Understanding gray track formation in KTP: Ti3+ centers studied from first principles

The magnetic signatures of ${\mathrm{Ti}}^{3+}$ centers in potassium titanyl phosphate (KTP) are studied within density-functional theory (DFT). The hyperfine tensor elements are very sensitive to the structural surrounding; the paramagnetic hyperfine splittings are used to evaluate the defect models. For each of the four experimentally observed electron paramagnetic resonance (EPR) spectra, we identify a defect model that reproduces the paramagnetic signature. All of them are electron donors, whereby one specific Ti atom can be identified, whose formal oxidation number is lowered from $4+$ in the ideal crystal to $3+$. The related charge redistribution leads to a strong polarization of the corresponding ${\mathrm{Ti}}^{3+}$ center. However, in three cases a second Ti atom, connected to the first by a mutual polarized O atom, is polarized too. Positively charged O vacancies at the lattice site O(10) are unique in leading to the formation of the only ${\mathrm{Ti}}^{3+}$ center that is stable at room temperature. This defect induces a defect state within the band gap, which may be excited during second harmonic generation (SHG) applications and thus is a plausible candidate to explain the so-called gray tracking, i.e., photochromic damage.