The structure of the incommensurate superlattices of 2H–TaSe2

Abstract

Abstract Reinterpretation of the neutron diffraction study of 2H–TaSe2 by Moncton, Axe and DiSalvo (1977) reveals an ambiguity in the sense of the displacements proposed for the commensurate superlattice structure. We attempt to resolve this ambiguity by electrostatic and short-range energy calculations of the phase dependence of the energy of the periodic structural distortion wave. There is a fine balance between Se–Se short-range repulsion, Ta–Se electrostatic and short-range repulsion and the CDW–Ta-ion interaction energy terms. The analysis reveals the phase dependence of the various terms and allows the different contributions to the stability of the distortion waves to be discussed more completely than was possible previously. It is shown that the phase of the PSD or CDW waves are determined, except for an ambiguity of π rad, by minimizing (harmonic) energy terms, which vary as the square of the PSD wave amplitude. This phasing is established above the normal/incommensurate onset temperature T0, but the π ambiguity is not resolved finally until the incommensurate/commensurate transition temperature Tc, by a minimization of the (anharmonic) energy terms which vary as the third power of the PSD wave amplitude. Our analysis leads naturally to a new structural model for the incommensurate superlattice structure for Tc≤T≤T0, involving contributions from both the preferred phase ø and the phase ø ±π which simply corresponds to changing the sign of all displacements. The model is the logical consequence of a ‘softening-mode’ phase transition and provides a very simple structural explanation for the observed incommensurate superlattice periodicity, 3·08α at onset, and its temperature dependence.