Transport and Structural Studies at Extreme Conditions of Pressure and Temperature

Saha, Pinku (2020) Transport and Structural Studies at Extreme Conditions of Pressure and Temperature. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The primary focus of this thesis is the development of laser heated diamond anvil cell (LHDAC) facility and the study of material properties at high pressures and temperatures (P-T) using X-ray diffraction (XRD), Raman spectroscopy and the temperature measurements. In this research project we report the instrumentation for development and calibration of the LHDAC facility, its application like studying Earth’s interior material and role of pressure induced strain in Transition Metal Dichangogenides (TMD’s) to introduce structural and as well as electronic transition. Melting temperature of cryogenically loaded Argon gas at a pressure of 17 GPa is estimated with an accuracy of ±25K following onset and freezing of laser speckle motion with increase and decrease of laser power, respectively. Micrometersized diamond crystals having regular facets have been synthesized from graphite powder at a pressure of 18 GPa and temperature 1785 K using LHDAC facility, which is confirmed by visual observation, micro-Raman measurements and Field emission Scanning electron micrograph. The radial temperature gradient across the sample surface is observed to be low during the transformation from graphite to diamond. The thermal conductivity of the most potential candidate of the Earths inner core, iron at high P-T is measured at steady state condition using LHDAC facility. The value is found to be in the range of 70 - 80 Wm⁻¹K⁻¹, almost constant with pressure, in the hcp (ϵ) phase of Fe. Melting of laser-heated hotspot while the rest of the sample is in the solid state lead to about 25 - 30% decrease in thermal conductivity value. Orthorhombic phase of Si-doped Fe carbide is synthesized at high P-T using LHDAC, followed by its characterization using Transmission Electron Microscopy (TEM), Raman spectroscopy, and XRD measurements. High-pressure XRD measurements on the synthesized sample are carried out up to about 104 GPa at room temperature for determination of its equation of state (EOS) parameters. The analysis of the XRD patterns reveal magnetic transitions driven by pressure induced anisotropic strain in the unit cell at 28 and 78 GPa without any structural transitions. Extrapolation of the density profile of our study to the inner core conditions agrees within 2-4% with respect to PREM data. The second part of my thesis deals with structural phase transitions and their effect on electronic properties of 2D-TMD’s at high pressures. In this process, we have carried out Raman and XRD studies on WS₂, MoSe₂, and ReS₂. All the results lead to importance of strain on the electronic properties. Careful analysis of ambient and high-pressure XRD and Raman spectroscopy data indicates the emergence of a triclinic phase at about 5.8 GPa in patches embedded in the parent hexagonal phase in the exfoliated WS₂. Raman mode values and their full width at half maximum of the monolayer sample show anomalous changes at about 27 GPa, the pressure where the sample completely gets converted to the triclinic structure indicating the importance of strain in structural as well as electronic properties of two-dimensional materials. The similar triclinic phase also observed in the exfoliated MoSe₂ at ambient condition. Pressure evolution of Raman modes and their full width at half maximum (FWHM) of prominent modes of the exfoliated sample show slope changes at about 13 GPa and 33 GPa. Slope change in the linear behavior of reduced pressure (H) with respect to Eulerian strain (fE) is observed at about 13 GPa. A minimum in the FWHM values of E¹₂g mode at the same pressure indicates to an electronic transition. Above 33 GPa the sample completely gets converted to the triclinic structure. High pressure X-ray diffraction studies in conjunction to the high pressure Raman investigations on ReS₂ show a structural phase transition at about 7 GPa followed by an anomalous behaviour in the range 15-25 GPa. Above 25 GPa the sample is found to be very much incompressible with respect to the ambient sample.

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