Paramagnetic centers related to Al, Sc, In, and Nb impurities in KTiOAsO4 crystals

Electron paramagnetic resonance (EPR) is applied to study Al-, Sc-, In-, and Nb-doped KTiOAsO4 (KTA) crystals. Paramagnetic hole centers O- are observed after ionizing irradiation of KTA crystals. These centers are, as a rule, unstable at room temperature and are slowly annealed for about two weeks. Oxygen ions are bridging two cations in KTA. Near the impurity, two p-orbitals of oxygen atoms participate in covalent bonding with cations, whereas the third p-orbital remains free and under the radiation effect captures the hole thus forming the paramagnetic center of Mn+-O--M(n-1)+ (here Mn+ is the lattice cation and M(n-1)+ is the impurity cation of Al, In, Sc, or Nb). In the centers investigated the specific principal direction of the g-factor g similar to 2 is normal to the Mn+-O--M(n-1)+ plane, and the main value of g(max) falls in this plane. The direction of the O--M(n-1)+ bond is close to the selected direction of the hyperfine interaction with the impurity ion. The models of six hole centers and the found parameters of EPR spectra are discussed.