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

• 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.

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.