Synthe0sis, Structure, and Ionic Conductivity of K 3 NdSi 6 O 15
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SYNTHESIS,
STRUCTURE,
AND IONIC CONDUCTIVITY OF K3 NdSi 6O 1 5
SOSSINA M. HAILE*, THEO SIEGRIST**, ROBERT A. LAUDISE**, AND BERNHARDT J. WUENSCH* *Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139 **AT&T, Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
ABSTRACT A structure earlier reported for K3 NdSi 6O1 5 suggests the possibility for fast-ion conduction by virtue of K+ ions which not only reside in connected channels, but which also display very large thermal vibration amplitudes. No properties of the phase had heretofore been examined. Crystals up to 2mm in length have been synthesized under a variety of hydrothermal conditions. Refinement of the structure with single crystal x-ray data to a residual of 8.3% confirms the composition and structure of our crystals and reveals a very high thermal vibration amplitude along c for K+ ions which reside in interconnected (001] channels. A reversible phase transformation has been discovered at 180*C with a heat of transformation of 4J/g. Preliminary measurements of the conductivity of the high temperature phase yield a preexponential factor of 3.4S/cm and an activation energy of 0.61ev along [001], indicating that this compound is not a particularly good fast-ion conductor.
INTRODUCTION The crystal structure of a new rare earth silicate, K3NdSi 6 0 1 5 , was reported in 1977 by Pushcharovskii et al. in a study primarily intended as a contribution to silicate crystal chemistry. No properties of the material were examined. Crystals had been synthesized hydrothermally, but the growth conditions were not provided. The structure contained silicate layers with connected channels, within which the alkali atoms were located. These ions, moreover, displayed large isotropic temperature factors, indicating that the alkali ions must reside in shallow potential minima. Both characteristics suggest a possibility of fast ion conduction, a property which, in addition, might be enhanced by crystal-chemical tailoring of this structure. The rare earth cation, for example, might be replaced by others of differing size or valence. The present paper reports synthesis of the phase under a variety of hydrothermal conditions to provide crystals of millimeter dimensions. We also report a refinement of the structure with single crystal methods to both confirm the composition of the phase and to examine the thermal vibration Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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autoclave. The largest crystals of K3 NdSi 6O 1 5 (2mm x 1mm x 0.2mm) were obtained using a low temperature of 335 0 C and a high temperature of 425*C. The nutrient consisted of 1.26g of Nd 2 0 3 and 2.69g of Si0 2 . The autoclave was filled to sixtythree percent of its capacity, giving an estimated pressure of 9 kpsi. A 2M solution was used, twice the concentration needed to supply the amount of potassium required for the stoichiometry of K3 NdSi 6O 1 5 . REFINEMENT OF THE CRYSTAL STRUCTURE OF K3 NdS
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