Yong-Nian Xu

Electronic Structure and Electrical Conductivity of Undoped LiFePO4

The electronic structure of LiFePO 4 underpins transport properties important to its use as a lithium storage electrode. Here we have calculated the electronic structure of LiFePO 4 in the ordered olivine structure by a first-principles method to determine (i) the effective mass of carriers and (ii) the nature of the band structure. The electrical conductivity in high purity undoped LiFePO 4 has also been measured experimentally. Spin-polarized calculations show a large electron effective mass and a much smaller but highly anisotropic hole effective mass, suggesting that hole-doped compositions should have the greater electronic conductivity. More surprisingly, the calculations show that this polyanion compound is a half-metal with spin-sensitive band structure, like some other oxides being studied for spintronics applications. This previously unappreciated aspect of the LiFePO 4 electronic structure may play a role in determining transport properties including those relevant to electrochemical applications.

Electronic structure of aluminum nitride: Theory and experiment

Recent efforts to develop electronic.,’ optical, and electro-optical’ -” components and applications based on wide band-gap III-V materials have generated considerable practical interest in the electronic structure of AlN. While a number of theoretical calculations5m10 of the band structure exist, the experimental work”-‘” available for comparison has been limited in either energy range or energy resolution. Extant experimental studies are mostly on thin films due to the difficulty of growing high-purity bulk crystals of 41N. Aside from techuological applications, the electronic structure of AlN is also of fundamental interest. Calculation of the electronic structure of ceramics is a developing area of theoretical physics and the nitrides are of particular importance because the bonding is more covalent than in the oxides. Experimental information about the electronic structure of AlN provides insight as to how theoretical models based on oxides should be adapted to covalent ceramics. Quantitative comparison of theory and experiment affords the best tool for gaining this insight. To this end, we present our results in terms of analytical critical point models of the interband transition strength, J,, , for both a first principles calculation and a vacuum ultraviolet (VW) optical measurement. Our method emphasizes the relationship among critical points grouping them into sets representative of transitions between pairs of bands, while other recent work” emphasizes individual critical points. A single crystal (W201), with a thermal conductivity of 275 W m -’ K-r grown by a modified Bridgman technique,r6 was studied. The oxygen content of the single crystal was 3GO ppm, as previously reported.‘” Polishing with diamond powders suspended in high-purity dry kerosene yields an oxide-free surface for reRectance measurements. Oxidized surfaces give spurious results and were avoided by nonaqueous polishing. The polished face of the single crystal was near normal to the c axis. The VIJV reflectance spectra were obtained with a laser plasma sourced VUV spectrophotometer, described elsewhere.9 Above the band gap at 6.2 eV, t.he spectra show two main features. A sharp peak appears at about 9 eV and a smaller, somewhat brozder peak appears at about l&15 eV. A small feature also appears at about 35 eV. Small features below the band gap are due to vacancies. The single crystal response was found to be representative of commercial polycrystalline substrates also studied.” The spectrum was adjustedd, consistent with an index of jz[ 1.25 eV] = 2.1 A Kramers-Kriinig (KK) analysi?” recovered the phase information, S[h,\r], and permitted calculation of other optical properties,” including the complex dielectric function shown in Fig. 1. The energy band structure of AlN was calculated from first principles, using the orthogonalized linear combination of atomic orbitals (OL.CAOj method with the local density approximation (LDA

Critical point analysis of the interband transition strength of electrons

Optical and electron-energy-loss spectroscopies are well established methods of probing the electronic structure of materials. Comparison of experimental spectroscopic results with theory is complicated by the fact that the experiments extract information about the interband transition strength of electrons, whereas theoretical calculations provide information about individual valence and conduction bands. Based on the observation that prominent features in the optical response arise from critical points in the joint density of states, critical point modelling was developed to gain an understanding of these spectral features in terms of specific critical points in the band structure. These models were usually applied to derivative spectra and restricted to the consideration of isolated critical points. The authors present a new approach to critical point modelling of the undifferentiated spectra and interpret the model in terms of balanced sets of critical points which describe the interband transition strength arising from individual pairings of valence and conduction bands. This approach is then applied to achieve a direct, quantitative comparison of theoretical and experimental data on aluminium nitride.

