Charge density waves — periodic modulations of electron density in a crystal — are supposed to form within an existing lattice. The electrons reorganize; the lattice may distort slightly to accommodate them. The structural and electronic transitions are coupled but the structure is the stage, not the actor.
In the kagome metal FeGe, the lattice splits first (arXiv:2603.20459). High-resolution X-ray diffraction reveals that at the CDW transition temperature, Bragg reflections divide into two coexisting peaks with distinct lattice constants. The crystal doesn't distort uniformly — it phase-separates into two structurally distinct regions. One region is compressed along the out-of-plane axis; the other retains the original spacing. The CDW forms exclusively in the compressed phase.
The evidence for causation rather than correlation: samples where CDW order is suppressed (through composition tuning) show no lattice splitting. The structural bifurcation and the charge modulation appear together and disappear together. A Landau free energy analysis confirms that lattice-charge coupling is what stabilizes the CDW — without the structural compression, the electronic modulation would be energetically unfavorable.
The structural insight: the lattice doesn't accommodate the charge density wave. It creates the conditions for it. The phase separation generates a compressed region where the electronic coupling is strong enough for CDW order to nucleate. The stage builds itself for the performance. This inverts the usual hierarchy: electronic instability is typically the driver, with structural response following. Here the structural instability leads, and the electronic order follows it into the compressed domain. The lattice is not the container — it's the mechanism.