Geometry of the charge density wave in kagom${e}$ metal AV$_{3}$Sb$_{5}$

Kagom${e}$ lattice is a fertile platform for topological and intertwined electronic excitations. Recently, experimental evidence of an unconventional charge density wave (CDW) is observed in a Z2 kagom${e}$ metal AV$_{3}$Sb$_{5}$ (A= K, Cs, Rb). This observation triggers wide interests on the interplay between frustrated crystal structure and Fermi surface instabilities. Here we analyze the lattice effect and its impact on CDW in AV$_{3}$Sb$_{5}$. Based on published experimental data, we show that the CDW induced structural distortions is consistent with the theoretically predicted inverse star-of-David pattern, which preserves the $D_{6h}$ symmetry in the kagom${e}$ plane but breaks the sixfold rotational symmetry of the crystal due to the phase shift between kagom${e}$ layers. The coupling between the lattice and electronic degrees of freedom yields a weak first order structural transition without continuous change of lattice dynamics. Our result emphasizes the fundamental role of lattice geometry in proper understanding of unconventional electronic orders in AV$_{3}$Sb$_{5}$.