P. Strobel

Crystallographic and magnetic structure of Li2MnO3

Abstract The structure of Li2MnO3 was refined on a single crystal: space group C2 m , a = 493.7(1), b = 853.2(1), c = 503.0(2) pm , β = 109.46(3)°, Z =4, R = 0.020, Rw = 0.027 for 619 independent reflections with I > 5σ. Distortions in the ordered rocksalt superstructure are induced by the cis positions of Mn4− in the O2 coordination octahedra. Powders of Li2MnO3 are slightly disordered with 12 ± 3% Mn on the Li sites. They are antiferromagnetic with TN = 36.5 K; the cell is doubled along c, corresponding to opposite moments on alternating Mn-containing layers along c. A comparison of structural data of rocksalt-related A2BO3 compounds (A = Li, Na; B = tetravalent metal) shows that many previously reported cells could be indexed in smaller monoclinic or hexagonal cells.

A study of the Cu-O chains in the high Tc superconductor YBa2Cu3O7 by high resolution neutron powder diffraction

Abstract The structure of orthorhombic YBa2Cu3O7 was refined in the temperature interval 5–320K, to an accuracy higher by a factor of about two compared to previous refinements. No major structural change is observed at the onset of superconductivity but the lattice dimensions and some of the structural parameters show small anomalies near 90K and 240K. All vibrational amplitudes are normal except for those of the 04 atoms across the Cu-O chain, which are large. Good agreement with the data is obtained by assuming that the 04 atoms are located on potential minima away by at least 0.08A from the chain axis at all temperatures. Annealing near 240K to look for a possible order-disorder transition revealed no structural changes.

Neutron and electron diffraction study of YBa2Cu22Cu1.77Fe.23O7.13

Abstract The compound of formula YBa 2 Cu 2.7 Fe 0.3 O 7.13 has been analyzed by neutron and electron diffraction techniques. The material is tetragonal with lattice parameters a = b = 3.8674(1), c = 11.6687(2) A and space group P4/mmm. The Fe cations substitute only the Cu cations located on the basal plane of the structure and can adopt three different types of coordination (tetrahedral, pyramidal and octahedral) depending upon the content and distribution of the extra oxygen atoms on the plane. Calculations of the effective valence of iron cations seem to indicate that Fe 3+ is present in tetrahedral coordination and Fe 4+ in pyramidal and octahedral coordination, while values of Cu 2.2+ and Cu 2.47+ were found for the copper cations located at (00z) and (000), respectively. The electron diffraction experiments show diffuse scattering planes parallel to (110) ∗ and (1 1 0) ∗ . Crosses of strong intensity are visible at reciprocal nodes located between the reciprocal lattice layers. This diffuse scattering is interpreted in terms of linear clusters of iron cations axtending along the [110] and [1 1 0] directions having a width of a few cations. The clusters are separated by domains of orthorhombic YBa 2 Cu 3 O 6+x having the same orientation or rotated of 90° one with respect to the other.

High-temperature oxygen defect equilibrium in superconducting oxide YBa2Cu3O7-x

Abstract The high-temperature superconductor YBa 2 Cu 3 O 7-x has been characterized by thermogravimetry in the 600–1100 K range under oxygen pressures from 10 -3 to 1 atm. The compound reversibly exchanges oxygen in the 0 2 ) vs. 1/T curves are linear with activation energies between 1.5 and 1.65 eV. These values are consistent with a simple interaction model involving the oxygen sites in the basal ab plane of the orthorhombic or tetragonal structure.

Structure and conductivity of Cu and Ni-substituted Bi4V2O11 compounds

The partially Cu- or Ni-substituted compounds (Bi4V2(1−x)M2xO11−3x;M=Cu, Ni) are highly oxygen-conducting. Three phases (α, β, γ) are observed in the unsubstituted compound; α is the low-conducting room temperature phase and γ the high-conducting phase at high temperature. Structure and conductivity are studied as a function of the substitution on the vanadium sites. Between 0 and 6% at room temperature, the Cu compound remains in the orthorhombic α phase and its ionic conductivity increases. A strong anisotropic conductivity is observed. For 0.07≤x≤0.12, the average structure is tetragonal (γ-type) at room temperature. The conductivity is very high and does not vary very much over this substitution range. Impedance spectroscopy measurements have also been carried out on the x=0.07 Ni-substituted compound. Commensurate or incommensurate superstructures are observed for all of these compounds.

