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The electronic and atomic structure of SrTiO 3 crystals below the antiferrodistortive phase transition observed at 105 K is calculated using the hybrid B3PW functional as implemented in the ab initio CRYSTAL-2003 computer code. Such a combination of non-local exchange and correlation permits the calculation for the first time of the TiO 6 octahedron rotational angle and the ratio c/a of tetragonal lattice constants in excellent agreement with experimental data. The level splitting of the bottom of the conduction band is found to be very small, <1 meV. The predicted phase-transition induced change of the optical gap from indirect to direct is confirmed by experimental photoconductivity data.

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X-and Q-band electron paramagnetic resonance (EPR) study is reported on the SrTiO 3 single crystals doped with 0.5-at.% MnO. EPR spectra originating from the S = 2 ground state of Mn 3+ ions are shown to belong to the three distinct types of Jahn-Teller centres. The ordering of the oxygen vacancies due to the reduction treatment of the samples and consequent formation of oxygen vacancy associated Mn 3+ centres are explained in terms of the localized charge compensation. The EPR spectra of SrTiO 3 : Mn crystals show the presence of next nearest neighbor exchange coupled Mn 4+ pairs in the <110> directions.

The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO3 crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm(-1). We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn-Teller Mn(3+) ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.