Evolution of the tridimensional organization of alkali metal doped polyacetylene complexes (MyCH) x with the doping level for M=Na,K,Rb,Cs

Abstract

The evolution of the tridimensional organization of (M yCH) x complexes with M=Na,K,Rb,Cs has been examined as a function of the doping level y. The highly crystalline starting polyacetylene, synthesized according to the Akagi procedure, was stretched to realize an elongation of 600 %. Such an orientation which allows the separation of the different families of X-ray reflections (001, hk0 and hkl) is necessary to appreciate the 3D order of the materials studied. For samples doped with heavy alkali metals, it appears that for molar doping levels y≤0.125, the polymer and the alkali metal lattices are commensurate : along the c-axis one alkaline atom is always associated with 4(CH) units (4.96Å) ; for y = 0.0625, a short distance order in (a,b) direction agrees however with a tetragonal system corresponding to Cmm or P 2 symmetries for the projection of the unit cell along the chain axis - the ideal formula for a stage 2 compound would be [M(C 4H 4) 4] x - for y = 0.125, polymer and alkali metal networks are commensurate and describe a centred tetragonal lattice corresponding to the formula [M(C 4H 4) 2] x. For doping levels higher than 0.125 the commensurability disappears. The periodicities of the alkaline species along the c-axis which depend on the size of the inserted counterion are 3.98, 4.02 and 4.07 Å for K, Rb and Cs respectively in saturated compounds. At the same time, the periodicity along the polymer chain is 1.250 Å. These data show that the formula [M(C 3H 3) 3] x is never obtained. For sodium doped compounds, an hexagonal symmetry is found according to other authors. We found that the periodicities of the sodium atoms along the c-direction are 4.96 Å (1 Na for 4(CH) units) and 4.20 Å respectively for lightly and heavily doped materials. It must be also noticed that the alkali metal doping improves the coherence lengths and decreases the mosaic along the fiber axis at least for heavily doped samples.