Temperature operated infrared nonlinear optical materials based on Tl4HgI6

In this work, we report on the synthesis and nonlinear optical features of novel temperature operated materials based on Tl4HgI6 single crystals. Nonlinear optical, differential scanning calorimetry (DSC) and mechanical properties of the titled crystals are studied within the temperature range 300-500 K. We have established that Tl4HgI6 single crystal show both positive and negative value of linear expansion temperature coefficient. The temperature region of rapid reduction coefficient for linear expansion is very small (only 2A degrees-5A degrees), which corresponds to occurrence of structural transformation for the Tl4HgI6 single crystal. Comparison between the temperature dependence of the photo-induced optical second harmonic generation for the fundamental 5.5 mu m wavelength is performed. We present an explanation for structural phase transition of the crystal Tl4HgI6 considering temperature changes of crystal lattice parameters and describing the transition from centrosymmetric phase P4/mnc, at ambient temperature to non-centrosymmetric P4nc at T > 458 K. The photo induced changes indicate a possibility of occurrence of intermediate non-centrosymmetry phase. Particular analysis is devoted to correlate with the temperature dependence of the DSC within the same temperature range. Physical mechanisms responsible for the observation of the dependences are given and they are based on the condensation of the soft phonon modes which are closely related with the anharmonic electron-phonon interactions responsible for the effects.