Negative Values Of Formation Energy Research Articles

First principle investigation of half metallic ferromagnetism and thermoelectric behavior of MgSm2(S/Se)4 spinels for spintronic and energy harvesting applications

Density functional theory investigates the spin-dependent structural, electronic, magnetic, and thermoelectric properties of MgSm2 × 4 (X = S, Se). The optimization uses PBEsol-GGA to compute the lattice constant, bulk modulus, and energies. The Heisenberg model has been applied to calculate the Curie temperature which shows ferromagnetism above room temperature. The negative values of formation energy confirm their thermodynamic stability. The half-metallic ferromagnetism (HMF) is observed from the metallic character in the spin-up channel and insulating behaviour in the spin-down channel. The density of states plots indicates that 4f-states of Sm, 3s states of Mg and 3p/4p states of S/Se play a crucial role in forming band edges. The computation of exchange energies and exchange magnetic constants has assisted in determining these spinels' ferromagnetic and 100 % spin polarized nature. The magnetic moments (MM) arise from the p-f hybridization of Sm and X atoms. The robust half-metallicity and high Curie temperature make the studied materials suitable for spintronic applications. In addition, calculating temperature dependent thermoelectric parameters reveals an extra degree of applications of these materials for thermoelectric devices.

DFT insights on the Be1-xCrxS alloys for optoelectronic and magnetic devices

In this work, the electro-optical and magnetic characteristics of Be1-xCrxS (x= 6.25%, 12.5% and 25%) are brought into investigation by employing full potential linearized augmented plane wave (FP-LAPW) scheme designed within density functional theory (DFT). The stability of the Be1-xCrxS alloy is justified by the negative values of formation energy. The band structures and density of states are examined by using GGA functional. Be1-xCrxS compound demonstrates the half-metallic (HM) ferromagnetic behavior for all doping concentrations; spin-up channel reveals the metallic character and other spin version displays the semiconductor (SC) behavior. The values of total magnetic moment (µB) are recorded as 4.0 8.0 and 16.0 µB for corresponding 6.25%, 12.5% and 25%, which mainly arises owing to Cr-3d state. Moreover, optical features including dielectric function ε(ꞷ), reflectivity, refraction, and absorption are explored within range of 0-10 eV. The maximum absorption of incident photons was found in ultraviolet (UV) span which implies their importance for optoelectronic applications. Results reveal that the studied alloy has potential applications in magnetic and optoelectronic gadgets.

Physical properties of elastically and thermodynamically stable magnetic AcXO3 (X = Cr, Fe) perovskite oxides: a DFT investigation

Spin-polarized calculations of mechanical, electronic structure, phonon, optical and magnetic properties of AcXO3 (X = Cr, Fe) perovskite oxides (POs) has been computed using the full-potential linearized augmented plane wave method. The modified Becke Johnson (mBJ) approximation has been utilized for exchange-correlation potential and implemented in the WIEN2k code. The negative values of formation energy and the positive fRequencies of the phonon modes show the stability of studied perovskite oxides. The mechanical stability is confirmed through the elastic parameters such as shear modulus (G), Bulk modulus (B), Poisson ratio (ν) and Cauchy pressure. The semiconductor nature with an indirect bandgap is observed for both compounds in both spin channels. The computed electron density contour plot describes the bonding nature of both compounds. The magnetic moments are calculated, which show the major involvement of Fe and Cr atoms in the overall magnetism of studied compounds. The optical response is also evaluated, showing the maximum absorption in the ultraviolet region. The overall analysis of the calculated properties shows that the studied oxide perovskites are suitable for spintronic and optoelectronic applications.

The study of new double perovskites K2AgAsX6 (X = Cl, Br) for energy-based applications

Halide double perovskites are the best alternative to Pb-halide perovskites. These materials play an important role in renewable energy generation. Therefore, we explore the physical properties of K2AgAsX6 (X = Cl, Br) double halide perovskites using full potential linearized augmented plane wave method . The structural parameters are calculated by optimization and analytical schemes. The negative values of formation energy confirm the thermodynamic stability, while Goldsmith’s tolerance factor guarantees the structural stability of both the double perovskites. The bandgaps of K2AgAsCl6 and K2AgAsBr6 are calculated as 2.10 and 1.48 eV, respectively by modified Becke and Johnson potential. Optical properties are examined in terms of the dielectric function, refractive index and absorption coefficients. The thermoelectric properties are calculated in terms of the electronic and thermal conductivities, Seebeck coefficients, power factor and figure of merit. Our study suggests the K2AgAsX6 perovskites for energy-related applications.

