Low temperature structures of the second stage cesium graphitide and effect of trace impurities
- PDF / 479,184 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 44 Downloads / 151 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Low temperature structures of the second stage cesium graphitide and effect of trace impurities L. Duclaux, I. Rannou, and F. B´eguin C.R.M.D., C.N.R.S.-Universit´e, 1B rue de la F´erollerie, 45071 Orl´eans Cedex 02, France
M. Lelaurain L.C.M.A., Universit´e Nancy I, B.P. 239, 54506 Vandoeuvre l`es Nancy, France (Received 8 May 1995; accepted 13 November 1995)
Stage 2 CsC24 graphite-cesium derivatives were synthesized and characterized using x-ray diffraction. Pure CsC24 specimens are single phase stage 2 in the range 77 –300 K. From the 00l scans, we observed on samples slightly polluted during the transfer in the glove box, the tridimensional segregation of stage 2 in a main CsC26 structure and an additional dense CsC20 structure. The 2D Laue diffraction photographs are the same as already reported, but are interpreted as a mixture of 2 3 2 R 0± lattice commensurate and 2.54 3 2.54 R 14.5± lattice incommensurate structures. In oxidized compounds, we identified another 2.23 3 2.23 R 25.5± 2D additional incommensurate structure corresponding to the composition CsC20 already evidenced by analysis of the 00l diffractograms. We suggest that oxygen impurities are at the origin of this particular stiff and dense structure at low temperature.
I. INTRODUCTION
The crystallo-chemistry of the graphite-alkali metal intercalation compounds was studied by R¨udorff and Schulze1 who suggested an ideal composition MC24 for the second stage derivatives (M : alkali metal). Parry found an order-disorder reversible transition in CsC24 close to 160 K2 using x-ray diffraction. At room temperature the cesium layer forms a liquid film modulated by the graphite host, and it gets ordered in a 2D structure at 165 K. In a detailed study of the order-disorder transition in CsC24 , Clarke reported an orientational ordering of the cesium atoms at 228 K.3 The low temperature diffraction pattern is composed of reflections from a commensurate (2 3 2 R 0±) and reflections from an incommensurate modulated phase. The low temperature 2D structure was interpreted by Clarke in terms of a modulated structure with discommensuration walls whereas for explaining the same diagrams, DiCenzo developed a model of relaxed incommensurate structure.3,4 However, these two models were unable to explain the commensurate reflections. The isobar of the cesium-graphite system shows that the stability domain of the second stage phase is limited by the compositions CsC23 and CsC27 .5 In a previous paper, we discussed the influence of the stoichiometry on the structure of the second stage compound6 by following the thermal evolution of the 00l lines for three compositions: CsC22 , CsC24 , and CsC28 . Our experiments gave no indication on the stability domain and we found that, whatever the composition, the formula for the second stage was CsC24 . Moreover, independently of 608
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 3, Mar 1996
Downloaded: 31 Mar 2015
the overall composition, the same evolution of the 0
Data Loading...
