Yuanhui Xu

Electronic and elastic properties of new semiconducting oP12-type RuB2 and OsB2

Using first-principles total energy calculations we investigate the structural, elastic and electronic properties of new hypothetical oP(12)-type phase RuB(2) and OsB(2). The calculations indicate that the oP(12)-type phase RuB(2) and OsB(2) are thermodynamically and mechanically stable. Remarkably, the new phases RuB(2) and OsB(2) are predicted to be semiconductors, and the appearance of band gaps is ascribed to the enhanced B-B covalent hybridization. Compared to metallic oP(6)-type RuB(2) and OsB(2) phases, the new phases possess similar mechanical properties and hardness. The combination of the probability of tunable electronic properties, strong stiffness and high hardness make RuB(2) and OsB(2) attractive and interesting for advanced applications.

Structural, Electronic, and Ferroelectric Properties of Compressed CdPbO3 Polymorphs

By means of first-principles calculations based on density functional theory (DFT) and hybrid functional, we studied the structural, electronic, and ferroelectric properties of the two recently synthesized high-pressure perovskite-type (orthorhombic, space group Pnma) and LiNbO(3)-type (rhombohedral, space group R3c) polymorphs of CdPbO(3). Besides providing structural and electronic results in good agreement with available experiments, our results are able to correctly describe the pressure-induced Pnma → R3c structural phase transition and most importantly predict the realization of proper ferroelectric behavior in LiNbO(3)-type CdPbO(3) with an electric polarization of 52.3 μC/cm(2). The proper covalent interaction mechanism driving the ferroelectric transition is discussed and explained in terms of the analysis of Born effective charges, potential-energy surfaces, charge density isosurfaces, and electric localization function.

Charge, orbital, and magnetic ordering in YBaFe2O5 from first-principles calculations.

First principles calculations using the augmented plane wave plus local orbitals method, as implemented in the WIEN2k code, have been used to investigate the electronic and magnetic properties of YBaFe2O5, especially as regards the charge-orbital ordering. Although the total 3d charge disproportion is rather small, an orbital order parameter defined as the difference between t2g orbital occupations of Fe2+ and Fe3+ cations is large (0.73) and gives unambiguous evidence for charge and orbital ordering. Strong hybridization between O2p and Fe e g states results in the nearly complete loss of the separation between the total charges at the Fe2+ and Fe3+ atoms. Furthermore, the relationship between the orbital ordering and charge ordering is also discussed. The dxz orbital ordering is responsible for the stability of the G-type antiferromagnetic spin ordering and the charge ordering pattern.

Comparative description of magnetic interactions in Sr2CuTeO6 and Sr2CuWO6

In this work, we comparatively explored the electronic structure and the low-dimensional magnetic interactions of double-perovskite compounds Sr2CuTeO6 and Sr2CuWO6 through first-principles calculations. The electronic structure calculations indicate that the Cu2+ (3d 9) site is the only magnetic active one, whereas Te6+ and W6+ remain in nonmagnetic states with d 10 and d 0 electronic configurations, respectively. The magnetic exchange interactions have been evaluated on the basis of the classical Heisenberg model. Both Sr2CuTeO6 and Sr2CuWO6 should be strong frustrated 2D magnetism, in excellent agreement with the experimental observations. Nevertheless, the nearest-neighbor antiferromagnetic interaction J 1 plays a determined role in constructing the Néel antiferromagnetic ordering within the square Cu2+ framework of Sr2CuTeO6. While, the next-nearest-neighbor antiferromagnetic interaction J 2 transcends the nearest-neighbor interaction J 1, establishes the collinear antiferromagnetic ordering in Sr2CuWO6. The discrimination has been explored and analyzed in detail using density of states, charge density as well as spin density analysis.

Cation ordering induced semiconductor to half metal transition in La2NiCrO6

La2NiCrO6, previously proposed to be a candidate of half metallic antiferromagnetism, is revisited using the first-principles calculation. Electron correlation is considered and cation ordering effects are studied by arranging Ni and Cr atoms along [111] and [001]. For the [111] case, which corresponds to an ordered double perovskite, a monoclinic structure is predicted to be the most stable. In contrast to the previous study, it is insulating from the calculation of electron structure. Attractively, the magnetic coupling of Ni and Cr is sensitive to electron correlation, i.e., it is antiferromagnetic in the GGA calculation, whereas the ferromagnetic state is favoured when electron correlation (U) is turned on. For the [001] case, it is ferromagnetic whether U is included or not. Interestingly, a semiconductor to half metal transition is expected according to the GGA + U method, and the half metallic character could be preserved under both compressive and tensile strain.

