Phase Relations in the KOH–GeO2–H2O System at 500°C and 0.1 GPa

The compounds crystallizing in the KOH–GeO2(quartz structure)–H2O system at 500°C, 0.1 GPa, and different GeO2 and KOH concentrations are R-GeO2 (rutile structure), K2Ge[6]Ge3[4]O9, and K3HGe4[6]Ge3[4]O16 · 4H2O (superscripts specify the oxygen coordination of Ge). With increasing KOH concentration, the phase composition of crystallization products changes in the sequence R-GeO2 + K2Ge[6]Ge3[4]O9 → K2Ge[6]Ge3[4]O9 + K3HGe4[6]Ge3[4]O16 · 4H2O → K3HGe4[6]Ge3[4]O16 · 4H2O. The matrix-assembly model and crystal-chemical analysis are used to examine the formation of K2Ge[6]Ge3[4]O9 (three-dimensional MTframework, where M = Ge[6] octahedron and T = Ge[4] tetrahedron) from K2M2T4 cyclic subpolyhedral structural units containing K atoms above and below the plane of the ring. The structural mechanism responsible for the formation of the T radical in the form of Ge3[4]O9 three-membered rings from orthotetrahedra and diorthogroups is elucidated.