Microstructural behaviour in the CDW states of NbTe₄and TaTe₄; domains, discommensurations and superlattice symmetry

The lattice distortions of NbTe4 and TaTe4 are investigated by electron microscopy and electron diffraction. Evidence is presented for a high-symmetry distortion, with a space group of P4/mcc for sublattices and commensurate superlattices in both materials. Satellite dark-field images show discommensuration arrays within the incommensurate state at NbTe4 which are replaced in a fascinating microstructural display during a slow, strongly first-order, lock-in transition by a network of commensurate domains. Similar commensurate domain networks are also imaged in room-temperature TaTe4. The distortions are discussed from the basis of crystal symmetry and simple landau free-energy arguments.