Structural Phase Separation Couples to Charge-Density-Wave Formation in Kagome Metal FeGe

The intertwining of charge, spin, and lattice degrees of freedom underlies the emergent properties of correlated materials. A recent prominent example is the kagome metal FeGe, which hosts a charge density wave (CDW) within a preexisting antiferromagnetic state. Although partial Ge-Ge dimerization is known to accompany CDW formation, the thermodynamic pathway through which long-range CDW order emerges remains unclear. Using temperature-dependent high-resolution x-ray diffraction measurements, we observed a robust splitting of the lattice reflection into two coexisting peaks with distinct lattice parameters in FeGe crystals exhibiting long-range CDW order at the CDW transition temperature. In addition, the long-range CDW order emerges concurrently and remains commensurate with a structural phase characterized by the smaller out-of-plane lattice parameter. Our observation provides direct evidence for a first-order structural phase transition coupled to long-range CDW order, which is absent in samples exhibiting short-range CDW order. A Landau model shows that strong charge-lattice coupling is a key factor in stabilizing the long-range CDW order. Our Letter reveals that the first-order structural phase transition is crucial for stabilizing long-range CDW order, suggesting lattice strain as a tuning parameter for controlling intertwined electronic states in kagome metals.