Crystal-Field-Tuned Spin-Flip Luminescence in NiPS₃

Layered magnetic materials provide the opportunity for fundamental investigations of magnetism in the two-dimensional limit. NiPS₃, a prototype member of this family, is antiferromagnetic below 155 K and exhibits sharp photoluminescence associated with a transition between a triplet ground state and a singlet excited state. The nature of the luminescent transition is a matter of current debate and so is an eventual fundamental link of this excitation to magnetism. We synthesize samples with metal and ligand substitutions altering the Néel transition temperature and measure the effects of these changes on temperature dependent photoluminescence. We perform crystal field and charge transfer multiplet calculations to explain the origin of the excitation and identify it as spin flip luminescence originating in Ni crystal field levels. Through observations of phonon side bands we confirm that it is localized at the Ni site as opposed to a possible coherent delocalized excitation between hybridized Zhang-Rice states. Alloying with Zn and Se shows a lack of correlation between the temperature dependence of the luminescence and the Néel temperature, belying its supposedly magnetic nature.