Helmer Fjellvåg

Theoretical investigation of magnetoelectric behavior in BiFeO3

The magnetoelectric behavior of BiFeO3 has been explored on the basis of accurate density functional calculations. We are able to predict structural, electronic, magnetic, and ferroelectric properties of BiFeO3 correctly without including any strong correlation effect in the calculation. Unlike earlier calculations, the equilibrium structural parameters are found to be in very good agreement with the experimental findings. In particular, the present calculation correctly reproduced experimentally-observed elongation of cubic perovskite-like lattice along the [111] direction. At high pressure we predicted a pressure-induced structural transition from rhombohedral (R3c) to an orthorhombic (Pnma) structure. The total energy calculations at expanded lattice show two lower energy ferroelectric phases (with monoclinic Cm and tetragonal P4mm structures), closer in energy to the ground state phase. Spin-polarized band-structure calculations show that BiFeO3 will be an insulator in Aand G-type antiferromagnetic phases and a metal in C-type antiferromagnetic, ferromagnetic configurations, and in the nonmagnetic state. Chemical bonding in BiFeO3 has been analyzed using partial density-of-states, charge density, charge transfer, electron localization function, Born-effective-charge tensor, and crystal orbital Hamiltonian population analyses. Our electron localization function analysis shows that stereochemically active lone-pair electrons are present at the Bi sites which are responsible for displacements of the Bi atoms from the centro-symmetric to the noncentrosymmetric structure and hence the ferroelectricity. A large ferroelectric polarization of 88.7 μC/cm is predicted in accordance with recent experimental findings, but differing by an order of magnitude from earlier experimental values. The strong spontaneous polarization is related to the large values of the Born-effective-charges at the Bi sites along with their large displacement along the [111] direction of the cubic perovskite-type reference structure. Our polarization analysis shows that partial contributions to polarization from the Fe and O atoms almost cancel each other and the net polarization present in BiFeO3 mainly (> 98%) originates from Bi atoms. We found that the large scatter in experimentally reported polarization values in BiFeO3 is associated with the large anisotropy in the spontaneous polarization.

Structural Changes and Coordinatively Unsaturated Metal Atoms on Dehydration of Honeycomb Analogous Microporous Metal-Organic Frameworks

Porous metal-organic framework compounds with coordinatively unsaturated metal sites on the inner surface of the pores promise to be valuable adsorbents and catalyst systems, either in industrial applications or as model systems to study interactions with guest molecules. The dehydration process of two isostructural microporous coordination polymers, [M2(dhtp)(H2O)2].8 H2O, termed CPO-27-M (M=Co, Zn; H(4)dhtp=2,5-dihydroxyterephthalic acid) was investigated by in situ variable temperature X-ray diffraction. Both compounds contain accessible coordination sites at the metal after complete removal of the solvent. However, despite the analogy of their crystal structures, they behave differently during dehydration. For CPO-27-Co, water desorption is a smooth topotactic process of second order with no concomitant space group change and no increase in microstrain, which is beneficial for the applicability of the material. Removal of the water propagates from the center of the channels outwards. The coordinating water molecule at the metal desorbs only when almost all the bulk water in the pores has disappeared. In contrast, discontinuities in the powder pattern of CPO-27-Zn indicate the occurrence of first-order transitions. The crystal structures of four of the five individual phases could be determined. The structure of the intermediate phase occurring just before the framework is completely evacuated was elusive in respect to full structure solution and refinement, but it is most probably related to the removal of the axis of threefold symmetry. The zinc-based material experiences a significant amount of strain.

A comparison study on Raman scattering properties of α- and β-MnO2

In this comment to a recent paper [Anal. Chim. Acta 585 (2007) 241-245], we report a comparison study on Mn oxide-related compounds with different crystallographic forms, which distinguish between beta-MnO(2) and alpha-MnO(2) type materials via Raman scattering (RS) spectroscopy. The tetragonal rutile-type beta-MnO(2) is characterized by a RS band at approximately 667cm(-1) of symmetry A(1g), whereas the alpha-MnO(2) type materials feature two main RS contributions at about 574 and 634cm(-1), belonging to A(g) spectroscopic species of a tetragonal hollandite-type framework. These data represent a clear signature for identifying beta-MnO(2) and alpha-MnO(2) type materials via RS spectroscopy.

