Luc Brohan

X-ray photoelectron spectroscopy and high resolution electron microscopy studies of Aurivillius compounds: Bi4−xLaxTi3O12(x=0, 0.5, 0.75, 1.0, 1.5, and 2.0)

In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, when in contact with the metal electrode, Pt, in ferroelectric nonvolatile memory. Since the fatigue phenomenon on a ferroelectric capacitor arises predominately from the pinning of domain walls on the metal-ferroelectric interface, the surface configurations of La-containing Bi4Ti3O12 compounds should be considered as one of the fatigue-free factors as well as the self-regulation of the Bi2O2 layer and the chemical stability of the perovskite slabs. In addition, the band gap of Bi3.25La0.75Ti3O12 is also estimated by UV absorption spectrum to be 3.9±0.1 eV.In order to determine the fatigue-free origin of the ferroelectric Bi3.25La0.75Ti3O12, a series of x-ray photoelectron spectroscopy and high resolution electron microscopy studies were performed on the polycrystalline Bi4−xLaxTi3O12 (BLTx, x=0, 0.5, 0.75, 1.0, 1.5, and 2.0) powders. From the XPS study, the surfaces of all La-containing compounds are found to consist of one outermost Bi-rich region and followed by a La-rich region, instead of the chemical stoichiometry in the bulk. An HREM study on Bi3.25La0.75Ti3O12 further confirms that this surface configuration arises from some intergrowth defects with a thickness of 5 nm appearing on the crystal edge, which is also observed in compounds with x=1.0 and 2.0, but not in the poor fatigue-resistant Bi4Ti3O12 (x=0). This La-induced defect locally changes the chemical composition of the crystal surface, which probably possesses a different physical characteristic as compared to the bulk. Consequently, it could can be the physical nature of the interface, whe...

TiO2(B) nanoribbons as negative electrode material for lithium ion batteries with high rate performance.

Nanosized TiO(2)(B) has been investigated as a possible candidate to replace Li(4)Ti(5)O(12) or graphite as the negative electrode for a Li-ion battery. Nanoribbon precursors, classically synthesized in autogenous conditions at temperatures higher than 170 °C in alkaline medium, have been obtained, under reflux (T ∼ 120 °C, P = 1 bar). After ionic exchange, these nanoribbons exhibit a surface area of 140 m(2) g(-1), larger than those obtained under autogenous conditions or by solid state chemistry. These nanoparticles transform after annealing to isomorphic titanium dioxide. They mainly crystallize as the TiO(2)(B) variety with only 5% of anatase. This quantification of the anatase/TiO(2)(B) ratio was deduced from Raman spectroscopy measurement. TEM analysis highlights the excellent crystallinity of the nanosized TiO(2)(B), crystallizing as 6 nm thin nanoribbons. These characteristics are essential in lithium batteries for a fast lithium ion solid state diffusion into the active material. In lithium batteries, the TiO(2)(B) nanoribbons exhibit a good capacity and an excellent rate capability (reversible capacity of 200 mA h g(-1) at C/3 rate (111 mA g(-1)), 100 mA h g(-1) at 15C rate (5030 mA g(-1)) for a 50% carbon black loaded electrode). The electrode formulation study highlights the importance of the electronic and ionic connection around the active particles. The cycleability of the nano-TiO(2)(B) is another interesting point with a capacity loss of 5% only, over 500 cycles at 3C.

New hollandite oxides: TiO2(H) and K0.06TiO2

The topotactic oxidation of the bronze KxTiO2 (x close to 0.25), with a hollandite structure, leads to lower x values. By complete oxidation of the parent bronze a new form of titanium dioxide TiO2(H) is obtained. Its structure is the iono-covalent framework of hollandite. By heating above 410°C, TiO2(H) transforms into anatase. Starting from a single crystal of the precursor bronze it is possible to obtain a partly disintercalated single-crystal product. In this way, K0.06TiO2 has been prepared and its structure determined.

