High Pressure Crystal Structure Prediction:Stability,Electronic And Mechanical Properties Of Sc-N,Au-Hg And Sn-N

Pressure is a fundamental thermodynamic variable that has a capacity to alter the interatomic interactions,electron density distribution,and bonding patterns of materials.These alterations in a material under pressure lead to phase transitions with unusual physical properties of the material.These phases cannot be recovered at ambient condition if the changes of properties are irreversible.Therefore,high-pressure study can be an effective way to discover new functional materials with exciting properties.In this thesis we have employed computational methods for crystal structure prediction with a particular emphasis on structures and stable phases that appear at high pressure.The first chapter of this thesis describes the background and applications of high-pressure,global optimization for crystal structure prediction and also elaborate the challenges in the field of crystal structure prediction.In the second chapter we extensively described the methodology of evolutionary approach for crystal structure prediction and also variable-composition method.Furthermore,we have described the basic of density functional theory(DFT)and approximate exchange-correlation functions that make DFT practical.In the third chapter we will investigate the Sc-N system under high pressure in the range of 0-110 GPa by using variable-compositions methodology implemented in Universal Structure Predictor:Evolutionary Xtallograpgy(USPEX)in conjunction with VASP Vienna Ab Initio Simulation Package(VASP).The calculation led to predication of new thermodynamically stable compounds,Sc4N3-I-43d,Sc8N7-C2/m,ScN3-P-1 and ScN5-P-1,and also phase transition of ScN5 from triclinic to monoclinic at 104 GPa.The results are important for understanding the structure of Sc-N compounds under high pressures and their elastic and electronic properties.In the fourth chapter we have extensively explored Au-Hg system under high pressure in the range of 0-300 GPa with and without spin-orbit coupling(SOC)inclusion.The variable-composition methodology has been used as employed in USPEX in combination with DFT.We have predicted several thermodynamically stable stoichiometric compounds of Au-Hg system,i.e.Au3Hg(space group 194,P63/mmc),AuHg(space group 62,Pnma),AuHg2(space group 15,C2/c),AuHg9(space group 38,Amm2),AuHg3(space group 44,Imm2)\Au3Hg2(space group 15,C2/c)and AuHg4(space group 15,C2/c)in the range of pressure 0-300 GPa.We also found a phase transition of AuHg3 from Imm2 to P63/mmc at 248.5 GPa and 246.5 GPa with and without SOC,respectively.We have also found that SOC does not influence the results much for Au-Hg system.In the fifth chapter we have highlighted the importance of high pressure to produce potential structures with good physical properties.Nitrides are always attractive and captivate much attention in research.In this chapter we have explored Sn-N system under pressure range 0-300 GPa with an objective to construct the complete phase diagram of Sn-N system.We have revealed two thermodynamically stable compounds:SnN2-Pa-3 and SnN4-P-1.The SnN2-Pa-3 has a wide band gap 2.7 eV and transformed to a metallic structure SnN2-I4/mcm at 100.5 GPa.We have also calculated the mechanical properties of the predicted compounds and we expect these results are significant to understand the Sn-N system under high pressure.