Meštrić, Hrvoje (2006)
Local structure and symmetry of paramagnetic ions in ferroelectric ceramics.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Polycrystalline iron-modified lead titanate (PbTiO3) and lead zirconate (PbZrO3) have been investigated by electron paramagnetic resonance (EPR) techniques. A multi-frequency approach has been employed to determine the fine structure interaction of the Fe3+ centre in both compounds. Experiments have been performed at 9 GHz, 94 GHz, 190 GHz, 240 GHz and 319 GHz. The EPR spectra of the Fe3+ ion in PbTiO3 exhibits axial symmetry. The fine structure parameter b02 of the spin Hamiltonian has been determined from highfrequency measurements. It amounts to b0 2 = 32.3(2) GHz. Fourth-order tetragonal and cubic splitting parameters were not necessary because the spectra could be explained sufficiently at all frequencies by invoking second-order parameters only. The Fe3+ centre in PbZrO3 has no symmetry (point group 1). The EPR spectra have been explained by a set of six different centres with low symmetry. Their spin Hamiltonian parameters have been averaged leading to the following mean values: b02 = 8.5(9) GHz and b22 = 2.8(12) GHz. In comparison to the iron centre in lead titanate, this reflects a considerably reduction of the fine structure interaction. Additionally, the Fe3+ spectra in PbZrO3 exhibit considerably broadened transitions. This observation has been explained by a multi-centre situation in PbZrO3. The local structure of the paramagnetic centres in both materials has been analysed via the Newman superposition model. The intrinsic parameters for the Fe3+ - O2+ pair have been taken from the measurements of Siegel and Müller. The NSM analysis on the basis of the size and sign of b02 shows that the Fe3+ ion in both compounds is substituted at the B-site (Ti4+/Zr4+) of the perovskite cell. Furthermore, only the structures assuming a directly coordinated oxygen vacancy agree with the experimental data. Hence, the incorporation of the Fe3+ ion in both PbTiO3 and PbZrO3 necessarily induces an oxygen vacancy in the surrounding oxygen octahedron. The proposed orientation of the Fe3+ - VO defect associate in PbTiO3 is along the crystallographic [001] axis. Any other orientation of the defect dipole would result in a fine structure tensor of lower than axial symmetry. In PbZrO3, the Fe3+ - VO defect associate can be oriented along any direction defined by the six oxygen atoms of the octahedron. This behaviour in PbZrO3 accounts for the six different paramagnetic centres observed by EPR. Ab initio DFT calculations for PbTiO3 furthermore confirm the proposed model. They show that the total energy for the arrangement along the crystallographic [001]-axis is below the energy predicted for the orientations along the [100] and [010]-axes. From two possible orientations along the [001]-axis, the structure with the vacancy at the nearest apical oxygen (O1) is energetically favoured. The NSM analysis offers also some information on the position of the Fe3+ ion relative to the oxygen octahedron. The b02 calculation in both compounds is compatible with two possible distinct positions for the Fe3+ ion. Their preference can be discriminated on the basis of: (i) a DFT calculation for PbTiO3, and (ii) a NSM calculation of b22 for PbZrO3. Both calculations show that Fe3+ in both compounds is shifted by roughly 10 pm away from the vacancy. The origin chosen refers to the original Ti4+/Zr4+ site. In every case the shift is towards the centre of the truncated oxygen octahedron. According to the NSM results, the Fe3+ ion doped in PbTiO3 is almost centred in the perovskite pseudocubic subunit, a behaviour which differs from the site preference of the substituted Ti4+ ion. This suggests that the unit cells with the Fe3+ ion cannot participate in the same way in microscopic polarisation effects as encountered for Ti4+. The present results confirm and extend the earlier investigations of Siegel and Müller on PbTiO3. In PbZrO3, the Fe3+ ion is moved also away from the oxygen vacancy. This result is valid for any of the six possible vacancy positions. The present analysis has been done for the first time. It can be compared with the PbTiO3 results. The same behaviour is found: Fe3+ is shifted towards the remaining oxygen pyramid. The identification of a multi-site iron centre with different orientations of the Fe3+ - VO defect dipole suggests that the reorientation of this dipole in lead zirconate occurs easier than in lead titanate. There the Fe3+ - VO dipole is oriented strictly along the crystallographic [001] axis

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