Zusammenhang zwischen Fehlordnungsgrad und Bildungstemperatur bei synthetischen Cristobaliten

Abstract

By hydrothermal hydrolysis of silicium nitride cristobalite with different degrees of disorder was formed metastably in the stability range of quartz. The synthesis conditions were 2 kbars $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ , temperatures between 200 and 610° C, and run durations of two days. Differential thermal analysis was applied to investigate order-disorder phenomena in the newly formed phases, since it turned out to be most sensitive to disorder effects; the results have been corroborated by infraredspectroscopic, electron optical, and X-ray investigations as well. In the DTA curve the degree of disorder of cristobalite can be recognized by four characteristic features: 1. the temperature of inversion (t i) from tetragonal (low-) to cubic (high-) cristobalite, 2. the width of the inversion temperature interval, 3. the shape of the inversion peak, and 4. the hysteresis between (t i) measured during heating and subsequent cooling. With increasing order, (t i) and the sharpness of the peak increase, whereas the inversion temperature interval decreases. The heating-cooling hysteresis, which is negative in poorly ordered crystals, becomes increasingly positive with improving degree of order. All the different methods of investigation have shown that the degree of order increases with increasing synthesis temperature if all other conditions are kept constant. The relatively low temperatures of formation can be understood as a consequence of high SiO2 supersaturations due to the rapid decay of Si3N4 under hydrous conditions. Substitution of NH 4 + ions in cristobalite which might behave like alkali ions has not been observed.