Studies on Ion Conducting Compounds with Cristobalite Related Structures
- PDF / 921,264 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 101 Downloads / 155 Views
STUDIES ON ION CONDUCTING COMPOUNDS WITH CRISTOBALITE RELATED STRUCTURES.
Jekabs Grins, S. Frosting and M. Nygren, Department of Inorganic Chemistry, Arrhenius Laboratory, University of Stockholm, S-106 91 Stockholm, Sweden
ABSTRACT
This article summarises ionic conductivity and structural studies in the sodium-ion conducting systems NaxM'X/ 2 M' 2-x/2 0 4, 2 a x 2 4/3 and (H',MI'') (Zn,Si), (Be,Si) and (Zn,Ge). The best conductors are found at x Z 1.75-1.85, with conductivities in the range 10-2_10-3 (Qcm)- 1 at 600 K. They exhibit stuffed cristobalite type structures, isotypic with the structure of orthorhombic Na2 BeSiO4 . Based on single crystal data, the Na 2 BeSiO4 structure has been determined and refined, both at 295 and 623 K. The structures of several other isotypic compounds have been refined by the Rietveld method, including Na1 . 8 (Zn 0 .sGa 0 . 2 )M04 with M = Si,Ge and Nal. 8 Beo.gSi 1 . 1 0 4 . The ionic conductivity properties of the materials are discussed in relation to the obtained structural data.
INTRODUCTION.
In the search for materials with high ionic conductivities, attention has partly been directed towards compounds with framework structures containing tunnels in which mobile ions are located, as e. g. Na 3 Zr 2 Si 2 POI 2 (NASICON). Following these guidelines we have studied the solid-electrolyte properties of a number of sodium-ion conducting silicate and germanate systems, containing phases with stuffed cristobalite type structures. From an ionic-conductivity point of view, an ideal stuffed cristobalite structure provides large cavities and interconnecting pathways for the conducting ions. The main part of our studies has concerned systems Na2 M'M''O4 -Na 2 M M''20% with (M',M'') = (Zn,Si), (Zn,Ge) and (Be,Si) [1-5]. The stoichiometry along these joins can be written as NaxM'x/ 2 M' '2-x/2 0 4 with 2 S x S 4/3. The sodium ion concentration is thus decreased by replacement of divalent M' atoms by tetravalent M'' atoms. A summary of conductivity data, phase relations and structural data for the best conductors is given below. Possible preferred pathways for sodium ion migration in the Na2 BeSiO4 structure are also described. PREPARATION AND PHASE ANALYSIS.
The materials were prepared by solid state reaction in air. The reaction products were characterized by their X-ray powder patterns obtained with a Guinier-Hhgg camera. Detailed descriptions of preparations and phase relations are given in Ref's. [1-5]. The best conductors in the systems 0 NaxM'x/2M''2-x/2 4 were found in the compositional range 2 2 x r1.75. Within this compositional range two different phases occur, both possessing cristobalite-related structures. The best conductors possess structures isotypic with the structure of orthorhombic Na2 BeSiO4 ; they are denoted as C-type phases below. Monophasic C-type samples suitable for conductivity measurements were obtained only Mat. Res. Soc. Symp. Proc. Vol. 210. @1991 Materials Research Society
658
within rather narrow compositional ranges ; 1.80SxS1.85 in the (Zn,Ge) s
Data Loading...
