  Phonon-derived behaviors are important indicators of novel phenomena in
transition metal trihalides, including spin liquid behavior, two-dimensional
magnetism, and spin-lattice coupling. However, phonons and their dependence on
spin structure and excitations have not been adequately explored. In this work,
we probe and critically examine the vibrational properties of bulk $CrCl_3$
using inelastic neutron scattering and density functional theory. We
demonstrate that magnetic and van der Waals interactions are essential to
describing the structure and phonons in $CrCl_3$; however, the specific spin
configuration is unimportant. This provides context for understanding thermal
transport measurements as governed by dynamical spin-lattice couplings. More
importantly, we introduce an efficient dynamic method that exploits
translational symmetries in large conventional unit cells that generates
insights into phonon dispersions, interactions, and measured spectra in terms
of quantum phase interference conditions. This work opens new avenues for
understanding phonons in layered magnets and more generally in conventional
cell geometries of a variety of materials.
