Christian Maunders

Effects of bond character on the electronic structure of brownmillerite-phase oxides, Ca2B′xFe2−xO5 (B′ = Al, Ga): an X-ray absorption and electron energy loss spectroscopic study

Brownmillerite-phase transition-metal oxides, Ca2B′xFe2−xO5 (B′ = Al, Ga), have been examined by use of X-ray absorption near-edge spectroscopy (XANES) and electron energy loss spectroscopy (EELS). These studies were performed to examine how the electronic structure was affected as a more (Ga) or less (Al) electronegative metal was substituted for Fe. The oxygen deficient perovskite-like structure is built up of alternating layers having either octahedrally or tetrahedrally coordinated metal atoms. Analysis of the Fe L-, Ga K-, and Al L-edge XANES and EELS spectra confirmed that the group III metals substitute primarily into the tetrahedral site, regardless of the size of the atoms. Through examination of O K-edge spectra, compared to LMTO calculated crystal orbital Hamilton population plots, the change in O–metal bond character with substitution was investigated. It was found that in Ca2GaxFe2−xO5, the peaks in the O K-edge spectra resulting from an excitation of O 1s electrons into hybridized, unoccupied, O 2p–Ga 4p/4s antibonding states decreased in energy and increased in intensity with greater values of x. This is a result of the formation of more covalent O–Ga bonds. No shifts in energy were observed in O K-edge spectra from Ca2AlxFe2−xO5 as the more ionic O–Al states are overlapped by stronger O–Ca antibonding states. This study shows that the electronic structure of these materials is tunable through selective substitution of metals into the tetrahedral site.

Synthesis and characterization of well aligned Ru nanowires and nanotubes

Arrays of well aligned Ru nanowires and nanotubes were prepared by electrodeposition through porous anodic aluminum oxide membranes. It is shown that Ru nanowires are formed at low cathodic deposition overpotential, while nanotubes are prepared at higher cathodic overpotential. The inner diameter of the nanotube can be controlled by the deposition potential, and it varies from 0 to 70% of the outer diameter as the deposition potential is changed from -0.30 to -0.45 V vs SCE.

Growth, structure, and properties of BiFeO/sub 3/-BiCrO/sub 3/ films obtained by dual cross beam PLD

Bi₂Fe_xCr_yO₆ epitaxial films have been grown on (100)-and (111)-oriented SrTiO₃ substrates using dual crossed beam pulsed laser deposition (DCB-PLD). The films crystal structure, chemical composition ferroelectric and magnetic properties were investigated and are reported.

Electron diffraction and microscopy study of the structure and microstructure of the hexagonal perovskite Ba3Ti2MnO9.

This paper reports a structural and microstructural investigation of the hexagonal perovskite Ba(3)Ti(2)MnO(9) using electron microscopy and diffraction. Convergent-beam electron diffraction (CBED) revealed the structure has the non-centrosymmetric space group P6(3)mc (186) at room temperature and at approximately 110 K. Compared with the centrosymmetric parent structure BaTiO(3), with space group P6(3)/mmc, this represents a break in mirror symmetry normal to the c axis. This implies the Ti and Mn atoms are ordered on alternate octahedral sites along the 0001 direction in Ba(3)Ti(2)MnO(9). Using high-resolution electron microscopy (HREM), we observed occasional 6H/12R interfaces on (0001) planes, however, no antiphase boundaries were observed, as were seen in Ba(3)Ti(2)RuO(9). Using powder X-ray Rietveld refinement we have measured the lattice parameters from polycrystalline samples to be a = 5.6880 +/- 0.0005, c = 13.9223 +/- 0.0015 A at room temperature.

Applications of aberration-corrected TEM-STEM and high-resolution EELS for the study of functional materials

The first applications of high-resolution aberration-corrected microscopy were demonstrated 10 years ago in key publications [1–3] and earlier conferences showing the potential impact of the technique in the study of materials. Aberration-corrected STEM imaging results demonstrating sub-A resolution were presented soon after [5,6].

Investigation of the valency distribution in Cu 1.2 Mn 1.8 O 4 using quantitative EELS near-edge structures analysis

Electron energy loss spectroscopy (EELS) is a well-established technique used to probe the electronic structure of complex materials. Improvements in spectrometer design and the introduction of monochromators combined with aberration correctors in modern transmission electron microscopes provide spectra of ultrahigh energy resolution (equivalent to synchrotron data) at the atomic level.

Electron Energy Loss Near-Edge Structures in Complex Perovskites

High-resolution electron energy loss spectroscopy (EELS) provides better sensitivity to fine structures related to bonding in complex materials [1,2]. Using monochromators and improved spectrometers it is possible to achieve 0.1eV resolution with symmetric zero-loss peak profiles leading to improved detection of both core-loss and low-loss features [3] in EELS spectra. This capability makes it possible to study systematically changes in bonding in materials that present interesting physical properties. Oxides with the perovskite structure provide an interesting playing field to study many fundamental changes in structure and physical properties when systematic substitutions of atoms in the crystals are made in a controlled fashion. Substitutions of Ti atoms in BaTiO3 with other transition metals provide ingenious ways to change the stability of the basic structure and induce phase transformations. For example, Mn substitutions for Ti in BaTiO3 stabilize the hexagonal phase at room temperature and it has been proposed that this is due to increased metal-metal bonding between adjacent face-sharing octahedra [4]. EELS measurements in Ba(Ti,Mn)O3, however, have shown that, with respect to the tetragonal structure, there are only subtle changes in the oxygen K edge fine structures of the hexagonal compound due to overlaps of the Mn 3d electrons and O 2p electrons while the Ti environment is not altered significantly [5]. Ru substitutions for Ti also lead to the stabilization of the hexagonal phase but there are no reports on the effect of the changes in the near-edge structures related to the structural changes. For this particular case, it is of great interest, to study the closely related hexagonal perovskite BaRuO3. Probing electronic structure changes in these two materials with respect to BaTiO3 makes it possible to gain insight on both the electronic structure of these closely related compounds and the origin of the structural changes. We have therefore used EELS with high energy resolution to probe the near edge structures in BaTiO3, Ba(Ti,Ru)O3 and BaRuO3.