Structural, electronic, magnetic, optical, thermoelectric and thermodynamic properties of R2Rh3Ge (R=Gd and Er)

The structural, electronic, magnetic, optical, thermoelectric and thermodynamic properties of the Gd2Rh3Ge and Er2Rh3Ge ternary rhombohedral Laves phases compounds were studied by using the density functional theory (DFT) based the full potential linearized augmented plane wave (FPLAPW) with the generalized gradient approximation (GGA). The lattice constants of two compounds are calculated by using the Perdew–Burke–Ernzerhof (PBE)-GGA approximation. The two systems have a metallic character. Density of states as well as magnetic moment confirm that these compounds are ferromagnetic metals. The ferromagnetic state was more stable than paramagnetic state. The dielectric function, reflectivity, energy loss function, absorption coefficient and optical conductivity were calculated. The heat capacities, the thermal expansion coefficient and the Debye temperatures have also been determined from the non-equilibrium Gibbs functions. The negative value of formation energy confirms the stability of these compounds.

Study of new lead-free double perovskites halides Tl2TiX6 (X = Cl, Br, I) for solar cells and renewable energy devices

In recent years, lead-free double perovskites have been found tremendously advantageous due to their unique optoelectronic and environmentally benign photovoltaic characteristics. Herein, we have explored optical, thermoelectric, and thermodynamical properties of Tl2TiX6 (where X ​= ​Cl, Br, I) using modified Becke and Johnson exchange potential. The calculated value of the Goldschmidt tolerance factor in the range 0.90–1.04 demonstrates the structurally stable cubic phase of our materials. Moreover, the negative value of formation energy together with positive phonon dispersion further reinforces the thermodynamically stable nature of the crystal structures. Further, a decrease in calculated bandgaps of Tl2TiCl6 (2.99 ​eV) was observed in Tl2TiBr6 (2.26 ​eV) and Tl2TiI6 (1.42 ​eV), obtained by changing Cl with Br and I anions, respectively. The first absorption bands 413 ​nm–310 ​nm for Tl2TiCl6, 496 ​nm–400 ​nm for Tl2TiBr6, and 775 ​nm–496 ​nm for Tl2TiI6 increase the potential of studied compounds for solar cells, and optoelectronic applications. In the end, the transport properties were examined based on Boltzmann transport theory via BoltzTrap code. Moreover, the reasonably high values of figure of merits (0.76 and 0.75) measured at room temperature further authenticate their utilization as thermoelectric generators.

Comprehensive study of ferromagnetic MgNd2X4 (X = S, Se) spinels for spintronic and solar cells device applications

Full potential LAPW + lo method is used for exploring electronic, structural and thermoelectric properties for MgNd2X4 (X = S, Se) spinels that are found to show ferromagnetic-semiconductor behaviour in the spinel structure. The investigated negative value of formation energy and positive value of phonon spectra computed using PBEsol GGA indicates the energetic and dynamical stability of the studied cubic ferromagnetic-semiconductors. We have used TB-mBJ potential functional for electronic and magnetic properties, which lead to a reliable account of electronic structure, demonstrating band occupancy in the spinels along with a clear explanation of density of states. The stability of ferromagnetic state in the studied materials is because of the exchange splitting of Nd cations based on p-d hybridization which is in accordance with the results obtained for electronic band structure and density of states. The exchange splitting of bands can be justified by the spin magnetic moment between anions and cations, and sharing of charge. The computed values of dielectric constant and their associated optical parameters are used to explain the optical active behaviour of the spinels under investigation; indicating that the two spinels studied in the present work are suitable for solar cells device applications. The calculation of thermoelectric properties is very useful for determining a material's potential use in waste energy recovery systems and many other innovative applications.

Comprehensive DFT investigation of Cd-based spinel chalcogenides for spintronic and solar cells devices

