Ab initio prediction of half-metallic ferromagnetism in Zn(TM)O2 (TM=Cr,Mn, Fe,Co, Ni) compounds

From the results of first principles tight-binding linear muffin-tin orbital (TB-LMTO) calculations, half-metallic ferromagnetism is proposed in Zn(TM)O₂ with a chalcopyrite structure. The calculated electronic and magnetic property shows that consistent with the integer value for the total magnetic moment, half metallicity is obtained for ZnCrO₂, ZnMnO₂, ZnFeO₂, ZnCoO₂ and ZnNiO₂. A careful analysis of the spin density reveals the ferromagnetic coupling between the p–d states and the cation dangling-bond p states, which is believed to be responsible for the stabilization of the ferromagnetic phase. The calculated heat of formation, bulk modulus and cohesive energy are reported.     

Energetics and electronic properties of Mg7TMH16 (TM=Sc, Ti, V, Y, Zr, Nb): An ab initio study

First-principles calculations have been performed on the face-centered cubic (FCC) magnesium-transition metal (TM) hydrides Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb). The cohesive energies are calculated to analyze the stability, and the obtained enthalpies of formation for hydrides Mg₇TMH₁₆ have been used to investigate the possible pathways of formation reaction. The calculated enthalpy changes show that the decomposition temperatures of Mg₇TMH₁₆ are lower than that of MgH₂. The electronic densities of states reveal that all the hydrides studied here exhibit metallic characteristics. The bonding nature of Mg₇TMH₁₆ is investigated, showing stronger covalent bonding between TM and H than between Mg and H.     

Photoluminescence of CdGeP2 and (Cd,Mn)GeP2

Photoluminescence of CdGeP₂ (112) single crystal and CdGeP₂ epitaxial film grown on GaAs (001) substrate have been studied and their spectral similarity found. Spectral bands associated with donor/acceptor transitions peak at close energies for both substances and all are lower than the energy gap of the chalcopyrite crystal.On the other hand, the growth of (Cd,Mn)GeP₂ ferromagnetic layer on CdGeP₂ (112) single crystal was performed to make it possible observation of PL from both the ferromagnetic layer and substrate. The green laser excitation (514, 532 nm) produces a proper photoluminescence similar to that in the undoped CdGeP₂ crystal and film. An extra emission from the ferromagnetic-nonmagnetic heterojunction occurs to extend up to photon energies exceeding E_g of the host semiconductor. The short wavelength photoluminescence is to be due to (Cd,Mn)GeP₂ dilute magnetic semiconductor (DMS). This fact states that Mn-doped II-IV-V₂ chalcopyrites are closer to II-VI DMS than to another group III-V DMS, where the heavy Mn-doping suppresses photoluminescence at all. Features of the observed short wavelength emission are discussing based on the temperature and spectral analyses.     

Spin-polarized calculations of electronic structures in ferromagnetic and antiferromagnetic Zn0.75TM0.25Se (TM=Cr,Fe, Co and Ni)

In this work, we aim to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of Zn_1−xTM_xSe (TM=Cr, Fe, Co and Ni) diluted magnetic semiconductors in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, and with 25% of TM. The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation (GGA) form. We treated the ferromagnetic and antiferromagnetic phases and we found that all compounds are stable in the ferromagnetic structure. Structural properties are computed after total energy minimization. Our results show that the cohesive energies of Zn_0.75TM_0.25Se are greater than that of zinc blende ZnSe. We discuss the electronic structures, total and partial densities of states, local moments and the p–d exchange splitting. Furthermore, we found that p–d hybridization reduces the local magnetic moment of TM and produces small local magnetic moments on the nonmagnetic Zn and Se sites. We found also that in the AFM phase the TM local magnetic moments are smaller than in the FM phase; this is due to the greater interaction of the TM d-up and d-down orbitals.     

First principles study of structural,magnetic and electronic properties of half-metallic CrO<Subscript>2</Subscript> under pressure

A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method within the local-density approximation is used to calculate the total energy, lattice parameter, bulk modulus, magnetic moment, density of states and energy band structures of half-metallic CrO₂ at ambient as well as at high pressure. The magnetic and structural stabilities are determined from the total energy calculations. From the present study we predict a magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM) state at 65 GPa, which is of second order in nature. We also observe from our calculations that CrO₂ is more stable in tetragonal phase (rutile-type) at ambient conditions and undergoes a transition to an orthorhombic structure (CaCl₂-type) at 9.6 GPa, which is in good agreement with the experimental results. We predict a second structural phase transition from CaCl₂- to fluorite-type structure at 89.6 GPa with a volume collapse of 7.3%, which is yet to be confirmed experimentally. Interestingly, CrO₂ shows half metallicity under ambient conditions. After the first structural phase transition from tetragonal to orthorhombic, half metallicity has been retained in CrO₂ and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a pressure of 65 GPa.     

