Growth and characterization of GeO2 single crystals with the quartz structure

A new method for growth of the alpha-GeO2 single crystals from recycled hydrothermal solutions is described. The most important feature of the method is connected with existence and relatively high solubility of metastable quartz-like alpha-GeO2 in the aqueous solutions at the temperatures lower than 180-200 degrees C, where the tetragonal (rutile-like) phase is thermodynamically stable. The heated aqueous solution is recycled according to the scheme: liquid --> vapor --> liquid. The partial evaporation of the solvent --> vapor condensation --> saturation of alpha-GeO2 in condensate --> condensate flow down to the initial solution --> crystallization of alpha-GeO2 on the quartz seeds are considered as the main successive stages of the process. Crystals of alpha-GeO2 with mass 300 g were obtained in a closed cylindrical steel vessel with the capacity of 2000 cm(3). The maximal crystal growth velocity (up to 0.5 mm/day) was fixed in the [0 0 0 1] direction and the following relationship between the velocities of crystal growth to different directions was found to be: V-[0 0 0 1] > V-[1 1 (2 over bar 1]) > V-[1 1 (2 over bar 0]) > V-[1 1 (2 over bar 0]) similar to V-[(1 over bar (1) over bar 2 1]) > V-[0 1 (1 over bar 1]) similar to V-[1 0 (1 over bar 1]) > V-[1 0 (1 over bar 0]). Crystals of Si-substituted alpha-GeO2 with SiO2 content up to 7.5 wt% were also obtained. They can be characterized by a decrease in lattice parameters and by the higher temperature GeO2 phase transition from the quartz structure to the rutile structure. The morphology and some physical properties of the crystals were determined.