Symmetry crossover in layered M PS 3 complexes ( M = Mn , Fe , Ni ) via near-field infrared spectroscopy

We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-sheet, and single-sheet members of the $M{\\mathrm{PS}}_{3}$ $(M=\\mathrm{Mn}, \\mathrm{Fe}, \\mathrm{Ni})$ family of materials and compare our findings with complementary lattice dynamics calculations. ${\\mathrm{MnPS}}_{3}$ and the Fe analog are similar in terms of their symmetry crossovers, from $C2/m$ to $P\\overline{3}1m$, as the monolayer is approached. These states differ as to the presence of a ${C}_{3}$ rotation around the metal center. On the other hand, ${\\mathrm{NiPS}}_{3}$ does not show a symmetry crossover, and the lack of a ${B}_{u}$ symmetry mode near 450 ${\\mathrm{cm}}^{\\ensuremath{-}1}$ suggests that ${C}_{3}$ rotational symmetry is already present, even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and those that take place under pressure.