Kinetics of the transformation of germanium oxide

Abstract

The kinetics of the transformation of germanium oxide have been studied in the temperature range, 800-1200"C, by X-ray measurements. Germanium oxide, in the pure state, does not transform by heating up to 1000°C, but it does transform in the presence of a catalyst. The kinetic data can be fitted reasonably well by the first-order equation and the energy of activation obtained in the present study is about 100 kcal/mole. The transformation rate is significantly affected by the surrounding atmosphere.The results can be formally interpreted in terms of a model set forth by Avrami, and are compared with the results for silica and titanium oxide.

A High-Resolution Diffraction and Spectroscopic Study of the Low-Temperature Phase Transformation of Hexagonal to Tetragonal GeO2 with and without Alkali Hydroxide Promotion

“…By far, the most widely used dopants in this respect are the carbonates of Li, Na, and K, ,,, less so Rb and Cs . There is common agreement that the phase transformation proceeds via the intermediate formation of a crystalline or glassy germanate compound and that the kinetics of this phase transformation is significantly affected by the experimental conditions (e.g., gas atmosphere, water vapor, catalyst loading). − ,, Activation energies between 170 and 400 kJ/mol depending on the catalyst used and the experimental parameters have been reported …”

“…Full transformation into the rutile-type modification has also been achieved by prolonged heating of the hexagonal phase at 1023 K for 700 h or by applying high pressures and temperatures (e.g., 2 kbar at 1000 K for 48 h). , More chemically oriented methods include the use of crystallization aids such as ammonium fluoride or alkaline compounds, which catalyze the structural rearrangement from the 4-fold tetrahedral to the 6-fold octahedral Ge coordination at high temperatures. ,− , Especially the latter provide a convenient and rather easy access to the tetragonal GeO 2 modification. By far, the most widely used dopants in this respect are the carbonates of Li, Na, and K, ,,, less so Rb and Cs . There is common agreement that the phase transformation proceeds via the intermediate formation of a crystalline or glassy germanate compound and that the kinetics of this phase transformation is significantly affected by the experimental conditions (e.g., gas atmosphere, water vapor, catalyst loading). − ,, Activation energies between 170 and 400 kJ/mol depending on the catalyst used and the experimental parameters have been reported …”

“…In order to study the physicochemical properties of the different phases, several methods have been proposed and applied to catalyze the transformation from the hexagonal to the tetragonal modification. − Without catalyst, it was stated that pure germanium dioxide does not undergo a phase transformation upon heating up to 1300 K . However, immediate formation of a low amount of tetragonal GeO 2 upon heating the hexagonal modification at 653 K and above at 10–30 kbar pressure has been reported, alongside inhibition of further growth of the rutile-type phase by encapsulating the grains of the hexagonal phase, subsequently causing internal strain .…”

“…However, immediate formation of a low amount of tetragonal GeO 2 upon heating the hexagonal modification at 653 K and above at 10–30 kbar pressure has been reported, alongside inhibition of further growth of the rutile-type phase by encapsulating the grains of the hexagonal phase, subsequently causing internal strain . Full transformation into the rutile-type modification has also been achieved by prolonged heating of the hexagonal phase at 1023 K for 700 h or by applying high pressures and temperatures (e.g., 2 kbar at 1000 K for 48 h). , More chemically oriented methods include the use of crystallization aids such as ammonium fluoride or alkaline compounds, which catalyze the structural rearrangement from the 4-fold tetrahedral to the 6-fold octahedral Ge coordination at high temperatures. ,− , Especially the latter provide a convenient and rather easy access to the tetragonal GeO 2 modification. By far, the most widely used dopants in this respect are the carbonates of Li, Na, and K, ,,, less so Rb and Cs .…”

Flux-Grown Piezoelectric Materials: Application to α-Quartz Analogues

“…The transformation from the rutile-like form to the α-quartz-type form has been reported to occur under normal atmospheric pressure in the 1024-1045 °C temperature range [11,35]. The kinetics of the allotropic transformation from a trigonal to a tetragonal structure is extremely low, and non-appreciable without the presence of any catalyst such as traces of GeO 2 rutile-like form, chlorides or water [11,[36][37][38][39].…”