Impurity centers and host microstructure in weakly doped SrTiO₃:Mn crystals: new findings

Electron paramagnetic resonance (EPR), optical absorption and low-frequency (100 Hz − 1 MHz) dielectric permittivity investigations on Verneuil as-grown and reduced SrTiO₃:Mn (0.1% at.) crystals were carried out to establish connection between the structure of Mn doping related centers and the SrTiO₃ dielectric properties. At room temperature the EPR spectrum of as-grown crystals evidences isotropic Mn⁴⁺ and Mn²⁺ centers. The Mn²⁺/Mn⁴⁺ ratio increases, according to the EPR spectra, by decreasing the temperature and the signals intensity of both centers strongly decreases approaching T∼120 K. Such behavior is attributed to antiferromagnetic interaction in MnO and MnO₂ nano-clusters. Also the dielectric permittivity behavior evidences samples inhomogeneity as well as absence of pronounced dielectric relaxation in as grown crystals. In reduced crystals the Mn²⁺ concentration increases and the dielectric relaxation emerges evidencing a polaronic-related nature of dipole reorientations.