The Tl-related paramagnetic centers of Tl2+ (6s1) and Tl0 (6p1) type were produced by low temperature X-ray irradiation in ferroelectric Rb2ZnCl4:Tl single crystals. Extensive electron paramagnetic resonance investigations were performed in order to elucidate their intrinsic properties and sensitivity as paramagnetic probes for structural phase transitions studies. Compared to the s-type Tl2+ centers, already used in many such studies, it was found that the p-type Tl0 centers were much more sensitive to the small variations in the local crystal field associated with these transformations. Their electron paramagnetic resonance spectra provided information about the unit cell tripling in the P21cn phase, as well as the symmetry lowering and lattice dynamics in the C1c1 phase. The temperature induced, continuous changes observed in the electron paramagnetic resonance spectra of both Tl0 and Tl2+ centers were explained by the influence of the soft modes responsible for the 74K structural phase transition.

EPR Probing of Low Temperature Structural Phases of Rb2ZnCl4 Crystals with Tl0 and Tl2+ Centers. M.Stefan, S.V.Nistor, E.Goovaerts, D.Schoemaker: Physical Review B, 2004, 69[10], 104107 (11pp)