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https://ptsldigital.ukm.my/jspui/handle/123456789/519634| Title: | First principles calculations for the structural, electronic, elastic, mechanical and thermodynamic properties of terbium oxide |
| Authors: | Samah Faisal Mahmoud Al-Qaisi (P63076) |
| Supervisor: | Geri Kibe Gopir, Dr. |
| Keywords: | Thermodynamics Fluids -- Thermal properties Materials at low temperatures Universiti Kebangsaan Malaysia -- Dissertations Dissertations, Academic -- Malaysia |
| Issue Date: | 25-Apr-2018 |
| Description: | Terbium oxide (TbO) being a technologically potential material with several interesting attributes requires in-depth understanding. In this view, the present thesis used the first principles density functional theory (DFT) to calculate the ground state structural, electronic, elastic, mechanical and thermodynamic properties of TbO. Five possible crystal structures of TbO were considered, namely rock salt (RS), cesium chloride (CsCl), zinc blende (ZB), wurtzite (WZ) and nickel arsenide (NiAs). Computer simulation was performed using the WIEN2k code with full potential linearized augmented plane wave (FP-LAPW) method within the local spin density approximation (LSDA) of DFT and the generalized gradient approximation (GGA). Structural evaluations were made in terms of lattice constant, bulk modulus, and their pressure derivatives. Furthermore, the electronic band structure including the band gaps of TbO were calculated via the modified exchange potential proposed by Becke and Johnson (mBJ). TbO was found to exhibit a stable ferromagnetic (FM) state with ambient RS crystalline structure. The effect of pressure on the physical properties of TbO was examined to predict the possible pressure induced crystallographic phase transformation. The structural phase transition is determined by calculating the Gibbs free energy (G) for the two phases. Eight phase transitions were predicted by the PBE-GGA and LSDA calculations that were performed for spin-polarized FM and non-spin-polarized NM phases. Both approximations that positive pressure values for the RS to CsCl, ZB to CsCl, WZ to CsCl, WZ to NiAs, ZB to NiAs and the NiAs to CsCl phase transitions indicated that a compression was needed for the RS structure to obtain the CsCl structure. Similar procedure has to be applied for all other cases to obtain the desired structural phase transformation. Conversely, negative pressure in the case of RS to ZB and RS to WZ transition implied that an expansion in the RS structure was necessary to achieve the ZB and WZ structure. Both LSDA and GGA calculations predicted the metallic nature of all five crystal structures. Conversely, mBJ potential based calculation revealed the metallic nature of RS, CsCl and NiAs together with the insulating nature of ZB (having spin up configuration), semiconducting makeup of ZB (with spin down configuration), insulating character of WZ (in spin up configuration), and the metallic behavior of WZ (in spin down configuration), the so called half metallic system. The RS TbO has the largest elastic constants and it was found to be mechanically strongest amongst the studied cubic structures, whereas, NiAs phase was found stronger than the WZ phase of TbO. The hardness and the stiffness of different TbO structures is determined using shear modulus and Vickers hardness calculations. The RS phase was found the hardest and the stiffest one from the cubic structures and NiAs phase was harder and stiffer than WZ phase of TbO. The types of bonding are predicted using Cauchy pressure and Poisson's ratio. The ZB, CsCl and WZ structures exhibited ionic bonds but the RS and NiAs structures of TbO are with covalent bonds. The ductility of different TbO structures is determined using Pugh's rule. The ZB, CsCl and WZ structures displayed ductility nature while the RS and NiAs structures manifested brittleness. Meanwhile, the computed sound velocities were averaged to estimate the Debye temperature for both the cubic and hexagonal structures of TbO.,'Certification of Master's/Doctoral Thesis' is not available,Ph.D. |
| Pages: | 168 |
| Call Number: | QC311.Q335 2018 tesis |
| Publisher: | UKM, Bangi |
| Appears in Collections: | Faculty of Science and Technology / Fakulti Sains dan Teknologi |
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