Distortion Of Oxygen Octahedra Research Articles

Size effects in chromium-doped PbTiO3 nanopowders observed by multi-frequency EPR

A multi-frequency (X, Q, and W band) electron paramagnetic resonance (EPR) study of Cr-doped PbTiO3 micro- and nanopowder samples was performed. Three Cr3+ centres were identified in tetragonal phase samples with different axial Zero Field Splitting (ZFS) parameters, C1, C2, and C3. The centre C1 is similar to that observed in previous X-band crystal and ceramic sample measurements. The superposition model by Newman and Urban was applied to translate the ZFS data into local displacements inside the distorted oxygen octahedra of the PbTiO3 lattice. In the tetragonal phase, only the centre C1 was observed. The powder spectra were fitted using a spin-Hamiltonian in which a Gaussian distribution of ZFS terms, characterized by a mean value D and distribution width deltaD, was assumed. The variation of D and deltaD with mean particle size was determined. A critical particle size, d(cr), exists, particles smaller than this size remain in the cubic phase for all temperatures, there is a size-driven tetragonal-to-cubic phase transition. Particles with d < d(cr) were found to give a new Cr3+ centre spectrum, C4, consisting of a single line with an isotropic g-factor, so allowing the cubic phase content to be quantified. Further, temperature-dependent EPR measurements were made, which allowed the variation in Curie temperature with mean particle size to be determined.

Octahedral distortions in the homometallic Fe ludwigite

An extended Hückel high spin band approach is used to investigate the effects of oxygen octahedral distortions in the Fe 3O 2BO 3 ludwigite. Owing to distortion, a 0.2 eV stabilizing gap (above the spin down Fermi level) is found to appear in a 1D sub-unit, formed by the strongly interacting Fe 3+–Fe 2+–Fe 3+ triad. Through a detailed analysis of the crystal wave functions, the gap is found to be a result of 3 d( σ)–3 d( π) orbital mixing, which generates a narrow band for the extra (spin down) Fe 2+ electron. Charge localization is obtained in the 1D sub-unit but not in the whole crystal (3D) calculation. It is suggested that the high barrier for electron hopping, experimentally found in the literature to occur around 220 K, be related to the 1D gap.

Electrical conductivity and local distortion of (Ba 0.5La 0.5) 2In 2O 5.5 doped with divalent or tetravalent cation in in-site

Total electrical conductivity of (Ba 0.5+ x La 0.5− x ) 2(In 1− x M x ) 2O 5.5 (M=Zr and Ce) and (Ba 0.5− x La 0.5+ x ) 2(In 1− x M x ′) 2O 5.5 (M′=Mg, Ca and Sr) solid-solution systems was measured as functions of temperature and oxygen partial pressure. The oxide-ion conductivity of all the solid-solution systems increased at first with an increase in the unit cell free volume. However, it showed a maximum value at a unit cell free volume and then decreased. All the solid-solution systems showed a similar dependence of the oxide-ion conductivity on the unit cell free volume, even if they have different average ionic valences in the A- and B-sites. From the results of the Raman spectra measurement, the decrease of conductivity in the larger unit cell free volume range is presumed to arise from the local distortion of oxygen octahedra.

Triaqua-1κO,2κO,3κO-hexakis-μ-chloroacetato-1:2κ4O:O′;2:3κ4O:O′;1:3κ4O:O′-μ3-oxo-triiron(III) nitrate trihydrate

The title compound, [Fe3O(C2H2ClO2)6(H2O)3]NO3·3H2O, contains an FeIII trimeric cation in which the three FeIII ions are at the corners of a slightly distorted triangle with an O atom in the centre. Two mono­chloro­acetate ligands bridge each pair of FeIII ions and a water mol­ecule coordinates each FeIII ion at the apex, opposite to the central O atom. Hence, the FeIII ions are surrounded by distorted oxygen octahedra. The distances between the FeIII ions and the central O atoms are considerably shorter than those between the FeIII ions and the other coordinated O atoms. The trimeric cations form a three-dimensional connected network via hydrogen-bonding interactions to extra-cluster water mol­ecules and nitrate anions.

