Thallium related paramagnetic centers of ${\mathrm{Tl}}^{2+}$ ${(6s}^1)$ and ${\mathrm{Tl}}^0{(6p}^1)$ type were produced by low temperature x-ray irradiation in ferroelectric ${\mathrm{Rb}}_2{\mathrm{ZnCl}}_4:\mathrm{Tl}$ single crystals. Extensive EPR investigations have been performed in order to elucidate their intrinsic properties and sensitivity as paramagnetic probes for structural phase transitions studies. Compared to the s-type ${\mathrm{Tl}}^{2+}$ centers, already used in many such studies, it was found that the p-type ${\mathrm{Tl}}^0$ centers are much more sensitive to the small variations in the local crystal field associated with these transformations. Their EPR spectra provided information about the unit cell tripling in the ${P2\mathrm{}}_1\mathrm{cn}$ phase, as well as the symmetry lowering and lattice dynamics in the $C1c1\mathrm{}$ phase. The temperature induced, continuous changes observed in the EPR spectra of both ${\mathrm{Tl}}^0$ and ${\mathrm{Tl}}^{2+}$ centers were explained by the influence of the soft modes responsible for the 74-K structural phase transition.

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