arXiv subjects

M. H. N. Assadi

Publications and source records attributed to M. H. N. Assadi.

12 recordsLinked to original sources

arXiv2020

Covalency a Pathway for Achieving High Magnetisation in $TMFe_2O_4$ Compounds

The interplay between covalency and magnetism is non-trivial and can be harnessed for designing new functional magnetic materials. Based on a survey using density functional calculations, we show that $TM\unicode{x2013}O$ bond covalency can increase the total magnetic moment of spinel compounds of $TMFe_2O_4$ composition ($TM = V-Ni, Nb-Pd$) which are isomorphic to the much-researched magnetite. Accordingly, $PdFe_2O_4$ was found to exhibit the highest magnetic moment of 7.809 $μ_B$ per formula unit which is approximately twice that of $Fe_3O_4$ with $T_c$ predicted to be well above ambient. We further propose a practical method for synthesising $PdFe_2O_4$.

cond-mat.mtrl-sci↗

arXiv2020

Molecular Dynamics Simulation of Vacancy Cluster Formation in β- and α-$Si_3N_4$

Molecular dynamics simulation is used to study vacancy cluster formation in $β$- and $α$-$Si_3N_4$ with varying vacancy contents (0 - 25.6 at%). Vacancies are randomly created in supercells, which were subsequently heat-treated for 114 nanoseconds. The results show that both $β$ and $α$ can tolerate vacancies up to 12.8 at% and form clusters, confirming previous experimental data indicating 8 at% vacancy in $α$-$Si_3N_4$. However, 25.6 at% vacancy in $β$ results in complete amorphization, while the same amount in $α$ results in a transformation of a semi-amorphous $α$ phase to a defective $β$ phase, leading to the removal of the clusters in newly formed $β$. This clearly explains why cluster vacancies are not experimentally observed in $β$, considering that $β$-$Si_3N_4$ ceramics are produced from $α$. Furthermore, the lattice parameters of both modifications increase with increasing vacancy content, revealing the cause of different lattice constants that were previously reported for $α$-$Si_3N_4$.

cond-mat.mtrl-sci↗

arXiv2020

Dominant role of orbital splitting in determining cathode potential in $O3$ $NaTMO_2$ compounds

Designing high potential cathodes for Na-ion batteries, which are comparable in performance to Li-ion cathodes, remains a challenging task. Through comprehensive density functional calculations, we disentangle the relationship between the cathode potential and the ionicity of $TM-O$ bonds in $O3$ $NaTMO_2$ compounds in which TM ions is a fourth- or fifth-row transition metal. We demonstrate that the magnetic exchange interaction and the local distortions in the coordination environment of TM ions play more significant roles in determining the cathode potential of the $TM^{3+}$ $\to$ $TM^{4+}+ e^-$ reaction than the ionicity of the $TM-O$ bonds in these compounds. These results indicate that designing cathode materials solely based on empirical electronegativity values to achieve high potential may not be a feasible strategy without taking into account a detailed structural assessment.

cond-mat.mtrl-sci↗

arXiv2019

In-plane antiferromagnetism in $Na_{0.5}CoO_2$ induced by $Sb_{Co}$ dopants

$Na_xCoO_2$ has a fascinating and complex magnetic phase diagram that can be further manipulated by doping. Here, we investigated the effect of electron doping on the magnetic coupling among $Co^{4+}$ ions in $Na_xCoO_2$ using density functional theory based on Hartree-Fock hybrid functional. We found that electron doping through substitutional $Sb_{Co}$ dopants flip the in-plane ferromagnetic coupling among $Co^{4+}$ ions in the undoped compound to antiferromagnetic. Electron doping through interstitial $Cu_{Int}$, however, does not have a similar effect as $Cu_{Int}$ dopant leaves the compound ferromagnetic, just like the case of the undoped compound. The results demonstrate the critical dependence of magnetic phase in $Na_{0.5}CoO_2$ on the dopant and its incorporation site.