Multi‐frequency EPR studyof Cr3+ doped lead titanate (PbTiO3) nanopowders

AbstractIn our multi‐frequency EPR study of Cr3+ doped PbTiO3 nanopowder samples with varying mean particle size, three paramagnetic Cr3+ centres (C1, C2, C3) with different axial fine structure (FS) parameters are identified, with one of them (C1) being compatible with former literature measurements on bulk and ceramics samples carried out in the X band. The superposition model by Newman and Urban was applied to translate the FS data into local displacements inside the distorted oxygen octahedra of the PbTiO3 lattice. When going to nanopowder samples with mean particle size below a critical value, other than the centre C1 are no longer detectable. Instead, a new one (C4) appears, testifying a pronounced size effect. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A model of phase transition induced antiphase boundaries in perovskite and layered perovskite oxides

A structural model of the antiphase boundaries (APBs) induced by phase transition in perovskite and layered perovskite oxides is put forward. Besides the distortion of oxygen octahedra, the existence of two types of ion with different sizes on A sites in perovskite blocks and their ordering in local areas might be the reasons for the occurrence of APBs. It could explain why APBs exist in SrBi2Ta2O9, Bi3.25La0.75Ti3O12, La2−xBaxCuO4, and La2/3Ca1/3MnO3, but not in Bi4Ti3O12 and Nd2NiO4 through their specific structure phase transitions. The formation of APBs might reflect the chemical inhomogeneity in local areas. Empirical results support this model.

High pressure synthesis, crystal structure and physical properties of a new Ni(ii) perovskite BiNiO3

A new triclinic perovskite, BiNiO3, has been synthesized at a high pressure of 6 GPa in an oxidizing atmosphere. Structure refinements based on synchrotron X-ray powder diffraction and bond-valence calculations revealed a disproportionation of the Bi ions into Bi3+ and Bi5+, both being located in distorted oxygen octahedra. Because of the presence of Bi5+, the oxidation state of Ni was +2, rather than +3 as had been expected; BiNiO3 thus showed insulating behavior with localized spins of S = 1.

Photoinduced Centers in PbZr 1−x Ti x O 3 Single Crystals

Measurements of ESR spectra in PbZr 1 m x Ti x O 3 , x h 0,06, single crystals were performed at T = (10 1 40) K before and after UV light illumination. Unavoidable impurities Fe 3+ , Pt 3+ , Ni 3+ and Ti 3+ were revealed respectively before and after light illumination. The components of orthorhombic symmetry g-tensor and its principal axes orientation were obtained from measurements of the spectra angular dependencies. It was shown that all paramagnetic ions substitute for Zr 4+ either in the center of oxygen octahedron (Pt 3+ , Ni 3+ , Fe 3+ ) or in off-center shifted along c-axis direction position (Ti 3+ ). Local distortion of oxygen octahedra was shown to be close to that in pure PbZrO 3 . Low symmetry of PZT and PbZrO 3 lattices manifest itself respectively in 8 and 4 magnetically nonequivalent positions of the paramagnetic ions.

Synthesis of Ca 0.25Cu 0.75TiO 3 and infrared characterization of role played by copper

Ca 0.25Cu 0.75TiO 3 compounds with a perovskite-type structure have been reported to display the double advantage for capacitance applications to show high dielectric constant and little dependence on temperature near room temperature. Powders have been prepared by using an organic gel-assisted citrate process. The infrared reflectivity spectra obtained on sintered ceramic discs are very different from those of simple cubic perovskites. Copper indeed is expected to form hybridized bonds with oxygen, contrary to calcium. As a result, instead of remaining equidistant from calcium or copper as in simple cubic perovskite, oxygen forms shorter bonds with copper inducing a strong distortion of oxygen octahedra. Spectra have been Kramers–Kronig transformed and fitted with the factorized form of the dielectric function. Results show that the origin of high dielectric constant is little phononic but rather due to a relaxational process with the characteristic frequency in the gigahertz range. Thin films grown by PLD on (100) SrTiO 3 substrate show preferential (100) orientation.

Oxygen Position, Octahedral Distortion, and Bond‐Valence Parameter from Bond Lengths in Ti1−xSnxO2 (0 ≤x≤ 1)

Based on the virtual crystal approximation (or Vegard's law), the bond lengths of Ti1−xSnxO2 were deduced from those of TiO2 and SnO2, to allow the oxygen position and octahedral distortion to be determined as a function of x. The oxygen positional parameter (u) increased linearly when the Sn4+ cation (which has a larger ionic radius) was substituted for the Ti4+ cation, whereas the octahedral distortion exhibited a nonlinear decay with increasing x in Ti1−xSnxO2. At the same time, the bond‐valence parameter, which relates bond valence to bond length, so that the central atom in the octahedron can retain a constant valence of +4.0, exhibited a correlation with u for Ti1−xSnxO2. The present results indicate that the different phonon/physical properties of TiO2 and SnO2 and/or their dependence on x in Ti1−xSnxO2 can be associated with different octahedral distortions.

