotron X-ray diffraction and Raman spectroscopy studies. External pressure was applied on the samples using the Mao-Bell type diamond anvil cell (DAC). The effect of the nanocrystalline nature on the phase transition pressure as well as bulk modulus were studied using Raman spectroscopy. The mode Grüneisen parameters were estimated from high pressure Raman studies. While scandates exhibited good structural stability under pressure, the rare earth oxides transformed to either monoclinic or hexagonal phases. The anharmonic behavior of these materials at low temperatures was studied using the variations in the phonon modes using liquid nitrogen from 80 to 440 K. Phonon softening as well as broadening of the Raman modes were observed with an increase in temperature from 80K onwards for most of the materials. Further, values of the explicit as well as implicit anharmonicity were also calculated. The three-phonon decay process was found to be predominating and anharmonic contributions were significantly higher than lattice contributions to the total anharmonic behavior. The effect of temperature variation on the Phonon lifetime was also explored. Further, Tm2O3 also demonstrated asymmetric phonon line shapes which are investigated and found to be occurring due to a weak Fano-interference effect. Before carrying out any measurement, calibration of the instrument being used is one of the most important aspects to ensure reliability of measurements. Preliminary work on Raman metrology, a recent and highly upcoming metrological area, was carried out. Raman metrology involves various aspects of Raman spectrometer calibration, resolution, intensity correction, effect of laser source wavelength etc. In the present work, the spectrometer calibration in the full spectral range of 100-3500 cm-1 was investigated. The various materials/chemicals as mentioned in ASTM guidelines have been used as calibrants and subsequent studies have been carried out. The spectral range was divided into low, middle, and high wavenumber ranges and the effects of calibration in each wavenumber range were studied. Also, the importance of right choice of calibrant has been highlighted with a need to ensure linearity in the spectrometer motors.