We have used dielectric spectroscopy to measure the frequency and temperature dependence of the lowfrequency ͑0.5 mHz-100 kHz͒ complex dielectric function () of thin C 60 films. A small charge transfer occurs between the C 60 and O 2 molecules which occupy interstitial spaces of solid C 60 exposed to oxygen ͑or ambient air͒. Due to the large size of the C 60 molecules, this small charge transfer creates large dipole moments, which in turn are coupled to the applied ac electric field via a diffusion-controlled relaxation mechanism. This gives rise to a significant increase in the permittivity 1 accompanied by a broad dielectric loss peak 2 observed at ϳ10-100 Hz. With increasing oxygenation, the interstitial sites become nearly fully occupied, interstitial hopping is inhibited, and the loss peaks, together with the enhanced polarization, disappear. Both tracer and chemical diffusion coefficients for the C 60 /O 2 system have been obtained purely from dielectric spectroscopy measurements.
The conditions for crystallization of a new β modification of barium diborate in the glass of stoichiometric composition are investigated. This modification is characterized using X-ray diffraction analysis. In order for the β -BaO · 2B 2 O 3 modification to crystallize, the glass should be purified from dissolved gases and have an undisturbed fire-polished surface. Otherwise, the α modification crystallizes under the same conditions. The melting temperature of the β -BaO · 2B 2 O 3 modification is determined to be T melt = 805 ° C. Upon heating, the β modification in the form of a powder transforms into the α modification, whereas the β modification in the form of a polycrystalline monolithic sample remains stable up to the melting temperature.
The thermal expansion and density of vitreous and polycrystalline barium diborates prepared by crystallization of monolithic glasses of stoichiometric composition are investigated in the temperature range 20-700 ° C. The glass crystallizes in the form of the α and β modifications depending on the heat treatment conditions. The α phase nucleates on the surface of the monolithic sample and grows inward the sample in the form of textured layers. The β phase nucleates in the melt bulk and grows in the form of spherulites. Textured polycrystalline barium diborate in the form of the α phase possesses a pronounced anisotropy of thermal expansion with respect to the direction of crystalline-layer growth. This is associated with the negative expansion along one of the crystallographic axes. The effective temperature coefficients of thermal expansion in the temperature range 20-600 ° C increase in the order: α phase-glass-β phase. Upon heating above 690 ° C, the β phase transforms partially into the α phase. This is accompanied by a considerable increase in the volume. The densities of the glass and polycrystalline samples in the temperature range 20-700 ° C increase in the order: glass-α phase-β phase.
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