LOW-TEMPERATURE STRUCTURES OF STAGE-2 CESIUM GRAPHITIDES WITH VARIOUSCOMPOSITIONS

Compounds with the compositions CsC22, CsC24, and CsC28 were synthesiz ed from H.O.P.G platelets and characterized by X-ray diffraction. All the specimens are basically 2nd stage phases. CsC24 appears as an almo st pure phase, whereas CsC22 and CsC28 contain respectively a trace of 1st stage CsC8, and a relatively large amount of 3rd stage phase. Fou rier analysis of the 00l reflexions shows that the composition range f or the 2nd stage compound seems to be very limited, and close to CsC24 . From the 00l scans realized in the range 300 K-77 K, a first order t ransition was observed for the 2nd stage phase at about 155 K, giving two new 2nd stage phases, respectively with Ic=9.33 Angstrom (main pha se), and Ic=9.58 Angstrom. This transition is attributed to the freezi ng of the cesium film leading to a 2D segregation inside a unique grai n. The domains could be separated by pleats distributed along a canted front.