C. B. Azzoni

Raman spectroscopy of AMn2O4(A = Mn, Mg and Zn) spinels

First order Raman spectra in the region 200–800 cm−1 have been collected for AMn2O4 (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO4 and MnO6 unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg1−xMnxMn2O4 solid solution; the effect of the spinel inversion has been also studied on MgMn2O4 quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Role of oxygen content on the transport and magnetic properties of La1−xCaxMnO3+δ manganites

Abstract In this paper we report about the synthesis and X-ray powder diffraction characterization of La 1− x Ca x MnO 3+ δ samples with x =0.1, 0.3 and −0.025≤ δ ≤0.054. Transport and magnetization measurements on these samples were performed as a function of the oxygen content both in over and under-stoichiometric regimes. We point out the role of the cation and anion vacancies and the obtained results suggest taking in relevant account the role of oxygen content when dealing with this kind of materials.

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.

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.

Frustration-driven structural distortion in VOMoO 4

Nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetization measurements, and electronic structure calculations in

On the low-temperature magnetic properties of CuO single crystals

{\mathrm{VOMoO}}_{4}

Magnetic and X-ray diffraction investigation on Mg1−xMn2+xO4 spinels

are presented. It is found that

EPR study of Gd3+ doped lead oxide based glasses

{\mathrm{VOMoO}}_{4}