38 research outputs found
Lattice Relaxation and Charge-Transfer Optical Transitions Due to Self-Trapped Holes in Non-Stoichiometric LaMnO Crystal
We use the Mott-Littleton approach to evaluate polarisation energies in
LaMnO lattice associated with holes localized on both Mn cation and
O anion. The full (electronic and ionic) lattice relaxation energy for a
hole localized at the O-site is estimated as 2.4 eV which is appreciably
greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on
the strong electron-phonon interaction in the former case. Using a Born-Haber
cycle we examine thermal and optical energies of the hole formation associated
with electron ionization from Mn, O and La ions in
LaMnO lattice. For these calculations we derive a phenomenological value
for the second electron affinity of oxygen in LaMnO lattice by matching the
optical energies of La and O hole formation with maxima of binding
energies in the experimental photoemission spectra. The calculated thermal
energies predict that the electronic hole is marginally more stable in the
Mn state in LaMnO host lattice, but the energy of a hole in the
O state is only higher by a small amount, 0.75 eV, rather suggesting that
both possibilities should be treated seriously. We examine the energies of a
number of fundamental optical transitions, as well as those involving
self-trapped holes of Mn and O in LaMnO lattice. The reasonable
agreement with experiment of our predicted energies, linewidths and oscillator
strengths leads us to plausible assignments of the optical bands observed. We
deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d)
transition of charge-transfer character, whereas the band near 2.3 eV is rather
associated with the presence of Mn and/or O self-trapped holes in
non-stoichiometric LaMnO compound.Comment: 18 pages, 6 figures, it was presented partially at SCES-2001
conference in Ann Arbor, Michiga
Identifying and dating the destruction of hydrocarbon reservoirs using secondary chemical remanent magnetization
Destructive processes are thought to be common in preâCenozoic oilâgas reservoirs. The timing, mechanism, and even identification of these processes, however, are difficult to clearly characterize, which obscures the evolution of such systems and the assessment of oil and gas reserves. Here, we reveal a new link between secondary chemical remanent magnetization acquisition and tectonically driven destruction of hydrocarbon reservoirs, which can be used to date the destructive processes and identify their tectonic controls. We performed a detailed paleomagnetic analysis of rocks from a typical destroyed reservoir (Majiang reservoir, China) and combined these data with scanning electronic microscope imaging and strontium isotope, total organic carbon, and clay analysis. We found that the Late Triassic syntilting secondary chemical remanent magnetizations of source and reservoir rocks resulted from the destructive processes driven by the Indosinian orogeny. We therefore argue that palaeomagnetic methods can be used to constrain destructive events within hydrocarbon reservoirs worldwide
Complex-Orbital Order in Fe_3O_4 and Mechanism of the Verwey Transition
Electronic state and the Verwey transition in magnetite (Fe_3O_4) are studied
using a spinless three-band Hubbard model for 3d electrons on the B sites with
the Hartree-Fock approximation and the exact diagonalisation method.
Complex-orbital, e.g., 1/sqrt(2)[|zx> + i |yz>], ordered (COO) states having
noncollinear orbital moments ~ 0.4 mu_B on the B sites are obtained with the
cubic lattice structure of the high-temperature phase. The COO state is a novel
form of magnetic ordering within the orbital degree of freedom. It arises from
the formation of Hund's second rule states of spinless pseudo-d molecular
orbitals in the Fe_4 tetrahedral units of the B sites and ferromagnetic
alignment of their fictitious orbital moments. A COO state with longer
periodicity is obtained with pseudo-orthorhombic Pmca and Pmc2_1 structures for
the low-temperature phase. The state spontaneously lowers the crystal symmetry
to the monoclinic and explains experimentally observed rhombohedral cell
deformation and Jahn-Teller like distortion. From these findings, we consider
that at the Verwey transition temperature, the COO state remaining to be
short-range order impeded by dynamical lattice distortion in high temperature
is developed into that with long-range order coupled with the monoclinic
lattice distortion.Comment: 16 pages, 13 figures, 6 tables, accepted for publication in J. Phys.
Soc. Jp
DEFECT INTERACTIONS AND ORDER-DISORDER IN TRANSITION METAL OXIDES
Nous discutons la structure de défaut des oxydes MnO, FeO, CoO et NiO en utilisant les résultats des calculs énergétiques. Nos résultats montrent que les modÚles simples qui ne prennent pas en considération l'agrégation des lacunes ne sont pas valables. Des larges aggrégats de lacunes peuvent se former ; ils sont stabilisés par la formation d'interstitiels d'ions métalliques. Les paramÚtres du champ de ligand jouent un rÎle important, puisqu'ils contrÎlent la charge de l'interstitiel ce qui, détermine finalement le mode d'agrégation des lacunes et il en résulte des différences notables entre les structures des défauts des différents oxydes. Notre étude attire l'attention sur les conséquences de l'association des lacunes sur les propriétés de transport et thermodynamiques de ces oxydes.The defect structure of the four oxides, MnO, FeO, CoO and NiO, is discussed using the results of calculations of defect energetics. Our results show that simple models, neglecting vacancy aggregation, are invalid. Large vacancy clusters may form ; they are stabilised by the accompanying formation of cation interstitials. Ligand field factors play an important role, as they control the interstitial charge ; this, in turn, determines the mode of vacancy aggregation and results in considerable differences between the defect structures of the different oxides. The discussion draws attention to the consequences of the association of vacancies for transport and thermodynamic properties of these oxides
Compositional Effects of Low-Pressure Impacts in Chondritic Meteorites: Oxygen Isotope Homogenization and Mg-Fe Diffusion in Matrix Olivine and Presolar Grains
Recent work has explored the effects of low-intensity impacts into porous chondrite precursors. We show oxygen-isotope homogenization of presolar grains and Mg-Fe diffusion in fine-grained matrix are potential consequences of this process
Mapping hydrocarbon charge-points in the Wessex Basin using seismic, geochemistry and mineral magnetics
This study reports a multidisciplinary approach to determining hydrocarbon charge-points and migration in the Wessex Basin, southern England. Geochemical analysis of reservoir core material (Bridport Sandstone and Inferior Oolite) using gas chromatography-mass spectrometry (GC-MS), suggests that the oil in the Wessex Basin is from a single source, and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years. Using seismic data, basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the Purbeck fault into the Sherwood Sandstone reservoir at Wytch Farm. One of these potential pathways is represented by cores termed Creech and the other Bushey Farm. To try to distinguish between the two potential pathways, cores were studied using mineral magnetic techniques. The magnetic signature was characterised using low-temperature (<50âŻK) magnetic measurements; this is because much of the magnetic signature was dominated by nanoparticlesâŻ<âŻ30âŻnm, which are thermally activated at room temperature and magnetically âtransparentâ. Wells that contained considerable amounts of hydrocarbons were dominated by nanometric magnetite (<30âŻnm). Such particles are small enough to migrate with the oil, through pore spaces, which are of the order ~100âŻnm. Wells located at the fringes of large hydrocarbon accumulation had enhanced pyrrhotite-dominated magnetic signals. Of the two potential migration pathways, the mineral magnetic results suggest that the oil migrated through Creech rather than through Bushey Farm
Effect of Low Intensity Impacts on Chondrite Matrix
Although the majority of carbonaceous chondrites have only experienced low-intensity (<5 GPa) impacts, we show that compacting initially highly porous matrix aggregates results in large temperature excursions even at low shock pressures