Fa-qin Dong

Effects of halogen substitutes on the electronic and magnetic properties of BiFeO3

To observe the high-temperature ferro-electricity at room-temperature, we used a halogen to create a 2p-hole for improving O–Fe p–d electronic transition, and calculated the systematic variations in electronic and magnetic properties using first-principle calculation. The systems prefer to charge disproportionate as Fe3+–O2−–Fe2+ with the highly localized halogen concentration increasing, where the O-site halogen creates a 2p-hole to disproportionate Fe charge from Fe3+-d5 orbital with a full-filled triple degeneracy orbits (t2g) orbital to Fe2+-d5–d0 with a partially filled t2g orbital. Whatever at room-temperature or high-temperature phase, the halogen substitutes, typically, for F (or Cl)-doping, induce the electrons to transfer from O-2p4 → unoccupied Fe3+-3d5 to O-2p4→Bi3+-6p3, while the ferromagnetic (FM)–anti-ferromagnetic (AFM) phase transits at about 1 atom per cell. Furthermore, to retain O–Fe electron transfer process, we applied the crystallographic anisotropy to produce the strong Fe–O orbital hybridization which offsets the effect of 2p-hole, and it causes more significant O-2p4 → unoccupied Fe3+-d5 electronic transitions at the valence band. This study opens a new perspective to the development of multiferroic devices with independent temperature.

DFT and two-dimensional correlation analysis methods for evaluating the Pu3+–Pu4+ electronic transition of plutonium-doped zircon

Understanding how plutonium (Pu) doping affects the crystalline zircon structure is very important for risk management. However, so far, there have been only a very limited number of reports of the quantitative simulation of the effects of the Pu charge and concentration on the phase transition. In this study, we used density functional theory (DFT), virtual crystal approximation (VCA), and two-dimensional correlation analysis (2D-CA) techniques to calculate the origins of the structural and electronic transitions of Zr1-cPucSiO4 over a wide range of Pu doping concentrations (c=0-10mol%). The calculations indicated that the low-angular-momentum Pu-fxy-shell electron excites an inner-shell O-2s(2) orbital to create an oxygen defect (VO-s) below c=2.8mol%. This oxygen defect then captures a low-angular-momentum Zr-5p(6)5s(2) electron to form an sp hybrid orbital, which exhibits a stable phase structure. When c>2.8mol%, each accumulated VO-p defect captures a high-angular-momentum Zr-4dz electron and two Si-pz electrons to create delocalized Si(4+)→Si(2+) charge disproportionation. Therefore, we suggest that the optimal amount of Pu cannot exceed 7.5mol% because of the formation of a mixture of ZrO8 polyhedral and SiO4 tetrahedral phases with the orientation (10-1). This study offers new perspective on the development of highly stable zircon-based solid solution materials.

Photovoltage response of (XZn)Fe2O4-BiFeO3 (X = Mg, Mn or Ni) interfaces for highly selective Cr3+, Cd2+, Co2+ and Pb2+ ions detection

High-photostability fluorescent (XZn)Fe2O4 (X=Mg, Mn or Ni) embedded in BiFeO3 spinel-perovskite nanocomposites were successfully fabricated via a novel bio-induced phase transfer method using shewanella oneidensis MR-1. These nanocomposites have the near-infrared fluorescence response (XZn or Fe)-O-O-(Bi) interfaces (785/832nm), and the (XZn)Fe2O4/BiFeO3 lattices with high/low potentials (572.15-808.77meV/206.43-548.1meV). Our results suggest that heavy metal ion (Cr3+, Cd2+, Co2+ and Pb2+) d↓ orbitals hybridize with the paired-spin X-Zn-Fe d↓-d↓-d↑↓ orbitals to decrease the average polarization angles (-29.78 to 44.71°), qualitatively enhancing the photovoltage response selective potentials (39.57-487.84meV). The fluorescent kinetic analysis shows that both first-order and second-order equilibrium adsorption isotherms are in line and meet the Langmuir and Freundlich modes. Highly selective fluorescence detection of Co2+, Cr3+ and Cd2+ can be achieved using Fe3O4-BiFeO3 (Langmuir mode), (MgZn)Fe2O4-BiFeO3 and (MnZn)Fe2O4-BiFeO3 (Freundlich mode), respectively. Where the corresponding max adsorption capacities (qmax) are 1.5-1.94, 35.65 and 43.7 multiple, respectively, being more competitive than that of other heavy metal ions. The present bio-synthesized method might be relevant for high-photostability fluorescent spinel-perovskite nanocomposites, for design of heavy metal ion sensors.

