Crystal structure of polyacetylene revisited: An x-ray study

We present new x-ray diffraction data on trans-(CH)x which shows that the blond alteration is out of phase on adjacent chains, and therefore the correct space group is P21n. Previous attempts to resolve this question by analyzing only the (00L) intensities have yielded ambiguous results; all experiments show some (001) intensity which is not allowed P21n. By analyzing indidual off-axis peak intensities, we show unambiguously that the bulk 3-D structure is inconsistent with P21a (in-phase bond alternation), and therefore the weak, sample-dependent (001) intensity probably results from uncorrelated local defect regions with in-phase bond alternation on adjacent chains.