F. W. Boswell

Charge density waves in some Nb and Ta chalcogenides

Abstract A brief description of the competing mechanisms important for the stability of composition- and temperature-dependent modulation phases of Nb 1− x Ta x Te 4 is given, with some emphasis on the microstructural changes at the two high temperature phase transitions in TaTe 4 . The charge density wave sliding in NbSe 3 is explained on the basis of a structural model which takes into account the easy switching between the two modulation wavevectors along the same trigonal prismatic columns. The dependence of the interpolytypic transitions and various charge density wave transition temperatures on the amount of intercalated Ag in 1T-TaS 2 is discussed.

Determination of the symmetry of charge-density-wave modulations in TaTe4 by HREM

Abstract TaTe4 is a quasi-one-dimensional crystal which at room temperature displays a periodic lattice distortion associated with charge-density waves [Boswell et al., J. Phys. C16 (1983) 1067]. The structure of TaTe4 has been studied in detail over the past few years, both by X-ray and electron diffraction and by electron microscopy. The literature, however, contains conflicting reports concerning the symmetry of the superlattice modulations. Eaglesham et al. [J. Phys. C18 (1985) 1] have investigated TaTe4 by CBED and determined the superstructure space group to be P4/mcc. Sahu and Walker [Phys. Rev. B32 (1985) 1643], on the other hand, predicted that the space group should be P4/ncc, on the basis of Landau theory calculations. Recently, Bronsema et al. [Acta Cryst. B43 (1987) 305] investigated the structure by X-ray diffraction and concluded that the symmetry is P4/ncc [4]. In the present study the modulations were investigated by high resolution electron microscopy (HREM). Images recorded in the [210] orientation confirm the absence of a simple mirror plane normal to the c- axis in the tetragonal structure. A comparison of experimental and computed images supports the model for the modulated structure deduced by X-ray diffraction.

The Instability of the NbTe2 Surface Structure

Low energy electron diffraction from clean NbTe 2 surfaces shows very diffuse reflections. The effect is attributed to an anisotropic heating of the irradiated Te-Nb-Te surface layer. Diffraction patterns for electron energies below 90 eV correspond to an overlapped contribution from numerous domains, belonging to three orientational variants. Electrons of higher energies stabilize the parent high-temperature CdI 2 structure. A similar effect is observed during scanning tunneling microscopy, where the expected surface corrugation is usually lost for tunneling currents of a few ten nA.

Advances in the Crystallographic and Microstructural Analysis of Charge Density Wave Modulated Crystals

Preface. Alternative Approaches to the Crystallographic Description of Charge Density Wave Modulated System A. Prodan, A. Budkowski. X-Ray Crystallographic Analysis of the Charge Density Wave Modulated Phases in the NbTe4 -- TaTe4 System H. Bohm. Charge Density Wave Phase Transitions and Microstructures in the TaTe4 -- NbTe4 System J.C. Bennett, F.W. Boswell. Transmission Electron Microscopy of CDW-Modulated Transition Metal Chalcogenides J.M. Corbett. Influence of Defects and Impurities on Charge Density Wave Systems H. Mutka. Analysis of Scanning Tunneling and Atomic Force Microscopy Images M.-H. Whangbo et al. Elucidating Complex Charge Density Wave Structures in Low-Dimensional Materials by Scanning Tunneling Microscopy H. Dai et al. Index of Subjects.

Enhanced CDW Transitions in Nb3X4 (X = S, Se, Te): Intercalation and Surface Effects

A x Nb 3 X 4 (A = In, Tl, ZnHg; X = S, Se, Te) compounds show CDW instabilities dependent on the type and concentration of intercalate. Tl or In intercalation flattens the Fermi surfaces and supports CDW formation. In the corresponding DOS spectrum the Fermi level is shifted towards coincidence with a small peak, derived mainly from the Nb dz 2 orbital. Localized modulated regions observed in STM images of Nb 3 X 4 at room temperature represent precursor effects to full CDW formation.

Scanning tunneling microscopy of defects in NbSe2

Defects in the quasi-two-dimensional compound NbSe2 were studied by scanning tunneling microscopy. Atomic vacancies as well as substitutional and interstitial defects, which require an accommodation of the surrounding chalcogen atoms, were identified in as-grown crystals. Defects were also introduced after growth, both by heating the crystals and by bombarding their surfaces with Ar+ ions. Heating the crystals for only 2 min to about 470 K causes in addition to Se vacancies local changes in the surface structure with reduced symmetry as compared to that of the bulk. Ar+ ions with energies and doses not exceeding 400 eV and 1013 ions/cm2 form large surface craters as well as Se vacancies accompanied by mobile adatoms.

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.