Growth and Spectra of YCOB and Nd:YCOB Crystals

Yttrium calcium oxoborate (YCOB) and Nd:YCOB crystals of high optical quality and large dimension have been grown by the Czochralski method. Measurements of refractive indices have confirmed that these crystals were negative biaxial. The absorption spectra were determined and the various spectrum parameters were calculated. Polarized absorption spectra revealed the strong anisotropic character of Nd:YCOB. Raman spectra indicate that the nonlinearity of the crystal is mainly attributed to planar BO3 triangles in the structure rather than to a distortion of oxygen octahedra.

Growth and Spectra of YCOB and Nd:YCOB Crystals

Yttrium calcium oxoborate (YCOB) and Nd:YCOB crystals of high optical quality and large dimension have been grown by the Czochralski method. Measurements of refractive indices have confirmed that these crystals were negative biaxial. The absorption spectra were determined and the various spectrum parameters were calculated. Polarized absorption spectra revealed the strong anisotropic character of Nd:YCOB . Raman spectra indicate that the nonlinearity of the crystal is mainly attributed to planar BO 3 triangles in the structure rather than to a distortion of oxygen octahedra.

Cooperative jahn-teller coupling in the manganites

The cooperative Jahn-Teller coupling between the Mn e(g) electrons and the oxygen octahedral distortions in LaMnO3 is studied using ab initio density-functional calculations and tight-binding models. The linear and quadratic vibronic coupling parameters are calculated using density-functional methods. It is shown that the cooperative Jahn-Teller coupling, primarily due to the interoctahedral electron hopping (band structure term), leads to the ordering of the octahedral distortion and simultaneously to orbital ordering. The coupling results in a two-minima adiabatic potential surface in the solid, instead of the three-minima "Mexican-hat" surface for the isolated octahedron.

Octahedral deformations and cationic displacements in the ferroelectric PbHf(0.8)Ti(0.2)O(3): a neutron powder diffraction study from 10 to 770 K

Neutron powder diffraction data, collected over the temperature range 10-770 K, have been analysed in order to make a detailed characterization of the sequence of phase transitions occurring in the Hf-rich ferroelectric PbHf(0.8)Ti(0.2)O3, titanium hafnium lead oxide. Over the whole temperature range this compound undergoes two phase transitions, which involve cationic displacements and octahedral deformations (tilt and/or distortion) leading to strongly distorted perovskite-type structures. The first transition appears around 415 K between two ferroelectric rhombohedral phases: a low-temperature nonzero-tilt phase F(RL) (space group R3c) and an intermediate zero-tilt phase FRH (space group R3m). The second one, detected around 520 K, is associated with a ferroelectric to-paraelectric transition between the FRH phase and the Pc cubic phase (space group Pm3m). From high-resolution neutron powder diffraction data (diffractometer 3T2-LLB, Saclay, France, lambda = 1.2251 A), the crystallographic structure of the three successive phases has been accurately determined at the following temperatures: T = 10 K (FRL): space group R3c, Z = 6, a(hex) = 5.7827 (1), c(hex) = 14.2702 (4) A, V(hex) = 413.26 (2) A3; T = 150 K (F(RL)): space group R3c, Z = 6, a(hex) = 5.7871 (1), C(hex) = 14.2735 (4) A, V(hex) = 413.98 (3) A3; T = 290 K (FRL): space group R3c, Z = 6, a(hex) = 5.7943 (1), C(hex) = 14.2742 (5) A, V(hex) = 415.04 (3) A3; T = 440 K (F(RH)): space group R3c, Z = 6, a(hex) = 5.8025 (1), c(hex) = 14.2648 (4) A, V(hex) = 415.94 (3) A3; T = 520 K (Pc): space group Pm3m, Z = 1, a(cub) = 4.1072 (2) A, V(cub) = 69.29 (1) A3. In addition, a neutron powder thermodiffractometry experiment, performed between 290 and 770 K (diffractometer D1B-ILL, Grenoble, France, lambda = 2.533 A), has been used to study in situ the temperature-induced phase transitions. From sequential Rietveld refinements, the temperature dependence of the cation displacements and the rotation and/or distortion of oxygen octahedra was derived.