Comparative studies of the electronic structure of LiFePO4, FePO4, Li3PO4, LiMnPO4, LiCoPO4, and LiNiPO4

We report a comparative study of the electronic structures of LiFePO4, FePO4, Li3PO4, LiMnPO4, LiCoPO4, and LiNiPO4. We used the spin-polarized ab initio orthogonalized linear combinations of atomic orbitals method for the calculation. Li3PO4 is a large-band gap insulator with a band gap of 5.75 eV. For other crystals containing the 3d-transition metals (TM), the localized 3d bands of the TM fall within the large insulting gap. In the cases of FePO4 and LiMnPO4, the band gaps between the occupied majority spin band and the unoccupied minority spin band are of the order of 0.27 and 1.91 eV, respectively. For LiFePO4 and LiCoPO4, our calculation shows that they are 100% spin-polarized half metals. In the case of LiNiPO4, the Fermi level is in the small gap separating t2g and eg minority bands. These results are discussed in the context of the number of 3d electrons in the TM series and the possibility of strong electron correlation effects in these crystals.

Selfconsistent band structures and optical calculations in cubic ferroelectric perovskites

Abstract The band structures of SrTiO3 BaTio3 and KNbO3 in the cubic perovsikte structure are calculated using the first-prinicipals selfconsistent orthogonalized linear combination of atomic orbitals method. In all three crystals, indirect band gaps of about 3 eV are obtained. The top of the valence band is at R and the minimum in the conduction band is at f. Effective charge calculation and the charg. 1 density maps reveal the highly ionic nature of the crystal bonding in these ferroelectric crystals. The optical conductivities and the frequency dependent dielectric functions up to 20 eV in all three crystals are also evaluated using the energy eigenvalues and wave functions obtained from the band calculation. The calculated spectra are found to be in good agreement with the existing experimental measurements.

Structure and properties of spinel Fe3N4 and comparison to zinc blende FeN

The structure and properties of iron nitride with a cubic spinel structure are predicted. Spin-polarized calculations show it to be weakly ferromagnetic with a total magnetic moment of 3.26 μB per formula unit. Total energy versus volume calculations yield a large bulk modulus of 304 GPa, which is attributed to the strong covalent bonding between Fe and N. These results differ significantly from those of stoichiometric FeN in a zinc blende structure.

Electron states of YAG probed by energy-loss near-edge spectrometry and ab initio calculations

Abstract Al K, Al L2,3, O K, Y L2,3, and Y M2,3 energy-loss near-edge structures (ELNESs) of Y3Al5O12 (YAG) were measured using a dedicated scanning transmission electron microscope equipped with a parallel electron energy-loss spectrometer system. An attempt was made to interpret the experimental edges with the help of calculated near-edge structures. An ab initio orthogonalized linear combination of atomic orbitals method, within the local density approximation (LDA), was used to calculate the electronic structure of YAG. Site-decomposed and symmetry projected local densities of states (LDOS) and photo-absorption cross-sections (PACS) were compared to experimental ELNES spectra. There is reasonable agreement between the five different, measured spectra and calculated near-edge structures. PACS calculations correlated better than LDOS with the experimental edges in terms of relative intensities of the peaks. Through a comparison with site-decomposed LDOS calculations, several features of the experimental...

Ab-initio calculation of excited state absorption of Cr4+ in Y3Al5O12

The Cr3+ and Cr4+ impurity states in Y3Al5O12 (YAG) crystal are studied by ab-initio supercell calculations using the density-functional theory. Calculations are carried out with Cr substitutions at the octahedral and tetrahedral Al sites including the effect of Ca co-doping. Optical transitions between various levels and to conduction band states are also calculated. A model for excited state absorption for Cr4+ in YAG is proposed.

Electronic structure of (Na½Bi½)TiO3 and its solid solution with BaTiO3

Abstract The electronic structure and bonding in the (Na15/32Bi½)TiO3 (NBT) crystal and its solid solution with BaTiO3 (BT) are studied by first-principles local density calculations. For the solid solution. an ordered superstructure (Na15/32Bi15/32Ba1/16)TiO3 with 320 atoms in a large celi is proposed. The results show that both NBT and NBT-BT are semiconductors with a band gap of about 1 eV. NBT is shown to be a harder crystal than PbTiO3 and PbZrO3 by having a larger calculated bulk modulus. This can be attributed partly to the increased covalent character of bonding in NBT and NBT-BT crystals. The calculated O K edges for a number of related crystals are presented and compared. It is suggested that electron energy loss near-edge structure measurements can be an effective tool to characterize the samples for different piezoelectric materials.

Vanadium substituted 2212 and 2223 superconducting ceramics

Abstract We report the successful fabrication of the ceramic superconductor Bi 2-x V x Sr 2 Ca 2 Cu 3 Oy and Bi 2-x V x Sr 2 CaCu 2 O 8 , with x ranging from a fraction to at least unity, showing that Vanadium behaves more than a dopent. The critical temperatures of these phases remain a few degrees higher than that of the pure bismuth case subject to similar fabrication processes. The 110 K phase in the predetermined superconductor (2223) is significantly enhanced when V is mixed in. Relevant results of band structure calculations are also presented to provide physical explanation of choosing the element Vanadium according to the EEM theory.