Hydrothermal synthesis of new alkali silicates II. Sodium neodymium and sodium yttrium phases

Hydrothermal techniques have been utilized to synthesize three sodium neodymium and seven sodium yttrium silicates as potential new fast ion conductors (FICs). We describe here the conditions which led to the crystallization of NaNdSi₆O₁₄, Na₃NdSi₆O₁₅· 5H₂O, NaNd₉(SiO₄)₆O₂, Na₃YSi₆O₁₅, Na₃YSi₇O₁₇, Na₇YSi₁₂O₂₉, Na₃YSi₃O₉, Na₅YSi₄O₁₂, Na₆YSi₁₃O₂₉(OH)₃, and NaYSi₆O₁₄, six of which are previously unreported. The compositions of new phases were determined from electron microprobe measurements and flame spectroscopy. Syntheses were carried out in aqueous solutions of NaOH, Na₂CO₃, NaF, NaHF₂, at pressures, temperatures, and molarities ranging from 0.5 to 1.4 kbar, from 350 to 600°C, and from 0 to 10M, respectively. Results obtained in these sodium systems are compared with those of the potassium neodymium silica system, reported in Part I. Consistent with the earlier observations, crystallization at high temperatures, high solution molarities and, to some extent, high pressures tended to yield phases of low silica content and low silicon to oxygen ratios, and hence low structural connectivity of the tetrahedra in the silica framework.