[en] An increase in the electrical conductivity is observed in the Pb_1-xSn_xTe:In/BaF₂ films subjected to laser radiation with a wavelength λ=336.8 μm (the frequency of ∼0.9x10¹² Hz) at the liquid-helium temperature; this increase cannot be accounted for by a heating of the sample. The observed photosignal-relaxation time does not exceed the RC time constant of the measurement circuit (this time constant amounts to fractions of seconds) and is much shorter than in the case of illumination within the fundamental absorption band of Pb_1-xSn_xTe:In. The results obtained relate to an increase in the low-frequency permittivity ε as a result of excitation (by photons of the submillimeter region) of one or two transverse optical phonons in the center vicinity of the Brillouin zone at the branch responsible for the ferroelectric phase transition. This circumstance brings about an increase in the space-charge-limited injection current that flows from the contacts without a generation of free charge carriers in the bulk