Order-disorder charge density wave instability in the kagome metal (Cs,Rb)V3Sb5

The origin of the charge density wave phases in the kagome metal compound AV(3)Sb(5) is still under great scrutiny. Here, we combine diffuse and inelastic x-ray scattering to identify a 3-dimensional precursor of the charge order at the L point that condenses into a CDW through a first order phase transition. The quasi-elastic critical scattering indicates that the dominant contribution to the diffuse precursor is the elastic central peak without phonon softening. However, the inelastic spectra show a small broadening of the Einstein-type phonon mode on approaching T-CDW. Our results point to the situation where the Fermi surface instability at the L point is of order-disorder type with critical growth of quasi-static domains. The experimental data indicate that the CDW consists on an alternating Star of David and trihexagonal distortions and its dynamics goes beyond the classical weak-coupling scenario and is discussed within strong-electron phonon coupling and non-adiabatic models. The mechanism of the charge density wave in kagome metals is under intense debate. Here, by using a combination of diffuse scattering and inelastic x-ray scattering, the authors show that the charge density wave transition in (Cs,Rb)V3Sb5 is of the order-disorder type.