Driving mechanism and dynamic fluctuations of charge density waves in the kagome metal ScV6Sn6

In contrast to the AV₃Sb₅ (A = K, Rb, Cs) family exhibiting the ubiquitous 2×2×2 charge density wave (CDW) order, only ScV₆Sn₆ in the RV₆Sn₆ (R = Sc, Y, La) family displays the unusual CDW with a √3×√3 in-plane ordering and a tripling of the unit cell along the c axis. Here, using first-principles density-functional theory calculations, we show that both the √3×√3×2 and √3×√3×3 CDW orderings can be driven by a Jahn-Teller-like effect where the Sn atoms residing in the kagome bilayers partially undergo delicately different schemes of interlayer dimerization, accompanied by charge redistribution between such Sn atoms and band-gap opening. Counterintuitively, whereas the √3×√3×2 phase is energetically more stable than √3×√3×3, the latter is thermodynamically stabilized above the CDW transition temperature TCDW by its higher configurational entropy contributed by degenerate fluctuating phases, and is kinetically selected below TCDW, as corroborated by experimental observation of the accompanying first-order phase transition. Our findings reveal the order-disorder nature of the √3×√3×3 CDW phase transition in ScV₆Sn₆, with broader implications for understanding charge orderings and CDW fluctuations in other kagome metals.