Doretta Capsoni

Ab initio structure determination of Li2MnO3 from X-ray powder diffraction data

The results of the structural determination of Li 2 MnO 3 from X-ray powder diffraction data and the refinement by the Rietveld technique are presented. The Li 2 MnO 3 structure has a monoclinic cell with space group C2/m (Z = 4) and cell parameters a = 4.9246(1), b= 8.5216 (1), c = 5.0245 (1) A, β=109.398 (1)°; the refinement with 14 structural parameters for 165 reflections in the pattern leads to R wp = 17.61%, R p = 13.25%, R B = 7.07% and S = 3.52. Such a solution agrees with a single-crystal structure determination previously reported in the literature and allows other hypotheses to be rejected.

On the thermal stability and defect structure of the solid solution LixNi1-xO

Abstract Li x Ni 1- x O solid solutions are both of scientific and technological interest as they have been proposed as cathodic material in MCFC. In this work TG, XRD and SEM measurements have been used to study the thermal stability of several solid solutions (of different initial composition) obtained starting from the reactive system Ni/Li 2 CO 3 . Composition changes, due to both lithium and oxygen loss, take place as a consequence of samples annealing. In some cases a sensible disagreement between the compositions deduced by TG and XRD data is showing, which is ascribed to the presence, in the final solid solutions, of lattice defects others than substitutional ones (namely cation vacancies). The TG and XRD measurements have been then worked out to obtain both the correct composition and vacancies concentration of the solid solutions. It is shown that solid solutions with similar lithium content but different vacancies concentration can be obtained depending on the preparation conditions.

Jahn-Teller transition in Al3+ doped LiMn2O4 spinel

Abstract Al-doped lithium manganese spinels, with starting composition Li 1.02 Al x Mn 1.98− x O 4 (0.00 x ≤0.06), are investigated to determine the influence of the Al 3+ doping on the Jahn–Teller (J–T) cooperative transition temperature T J–T . X-ray powder diffraction (XRPD), nuclear magnetic resonance, electron paramagnetic resonance, conductivity and magnetic susceptibility data are put into relation with the tetrahedral and octahedral occupancy fraction of the spinel sites and with the homogeneous distribution of the Al 3+ ions in the spinel phase. It is observed that Al 3+ may distribute between the two cationic sublattices. The J–T distortion, associated with a drop of conductivity near room temperature in the undoped sample, is shifted towards lower temperature by very low substitution. However, for x >0.04 T J–T it increases with increasing x , as clearly evidenced in low temperature XRPD observations. A charge distribution model in the cationic sublattice, for Al substitution, is proposed to explain this peculiar behavior.

Thermal stability and structural transition of metastable Mn5O8: in situ micro-Raman study

The Raman spectrum of the metastable Mn5O8 phase, obtained from slow oxidation of Mn3O4 at low temperature, is presented and analysed for the first time and the thermal stability is monitored by the changes in Raman spectra due to laser-induced thermal treatments. A structural transformation toward the spinel phase Mn 3O4 is observed at temperature higher than 1000 K. Other Mn oxides, characterised by intermediate Mn oxidation states, are not detected below or during the transition. A compositional model of the sample grains is also proposed by comparing Raman data with X-ray diffraction and scanning electron microscopy measurements. q 1999 Elsevier Science Ltd. All rights reserved.

Inhibition of Jahn–Teller cooperative distortion in LiMn2O4 spinel by transition metal ion doping

The aim of this study is to determine the minimum amount of dopant that prevents the occurrence, near room temperature, of a Jahn–Teller (J–T) transition in the M-doped lithium manganese spinel of composition Li1.02MxMn1.98−xO4 with 0.00<x⩽0.06 and M = Ni2+, Co3+, Cr3+ or Ti4+. EPR spectra and magnetic susceptibility data are related to the valence state of M and Mn, and the homogeneous distribution of the dopant. We find that the spinel framework is remarkably sensitive to displaying low electronic and magnetic changes in its cationic sublattice due to cation substitution. The J–T distortion, which is associated with a sudden drop in conductivity with decreasing temperature, is suppressed by substituting 3% of Mn with Co3+ or Cr3+, or by adding an even smaller amount of Ni2+ (x = 0.02, or 1% substitution). However, this inhibition occurs only in samples with a ratio r = [Mn4+]/[Mn3+]1.18, i.e., a value larger than the ratio r = 1.106 we have with no doping (x = 0). As a consequence, doping with the tetravalent cation Ti4+, which always decreases the r value, does not suppress the J–T transition. We suggest that both the dopant ion and the Li+ in excess over the stoichiometric composition are located in 16d sites. The removal of the J–T transition in the Co3+ (x = 0.06) sample is also due to local disorder.

