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Alkali Ion Conducting Silicates

Alkali rare earth silicates, in which the alkali ion serves as the mobile charge carrier, are interesting both scientifically and technologically as fast ion conductores because of the unique possibilities for crystal chemical tailoring, the chemical stability of silicates, and the large number of phases available for investigation. This system also provides a set of model compounds for the ultimate goal of solid state chemist to predict structure from a given composition and set of synthesis conditions. Because of the tendency of compounds with high silicate content to melt at elevated temperatures, long before sintering takes place, and form stable glasses, all of the preparative work has been conducted via hydrothermal synthesis. This method employs high temperatures (350-600C) and pressures (0.3-1.4kbar) to dissolve a normally insoluble precursor material and induce precipitation the desired crystalline phase. To date we have established that synthesis under conditions of very high temperature, high solution molarity, and to some extent high pressures, tends to produce structures with low silica content (relative to the precursor material) and hence low connectivity of the SiO4 tetrahedra. In addition, layered silicates with the complex anion, MSi6O15 (M = 3+ or 4+ cation), exhibit a high degree of layer corrugation for large M cations such as Nd and Ce, in comparison to structures formed by Zr and Ti. The large channels formed between highly corrugated layers may serve as pathways for fast alkali ion transport.

Selected Publications

  • S.M. Haile and B.J. Wuensch, "X-ray Diffraction Study of K3NdSi7O17 -- a New Framework Silicate with a Linear Si-O-Si Bond," Acta Cryst. B56 (2000) 773-779.
  • S.M. Haile and B.J. Wuensch, "Structure, Phase Transitions and Ionic Conductivity of K3NdSi6O15•xH2O I. a-K3NdSi6O15•2H2O and its Polymorphs," Acta Cryst. B56 (2000) 335-348.
  • S.M. Haile and B.J. Wuensch, "Structure, Phase Transitions and Ionic Conductivity of K3NdSi6O15•xH2O II. The Structure of b-K3NdSi6O15," Acta Cryst. B56 (2000) 349-362.
  • S.M. Haile, B.J. Wuensch and T. Siegrist, "The Structure and Conductivity of K8Nd3Si12O32OH -- A New Layered Silicate,"  J. Solid State Chem. 148 (1999) 406-418.
  • S.M. Haile and B.J. Wuensch, "Comparison of the crystal-chemistry of selected (MSi6O15)-based silicates," Amer. Min., 82 (1997) 1141-1149.
  • S.M. Haile, B.J. Wuensch, R.A. Laudise and J. Maier, "The Structure of Na3NdSi6O15•2H2O-- A Layered Silicate with Paths for Possible Fast Ion Conduction," Acta Cryst. B53 (1997) 7-17.
  • S.M. Haile, B.J. Wuensch, R.A. Laudise and J. Maier, "The Structure of Na3YSi6O15 -- A Unique Silicate Based on Discrete Si6O15 Units and a Possible Fast Ion Conductor," Acta Cryst B51 (1995) 673-680.
  • S.M. Haile, B.J. Wuensch and R.A. Laudise, "Hydrothermal Synthesis of New Alkali Silicates II. Sodium Neodymium and Sodium Yttrium Phases," J. Cryst. Growth 131 (1993) 373-386.
  • S.M. Haile, B.J. Wuensch, T. Siegrist and R.A. Laudise, "Hydrothermal Synthesis of New Alkali Silicates I. Potassium Neodymium Phases," J. Cryst. Growth 131 (1993) 352-372.
  • S.M. Haile, B.J. Wuensch, T. Siegrist and R.A. Laudise, "Conductivity and Crystallography of New Alkali-Rare Earth Silicates Synthesized as Possible Fast Ion Conductors," Solid State Ionics 53-56 (1992) 1292-1301.
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