Updated references for the structural, electronic, and vibrational properties of TiO2(B) bulk using first-principles density functional theory calculations.

To get updated references on the structural, electronic, and vibration properties of the metastable TiO(2)(B) compound, infrared and Raman spectra of TiO(2)(B) are computed within the density functional theory framework and all active modes are assigned. Phonons and their possible coupling with the macroscopic electric fields resulting from the long-range interactions of instantaneous local dipoles (due to nuclear vibrations) in polar solids are taken into account through supercell calculations and longitudinal optical-transversal optical splitting corrections. Full structural relaxations using conventional density functional theory and hybrid functionals with localized Gaussian-type orbitals or plane-wave basis sets reveal a similar deviation of the local Ti environment compared to the TiO(2)(B) structural refinements reported so far. Such deviations are shown to be significant from those computed for anatase using the same method, thus yielding distinguishable spectroscopic responses for the two polymorphs.

Characterization of perovskite systems derived from Ba2In2O5□: Part I: the oxygen-deficient Ba2In2(1-x)Ti2xO5+x□1-x (0≤x≤1) compounds

Ba2In2(1−x)Ti2xO5+x□1−x (0≤x≤1) compounds have been prepared by solid state reaction. At room temperature (RT), when x increases, the progressive filling of oxygen vacancies, concomitant with the substitution of Ti for In, first induces (for 0<x≤0.075) a disorder in the plane of oxygen vacancies observed in Ba2In2O5□. Then, it suppresses the distortion to orthorhombic symmetry; for 0.075<x≤ 0.15, the symmetry becomes tetragonal and a formation of domains is observed. For 0.15<x<1, all members adopt a disordered cubic perovskite (DCP) structure at RT. Conductivity measurements between 450 and 800 °C show that the change from brownmillerite to tetragonal structure when x increases from 0.075 to 0.1 induces a drastic decrease of the activation energy. The highest oxide-ion conductivity is observed for 0.1≤x≤0.33: ∼0.5×10−2 S cm−1 at 700 °C.

Characterization of perovskite systems derived from Ba2In2O5□: Part II: The proton compounds Ba2In2(1−x)Ti2xO4+2x(OH)y [0≤x≤1; y≤2(1−x)]

Abstract The proton compounds Ba 2 In 2(1− x ) Ti 2 x O 4+2 x (OH) y [0≤ x ≤1; y ≤2(1− x )] were prepared by reacting Ba 2 In 2(1− x ) Ti 2 x O 5+ x □ 1− x (0≤ x ≤1) phases with water vapor at ∼200 °C. For 0≤ x ≤0.20, the filling of oxygen vacancies is almost complete. For larger x values, it decreases significantly down to ∼30% only for x =0.7. The crystal structure of the end member Ba 2 In 2 O 4 (OH) 2 ( x =0) was reinvestigated by a combination of techniques including 1 H and 2 D NMR and electron, X-ray and neutron diffraction. The actual cell is eight times larger than that previously published. The structure analysis confirms that the sheet consisting of parallel chains of In(2)O 4 tetrahedra and parallel rows of oxygen vacancies in the parent structure of Ba 2 In 2 O 5 □ has been converted into a In(2)O 6 octahedral perovskite-like sheet. It demonstrates that the protons are bonded only to the O atoms around this In(2) site. When x increases, the change in reduced perovskite cell volume, concomitant with the water uptake, remains small for x x values. The proton conductivity was measured between room temperature and 180 °C. The highest conductivity at 180 °C, σ 180 ≈10 −6 S cm −1 , is observed for x ∼0.3.

