Structure, phase transitions and ionic conductivity of K3NdSi6O15·xH2O. I. α-K3NdSi6O15·2H2O and its polymorphs

S. M. Haile*, B. J. Wuensch

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

31 Scopus citations

Abstract

Hydrothermally grown crystals of α-K3NdSi6O15·2H2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m-3. Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si2O52-) layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure.

Original languageEnglish (US)
Pages (from-to)335-348
Number of pages14
JournalActa Crystallographica Section B: Structural Science
Volume56
Issue number3
DOIs
StatePublished - Jun 2000

ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)

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