Marcella Bini

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.

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.

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.

Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials

Transition metal-based lithium orthosilicates (Li2MSiO4, M = Fe, Ni, Co, Mn) are gaining a wide interest as cathode materials for lithium-ion batteries. These materials present a very complex polymorphism that could affect their physical properties. In this work, we synthesized the Li2FeSiO4 and Li2MnSiO4 compounds by a sol-gel method at different temperatures. The samples were investigated by XRPD, TEM, 7Li MAS NMR, and magnetization measurements, in order to characterize the relationships between crystal structure and magnetic properties. High-quality 7Li MAS NMR spectra were used to determine the silicate structure, which can otherwise be hard to study due to possible mixtures of different polymorphs. The magnetization study revealed that the Néel temperature does not depend on the polymorph structure for both iron and manganese lithium orthosilicates.

Preparation and characterization of carprofen co-crystals

Carprofen co-crystals with selected co-formers were prepared by solvent evaporation and wet/dry grinding methods. Their effective formation was investigated by thermal analysis, FT-IR, X-ray single crystal and powder diffraction and SEM-EDS. This last technique has been applied for the first time to co-crystals since it provides unambiguous confirmation of co-crystal formation. Among the investigated co-formers we studied, only 4,4′-dipyridyl yields co-crystals. Two different crystal structures are obtained when the molar ratio of carprofen : 4,4′-dipyridyl is 2 : 1 (triclinic cell) and 1 : 1.5 (monoclinic cell). The asymmetric triclinic cell (Z = 2) contains two carprofen and two half 4,4′-dipyridyl moieties while the monoclinic cell (Z = 4) contains a single carprofen, and one and a half 4,4′-dipyridyl moieties. Several hydrogen-bond supramolecular synthons can be identified in the solid state. For both the 2 : 1 and 1 : 1.5 co-crystals, the main hydrogen-bond interaction consists of an O–H⋯N heterosynthon involving, as a donor, the COOH group of carprofen and, as a H-acceptor, the nitrogen of a 4,4′-dipyridyl molecule. The two co-crystals have characteristic FT-IR spectra and slightly different melting temperatures. X-Ray powder diffraction patterns of the 1 : 1 and 1 : 2 compositions reveal a mixture of phases, whose amount is quantified with Rietveld analysis.