First-principles study of electronic structure, chemical bonding, and optical properties of cubic SrHfO3

Electronic structure and optical properties of SrHfO₃ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. From the density of states (DOS) as well as charge density studies, we find that the bonding between Sr and HfO₃ is mainly ionic and that HfO₃ entities bond covalently. The complex dielectric functions are calculated, which are in good agreement with the available experimental results. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

A first-principles study on the electronic structure of one-dimensional [TM(Bz)]∞ polymer (TM= Y, Zr, Nb, Mo, and Tc)

A systematic density functional theory (DFT) study has been performed to investigate the electronic and magnetic properties of one-dimensional sandwich polymers constructed with benzene (Bz) and the second-row transition metal (TM = Y, Zr, Nb, Mo, and Tc). Within the framework of generalized gradient approximation (GGA), [Tc(Bz)]_∞ is a ferromagnetic half-metal, and [Nb(Bz)]_∞ is a ferromagnetic metal. With the on-site Coulomb interaction for 4d TM atoms being taken into account, [Tc(Bz)]_∞ keeps a robust half-metallic behavior, while [Nb(Bz)]_∞ becomes a spin-selective semiconductor. The stability of the half-metallic [Tc(Bz)]_∞ polymer is discussed based on magnetic anisotropy energy (MAE). Compared with 0.1 meV per metal atom in [Mn(Bz)]_∞, the calculated MAE for [Tc(Bz)]_∞ is 2.3 meV per metal atom. Such a significantly larger MAE suggests that Tc(Bz)]_∞ is practically more promising than its first-row TM equivalent.     

The first principle study on the electronic structure and spin ordering of the rare earth palladium sulfide, EuPd3S4

We have performed relativistic first-principles full-potential linearized augmented plane wave (FLAPW) calculation for rare earth palladium sulfide EuPd₃S₄ in the ferromagnetic and antiferromagnetic states. The density of 4f electrons of Eu is taken from a local-spin-density approximation self-interaction correction (LSDA-SIC) atomic calculation. EuPd₃S₄ is found to exhibit antiferromagnetic ordering in its ground state. The charge, orbital, magnetic moment and spin ordering are explained with the electronic structure, the orbital-projected density of states and the total energy study. EuPd₃S₄ is found to be stable in the body-centered Type-I antiferromagnetic state, in agreement with experimental results. Different Eu states are found in antiferromagnetic ordering. The magnetic moments of different states obtained through spin-polarized calculation are also in good agreement with experimental results. The phenomena observed are explained by the orbital hybridization of Eu and Pd ions as compared with the free ions.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Theoretical investigations on ZnCdO2 and ZnMgO2 alloys: A first principle study

We report on the calculations related to the electronic structure of ZnO, CdO, MgO, ZnMgO₂ and ZnCdO₂ in the wurtzite, rocksalt and chalcopyrite structures. From this study we found that ZnO and MgO are of direct band semiconductor, CdO is of semi metallic in nature. ZnMgO₂ and ZnCdO₂ are direct band semiconductors. From the energy considerations, we found that ZnMgO₂ and ZnCdO₂ are more stable in chalcopyrite structure rather than in rocksalt structure. Using the calculated band gap values, the bowing parameter for ZnMgO₂ and ZnCdO₂ is deduced and found to be in agreement with the reported value.     

First-principles prediction of coexistence of magnetism and ferroelectricity in rhombohedral Bi2FeTiO6

First principles calculations based on the density functional theory within the local spin density approximation plus U(LSDA + U) scheme, show rhombohedral Bi₂FeTiO₆ is a potential multiferroic in which the magnetism and ferroelectricity coexist. A ferromagnetic configuration with magnetic moment of 4μB per formula unit has been reported with respect to the minimum total energy. Spontaneous polarization of 27.3 μC/cm², caused mainly by the ferroelectric distortions of Ti, was evaluated using the berry phase approach in the modern theory of polarization. The Bi-6s stereochemical activity of long-pair and the ‘d⁰-ness’ criterion in off-centring of Ti were coexisting in the predicted new system. In view of the oxidation state of Bi³⁺, Fe²⁺, Ti⁴⁺, and O²⁻ from the orbital-resolved density of states of the Bi-6p, Fe-3d, Ti-3d, and O-2p states, the valence state of Bi₂FeTiO₆ in the rhombohedral phase was found to be Bi³⁺₂Fe²⁺Ti⁴⁺O₆.     

