Abstract
The temperature dependence of the infrared spectra of Ba(NO3)2 is reported in the whole ordered-disordered phase, from about 25°C up to 400°C. The study includes the temperature dependence of the infrared-active lattice modes. The frequency ν1 denotes the totally symmetric NO stretching mode, ν2 the bending mode, ν3 the asymmetric stretching mode, ν4 the asymmetric in-plane bending mode, 2ν1, 2ν3 the first overtones, and (ν1 + ν4) and (ν1 + ν3) the combination modes. We have measured and interpreted the IR spectral band shapes, intensities and frequencies of the internal modes as a function of temperature. The results reveal that the transformation from the ordered state (phase II) to the disordered state (phase I) of Ba(NO3)2 is indicated by clear variations in the spectral parameters for (ν1 + ν4), 2ν1, (ν1 + ν3) and 2ν3. The temperature dependence of the relaxation parameters confirms the gradual transformation due to the growing of the disorder in the crystal structure which appears at about 300°C and is associated with a thermal energy of about 0.012 eV.