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The results of neutron diffraction researches in the temperature region 10 - 290 K as well as thermoelectric, magnetic and electric properties of solid solution Pb 0,8 Sn 0,2 Te of high structural perfection within temperature range (4,2-25) K in magnetic fields up to 4T are represented. The analysis of experimental data testifies to the existence of phase transformations at Tc 1 = (35 ± 5) K, Tc 2 = (110 ± 5) K, Tc 3 = (140 ± 5) K and Tc 4 = (260 ± 5) K. The temperature hysterenis of thermoelectric power is observed in the interval Tc 1 2 . It testifies to the ferroelectrics transition of I kind and lowering of crystal symmetry. Phase transformations at Tc 3 and Tc 4 are transitions of presumable the II kind. The structural perfection improvement of single crystals Pb 0,8 Sn 0,2 Te results in the shift of the first phase transition temperature Tc 1 from 20 - 25 K to Tc 1 = 35 K. This shift can be described by Dingle temperature - T D , which is determine a degree of crystal imperfection at T = O K.