Novel solvents for the single crystal growth of germanate phases by the flux method

A series of alkali-borate (Na2O·B2O3, Na2O·1.5B2O3, Na2O·2B2O3 and Li2O·B2O3), and a series of alkali-molybdate (Na2O·1.5MoO3, Na2O·2MoO3 and Li2O·MoO3) solvents were studied with a view to find out the suitable conditions for growing single crystals from germanate phases by the flux method. The ternary systems solvent-CaO–GeO2 were investigated and crystallization temperature and crystalized phase were determined. As a main result the concentration and temperature regions of crystallization of calcium germanates (Ca5Ge3O11, CaGeO3) and of some alkali germanates (Li2GeO3, Li2CaGeO4, Na2CaGe6O14) were experimentally determined for a first time. The conditions found for Ca2GeO4 growth are significantly more favorable than those known so far. Additionally during this study, two novel unknown phases were obtained. X-ray diffractograms of the novel phases are presented. The obtained conditions are a basis for successful single crystal growth by the flux method for all mentioned germanate compounds. •Several high temperature solvents for germanate compounds have been investigated.•Temperature and concentration regions of important germanate phases were determined.•Significant more suitable conditions for flux single crystal growth were obtained.•New oxide compounds into the investigated systems were detected to be crystallized.