cond-mat.mtrl-sci↗

arXiv2019

Native point defects in $Ti_3GeC_2$ and $Ti_2GeC$

Using density functional theory, we calculated the formation energy of native point defects (vacancies, interstitials and antisites) in MAX phase $Ti_2GeC$ and $Ti_3GeC_2$ compounds. Ge vacancy with formation energy of 2.87 eV was the most stable defect in $Ti_2GeC$ while C vacancy with formation energy of 2.47 eV was the most stable defect in $Ti_3GeC_2$. Ge vacancies, in particular, were found to be strong phonon scattering centres that reduce the lattice contribution to thermal conductivity in $Ti_2GeC$. In both compounds, the reported high thermal and electrical conductivity is attributed to the electronic contribution that originates from the high density of states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2019

Restoration of long range order of Na ions in $Na_xCoO_2$ at high temperatures by sodium site doping

We have systematically investigated the $Na_xCoO_2$ system doped with Cu, Y, Sn, W, Au and Bi for $x$ = 0:5; 0:75 and 1.00 using density functional theory. Sn, W, and Bi always substitute a Co while Au always substitutes a Na regardless of Na concentration. However, for Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity.

cond-mat.mtrl-sci↗

arXiv2019

Dependence of Dopant Geometry on Na Concentration in $Na_{x}CoO_{2}$

In this work, we investigated the behaviour of Sb dopants in $Na_{x}CoO_{2}$ for Na concentrations of $x = 0.75, 0.875$ and $1.00$ by density functional theory. We chose $Na_{x}CoO_{2}$ with higher Na concentration of $x > 0.75$ because it has excessively higher thermo-power thus it is appealing for practical applications. The rationale for choosing Sb was its exceedingly higher atomic mass than all elements of the host crystal which enable Sb to rattle phonons considerably.

cond-mat.mtrl-sci↗

arXiv2019

Interplay between Magnetism and Na concentration in $Na_xCoO_2$

Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of $Na_xCoO_2$ ($x$ = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in $NaCoO_2$ and $Na_{0.875}CoO_2$ share the basal coordinates with O ions. However, as $x$ decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na$-$Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the $Na_{0.75}CoO_2$ system, while all other Na deficient systems had a weaker ferromagnetic tendency. The results on magnetism were in excellent agreement with the experiments.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic, electrochemical and thermoelectric properties of $P2 - Na_x(Co_{7/8}Sb_{1/8})O_2$

We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped $Na_xCoO_2$ ($x = 1, 0.75$ and $0.50$). $Sb_{Co}$ dopants adopt +5 oxidation state in $Na_xCoO_2$ host lattice for all Na concentrations ($x$). Due to high oxidation states, $Sb^{5+}$ strongly repels Na ions and therefore it decreases the electrochemical potential (vs. Na/Na$^+$). The electrons introduced by $Sb^{5+}$ localize on nearby Co ions creating $Co^{2+}$ species which are absent in undoped $Na_xCoO_2$. $Co^{2+}$ ions reduce the spin entropy flow decreasing the Seebeck coefficient in the Sb doped compounds. The results can be generalized to other dopants with high oxidation state.

cond-mat.mtrl-sci↗

arXiv2019

Selecting the suitable dopants: electronic structures of transition metal and rare earth doped thermoelectric sodium cobaltate

Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co with formation energy $-2.35$ eV, $2.08$ eV when Fermi level equals to the valence band maximum. While Eu is more stable when it substitutes Na having formation energy of $-2.64$ eV. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high-performance sodium cobaltate based multifunctional materials.

physics.chem-ph↗

arXiv2019

Dopant incorporation site in sodium cobaltate's host lattice: A critical factor for thermoelectric performance

$Na_xCoO_2$ that comprises of alternating Na and $CoO_2$ layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped $Na_xCoO_2$ ($x$ = 0.50; 0.625; 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA+$U$ method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid like Na layer, therefore, improving the crystallinity and decreasing the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.

cond-mat.mtrl-sci↗

arXiv2010

Intrinsic Ferromagnetism in Eu doped ZnO

We report room temperature ferromagnetism in as-implanted Eu doped ZnO (ZnO:Eu). To address the origin of ferromagnetism ab initio calculations of ZnO:Eu system are performed. Results show that the ferromagnetism is induced by ZnO point defects as Eu ions in perfect ZnO tend to align antiferromagnetically.

cond-mat.mtrl-sci↗