OPTICAL AND PARAMAGNETIC PROPERTIES OF TITANIUM CENTERS IN ZNS

ZnS:Ti crystals grown by different methods are investigated at low tem peratures (2 K less than or equal to T less than or equal to 80 It) by electron paramagnetic resonance (EPR) and optical spectroscopies, mai nly with regard to their photoluminescence properties, including excit ation and sensitisation spectra. The more familiar lattice-neutral Ti2 + (d(2)) ion on a cubic site (AN) presents an isotropic EPR signal wit h g = 1.928 observable up to 80 K. Signals of axial PN and AS sites ar e 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-phono n lines (NPL) of centres in various environments, with the AN site rep resented at 3613 cm(-1), and the T-3(1)(F) and T-3(1)(P) bands in exci tation spectra. Substitutional Ti3+(d(1)) is identified by an anisotro pic EPR spectrum at T 3.5 K, indicating a quasistatic Jahn-Teller effe ct at the doubly degenerate E-2(D) ground state, while in the first ex cited vibronic state a quasidynamic JT effect in the strain-split vibr onic 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 an d AS sites in the T-2(2)(D) --> E-2(D) transition. Charge-transfer pro cesses are described in a one-particle model, depicting the Ti3+/Ti2donor level at 12900 cm(-1) below the conduction-band edge.