EPR studies of transtion metal impurity ions in strontium titanate

The topic of this thesis is the study of paramagnetic impurity metal ions in single crystals of strontium titanate (SrTiO₃) by means of EPR spectroscopy. SrTiO₃ belongs to the perovskite family. Doping of the crystal with transition metal ions in general gives rise to a replacement of the Ti⁴⁺ ion by the metal ion in question. For various transition metal impurity ions, it has been found that these ions can exist in different valence states in SrTiO₃.
The variation in the charge state of the metal ion is possible because of the relatively high dielectric constant of SrTiO₃ (ε ≅ 310 at room temperature and ε ≅ 24,500 at liquid helium 2 temperature). As a result the Coulomb interactions are kept relatively weak and there is often no sign for local charge compensation. The metal ion valence state may be altered in a variety of ways: By heat treatment, oxidation and reduction treatments, neutron irradiation and light irradiation, the band gap for SrTiO₃ being 3.2 eV.
One of the purposes of this investigation is to study different photochromic valence state modifications of a number of metal impurity ions i.e., non-cubic Fe⁵⁺, tetragonal V⁴⁺ and cubic Mo³⁺ with the help of EPR spectroscopy.