Negligible spin-orbit coupling effect in the Mott-insulating antiferromagnet KRuO4

A comprehensive investigation of the electronic and magnetic properties of KRuO4 has been performed using the first-principles calculations in order to clarify the importance of Coulomb interaction and spin-orbit coupling effect. The results indicate that its ground state is of a G-type Mott-insulating antiferromagnet with nearest-neighbor antiferromagnetic coupling, and the computed magnetic moment of Ru7+ ion is 0.50 μB, in nice agreement with the observed value of 0.57(7) μB. In addition, the electronic structure near the Fermi level is dominated by strong hybridized Ru 4d and O 2p states. In sharply contrast with KOsO4, the significantly weaker spin orbit coupling of Ru 4d electrons has negligible impact on the electronic and magnetic properties of KRuO4, and the orbital contribution to the total moment is minor. On the other hand, the on-site Coulomb repulsion affects the band structure significantly, and is indispensable for appraising the electronic properties, opening the band gap, establishing th...

Pressure induced structural and spin state transitions in Sr3Fe2O5

We report on a first-principles study of the effect of pressure on the structural, electronic and magnetic properties of the two-legged spin ladder structure Sr3Fe2O5, using density functional theory within the generalized gradient approximation (GGA)+U method. The theoretical results showed that a first order structural transition with the space-group change from Immm to Ammm is found around 33 GPa, which is in fair agreement with the experimental value (30 GPa). Furthermore, a spin state cross from the high spin state (S = 2) to the intermediate spin state (S = 1) has been demonstrated in the four-fold square-planar FeO4 coordination, when further pressure is applied. The spin collapse is accompanied by a magnetic configuration transition (antiferromagnetic to ferromagnetic) and an electronic transition (insulating to metallic). However, the predicted pressure for spin state transition is considerably larger than the experimental value. The reason for this discrepancy originates from the constant Hubbard U value we adopted. Furthermore, the transition mechanism underlined has been uncovered in terms of density of states analysis and the evolution of the lattice parameters under the pressure.

Charge ordering and magnetic frustration in CsFe 2 F 6

The structural, electronic and magnetic properties of a charge-ordered iron fluoride material CsFe2+Fe3+F6 have been explored by density functional theory calculations based on the generalized gradient approximation  +  U approach, which was implemented in the VASP code. The material exhibits a 3D pyrochlore-related structure which consists of corner-shared Fe2+F6 and Fe3+F6 octahedra. Our results confirm that CsFe2F6 is a Mott-Hubbard insulator, and bears a magnetically frustrated ground state in which the localized 3d electrons are antiferromagnetically coupled between the homogeneous Fe ions (Fe3+-Fe3+ along the b axis, and Fe2+-Fe2+ along the a axis), while interactions between the heterogeneous Fe ions (Fe3+-Fe2+ along the c axis) are frustrated, consistent with Goodenough-Kanamori superexchange interactions. Although the disproportionation of the total 3d charge is extremely low, explicit evidence is provided on the charge ordering by an order parameter, which is defined as the difference in minority d yz orbital (in the local coordinates) occupations between the Fe3+ and Fe2+ cations. In addition, spin ordering and the spin-orbit coupling effect play an insignificant role in the charge ordering and the preferential occupation of the d yz orbital scenario in CsFe2F6.

First-principles study of the electronic and magnetic properties of the spin-ladder iron oxide Sr3Fe2O5

The electronic and magnetic properties of the novel spin-ladder iron oxide Sr3Fe2O5, containing an unusual square-planar coordination around high-spin Fe2+ cations, were investigated using the generalized gradient approximation plus the Coulomb interaction correlation method. Our results demonstrated that the G-type antiferromagnetic configuration is the ground state, which is in excellent agreement with experimental neutron powder diffraction and Mossbauer spectroscopy measurements, as well as available theoretical results in the literature, albeit with slightly larger computed magnetic moments. Moreover, the outstanding discrepancy between the two-dimensional crystal structure and the three-dimensional electronic/magnetic properties was resolved via the special localization and orientation of electronic/spin charge in real space, i.e., the dz2 orbital ordering of the down-spin Fe 3d electron.

First-principles study on the electronic structure and magnetism of layered oxyselenide La2Mn2Se2O3

The electronic structure and magnetism of layered oxyselenide La(2)Mn(2)Se(2)O(3) have been studied by using first-principles calculations within the generalized gradient approximation (GGA) and GGA + U methods. The G-type antiferromagnetic (AF) state is calculated to be the most stable phase among the various magnetic configurations of interest, irrespective of the choice of the functional used, which is in good agreement with the experiments. In contrast to La(2)Fe(2)Se(2)O(3) and La(2)Co(2)Se(2)O(3), in which the AF states show metallic behavior under the GGA method, we predict the ground state of La(2)Mn(2)Se(2)O(3) is a semiconductor with an indirect band gap of ∼0.52 eV via the GGA calculations. This is closely related to a closed shell configuration and large exchange splitting (∼3.5 eV) in the Mn 3d states. Moreover, the magnetic properties are also discussed in terms of the calculated Heisenberg spin exchange constants, suggesting that La(2)Mn(2)Se(2)O(3) is a strong two-dimensional magnetically frustrated system.