Dc conductivity measurements were performed as a function of temperature on unplasticized poly(vinyl chloride) and on PVC plasticized with various amounts of dioctylphthalate. The conductivity curves consist of two or three straight‐line segments denoted I, II, and III with increasing of temperature. The intersection of segments I and II occurs at the glass‐transition temperature Tg. The slope in region I is independent of the DOP concentration, while the slope in region II decreases slowly with an increase in the amount of DOP. No dependence of the conductivity on the molecular weight was found. From the conductivity curves, activation energies were evaluated below and above Tg. These satisfactorily coincide with those determined by dielectric loss or by electrical transient phenomena. A dc conduction mechanism is proposed based on electronic hopping favored by the micro‐Brownian motions responsible for dielectric losses. These motions involve smaller chain lengths below than above Tg. The experimental results are discussed and interpreted in terms of the proposed mechanism.

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Simultaneous measurements are made of light scattering and ionic conductivity as a function of temperature in the range 20 to 650 "C on Ba-and Cd-doped NaCl single crystals submitted to different heat treatments. I n case of Ba-doped crystals, the results are interpreted by changes in the decoration of the precipitates with cation vacancies; in case of Cd-doped crystals by dissolution and reformation of the precipitates as a consequence of appropriate heat treatments.Des mesures simultanees de diffusion de la lurnidre et de conductibilit6 ionique en fonction de la temperature entre 20 et 650 OC ont BtB effectubes sur des monocristaux de NaCl dopes au Ba et au Cd et soumis aux diffbrents traitements thermiques. Les rhsultats ont BtB interpreths dans le cas des cristaux dopks au barium, par la modification de la d6co-ration des pr6cipitks par des vacances de cations et dans le cas des cristaux dopes au cadmium par la dissolution et la formation des prbcipites & la suite de traitements thermiques appropriks.

The photostimulated electron emission spectra of CaF,, SrF,, and BaF, crystals, coloured by electron bombardment, are analysed. The comparison with optical absorption spectra leads to some conclusions with respect to the kind of colour centres corresponding to the various bands observed. The results verify a relation of the Mollwo-Ivey type. Some complex bands are resolved. Sont analysBs les spectres d'Bmission electronique photostimulee des cristaux de CaF,, SrF, et BaF, color& par bombardement Blectronique. La comparaison avec les spectres d'absorption optique conduit A certaines conclusions concernant le genre de centres colorBs correspondant aux diverses bandes observbes. Les rBsultats verifient une relation du type Mollwo-Ivey. Certaines bandes complexes sont rBsolues.

Some new results concerning the appearance of a periodic structure in a r/2-twist cell filled with MBBA under d.c. excitation are reported. This structure consists of rectilinear domains whose orientation varies with the applied voltage around the normal to the molecular orientation in the middle of the cell and this variation covers a large range of angles even for fixed polarity of the excitation. When the polarity is changed, the sense of the domain orientation is also changed.