Channel structures in alkali-metal-doped conjugated polymers: Broken-symmetry two-dimensional intercalation superlattices

Conjugated polymers doped with alkali metals typically exhibit ordered two-dimensional (2D) superlattices of linear chains and alkali-metal channels, analogous to guest-host monolayer sequences in layer intercalates. A common feature is a superlattice periodicity that varies with dopant concentration. Unique to doped polymers, the local 2D symmetry that defines the superlattice building block can vary with concentration and with the relative sizes of the dopant and chain projection normal to its axis. We present x-ray-diffraction measurements of stage-2 and stage-1 K-doped trans-polyacetylene, and of stage-1 Cs-doped polyparaphenylene vinylene (PPV). In all three cases, detailed profile fits are inconsistent with the highly symmetric intercalation channels previously proposed. We give evidence for new structural models in which the symmetries are broken by rotations and translations of the polymer chains. In the case of stage-2 K-(CH${)}_{\mathit{x}}$, this takes place by a rotation and translation of (CH${)}_{\mathit{x}}$ chains, lowering the 2D lattice symmetry from P4mm to P4. In stage-1 K-(CH${)}_{\mathit{x}}$, translational distortions reduce the symmetry from P4mm to P4gm, while in stage-1 Cs-PPV a similar local symmetry reduction arises from chain rotations that do not affect the space-group symmetry. We offer several candidates as the driving force for the broken symmetries and discuss the implications for phase-diagram and band-structure calculations.