Paramagnetic centers in nanocrystalline TiC/C system

Electron paramagnetic resonance is applied to study the defect centers in nanocrystalline titanium carbide dispersed in carbon matrix (TiCx/C) synthesized by the non-hydrolytic sol-gel process. The presence of Ti3+ paramagnetic centers is identified below 120 K along with a minor contribution from localized defect spins coupled with the conduction electron system in the carbon matrix. The temperature dependence of the resonance intensity of the latter signal indicates weak antiferromagnetic interactions. The presence of paramagnetic centers connected with trivalent titanium is suggested to be the result of chemical disorder, which can be further related to the observed anomalous behavior of conductivity, hardness, and corrosion resistance of nanocrystalline TiCx/C. © 2007 Elsevier B.V. All rights reserved

Ageing effect in nanocrystalline TiCx/C studied by EPR

TiC/C nanocrystalline material: titanium carbide TiC dispersed in a carbon matrix has been prepared by a nonhydrolytic sol-gel process. Temperature dependence of the electron paramagnetic resonance (EPR) spectra of this material has been studied in the 3.5-120 K range. Two very different EPR lines have been recorded in fresh sample at temperatures below 120 K arising from the Ti(III) complex (broad and asymmetric line) and conduction electrons (very narrow line). In the same aged sample (1 year old) the magnetic anisotropy of Ti(III) line has increased while a narrow line attributed to conduction electrons has vanished. The existence of the paramagnetic centers connected with trivalent titanium ions could the result of disordering processes. The increase of anisotropy in Ti(III) line could be connected with the oxidation processes. The temperature dependence of the integrated intensity of the broad line revealed the presence of titanium antiferromagnetic dimers. The disappearance of a narrow EPR line suggests that the oxidation process (ageing effect) could influence also the electrical properties of titanium carbide. © 2008 Elsevier B.V. All rights reserved

Magnetic resonance and dielectric studies of a nonlinear La 3Ga5.5Ta0.5O14 single crystal doped with erbium

Single crystal of erbium doped La3Ga5.5Ta 0.5O14 grown by the Czochralski method have been investigated by electron paramagnetic resonance and dielectric spectroscopy methods. Dielectric permittivity ε measurements performed in 90-440 K temperature range have shown negligible dispersion for 1 kHz - 1 MHz frequencies and a Curie-Weiss type behaviour with C = 47700 K and θ = -340 K. Electron paramagnetic resonance studies have revealed the presence of two different paramagnetic, monoclinic centres. The calculated g factor values are: g1 = 1.449, g2 = 11.534, g3 = 4.24 for the main M1 centre and g1 = 1.982, g2 = 4.169, g 3 = 4.25 for the weaker M2 centre. The temperature dependence of EPR line intensity for centre M1 and M2 is quite different - while lines attributed to M1 could only be observed at low temperatures, below 20 K, lines of M2 centre persisted up to 200 K. The M1 centre is connected with Er3+ ion entering substitutionally into La3+ site, while M2 is probably connected with 3d ions at the same site, unintentionally introduced into the material as an admixture