The Temperature Dependent Shear-Strain of the (NbSe$\bf_4$)$\bf_{10}$I$\bf_3$ Compound, a Quasi-One-Dimensional Charge Density Wave System, below the Peierls Transition

An X-ray study of the quasi-one-dimensional charge density wave (CDW) system (NbS$_4$)$_{10}$I$_3$ as a function of temperature from room temperature down to 130 K has been performed by taking oscillation and zeroth level Weissenberg photographs. A reversible transformation of the room temperature tetragonal lattice into four symmetrically-related monoclinic domains has been observed at the Peierls transition temperature, $T_{\rm P}\approx 285$ K, with a continuous change of the monoclinic angle from 90° to 92.1° at 130 K. The monoclinic deformation of the tetragonal lattice, i.e. the shear-strain along the chain axis seems to be an elastic response to the long wavelength transverse modulation of the 3D-CDW order which can be qualitatively explained by taking into account the Coulomb interaction between CDWs on neighbouring chains up to the next-nearest-neighbours