Solid State Ionics 53-56 (1992) 1292-1301 North-Holland SOLID STATE IONICS Conductivity and crystallography of new alkali rare-earth silicates synthesized as possible fast-ion conductors S.M. Haile ~, B.J. Wuensch Department of Materials Science and Engineering, Massachusetts Institute o/'Technology, ('ambrtdge. MA 02139. L'X 1 T. Siegrist and R.A. Laudise AT&T. Bell Laboratories, Murray Hill NJ 07974. USA Eight potassium neodymium silicates have been synthesized as possible fast-ion conductors. Hydrothermal growth experiments in the high-silica region of the SiO2'HaO'NdzO3'K20 system yielded crystals of the followingphases (five of which are previously unreported): K3NdSi6015, K8Nd3SilaO32OH, KioNd4Si14039, K4Nd2SisO21, K3NdSisOIg, K12Nd2Si]8045, KsNd~Si2oOaT. and KNdg(SiO4)602. The compositions and crystallographic data were determined using electron microprobe measurements and precession X-ray photographs, respectively. Single-crystalintensity data for phases K3NdSi6OI5and KsNd3SilzO32OH were ob- tained with a four-circle diffractometer. Of these eight phases, six have been obtained as crystals large enough to permit conduc- tivity measurements along at least one crystallographic axis. Conductivities were measured from 300 to 900 °C in an air atmo- sphere using either complex impedance techniques or single-frequency ac methods. Our results show that activation energies range from 0.3 to 2.2 eV, that the conduction process does not strictly follow the Arrhenius equation, and that these silicates are signif- icantly anisotropic with respect to conductivity. In the K3NdSi6Ot5 phase this anisotropy corresponds well to expectations based on structural considerations. I. Introduction Many alkali rare-earth silicates, in which the alkali ion serves as the mobile species, exhibit high ionic conductivity. Examples include Na2ZnSiO4 [1 ], NasYSi40~2 [2], and the solid solution series Na~+~,ZrzSi,P3_xO~2, otherwise known as NASI- CON [ 3 ]. In this work we report the crystallography and conductivity of a number of potassium neodym- ium silicates synthesized in an attempt to find new fast-ion conductors. We have measured the proper- ties of hydrothermally grown single crystals in order to study further the relationship between ionic con- ductivity and structure. This work was motivated, in part, by a report by Pushcharovskii et al. [4] de- scribing the structure of K3NdSi60~5. A close ex- amination of their results suggested that this mate- Present address: Max-Planck-lnstitut fiir Festk6rperfor- schung, Heisenbergstr. 1, D-7000 Stuttgart 80, Germany: au- thor to whom all correspondence should be addressed. rial, based on structural considerations, was a likely fast-ion conductor. Alkali rare-earth silicates are particularly suited to an investigation of the relationship between con- ductivity and structure because of the possibility of very precisely tailoring the crystal chemistry. For ex- ample, the mobile alkali species can be replaced without significantly affecting the structure or, al- ternatively, the structure can be changed very grad- ually by making isovalent substitutions on the rare- earth site. Non-isovalent substitutions will affect the defect chemistry while leaving the silicate framc- work structure intact. Additionally, as the vast ma- jority of silicate compounds have non-cubic sym- metries, measurement of the complete conductivity tensor, a,j (using single crystals), provides a means of relating structural features to conductivity within a single compound. This has the added benefit of isolating structural effects from defect chemistry or dopant/impurity level effects. Lastly, the alkali atom in complicated silicate structures typically occupies 0167-2738/92/$ 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.