  Kagome metals are widely recognized as versatile platforms for exploring
novel topological properties, unconventional electronic correlations, magnetic
frustration, and superconductivity. In the $R$V$_6$Sn$_6$ family of materials
($R$ = Sc, Y, Lu), ScV$_6$Sn$_6$ hosts an unusual charge density wave ground
state as well as structural similarities with the $A$V$_3$Sb$_5$ system ($A$ =
K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with
first-principles lattice dynamics calculations to reveal the charge density
wave state in ScV$_6$Sn$_6$. In the low temperature phase, we find a five-fold
splitting of the V-containing totally symmetric mode near 240 cm$^{-1}$
suggesting that the density wave acts to mix modes of $P$6/$mmm$ and
$R$$\bar{3}$$m$ symmetry - an effect that we quantify by projecting phonons of
the high symmetry state onto those of the lower symmetry structure. We also
test the stability of the density wave state under compression and find that
both physical and chemical pressure act to quench the effect. We discuss these
findings in terms of symmetry and the structure-property trends that can be
unraveled in this system.