In light of existing literature of stable ferromagnetic phase, the thermoelectric and optical behavior of the Cd-based spinel chalcogenides i.e. CdCr2X4, (X = S, Se, Te) were inspected and their structural and electronic characteristics were examined in terms of density functional theory calculations. The negative values of formation energy of said spinels were calculated to confirm their ferromagnetic phase stability. Optimization of band structure and calculations of density of states confirmed their ferromagnetic nature. For both channel, the band structure exhibited the semiconducting behavior. The exchange constants (N0α and N0β) were investigated with the help of exchange splitting energies, which were explored from the density of states. The splitting of 3d-states of Cr were explained on the basis of larger value of N0β as compared with N0α. Owing to the strong p-d hybridization indicated by N0β = −0.11, −0.12 and −0.15 and N0α = 0.14, 0.32 and 0.44 for CdCr2S4, CdCr2Se4 and CdCr2Te4, respectively, the necessary condition for ferromagnetic system is satisfied showing that exchange field dominates over the crystal field that induces the ferromagnetism in these spinels. Moreover the p-d hybridization was used to decrease the magnetic moment at Cr ions, by using a fraction of magnetic moment at non-magnetic sites. Further, optical characteristics were investigated in terms of photon energy 0–6 eV showing less dispersion of light and refractive index ranges up to 1–1.5 in visible region, thus suggesting spinels as potential candidates for opto-electronic applications. The thermoelectric performance observed in the temperature range of 200–600K which indicate positive Seebeck coefficients of ∼250 CdCr2S4, CdCr2Se4 while a negative Seebeck coefficients of −112 for CdCr2Te4 at room temperature. Further the power factors increased from S to Te as well as with increase of temperature which shows the potential of these chalcogens for thermoelectric power generators.

Structural, thermal, elastic, electronic and magnetic properties of cubic lanthanide based perovskites type oxides PrXO3 (X = V, Cr, Mn, Fe): insights from ab initio study

Different properties such as the structural, elastic, thermal, electronic, magnetic as well as the Curie temperature and the formation energy of the cubic perovskites PrXO3 compounds (X = V, Cr, Mn, and Fe) are studied by using the density functional theory based on the full-potential linearized augmented plane wave with local orbitals method with the generalized gradient approximation (GGA) for the exchange correlation potential as applied in WIEN2k code. The GGA + U approximation is also used to treat the f-states of Pr atoms and d-states of X atoms. We have also applied the analytical techniques for the structural parameters. The calculated structural parameters by both methods are in good agreement with experimental results. The calculated critical radii of the compounds exhibit ion conductivity as well as oxygen migration. The PrVO3, PrCrO3 and PrFeO3 compounds have a ductile nature, while PrMnO3 is brittle. The calculated electronic properties reveal the metallic nature for the studied compounds. Double cell optimizations, density of states as well as magnetic moment confirm that these compounds are ferromagnetic metals. The negative value of formation energy confirms the stability of these compounds. Large and small values of Curie temperature in these compounds show strong and weak interaction among the magnetic atoms, respectively.

Exploring adsorption behavior of ethylene dichloride and dibromide vapors on blue phosphorene nanosheets: A first-principles acumens

The interrelation of toxic vapors ethylene dichloride (EDC) and ethylene dibromide (EDB) with the sensory base material blue phosphorene nanosheet (BLPNS) is studied using ab-initio method. The formational stability of BLPNS is ensured by the negative value of formation energy. Prior to the adsorption studies, we calculated the formation energy of BLPNS to ensure its stability, which is calculated to be −5.194eV/atom and found stable. The main motive behind the present work is to detect these toxic vapors using BLPNS. The intercommunication between the targeted vapors and the base material has been analyzed using the aid of adsorption energy, Bader charge transfer, energy band gap, and variation of band gap along with energy bands and DOS spectrum. The energy gap of isolated BLPNS is observed to be 1.621eV. However, the adsorption of EDC and EDB modulates the energy gap of BLPNS. The nature of assimilation is noticed to be of physisorption, which facilitates desorption of EDC and EDB molecules much easier. The successful outcome of the present research validates that BLPNS can be deployed as a prominent sensor for detection of EDC and EDB effectively.

Structural, electronic and magnetic properties of new full Heusler alloys Rh2CrZ (Z = Al, Ga, In): First-principles calculations

First-principles calculations based on the density functional theory (DFT) within full-potential linearized augmented plane wave method (FP-LAPW) were carried out to investigate the structural, electronic and magnetic properties of Rh2CrZ (Z = Al, Ga, In) Heusler alloys. The electron exchange-correlation energyis described by the generalized gradient approximation (GGA-PBE), Tran and Blaha modified Becke-Johnson (TB-mBJ) potential and GGA + U (U is Hubbard correction). The present work reveals the stability of Rh2CrZ (Z = Al, Ga, In) Heusler in Cu2MnAl-type structure. The negative value of formation energy in Rh2CrZ (Z = Al, Ga, In) alloys indicates that these compounds can be synthesize experimentally. Rh2CrZ (Z = Al, Ga) are half metal ferromagnets and expected to be potential candidatefor application in spintronics. However, Rh2CrIn is nearly half metallic behavior.

