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Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs

Haile, S. M. and Wuensch, B. J. (2000) Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs. Acta crystallographica. Section B, Structural science, 56 (3). pp. 335-348. ISSN 0108-7681. doi:10.1107/S0108768199015785.

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Hydrothermally grown crystals of α-K_3NdSi_6O_(15)·2H_2O, 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(F^2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si_2O_5^(2−))_∞ 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.

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Haile, S. M.0000-0002-5293-6252
Additional Information:© 2000 International Union of Crystallography. Received 26 April 1999. Accepted 6 December 1999. The authors gratefully acknowledge the contributions of several colleagues to this work. Dr Karl Peters of the Max Planck Institut (MPI) für Festkörperforschung kindly measured single-crystal intensity data, Herr Florian Viczian (MPI) performed high-temperature powder diffractometer experiments, Herr Willie Rötenbach (MPI) collected high-temperature Guinier film data and Frau Nubia Carioca (MPI) conducted the thermal analysis and mass spectroscopy. Mike Jercinovic of the Massachusetts Institute of Technology (MIT) carried out microprobe analyses. The authors also thank Professor Harry Tuller (MIT) for providing access to his electrical characterization laboratory. SMH thanks Dr Joachim Maier for hosting her visit to the Max Planck Institut für Festkörperforschung, where some portions of this research were carried out. This work is dedicated to the memory of the late Robert A. Laudise, a brilliant scientist and exemplary human being who provided invaluable guidance in the method of hydrothermal synthesis. His friendship and inspiration are deeply missed by those who were fortunate to know him.
Subject Keywords:Phase transitions; Ionic conductivity; Polymorphism
Issue or Number:3
Record Number:CaltechAUTHORS:20160914-083031187
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Official Citation:Haile, S. M. and Wuensch, B. J. (2000), Structure, phase transitions and ionic conductivity of K3NdSi6O15·xH2O. I. α-K3NdSi6O15·2H2O and its polymorphs. Acta Crystallographica Section B, 56: 335–348. doi: 10.1107/S0108768199015785
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70331
Deposited By: Ruth Sustaita
Deposited On:29 Sep 2016 23:33
Last Modified:11 Nov 2021 04:27

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