Raman Investigation of the 2D Ferromagnet AgVP₂Se₆

Metal thio/seleno-phosphates (MTPs) are 2D magnets which offer rich optoelectronic properties. Prior studies on antiferromagnetic ternary MTPs probed Ising-type magnetism and magnon-phonon hybridization using magneto-Raman spectroscopy. Recent exploration of the quaternary MTPs AgVP₂Se₆ demonstrated ferromagnetism below 18 K; however, spin-phonon coupling in this material remains unexplored. We conduct temperature- and polarization-resolved Raman spectroscopy of AgVP₂Se₆ crystals grown by two methods: chemical vapor transport (CVT) and horizontal flux growth. The unusually low symmetry of this material leads to many Raman active modes, all of which are accounted for by DFT calculations. We find no modification of the Raman spectrum through the magnetic transition but do observe a change at ~100 K which suggests a previously unobserved structural phase transition. Interestingly, DFT calculations at T=0 converge to a lower symmetry (P1) than reported in the literature (C2). The spectra of CVT- and flux-grown samples exhibit clear differences; flux samples show fewer modes and broader peaks, which we attribute to increased stacking disorder. Our work reveals important connections between growth method and magnetic state that will guide future explorations of this exciting material. Additionally, our DFT calculations explain the experimentally reported dramatic difference between the activation gap and the optical gap.