Order-disorder transition in Na0.25TiO2 bronze: Thermodynamic and crystallographic studies

A stoichiometric Wadsley bronze Na0.25TiO2 has been prepared for the first time. The perovskite-like sites of the TiO2 framework are fully occupied by sodium ions. The space group, at room temperature, is P2a or Pa and the lattice parameters are a = 12.177(3)A, b = 3.875 × 4 = 15.50(2)A, c = 6.480(3)A, β = 107.08(2)°. As compared with those of the well-known Wadsley bronze of C2m symmetry, X-ray and electron diffraction patterns exhibit extra reflections at about b∗4. At T3Dc = 430 K, a second-order transition appears. The smooth variation of the η-order parameter and the anomalous specific heat have been studied. The critical exponents α ⋍ α′ = 0.110 ± 0.003 and β = 0.324 are in good agreement with those calculated from renormalization group method for d = 3 and n = 1 (d = dimensionality and n = number of order parameter components).

Transport properties and magnetic behavior in the polycrystalline lanthanum-deficient manganate perovskites (≈La1−xMnO3)

Abstract The rhombohedral lanthanum manganates ≈La1 − xMnO3 (x = 0.0, 0.05, 0.1) were prepared by a soft chemistry route to test first as catalysts. These three weak ferromagnets exhibit a structural transition at Tt slightly below Tc. ≈La0.9MnO3 undergoes a group-subgroup R3c ↔ R3c transition most likely of second order. ≈LaMnO3 does not show any insulator-to-metal transition, but does show a semiconductor-semiconductor at Tt ~ 130 K. Analysis of the transport data (conductivity and thermopower) indicates hopping of polarons at both low and high temperature. An effective magnetic moment of 3.50 μB/Mn is deduced from the spontaneous magnetization value at 5 K. The slight difference from the spin-only value (3.67 μB) is assigned to a spincanting effect which is reflected in the divergence of the FC and ZFC susceptibility curves at low fields (H ≤ 10−2 T) and low temperature. Compared with recent results on thin films, the compositions x = 0.05, 0.1 are expected to show magnetoresistive effects as well as a semiconductor-to-metal transition. If it is verified on bulk samples which all contain a similar Mn4+ content of 35%, the structural transition at Tt could be considered as magnetically driven.

Intermediate band in the gap of photosensitive hybrid gel based on titanium oxide: role of coordinated ligands during photoreduction

UV light induced chemical modifications of a new hybrid organic–inorganic gel, based on titanium oxide and obtained by a soft chemistry method, are studied by XPS and UV-visible spectroscopy. XPS experiments, with an in situ UV illumination design, were performed in order to study the different chemical modifications of the gel. The Ti4+ to Ti3+ reduction, as well as the associated change in the ligands coordinating titanium cations, are accompanied by a deprotonation of the molecules in the vicinity of the inorganic part. In the meantime, an intermediate band arises in the gap just below the Fermi level. The appearance of this band correlates with the light absorption properties of the gel as measured by UV-visible spectroscopy. Following on from these experiments, a complete band diagram of the electronic structure is proposed in order to clarify the position of this intermediate band. These distinctive properties suggest that the nanostructured hybrid gel could be used as a new class of intermediate band material that can be synthesized at low cost for photovoltaic applications.

Evidence for a monoclinic distortion in the ferroelectric Aurivillius phase Bi3LaTi3O12

Abstract An alternative chemical approach was used to prepare the ferroelectric Aurivillius phase Bi 3 LaTi 3 O 12 (BLT1.00) at low temperature (450°C) leading to well-crystallized compounds at 750°C. The existence of 90°-oriented twin domains, characteristic of a ferroelectric state, was revealed in selected area electron diffraction and microdiffraction studies. Though the 3D reconstruction of the reciprocal lattice enables to propose the non-centrosymmetrical orthorhombic space group (SG) B 2 cm , a further convergent-beam electron diffraction study was undertaken in order to ascertain the symmetry. The monoclinic symmetry was clearly identified. BLT1.00 crystallizes in the polar SG B 1 c 1 (no. 7, non-standard setting of P 1 c 1) instead of I 4/ mmm , as previously reported.