High-Temperature Flux Growth as a Tool for the Preparation of Mixed-Framework Metal-Y Silicates: A Systematic Evaluation of the Influence of Experimental Parameters

We report new aspects of the application of the flux-growth method for the preparation of single crystals of silicates, in particular, mixed-framework (octahedral-tetrahedral) metal-Y silicates, in air. The detailed investigations involve flux-growth syntheses in the system BaO- K2O-Y2O3-SiO2-MoO3, in which various important run parameters have been varied in a systematic way (heating and cooling rates, Tmax and duration of holding step, continuous or stepwise cooling, total duration of run, amount of MoO3 solvent, Y2O3/SiO2 molar ratio, size and filling volume of platinum crucible). The results demonstrate that the crystallization of various silicate structure types are strongly controlled by the Y2O3/SiO2 molar ratios and the amount of MoO3 solvent in the precursor mixtures. A decrease of the Y2O3/SiO2 ratio, i.e., an increasing amount of SiO2, promotes an increasing structural complexity of the silicates; nesosilicates crystallize at comparatively low SiO2 concentrations, sorosilicates at intermediate ones, and framework silicates finally form only at high SiO2 concentrations. An increasing MoO3 concentration in the flux mixture also causes the growth of silicates with increasing SiO4 connectivity. A range of minimum and maximum MoO3 concentrations exist in which crystals of these framework silicates can be synthesized. Too high concentrations of Mo6+ cations in the melt handicap the crystal growth of Ba/K-Y-silicates. As expected, lower cooling rates lead to better-developed crystals and a higher crystal yield. Unexpectedly, both the total mass loss by evaporation and the crucible size have no influence on the crystal growth of the silicates. Also, a holding step at Tmax is not necessary for the crystallization of the silicates. The crystals obtained were characterized by scanning electron microscopy, chemical analyses, and single-crystal and powder X-ray diffraction (including Rietveld refinement).