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, <i>T</i>_max 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 <i>T</i>_max 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)