A first-principles study on the electronic structure of one-dimensional [TM(Bz)]∞ polymer (TM= Y, Zr, Nb, Mo, and Tc)

A systematic density functional theory (DFT) study has been performed to investigate the electronic and magnetic properties of one-dimensional sandwich polymers constructed with benzene (Bz) and the second-row transition metal (TM = Y, Zr, Nb, Mo, and Tc). Within the framework of generalized gradient approximation (GGA), [Tc(Bz)]_∞ is a ferromagnetic half-metal, and [Nb(Bz)]_∞ is a ferromagnetic metal. With the on-site Coulomb interaction for 4d TM atoms being taken into account, [Tc(Bz)]_∞ keeps a robust half-metallic behavior, while [Nb(Bz)]_∞ becomes a spin-selective semiconductor. The stability of the half-metallic [Tc(Bz)]_∞ polymer is discussed based on magnetic anisotropy energy (MAE). Compared with 0.1 meV per metal atom in [Mn(Bz)]_∞, the calculated MAE for [Tc(Bz)]_∞ is 2.3 meV per metal atom. Such a significantly larger MAE suggests that Tc(Bz)]_∞ is practically more promising than its first-row TM equivalent.      

Observation of self-regulating response in LixMyMn2−yO4 (M=Mn, Ni): A study using density functional theory

Density functional theory is used to understand the response of the transition metal-oxygen octahedra in Li_xMn₂O₄ and Li_xNi_0.5Mn_1.5O₄ to lithium intercalation and de-intercalation. Electronic structure computations on these compounds for x=0, 0.5 and 1 indicate that the 3d DOS of Mn is almost unaffected to variations in x. On the other hand, the oxygen 2p-DOS and to a lesser extent Ni 3d DOS are found to be sensitive to perturbation. The observations are explained on the grounds of self-regulating response, characteristic of systems having localized d states that communicate with a covalent manifold.     

The electronic structure and magnetism of GdSi2 by first-principles study

We have investigated electronic and magnetic properties of hexagonal, tetragonal, and orthorhombic GdSi₂, using the full-potential linearized augmented plane-wave method based on general gradient approximation for exchange-correlation potential. Antiferromagnetic (AFM) states of the GdSi₂ are found from total energy calculations to be energetically more stable, compared to ferromagnetic (FM) states in all of the considered present crystal structures. It is in good agreement with an experimental result. The calculated magnetic moments of valence electrons of the Gd atoms are 0.16, 0.14, and 0.14 μ_B for hexagonal, tetragonal, and orthorhombic crystal structures in AFM states, respectively, and the Si atoms are coupled antiferromagnetically to the Gd atoms irrespective of crystal structure even though their magnitudes are negligible.     

The effect of cation doping on spinel LiMn2O4: a first-principles investigation

The effect of the cation doping on the electronic structure of spinel LiM_yMn_2−yO₄ (M=Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn₂O₄. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiM_yMn_2−yO₄ can be ascribed to the lower O-2p level introduced by the doping cation M.     

The electronic structure and magnetic properties of metallic antiferromagnetic Co[(CH3PO3)(H2O)] studied by first-principles calculations

The electronic structure, the metallic and magnetic properties of metal phosphonate Co[(CH₃PO₃)(H₂O)] have been studied by first-principles calculations, which were based on the density-functional theory (DFT) and the full potential linearized augmented plane wave (FPLAPW) method. The total energy, the spin magnetic moments and the density of the states (DOS) were all calculated. The calculations reveal that the compound Co[(CH₃PO₃)(H₂O)] has a stable metallic antiferromagnetic (AFM) ground state and a half-metallic ferromagnetic (FM) metastable state. Based on the spin distribution obtained from calculations, it is found that the spin magnetic moment of the compound is mainly from the Co²⁺, with some small contributions from the oxygen, carbon and phosphorus atoms, and the spin magnetic moment per molecule is 5.000μ_B, which is in good agreement with the experimental results.     

First-principles study of the electronic and optical properties of rutile TiO2

The electronic, structural properties and optical properties of the rutile TiO₂ have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our results including lattice parameter, bulk modulus, density of states, the reflectivity spectra, the refractive index and band gap are compared with the experimental data. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0).     

Ab initio calculations and experimental determination of the structure of Cr2AlC

We have calculated the equilibrium volume and the density of states (DOS) of Cr₂AlC for antiferromagnetic (AFM), ferromagnetic (FM) and paramagnetic (PM) configurations by ab initio total energy calculations. Based on a comparison of the cohesive energies as well as the DOS for all three magnetic configurations we have identified the FM configuration to be metastable. Furthermore, we report the structural characterization of polycrystalline Cr₂AlC thin films grown by magnetron sputtering. Our calculated interplanar distances and equilibrium volume for the PM and AFM configurations are in good agreement with our experiment. The charge density distribution suggests that the chemical bonding between Cr and C in Cr₂AlC is very similar to the one in cubic CrC.