Toward a Theory of Orbiton Dispersion in LaMnO3

At 750K, LaMnO_3 has a cooperative Jahn-Teller (JT) distortion, with Mn atoms in distorted oxygen octahedra. This lifts the degeneracy of the singly-occupied $e_g$ orbitals of the Mn$^{3+}$ ions, which then become orbitally ordered. We use a minimal model to describe the ordered phase at T=0. The on-site Coulomb repulsion $U$ is set to infinity. There are two electronic orbitals and three oxygen vibrational coordinates per unit cell. In addition to spin excitations and phonons, the model has electronic excitations consisting of mis-orienting orbitals on Mn ions. Neglecting coupling to the oxygen displacements, the gap to such excitations is $2\Delta=16g^2/K$ where $g$ is the electron-phonon coupling and $K$ is the oxygen spring constant. When static oxygen displacements are coupled, this excitation becomes a self-trapped exciton with energy $\Delta$, half the JT gap. Adding dynamic oxygen displacements in one-phonon approximation introduces dispersion to both the (previously Einstein-like) phonons and the orbital excitons (``orbitons''). One of the phonon branches has zero frequency at $\vec{k}=(0,0,\pi)$. This is the Goldstone mode of the JT broken symmetry.

Ab initio structures of (M2) and (M3) VO 2 high pressure phases

(M2) and (M3) VO 2 high pressure phases have been obtained at 65kbars by Chamberland. Both phases crystallize in the monoclinic system, (M2) with the space group C2/m and unit cell dimensions a = 9.083A, b = 5.763A, c = 4.532A, β = 90.3 ° whereas (M3) belongs to P2/m with a = 4.506A, b = 2.899A, c = 4.617A and β = 91.79 °. Based on this data ab initio structures have been elaborated and adjusted to fit their experimental powder patterns. (M2) structure exhibits two crystallographically different infinite parallel [VO 4] n 4n− strings of VO 6 edge shared octahedra interconnected by apices alike in the rutile structure but the oxygens are hexagonally close packed. The (M3) variety shows also two different [VO 4] n 4n− strings but the general network now is rutile like slightly distorted. Vanadium atoms are situated in distorted oxygen octahedra, the V O bond lengths ranging from 1.66A to 2.15A with the V 4+-V 4+ pairing, V1-V1 = 2.70A and V2-V2 = 2.89A in (M2). In (M3) the V O distances range from 1.75A to 2.10A and V1-V1 = V2-V2 = 2.90A. The homopolar V 4+ pairs evidenced in the (M2) form and the general unsymmetrical arrangement of oxygen about V 4+ are in excellent agreement with the unusual physical properties of these two high pressure varieties.

Structural Analysis of Iron and Manganese Species in Iron- and Manganese-Promoted Sulfated Zirconia

The structural analyses of Fe, Mn−SO42-/ZrO2 (FMSZ), Fe−SO42-/ZrO2 (FSZ), Mn−SO42-/ZrO2 (MSZ), and Fe−Mn/ZrO2 (FMZ) were carried out by means of XAFS, XRD, and Raman spectroscopy. Local structure around Fe and Mn of each catalyst, which were at a working state for n-butane isomerization and in vacuo, were studied by in situ XAFS. The valence of the Fe atom of all the catalysts was invariantly trivalent. Coordination environment around Fe of all the catalysts was quite similar to each other. Fe atoms are present inside the bulk phase of ZrO2, the system of which forms interstitial-type solid solution and located at the center of distorted oxygen octahedra. Reaction gas does not directly make contact with Fe atoms because the introduction of reaction gas did not cause any change of coordination environment and the valence. The Mn species on FMSZ and MSZ were present as MnSO4, and their coordination environment was reversibly affected by exposure to n-butane. Without sulfate ions, Mn species are present on t...

On variety of the Ca coordination in the incommensurate structure of synthetic iron-bearing akermanite, Ca2(Mg0.55,Fe0.45)Si2O7.

