Thermal behavior, magnetic properties, ESR, XPS, Mössbauer and NEXAFS study of Fe-doped CaCu3Ti4O12 ceramics

Abstract

The methods of magnetic dilution, thermal analysis, ESR (Electron Spin Resonance), Mössbauer and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectroscopy were used to study iron-doped solid solutions of calcium copper titanate CaCu3Ti4-4xFe4xO12-δ (0.005 ≤ x ≤ 0.08, 0.2) and CaCu3-3xFe3xTi4O12+δ (0.005 ≤ x ≤ 0.06) synthesized by the standard ceramic method by replacing copper or titanium ions with iron. According to X-ray diffraction data, the samples of the both series contained no iron-based impurity phases. A characteristic feature of the Mössbauer spectra of the samples of both series was the doublet part in the form of an asymmetric triplet with an intensity ratio of 1:3:1. The analysis of the Mössbauer spectra of the samples of the both series showed the presence of three doublets in them with different quadrupole splitting and isomeric shift values of 0.3–0.4 mm/s, which is typical for Fe3+ ions in an octahedral coordination with varying degrees of distortion. According to the Mössbauer data and ESR spectroscopy, in the both series of solid solutions, the substitution (Fe→Ti or Fe→Cu) occurred mainly in the octahedral positions of titanium by means of heterovalent substitution Fe3+→Ti4+. In the ESR spectra, the increase in the iron content in the samples of the both series led to broadening of the Lorentz absorption line of Cu(II) ions with g ≈ 2.15 and to decrease in the g-factor. NEXAFS and XPS revealed that the doping of CaCu3Ti4O12 (CCTO) with iron ions had no significant effect on the electronic state and polyhedral environment of Ca, Cu and Ti ions in CCTO, regardless of the degree and nature of the doping. The charge state of iron ions in the solid solutions corresponded to Fe(III), iron ions predominantly occupy the positions of titanium. According to the magnetic dilution data, the antiferromagnetic exchange appeared between the nearest iron (III) ions in the solid solutions of calcium copper titanate. The clusters of iron (III) ions interacted with Cu(II) ions through the formation of ferromagnetic exchange. There were no significant differences in the magnetic behavior of the iron-doped solid solutions of both series. According to the thermogravimetric data, there was no mass loss of the samples below the melting temperature (∼1033 °C). The DSC (Differential Scanning Calorimetry) curves of the both series of the solid solutions revealed the exothermic effect at 233–300 °C and a number of endothermic processes at temperatures above 946 °C, near the melting temperature of the samples.