Local position ofFe3+in ferroelectric BaTiO3

The EPR spectra of ${\mathrm{Fe}}^{3+}$ in the three ferroelectric phases of BaTi${\mathrm{O}}_3$ could be interpreted on the basis of the Newman superposition model. In the latter, the position of the nearest (oxygen) ligands determines the EPR spectrum. It is found that the ${\mathrm{Fe}}^{3+}$ participates by less than an order of magnitude in the collective motion of the ${\mathrm{Ti}}^{4+}$ ions out of their inversion symmetric position: by centering the ${\mathrm{Fe}}^{3+}$ and using previously determined intrinsic oxygen coordinates and parameters of the Newman model, the sign and magnitude of the EPR ${b_2}^0$ terms could be obtained for (a) the tegragonal phase, (b) the tetragonal $〈001〉$ spectrum perpendicular to the $〈110〉$ polarization in the orthorhombic phase, and (c) the very small $〈111〉$ splitting in the rhombic phase; the electronic polarization being of the same magnitude in all three phases. The centered ${\mathrm{Fe}}^{3+}$ also yields correct EPR ${b_2}^0$ parameters for PbTi${\mathrm{O}}_3$ and KNb${\mathrm{O}}_3$. The investigation shows that the ${\mathrm{Fe}}^{3+}$ is a very sensitive probe of the intrinsic oxygen positions. The small coupling of the ${\mathrm{Fe}}^{3+}$ to the off-center ferroelectric motion is ascribed to its half-filled $3d$ shell.