Chemical pressure and charge-density waves in rare-earth tritellurides

We report the results of transmission electron microscopy on the layered rare-earth tritellurides R${\mathrm{Te}}_3$ (R=La,Sm,Gd,Tb,Dy,Ho,Er,Tm). Through electron diffraction we have identified superlattice reflections indicating the presence of incommensurate distortions, consistent with sinusoidal atomic displacements in the square Te sheets. The superlattice wave vector corresponds to the maximal Fermi-surface nesting wave vector determined from extended Hückel tight-binding band calculations. The charge-density wave (CDW) is stable under the volume decrease obtained by substituting the heavier rare earths, and the distortion wave vector scales with the lattice parameters across the rare-earth series. Our results indicate that the rare-earth tritellurides host small-amplitude Fermi-surface-driven distortions. We find no evidence for substantial deviation from sinusoidal atomic displacements, in contrast to the large commensurate distortions and ordered vacancy structures found in other rare-earth polychalcogenide phases. Our observations establish the rare-earth polychalcogenides as a family of CDW hosts in which a variety of structural distortions occur with variation of chalcogen type and stoichiometry, despite a very simple but universal chalcogen sheet band structure.