Half-metallicity in new Heusler alloys NaTO2 (T=Sc, Ti, V, Cr, and Mn): A first-principles study

On the basis of the full-potential linearized augmented plane wave (FPLAPW) method within density functional theory (DFT), electronic structure and magnetic properties of Heusler alloys NaTO2 (T = Sc, Ti, V, Cr, and Mn) were investigated. The negative values of formation energy showed that these compounds can be experimentally synthesized. Results showed that in all compounds, AlCu2Mn-type structure was the most favorable one. The NaTO2 (T = Sc, Ti, V, Cr, and Mn) alloys were HM ferromagnets except NaScO2 (in both structures which were nonmagnetic semiconductors) and NaVO2 (in AlCu2Mn-type structure which was a magnetic semiconductor). The origin of half-metallicity was also verified in HM alloys. NaCrO2 and NaVO2 alloys had higher half-metallic band gaps in comparison with Heusler alloys including and excluding transition metals. The total magnetic moments of HM NaTO2 (T = Ti, V, Cr, and Mn) alloys obeyed Slater-Pauling rule (Mtot = Ztot-12). Among NaTO2 (T = Sc, Ti, V, Cr, and Mn) alloys, NaCrO2 had the highest robustness of half-metallicity with variation of lattice constant in both structures.

First principle calculations on structural, electronic, and magnetic properties of CdMAs2 (M = Sc, Ti, V) chalcopyrites

Structural, electronic, and magnetic properties of ternary CdMAs2 (M = Sc, Ti, and V) compounds in the chalcopyrite structure have been studied using full-potential linearized augmented plane wave method based on density functional theory. We present a detailed study of electronic band structure, density of states, and magnetic moment of all three compounds within local spin density approximation and generalized gradient approximation. CdMAs2 compounds are derived from chalcopyrite structured CdGeAs2 with the substitution of transition metal (TM) atoms at Ge site. Negative values of formation energy signify that these materials are stable in chalcopyrite structure. Spin-polarized calculations show that the substitution of TM atoms at the group IV site influences the appearance of ferromagnetic state (FM) in CdScAs2 and CdVAs2 compounds. FM in CdScAs2 and CdVAs2 compounds is mainly due to the strong spin polarization of 3d states of M cations and 4p states of As anion. CdVAs2 also exhibits half metallic ferromagnetism with an integer magnetic moment of 1.00μB per formula unit. However, there is no effective spin-polarization of energy states at the Fermi level in CdTiAs2 compound and shows a non-magnetic behaviour.

Ferromagnetic ordering in chemically synthesized ZnS:Mn diluted magnetic semiconductor: A density functional theory explanation

We have carried out DFT calculations to investigate the origin of room temperature ferromagnetism in experimentally obtained Mn doped ZnS magnetic semiconductor. For experimental investigations, the formation of the cubic ZnS structure of ∼4 nm in size is confirmed by XRD and TEM results. Theoretical calculations reveal that Mn doped ZnS system possesses half metallic characteristics in addition to strong p–d hybridization between the d-states of Mn atom and the p-states of surrounding S atoms, which leads to the strong ferromagnetic coupling and is experimentally realized in the M–H curves of Mn doped ZnS nanoparticles. The ferromagnetic coupling between Mn–Mn atoms are studied by using two dopant model, replacing two Zn atoms by two Mn atoms in ZnS supercell, which depicts that ferromagnetic state is always ground state. In calculating formation energies for single and two Mn dopant ZnS systems, negative values of formation energy are obtained indicating of higher stability of the system.

Mechanical and magneto-electronic properties of half-metallic ferromagnetism in Ti-doped ZnSe and CdSe alloys: Ab initio study

In this article we have studied the structural, elastic, electronic and magnetic properties of Zn1-xTixSe and Cd1-xTixSe alloys at (x=0.25, 0.50, 0.75) using first principles density functional theory calculations with local spin density approximation (LSDA) and generalized gradient approximation plus Hubbard parameter (GGA+U) as exchange-correlation potential. The physical properties of both alloys were investigated in the zinc-blend phase. The structural parameters at equilibrium are consistent with experimental and earlier theoretical predictions. The elastic constants are also computed and compared with the literature. The DOS curves of Zn1-xTixSe and Cd1-xTixSe alloys for all the concentrations show the existence of hybridization among Ti (3d) and Se (4p) states. The calculated exchange constants N0α(s-d) and N0β (p-d) are useful to determine the contribution in the valence band and conduction band and are also shows the magnetic character of these alloys. In addition, the p-d hybridization in the PDOS reduces local magnetic moment of Ti from its free space charge of 2µB and results small magnetic moments on the nonmagnetic Zn, Cd and Se sites. The calculated negative values of formation energy (Ef) reveal that all the Zn1-xTixSe and Cd1-xTixSe alloys are thermodynamically stables. A larger/Smaller value of Curie temperature (TC) for all the Zn1-xTixSe and Cd1-xTixSe alloys shows the strong/low interaction among the magnetic atoms respectively.