Structural and electrochemical study of lithium insertion into γ-Fe2O3

Abstract Lithium insertion into the defect spinel γ-Fe 2 O 3 was investigated both chemically and electrochemically. A combination of slow potential-controlled insertion and in situ X-ray diffraction shows three successive reactions: (i) filling the octahedral vacancies up to x (Li) ≈ 0.25, (ii) a spinel→rocksalt-type first-order transition at increasing x (Li) (equilibrium potential 1.60 V versus Li + /Li), (iii) filling the remaining octahedral vacancies in the rocksalt phase. Structure refinements at x (Li)=0.86±0.01 from both X-ray and neutron diffraction data showed a remaining occupation of tetrahedral sites equal to ≈0.10.

Cation ordering in Li2Mn3MO8 spinels: structural and vibration spectroscopy studies

Abstract Lithium-manganese oxide spinels with 1/4 manganese replaced by Mg, Ti, Co, Ni, Cu, Zn and Ga, yielding formula LiMn1.5M0.5O4 (or Li2Mn3MO8) have been prepared. Cationic ordering was known previously for M=Mg and Zn, resulting in a superstructure with primitive cubic symmetry. Given the poor chemical contrast of X-ray diffraction between Mn and Ti, Co, Ni, Cu or Ga, neutron diffraction studies were carried out. Evidence of cation ordering is found for M=Ni and Cu, but not for Ti, Co or Ga. These results are confirmed by FTIR and Raman spectroscopies. Doubly-substituted samples (Li0.5M0.5)[Mn1.5M0.5]O4 (overall formula LiMn3M2O8) were also prepared for M=Mg and Zn. These do not form the primitive superstructure, a result ascribed to the lower manganese valence with respect to LiMn1.5M0.5O4. Zn-containing spinels give rise to an extensive Li/Zn cation inversion, which also shows up as additional high-frequency bands in IR and Raman spectroscopies. This investigation shows that the cell volume is determined by the average octahedral-site cation radius, and that the main driving force for octahedral cation ordering is the charge difference between Mn and M atoms.

Phase diagram of the system Bi1.6Pb0.4Sr2CuO6-CaCuO2 between 825°C and 1100°C

Abstract The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C–1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+ x . It involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus (∼855°C), the sequence of stable { n } phases is {1}, {1}+{2}, {2}, {2}+{3}, and {3}. The {3} phase (110K) exists as the dominant phase between ∼835°C and ∼875°C for 3 n t >∼900°C) mainly contains the transformation and melting lines of CuO, (Ca, Sr) 2 CuO 3 , and (Ca, Sr) 14 Cu 24 O y . The liquids involves a eutectic point at (∼930°C, n =2) and a critical end point of miscibility of the two liquids L 1 (bismuth rich) and L 2 . The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.

Lithium intercalation in LiMgMnO and LiAlMnO spinels

LiAlMnO4 and LiMg0.5Mn1.5O4 have been investigated as replacements of LiMn2O4 for lithium intercalation below 3 V. The decomposition of acetate and carbonate precursors was studied by thermogravimetry. Solid state reactions yielded ‘LiAlMnO4’ with manganese valence < 4 and always containing impurity phases such as LiAl5O8 or LiMn2O4. The intercalation behaviour was studied potentiostatically in lithium cells using both liquid and solid electrolytes, and by X-ray diffraction on intercalated cathodes. The structural and electrochemical behaviour of ‘LiAlMnO4’ is very similar to that of LiMn2O4, and gives lower capacities on cycling. Lithium intercalation in LiMg0.5Mn1.5O4 includes three reduction steps between 2.8 and 1.6 V, corresponding to the intercalation of ca. 0.58, 0.22 and 0.25 Li atoms, respectively. The total initial capacity is 180 mAh/g, but also drops on cycling, mainly due to the collapse of the second step corresponding to the critical threshold around Mn+3.5, where the tetragonal distortion due to the Jahn—Teller effect takes place.

Thermal behaviour of Bi4V2O11 : X-ray diffraction and impedance spectroscopy studies

Abstract Bi 4 V 2 O 11 powdered samples and single crystals were studied by high temperature X-ray diffraction and impedance spectroscopy to characterize the phase transitions. From high temperature X-ray diffraction on powders and single crystals, the α ⇆ β and β ⇆ γ reversible phase transitions were observed. The β ⇆ γ one is ferroelastic ⇆ paraelastic but surprisingly the α ⇆ β transition also exhibits a ferroelastic character, with a 90 ° switching of the a and b axis on cooling and/or, more scarcely, on heating. Impedance spectroscopy measurements were carried out using platelet shaped single crystals with well developed (001) faces. The corresponding σ ∥ (001 plane) and σ ⊥ ( c direction) bulk conductivities were obtained and compared with values from ceramic pellets, σ ∥ values are close to those characterizing the pellets, and the anisotropy of the conductivity is evidenced by σ ∥ values about 2 orders of magnitude larger than σ ⊥ ones. Slope changes observed in Arrhenius plots are in agreement with the phase transitions.