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The incommensurate phases in the Ta1-xNbxTe4 charge-density wave system

A series of displacively modulated mixed crystals Ta1-xNbxTe4 (0<or=x<or=1) exist in which the modulation wavevector q varies as a function of composition. The nature of the compositional dependence of q is studied in greater detail and evidence is presented for a stepwise variation of q with x. Electron diffraction experiments at room temperature reveal that the modulation periodicity remains commensurate until a threshold dopant concentration (x approximately=0.3) is exceeded and then jumps discontinuously to an incommensurate value. Further jumps in q are observed as the dopant concentration is increased. Models of the superstructures of the doped crystals corresponding to plateaux in the dependence of q on x are studied by comparison of the observed and calculated electron diffraction patterns. The results indicate that the mixed crystals may be described as a series of long-period commensurate superstructures. In addition, satellite dark-field microscopy has revealed the presence of defects in the modulation structures of the doped crystals, including antiphase boundaries and discommensuration arrays. The effects of substitutional doping on the phase transitions occurring on heating above room temperature in TaTe4, and on cooling in NbTe4, are also examined. The observations are interpreted in terms of the charge-density wave origin of the modulations.