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The high‐temperature phase transition III—I of Na2SO4 is studied by infrared spectroscopy. A sudden distortion in the SO4 ion is detected at 267 °C by a careful study of the temperature dependence of the IR bands of Na2SO4 during this phase transition. The results show that phase III is activated by an energy of 0.56 eV while phase I is associated with an energy barrier of 1.16 eV. The infrared analysis is successfully used here to confirm the phase changes which are detected before by electric properties, DTA, and other means. The IR study reveals that infrared spectroscopy only may be used for determining the relevant transition temperature.

A complete systematic IR study (400 to 4000 cm−1) of AgNO3 is carried out over a wide range of temperature (30 to 195 °C) and new results are reported for the order–disorder phase transition II–I. The study includes measurements and interpretations of the IR spectral band shape, intensities, and frequencies of the internal modes as functions of temperature. A special attention is paid to the bending mode v2 and the combination mode (v1 + v4). The results reveal that the transformation from the order state (phase II) to the disorder state (phase I) of AgNO3 is monitored by clear variations in the spectral parameters for v2 and (v1 + v4) modes. According to the present IR analysis, the energy barrier of the reorientation of the NO 3− ions is found to be 0.25 eV in the ordered phase II and 0.14 eV in the disordered phase I. Also, the phase transition process is activated by an energy of 0.11 eV.

A b s t r a c t . A complete infrared study of the mixed crystal system of AgNO, and NaNO, is carried out in the region 400-4000 em-'. The study includes internal fundamental normal vibrations of the NO-, ion in the ordered and disordered phases of AgNO, a t different values of the sodium concentration, the I.R. spectrit, spectral band shape intensities, and frequencies of t h r intrrnal modes as functions of the sodium ion concentration. Special attcntion is paid to bending mode, combination mode, asymmetric stretching mode, and over-tone. The rotation$ energy barrier is determined a t different concentrations of the sodium ions in the systrm. Eine Untersuchung der Phaseniibergange im System [Agl-,(Na),]NOs mittels InfrarotspcktroskopieI n h a l t s iibersicht. Es wird eine vollstandige IR-Untrrsurhitng drs Misc1ikrist;~llsystems aus AgNO, und NaNO, im Brreich zwischen 400 und 4000 rm-l durchgrfuhrt. Sie umfnI3t die inneren Grundschwingungen des NO-,-Ions in der geordneten nnd ungeordnetcn AgN0,-Phase und die 1R-Spektrencharakteristika bei unterschiedlichrn Na-Konzentrstionen. Dir Rotntionsencrgirbarrirre wird fur unterschiedliche Na-Konzentrationen im System bestimmt.

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