DFT and two-dimensional correlation analysis for evaluating the oxygen defect mechanism of low-density 4f (or 5f) elements interacting with Ca-Mt

Understanding how f-shell electrons affect clay minerals is important in an ideal buffer/backfill material. Hitherto, however, there have been few reports that quantitatively simulated the effects of low-density 4f (or 5f) electrons on oxygen defects. Here, we used density functional theory (DFT) and two-dimensional correlation analysis (2D-CA) techniques to calculate the origins of the oxygen defect and electronic transitions of f-shell electrons/Ca type montmorillonite (Ca-Mt) system. We determined the number effect of f-shell electrons to explain the oxygen defects of aluminium–oxygen octahedron and silicon–oxygen tetrahedron at the valence band, which is consistent with the orbital fluctuation results. This study offers a new method for explaining the oxygen defect mechanism.

Detection mechanism of perovskite BFO (1 1 1) membrane for FOX-7 and TATB gases: molecular-scale insight into sensing ultratrace explosives

Perovskite bismuth ferrite-BFO (1 1 1) membranes, as potential-sensitive electrochemical sensors, are investigated for the detection of high-energy-density materials by molecular dynamics (MD) and density functional theory (DFT) calculations. For the detection mechanism of the sensitive 1, 1-diamino-2, 2-dinitroethylene (FOX-7) gases, both a cation bridge and electrostatic models can be used to explain the STM signatures as 0.02–0.04 V (single) and 0.03~0.05 V (coverage) over a wide range (0–0.1 V) of bias voltages. For insensitive 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) gases interacting with the surface of a BFO (1 1 1) membrane, the charge signature can be as high as 0.08 V (coverage: 0.06 V). Analysis indicates a significant difference from the detection mechanism observed for FOX-7 gases; that is, the molecularly intact bidentate bridge configuration with only – bonds binds to both Fe and Bi atoms. These differences are attributed so that the surface O2− of BFO will capture a part of the surface electron of the –NO2 group, creating a 2p-hole defect (h+) which annihilates a spinning upward (↑) Fe3+, forming a spinning downward (↓) Fe2+. The –NO2 electron decreases 0.35 e (single FOX-7; coverage FOX-7: 0.24 e) and 0.56 e (single TATB; coverage TATB: 0.06 e). Such a system could open up new ideas in the design and application of BFO-based sensors.

DFT simulation on the temperature-dependent electronic transition of V (Nb or Ta) substituted NiMn2O4

Previously, we reported that the d–p (Mn-3d–O-2p) orbital hybridization induces Mn valence change (Mn3+→Mn4+) in the octahedron. The electron transfer mechanism can be controlled by modifying the Mn-3d orbital in the octahedron. Here, we used the density functional theory (DFT) with generalized gradient approximation (GGA) and two-dimensional correlation analysis (2D-CA) techniques to calculate the electron transfer mechanism of the V (Nb or Ta) substituted NiMn2O4 (NMO) in the temperature range of 50–1500 K. The results show that the heat accumulation accelerates the O-2p4 orbital splitting, inducing charge disproportionation. The V-3d3 substituted Mn increases the intensity and of the partial density of state (PDOS) at conduction band (1–3 eV), this enhances the V-3d3–O-2p4 p–d σ∗ orbital. The Nb-4d3/Ta-5d3 substituted Mn reduces the intensity of the PDOS at conduction band (1–5 eV), this weakens the Nb-4d3/Ta-5d3–O-2p4 p–d σ∗ orbital. This study effectively analyzes the microscopic changes of the electron transfer caused by the heat accumulation, provides a theoretical basis for the design of NMO-based negative temperature coefficient (NTC) thermistors.