Evolution of charge density wave soft phonon modes in PdxErTe3

We investigated the lattice dynamics of quasi-two-dimensional Pd-intercalated ErTe3 in relation to its charge density-wave (CDW) transitions by means of x-ray diffuse and meV-resolution inelastic x-ray scattering. In pristine ErTe3, CDW order develops at orthogonal in-plane wave vectors qc 1 = (0,0,0.29) (the c-CDW) and qa 2 = (0.31,0,0) (the a-CDW), with transition temperatures Tc 1 = 270 K and Ta 2 = 160 K, respectively. Remarkably, we observe diffuse x-ray scattering already near the higher transition temperature Tc 1 along a-CDW but at a slightly different wave vector qa 1 = (0.29,0,0). Inelastic x-ray scattering for Pd0.01ErTe3 shows that a partial phonon softening at qa 1, underscoring the strong competition between ordering tendencies along the nearly equivalent in-plane axes of the orthorhombic lattice. For intercalation levels x ⩾ 0.02, the a-CDW state is suppressed. Nevertheless, a similar correlation between phonon softening and diffuse scattering persists along the [100] direction, again observed at qa 1 = (0.29,0,0) and Tc 1. These findings suggest that the a-CDW is fully suppressed for x ⩾ 0.02, and that the residual diffuse scattering at qa 1 originates from the partial phonon softening associated with the c-CDW, reflected by the near equality of the absolute size of qc 1 and qa 1.