Hydrothermal synthesis of new alkali silicates I. Potassium neodymium phases

Hydrothermal techniques have been used to synthesize potassium neodymium silicates as potential new fast ion conductors (FICs). Alkali silicates are interesting both scientifically and technologically as FICs because of their typically open framework structures and the unique possibilities for crystal chemical tailoring. We describe here the growth conditions for eight phases, K₃NdSi₃O₈(OH)₂, K₃NdSi₇O₁₇, K₅NdSi₉O₂₂, K₈Nd₃Si₁₂O₃₂OH, K₃NdSi₆O₁₅ (Bb **), K₃NdSi₆O₁₅ (Pbam), K₂NdSi₄O₁₀OH, and KNd₉(SiO₄)₆O₂ the first four of which have no known isomorphs. Syntheses has been carried out employing an aqueous solution of KOH, K₂CO₃, K₂B₄O₇, KF, or KHF₂ as the solvent, and either a glass of high silica content, 4K₂O-Nd₂O₃-17SiO₂, or a mixture of Nd₂O₃, vitreous SiO₂ and K₂CO₃ as the precursor material. Pressures, temperatures, and solution molarities utilized ranged from 0.3 to 1.4 kbar, from 350 to 600°C, and from 0 to 10M, respectively. Experiments, performed both isothermally and under the influence of a temperature gradient, proceeded for 5 to 20 days. Phases were identified using electron microprobe composition measurements and X-ray single-crystal and powder diffraction methods. Most phases were produced under well-defined conditions and synthesis was characterized by a tendency to crystallize compounds of low silica content and hence low connectivity of the SiO₄ tetrahedra in the crystal structure at high molarities, high temperatures, and, to some extent, high pressures. Of the eight phases listed, six were obtained as crystals large enough for X-ray structure determination and conductivity measurements along at least one crystallographic axis.