Hydrothermal synthesis of new alkali silicates I. Potassium neodymium phases

Abstract

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, K_3NdSi_3O_8(OH)_2, K_3NdSi_7O_(17), K_5NdSi_9O_(22), K_8Nd_3Si_(12)O_(32)OH, K_3NdSi_6O_(15) (Bb * *), K_3NdSi_6O_(15) (Pbam), K_2NdSi_4O_(10)OH, and KNd_9(SiO_4)_6O_2 the first four of which have no known isomorphs. Syntheses has been carried out employing an aqueous solution of KOH, K_2CO_3, K_2B_4O_7, KF, or KHF_2 as the solvent, and either a glass of high silica content, 4K_2O-Nd_2O_3-17SiO_2, or a mixture of Nd_2O_3, vitreous SiO_2 and K_2CO_3 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 10 M, 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 SiO_4 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.

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