Browsing by Author "Ghellab, T."

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Effect of Octahedral Cation on Electronic, Magnetic and Optic Properties of Cox2o4 (X = Cr, Mn and Fe) Spinel Compound
(Taylor & Francis Ltd, 2022) Hetache, N.; Charifi, Z.; Ghellab, T.; Baaziz, H.; Soyalp, F.
The magnetic, structural and optical properties of CoX2O4 (X = Cr, Mn and Fe) spinels are calculated using GGA + U approximation. The effect of the octahedral cation X on the properties of these spinels are analyzed. In order to better understand the electronic aspect of these compounds we studied the issue between the relative forces of the exchange effect and the crystal field effect through a complete analysis of the densities of electronic states. Obtaining the correct ground state is only possible if the electron-electron interactions between magnetic cations are introduced. When the X cations are changed, the crystalline structure changes totally from cubic normal spinel for CoFe2O4 to tetragonal normal spinel one for CoMn2O4 to inverse spinel for CoCr2O4. The electronic properties of our spinels are significantly different, an increase in the band gap from Fe to Mn to Cr compounds is obtained. Magnetic exchange interactions are strongly affected by sub-lattices occupation in the inverse phase of CoFe2O4 and significant structural distortion of the CoMn2O4 compound. The analysis of structural parameters and electronic structures plays a role on the trends of magnetic exchange interactions. We have noticed that the iron states in CoFe2O4 are extremely localised making this spinel very different from the X cation states in the other two spinels. So the variation in X cations allows us to confirm the trend in the properties of CoX2O4. The prediction of optical properties is possible and it allowed us to calculate different optical parameters. We have noticed that epsilon(1)(0) decreases with increasing band gap.
Electronic, Optical, and Thermoelectric Properties of Vacancy-Ordered Double Perovskite K2snx6 (X = Cl, Br, I) From First-Principle Calculations
(Institute of Physics, 2024) Zikem, A.; Baaziz, H.; Ghellab, T.; Charifi, Z.; Soyalp, F.
The present study explores the structural, optoelectronic, and thermoelectric properties of potassium tin halide vacancy-ordered double perovskites K2SnX6 (X = Cl, Br, and I) in their stable monoclinic phase. Our study uses first-principles calculations based on density functional theory (DFT). Electronic band structures reveal direct band gaps for K2SnCl6 and K2SnBr6, while K2SnI6 exhibits an indirect band gap. Theoretical computations utilising the modified Becke-Johnson potential (mBJ-GGA) demonstrate that the optical band gaps of K2SnCl6, K2SnBr6, and K2SnI6 decrease in the following order: 2.581 eV, 1.707 eV, and 4.126 eV, respectively. These values render the materials suitable for photovoltaic applications. Analysis of dielectric functions, absorption coefficients, and refractive indices demonstrates their potential as light-absorbing materials. We evaluate the thermoelectric properties, including electronic and lattice thermal conductivities, Seebeck coefficients, and power factors, which lead to favorable thermoelectric performance. The maximum figure of merit (ZT) values of 0.58, 0.69, and 0.50 are achieved for K2SnCl6, K2SnBr6, and K2SnI6, respectively, at 500 K. These findings highlight the potential of these materials for applications in solar cells and thermoelectric devices, emphasising their effectiveness at elevated temperatures. © 2024 IOP Publishing Ltd.
First Principles Study of Hydrogen Storage Material Nabh4 and Lialh4 Compounds: Electronic Structure and Optical Properties
(Iop Publishing Ltd, 2016) Ghellab, T.; Charifi, Z.; Baaziz, H.; Ugur, S.; Ugur, G.; Soyalp, F.
A comprehensive study of structure, phase stability, optical and electronic properties of LiAlH4 and NaBH4 light-metal hydrides is presented. The calculations are carried out. within density functional theory using the full potential linear augmented plane wave method. The exchange-correlation potential is treated within the local density approximation. and the generalized gradient approximation (GGA) to calculate the total energy. Furthermore, the Engel-Vosko GGA. approach is employed to compute. electronic and optical properties such as reflectivity spectra. The phases a, beta and gamma of LiAlH4 and NaBH4 hydrides are investigated, the phase transition from the beta to the. high-pressure gamma phase is determined for NaBH4. and is accompanied by a 1% volume decrease. For LiAlH4, no phase transition is detected. The materials under consideration are classified as wide band gap compounds. From the analysis of the structures at different phases, it is deduced that the hydrides show strong covalent interaction between B (Al) and H in the [BH4](-) ([AlH4](-)) anions and ionic bonding character between [BH4](-) and Na+ for NaBH4, and [AlH4](-) and Li+ for LiAlH4. The complex dielectric function, absorption coefficient and the reflectivity spectra are also computed and analyzed in different phases.
The Study of Structural, Electronic and Thermoelectric Properties of Ca1-Xybxzn2sb2 (X=0, 0.25, 0.5, 0.75, 1) Zintl Compounds
(World Scientific Publ Co Pte Ltd, 2021) Mili, I; Latelli, H.; Ghellab, T.; Charifi, Z.; Baaziz, H.; Soyalp, F.
Based on the electronic structure, the physical properties of Ca1-xYbxZn2Sb2 (x = 0, 0.25, 0.5, 0.75, 1) Zintl compounds are studied. The transport properties can be significantly changed by varying the composition x. The materials under study are more metallic with increasing x and behaves like a semiconductor when x decreases. It is found that CaZn2Sb2 exhibits a larger thermopower magnitude (S = 241 mu V/K at T = 700 K) and the Seebeck coefficient decreases as x increases. The calculated figure of merit factor of YbZn2Sb2 is found to be low, this is explained by the fact that its structure is very compact and its bandgap is small which lead to high electrical and thermal conductivity due to high carrier concentration (n = 1:25.10(20) cm(-3) at T = 300 K). On other hand a narrow-gap (0:46 eV for CaZn2Sb2), provides a balance between a high Seebeck coefficient and low electronic thermal conductivity, with a slight increase in the carrier concentration when the temperature increases (3:87.10(19) cm(-3) at 600 K). As a consequence, CaZn2Sb2 compound is predicted to have good performance for thermoelectric applications. The electrical (sigma) and the thermal (K) conductivity for CaZn2Sb2 compound in both directions (along x and z-axes) are calculated. It is obtained that (sigma(xx)) is 120% of (sigma(zz)) at high-temperature, whereas S-zz Seebeck coefficient was higher than S-xx especially at T = 300 K (S-zz = 246 mu V/K; S-xx = 213 mu V/K). The large value of S-zz showed that the transport is dominated by zz-axis.