HYDROTHERMAL SYNTHESIS OF NEW ALKALI SILICATES .1. POTASSIUM NEODYMIUM PHASES

Hydrothermal techniques have been used to synthesize potassium neodymi um silicates as potential new fast ion conductors (FICs). Alkali silic ates are interesting both scientifically and technologically as FICs b ecause of their typically open framework structures and the unique pos sibilities 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. Syn theses has been carried out employing an aqueous solution of KOH, K2CO 3, K2B4O7, KF, or KHF2 as the solvent, and either a glass of high sili ca content, 4K2O-Nd2O3-17SiO2, or a mixture of Nd2O3, vitreous SiO2 an d K2CO3 as the precursor material. Pressures, temperatures, and soluti on molarities utilized ranged from 0.3 to 1.4 kbar, from 350 to 600-de grees-C, and from 0 to 10 M, respectively. Experiments, performed both isothermally and under the influence of a temperature gradient, proce eded for 5 to 20 days. Phases were identified using electron microprob e composition measurements and X-ray single-crystal and powder diffrac tion 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 tetrahed ra 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 a nd conductivity measurements along at least one crystallographic axis.