Solid-state characterization of paracetamol metastable polymorphs formed in binary mixtures with hydroxypropylmethylcellulose

Two metastable polymorphs of paracetamol (forms II and III) were prepared by appropriate thermal methods from binary mixtures containing 10% (w/w) of hydroxypropylmethylcellulose. By controlling the reheating step, it was possible to address the recrystallization of the drug either into form II or III. Moreover, it was observed that form III transforms either into form II or I depending on the preparation method. The physical characterization of the polymorphs was performed by means of micro-Fourier transform infrared spectroscopy (MFTIR) and powder X-ray diffractometry (PXRD), both temperature controlled.

LiMn2O4 low-temperature phase: synchrotron and neutron diffraction study

The structural evolution of LiMn2O4 spinel was followed from 320 K down to 10 K. The structural transformation, recently studied down to 230 K [Rodriguez-Carvajal, Rousse, Masquelier & Hervieu (1998). Phys. Rev. Lett. 81, 4660–4663], takes place near room temperature with a significant hysteresis: the high-temperature cubic phase transforms to a superstructure orthorhombic cell. The present study indicates that the nuclear structure is stable down to 10 K, while neutron diffraction patterns below 80 K show the rise of a magnetic ordering in the spinel phase. From Mn—O bond-length analysis of the MnO6 octahedra, a temperature-independent charge ordering in the structure can be deduced.

On the role of lithium carbonate in the preparation of doped nickel oxide cathodes for molten carbonate fuel cells

Abstract Clear evidence was obtained by dilatation and density measurements that the amount of lithium carbonate in the starting mixture sensibly affects the sintering behaviour of mixed lithium-nickel oxide cathodes. Porosimetric, diffractometric and microscopic measurements were then performed to understand and put on a quantitative basis such an influence. It is shown that, when the sintering temperature is low enough to prevent lithium oxide loss from the mixed oxide, cathodes total porosity increases with increasing lithium content up to a limiting composition beyond which a porosity decrease takes place. Lithium oxide loss, on the other hand, leads to a total porosity decrease which is related both to the initial lithium content and to the amount of the lithium oxide lost. Such a behaviour can be explained on the basis of the different mechanism by which the mixed oxide is obtained depending on the relative amounts of nickel and lithium carbonate in the starting mixture. Moreover it is shown that the observed total porosity changes are mainly a consequence of microporosity changes.

Electron paramagnetic resonance investigation of polycrystalline CaCu3Ti4O12

Electron paramagnetic resonance (EPR) measurements on pure polycrystalline CaCu3Ti4O12 have been performed and are discussed within a crystal-field approach. A symmetric signal centred at g = 2.15 is observed for T>25 K, with no evidence of hyperfine structure. At this temperature an antiferromagnetic transition is observed as confirmed by static magnetization data. Cu defective and 2% doped (V, Cr, Mn, La) samples were also prepared and considered, mainly to understand the nature of the observed paramagnetic centre. Substitutions in the octahedral sites, causing variations of the configuration in CuO4–TiO6–CuO4 complexes, change the magnetic and EPR features. To justify the EPR response a strong copper-hole delocalization is suggested.

Pair distribution function analysis and Mössbauer study of defects in microwave-hydrothermal LiFePO 4

Olivine-type LiFePO4 is nowadays one of the most important cathode materials of choice for high-energy lithium ion batteries. Its intrinsic defectivity, and chiefly the so-called lithium ironanti-site, is one of the most critical issues when envisaging electrochemical applications. This paper reports a combined diffractometric (Synchrotron Radiation XRD with Rietveld and PDF analyses) and spectroscopic (Mossbauer) approach able to give a thorough characterization of the material defectivity. Such analytical procedure has been applied to a sample prepared following an innovative microwave-assisted hydrothermal synthesis route that, in a few minutes, allowed us to obtain a well crystallized material. PDF analysis, which is applied for the first time to this type of battery material, reveals the presence of disorder possibly due to Li/Fe exchange or to a local symmetry lowering. A 5% amount of iron on the lithium site has been detected both by PDF as well as by Mossbauer spectroscopy, which revealed a small percentage of Fe3+ on the regular sites.