CW EPR Investigation of Red‐Emitting CaS:Eu Phosphors: Rationalization of Local Electronic Structure

Abstract A series of commercial and prepared CaS:Eu²⁺ rare earth activated phosphors are investigated following different post‐synthetic treatments. A number of species directly related to the function of the material are characterized using electron paramagnetic resonance (EPR) spectroscopy. Isolated Eu²⁺ sites are identified and associated with the substitutional doping for Ca²⁺ in the lattice which are responsible for the 645 nm emission of interest. Another inactive Eu²⁺ site based within a “EuO” type phase aggregated on the surface of the material is also identified, as well as competitive F⁺ center defects that are known to reduce emission from the Eu²⁺ sites. Intrinsic Mn²⁺ impurities are identified and used as local order probes to determine changes in ordering and symmetry upon cryo‐milling and heating treatments of the samples. X‐ray photoelectron (XPS) and photoluminescence (PL) spectroscopies are also conducted to complement the local structure observations. The reported data is useful in understanding how the nature of the lattice affects ground state electronic structure of functional defective sites, for the development of efficient and selective materials.