Optical and paramagnetic properties of titanium centres in ZnS

ZnS:Ti crystals grown by different methods are investigated at low temperatures (2 K \(\) K) by electron paramagnetic resonance (EPR) and optical spectroscopies, mainly with regard to their photoluminescence properties, including excitation and sensitisation spectra. The more familiar lattice-neutral Ti2+(d 2 ) ion on a cubic site (AN) presents an isotropic EPR signal with g=1.928 observable up to 80 K. Signals of axial PN and AS sites are detected in the same temperature range. This Ti2+ ion exhibits the \(\) transition in emission, structured by no-phonon lines (NPL) of centres in various environments, with the AN site represented at 3613 cm-1, and the 3T 1 (F) and 3T 1 (P) bands in excitation spectra. Substitutional Ti3+(d 1 ) is identified by an anisotropic EPR spectrum at T = 3.5 K, indicating a quasistatic Jahn-Teller effect at the doubly degenerate 2E(D) ground state, while in the first excited vibronic state a quasidynamic JT effect in the strain-split vibronic 2E/2A 2 manifold is found. A new luminescence band centred at 4500 cm-1 with NPL structures near 5000 cm-1 represents AN, PN and AS sites in the \(\) transition. Charge-transfer processes are described in a one-particle model, depicting the Ti3+/Ti2+ donor level at 12 900 cm-1 below the conduction-band edge.