Equation of state of magnetite and its high-pressure modification: Thermodynamics of the Fe-O system at high pressure

Abstract Fe3O4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a highpressure modification, h-Fe3O4, is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn2O4-type structure, space group Pbcm) recently proposed for h-Fe3O4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi2O4- type structure (space group Bbmm). Bulk modulus values for magnetite, KT0 = 217 (2) GPa, and h-Fe3O4, KT0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe3O4 gives an increased stability of wüstite relatively to a two-phase mixture of iron and h-Fe3O4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.

Crystal Structures of Titanate Nanotubes: A Raman Scattering Study

Crystal structures of titanate nanotubes prepared from a NaOH treatment of TiO(2) with subsequent acid washing were discussed from a viewpoint of vibrational spectroscopy. The correlation between the vibrational feature and the polymerization nature of the TiO(6) octahedron was established by analyzing Raman scattering data of crystalline TiO(2) (anatase and rutile) and layered protonic titanates. Then, the polymerization nature of TiO(6) octahedra in the titanate nanotubes was identified by comparing their Raman scattering spectra with those of the crystalline TiO(2) and layered protonic titanates. It demonstrated that the titanate nanotubes consist of two-dimensional TiO(6) octahedral host layers with a lepidocrocite (gamma-FeOOH)-type layered structure. This conclusion was confirmed further by considering the Raman scattering properties of a restacked titanate prepared by assembling TiO(6) octahedral layers derived from the original scroll-like titanate nanotubes. Our findings offered a convenient approach to validate the crystal structures of the products from an alkaline treatment of TiO(2) under different experimental conditions.

Magnetic properties of the one-dimensional Ca3Co2O6

Abstract The magnetic properties of Ca 3 Co 2 O 6 have been studied by magnetic susceptibility and neutron diffraction methods from 5 K to room temperature. Magnetic ordering occurs below 24±2K. Above 80 K, the Curie-Weiss law is followed and an effective magnetic moment of 5.7±0.2 μ B was found. The magnetic structure of Ca 3 Co 2 O 6 has been refined from Rietveld analysis of powder neutron diffraction data at 10 K. The one-dimensional chains in Ca 3 Co 2 O 6 were found to have alternating low magnetic moment (0.08±0.04 μ B ) for the octahedral cobalt and high magnetic moment (3.00±0.05 μ B ) for the trigonal prismatic cobalt ions. Magnetic moments close to zero were indicated in the ab -plane. Ca 3 Co 2 O 6 is ferrimagnetic at low temperatures, with ferromagnetic ordering dominating within the CoO chains. In high magnetic fields, above around 2.7 MA m −1 , a field-induced transition from ferrimagnetic to ferromagnetic structure occurs, implying turning of the spins in one chain.

Pressure-induced structural transitions in MgH2.

The stability of MgH2 has been studied up to 20 GPa using density-functional total-energy calculations. At ambient pressure alpha-MgH2 takes a TiO2-rutile-type structure. alpha-MgH2 is predicted to transform into gamma-MgH2 at 0.39 GPa. The calculated structural data for alpha- and gamma-MgH2 are in very good agreement with experimental values. At equilibrium the energy difference between these modifications is very small, and as a result both phases coexist in a certain volume and pressure field. Above 3.84 GPa gamma-MgH2 transforms into beta-MgH2, consistent with experimental findings. Two further transformations have been identified at still higher pressure: (i) beta- to delta-MgH2 at 6.73 GPa and (ii) delta- to epsilon-MgH2 at 10.26 GPa.

Electronic structure and optical properties of ZnX (X=O, S, Se, Te): A density functional study

Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and

Accurate structure of LiAlD4 studied by combined powder neutron and X-ray diffraction

\mathrm{LDA}+U

Growth of manganese oxide thin films by atomic layer deposition

approaches. Calculations of the optical spectra have been performed for the energy range