Spin-lattice and spin-electronic interactions in the van der Waals semiconductor C o2 P2 S6

We report the results of a temperature-dependent reflection spectroscopy study, across a wide energy range (∼0.01 to 3 eV), of unsubstituted and 40% Zn substituted for Co, for the transition-metal phosphorus trichalcogenide, Co2P2S6. We observe a transition from a paramagnetic to antiferromagnetic state with a Néel temperature at TN∼120 K that is completely suppressed in Zn-substituted samples. At 300 K we identify four narrow (∼1 meV) infrared active phonon modes while at 70 K (below TN) we observe that the low-energy phonons, dominated by Co motion, resolve into two modes. These low-energy modes are asymmetric, indicating coupling to a broad electronic continuum. We also report a broad (∼30 meV) low-temperature infrared absorption band that appears near TN that we suggest is determined by a multiphonon-assisted 2-magnon absorption process. At 300 K in higher-energy spectra, we observe considerable absorption starting from 0.20±0.02 eV which we associate with inter-Co2+ ion 3d transitions. At temperatures below TN the number of electronic absorption bands increases from 4 to 6, indicating a lowering of the symmetry around the Co2+ ions. On substituting 40% Zn for Co, the antiferromagnetic transition is suppressed along with temperature-dependent changes in the phonon and electronic spectra. The temperature-dependent spectral changes indicate strongly correlated behavior between the infrared active lattice vibrations, the electronic excitations, and magnetism in unsubstituted Co2P2S6.