Optical and paramagnetic properties of titanium centres in ZnS

ZnS:Ti crystals grown by different methods are investigated at low temperatures (2 K less than or equal to T less than or equal to 80 It) 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 T-3(2)(F) --> (3)A(2)(F) 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 T-3(1)(F) and T-3(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 E-2(D) ground state, while in the first excited vibronic state a quasidynamic JT effect in the strain-split vibronic E-2/(2)A(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 T-2(2)(D) --> E-2(D) transition. Charge-transfer processes are described in a one-particle model, depicting the Ti3+/Ti2+ donor level at 12900 cm(-1) below the conduction-band edge.