DFT study of Mg2TiO4 and Ni doped Mg1.5Ni0.5TiO4 as electrode material for Mg ion battery application

Mg2TiO4 is a spinel material, however it is electrochemically inactive. We have simulated Ni doping in its tetrahedral network by partially replacing Mg+2 (0.5 Mg+2) ions with Ni+2 to form Mg1.5Ni0.5TiO4. Mg2TiO4, Mg1.5Ni0.5TiO4 and its de-intercalated end product MgNi0.5TiO4 are studied through analysis of structural parameters and density of states. All the three materials produce almost similar x-ray diffraction pattern ensuring structural stability during charge–discharge. After doping of Ni, Mg1.5Ni0.5TiO4 becomes electrochemically active, as, only Ni states are found to be contributing charges (electrons) to the conduction band of MgNi0.5TiO4 after de-intercalation. This redox activity is also supported from the magnetic moments of Ni indicating change in valance from +2 to +3 on de-intercalation. Calculation of de-intercalation voltage for de-intercalation of tetrahedral Mg from Mg1.5Ni0.5TiO4 indicates a value of 4.22 V. This high voltage with electrochemical capacity 151 mAh g−1 would generate energy density of 637.2 W–h kg−1. Simulation of formation energy (Ef) of Mg1.5Ni0.5TiO4 indicates that with rutile TiO2 the phase formation is probable with slightly negative value of formation energy, however, with anatase TiO2 the phase formation is highly probable with high negative value of formation energy (Ef).

Electronic structure, magnetic and optical properties of Co2TiZ (Z = B, Al, Ga, In) Heusler alloys

Electronic structure, magnetic and optical properties of Co2TiZ (Z=B, Al, Ga, In) Heusler alloys have been computed by density functional theory implemented in WIEN2k within generalized gradient approximation for exchange correlation functions. Lattices constants, bulk modulii, energy gaps, spin polarization and density of states have been calculated. Negative value of formation energy of these alloys evidences their stability. Spin polarization of Co2TiZ (Z=Al, B, Ga, In) are 100%, 99%, 97% and 80% respectively and Co2TiAl shows true half metallic ferromagnetism. Co2TiZ, for Z=B, Al, Ga, shows stable half metallically over a wide range of pressure making them suitable for fabricating thin films for spintronics applications. Optical parameters such as complex dielectric function, refractive index, reflectivity, absorption, extinction coefficient, optical conductivity have also been calculated.

Ti-5Al-5Mo-5V-3Cr and similar Mo equivalent alloys: First principles calculations and experimental investigations

This paper reports the stability of Ti-5Al-5Mo-5V-3Cr and similar Mo equivalent alloys using first principles calculations. The lattice constant and modulus values are calculated using X-ray diffraction and ultrasonic techniques, respectively. All the four alloys display negative values of formation energy per atom and the alloy Ti-5Al-5Mo-5V-3Cr is most stable among these. The lattice constant, modulus and hardness values of these alloys lie in low (Ti-5Al-5Mo-5V-3Cr, Ti-5Al-5Mo-8.6V-1.5Cr) and high (Ti-5Al-3.5Mo-7.2V-3Cr, Ti-5Al-3.5Mo-5V-3.94Cr) regimes. All the alloys display ductile behavior based on shear and bulk modulus ratios.

Effects of the vacancy and doping on the half-metallicity in La0.5Sr0.5MnO3

The structural, magnetic and electronic properties of the perfect and defective La0.5Sr0.5MnO3 are investigated by performing the first-principle calculation. The negative value of formation energy implies the formation process of N atom substituting O atom in La0.5Sr0.5MnO3 are exothermic, indicating this structure could stably exist. Both the perfect and defective La0.5Sr0.5MnO3 are found to be half-metallic ferromagnets and potential in spintronic devices. For La0.5Sr0.5MnO3, the gap width of 2.902eV is extremely wide. While the gap width decrease with existing the defect and the gap width decreases according to the order of the perfect, VO and DN. There are different mechanisms determining the band gaps of the perfect and defective La0.5Sr0.5MnO3.