The incommensurate modulation in a synthetic iron-bearing akermanite, Ca2(Mg, Fe )Si207, has been determined at room tempe~ature by the five-dimensional refinements of the structure. The basic structure is tetragonal P4 21m with unit-cell dimensions a =7.8679(3), c ~5.0144(2)A, V=31OAl(2)A3, 2=2, M=286.69, Dx =3.067Mg . m-3, MoKa with graphite monochromator, J1 =3.199mm-l, F(000)=284A9, Mg/(Mg+Fe)=0.554(6), R=0.065 for 770 unique reflections. The modulated structure is also tetragonal P~~~km, kt = 0.295(2)x (a*+b*), k2=0.295(2)x(-a*+b*), where kt. k2 are the wave vectors and a*, b* the reciprocal lattice vectors of the basic structure. R=0.136 for 3965 unique reflections. The modulation is caused mainly by the shifts of Ca and 0 atoms, and substitutional modulation at (Mg, Fe) sites was not detected in the present studies. Six-coordinated Ca forming a distorted oxygen octahedra exists in the modulated structure and most of them form clusters together with the flattened (Mg,Fe )04 tetrahedra. The number of the clusters in the present material is less than that in Co-akermanite. The reason may be attributed to the disorder of the modulation amplitudes and wavelength.

The substitution effect of Ca 2+ ion on the physical properties in nonstoichiometric Dy 1− xCa xCoO 3− y system

The Dy 1− x Ca x CoO 3− y (0.00⩽ x ⩽1.00) compounds have been prepared by a drip pyrolysis technique. The electronic structure and the geometric environment of the solid solutions are determined with Co K-edge X-ray absorption near edge structure (XANES). In the absorption edge region, it has been observed that three peak groups correspond to the 1s → 4p main transition allowed by the electric dipole matrix, the shakedown transition of ligand to metal charge transfer process, and 1s → 3d quadrupole allowed transition, respectively. In our comprehensive crystallographic investigation of the system, it is found that the DyCoO 3 compound has a typical GdFeO 3 structure with the space group of Pbnm. The compounds of x = 0.75 and 1.00 are formed with more than twice the size of an ideal perovskite unit cell and those of x = 0.25 and 0.50 include a microscopic binary phase of CaCoO 2.50. The electrical conductivities depend on the Ca 2+ ion content which affects the distortion of oxygen octahedra along the c-axis and then the transition of π ∗ to σ ∗ energy level. As a result of magnetic measurement, it is found out that low spin states of Co 3+ ions are converted to high spin totally or partially around 550 K in the compounds of x = 0.00−0.50. The constant ratios of low-high spin states at x = 0.75 and 1.00 are preserved in the given temperature region.

Crystal structure and magnetic properties of CuMoO 4 at low temperature (γ-phase)

The crystal structure and magnetic properties of a new modification of copper molybdate (γ-CuMoO 4) are reported, which is stable below 200 K. Single-crystal X-ray diffraction revealed triclinic symmetry ( P 1 ̄ ) with a = 9.699(9) A ̊ , b = 6.299(6) A ̊ , c = 7.966(7) A ̊ , α = 94.62(4)°, β = 103.36(4)°, γ = 103.17(4)° at 180 K and Z = 6. All Cu- and Mo-ions are surrounded by distorted oxygen octahedra. In contrast to the high-pressure modification CuMoO 4-II, which is of a triclinic distorted wolframite structure, the octahedra are not closed packed. This is also in contrast to the triclinic ( P 1 ̄ ) room-temperature phase (α) at standard pressure with tetrahedrally coordinated Mo-ions and with a = 9.901(3) A ̊ b = 6.786(2) A ̊ , c = 8.369(3) A ̊ , α = 101.13(1)°, β = 96.88(1)° and γ = 107.01(1)°. The new crystal structure of γ-CuMoO 4 is extensively discussed and compared with those of AlNbO 4 and α-CoMoO 4. The transition between α- and γ-phase is of first order and a wide temperature range of coexistence is observed in powder experiments. The relative amount of both phases depends on both temperature and cooling rate. No long range magnetic ordering was observed down to 1.8 K by SQUID measurements. The magnetic properties of γ-CuMoO 4 can be explained by a chain-like arrangement of copper-moments deduced from the crystal structure. If only interactions within such chains are considered, the calculations based on an Ising-model are in good agreement with experimental data.