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The influence of the layered structure on the temperature dependencies of the specific heat, thermal conductivity, and electric resistivity of photosensitive bismuth oxyhalides BiOX (X = Cl, Br and I) single crystals were investigated in the temperature range 2 K ≤ T ≤ 300 K. The low temperature specific heat data for BiOX demonstrated the presence of two-dimensional behavior with increased anisotropy for the BiOI sample. Moreover, the thermal conductivity values for BiOI are three times higher than for BiOCl and BiOBr, and this observation is consistent with increased anisotropy arising from the contribution of optical out-of-plane phonons. The influence of green (λ = 532 nm) and red (λ = 640 nm) laser excitations on the electrical conductivity properties has been investigated. Each photoconductivity curve reveals transitions from low to high sensitivity. The depletion temperature region for BiOX single crystals corresponds to the peak in temperature dependence of the charge carrier mobility.

The low-temperature specific heat of layered tetragonal bismuth oxyhalide crystals, prepared by the method of chemical gas transport reactions, is measured. The specific heat is found to have Debye behavior at temperatures below 4 K. The Debye temperatures θD of BiOCl (205 K), BiOBr (168 K), and BiOI (146 K) single crystals are determined. The entropy and Debye temperatures of these compounds are found to depend linearly on the corresponding correlations parameters. The low values of the Debye temperatures with the rather high values of the upper boundaries of the phonon spectrum of these compounds may be caused by a high density of phonon states at low frequencies and a substantial deviation of their phonon spectrum from a Debye spectrum at frequencies above 10 cm−1.

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Oxyhalogenides of bismuth BiOX (X=Cl, Br and I) are very interesting materials which nd various applications as X-ray luminescent screens, anti-Stokes (frequency upshift) converters, luminophors, and photoconducting analyzers of linear polarized radiation. Since each primitive cell consist of six monoatomic sites, the structure of the reduced representation of the 15 normal modes of vibration is: Γ = 2A1g(Ra.) + B1g(Ra.) + 3Eg(Ra.) + 2A2u(i.r.) + 2Eu(i.r.), in which the vibrations of A1g, B1g and Eg species are active in the Raman spectrum and those of A2u and Eu species are active in the infrared (i.r.) spectrum. The Raman active modes are observed in frequency range 55 225 cm −1 , 50 185 cm −1 , and 45 175 cm −1 for the BiOCl, BiOBr and BiOI single crystals respectively.