Effect of Rare Earth Element Codoping on the Mn Red Emission in Ca and Sr Thiogallates: Focusing on Its Mechanism through Electron Spin Resonance Study of Single Crystals

The weak Mn2+ involved red emission in CaGa2S4 can be greatly enhanced by codoping with a suitable rare earth element (REE). To clarify the involved physical mechanism, the actual Mn sites in the host crystal are investigated through electron spin resonance (ESR) measurements of single crystals. Three different patterns of ESR signals with different angular dependences are observed by rotating the magnetic field around the three crystal axes. As a result, three independent sites occupied by the Mn2+ ions are confirmed. Based on the experimental finding that the hyper fine constants (HFS) for the three sites are nearly the same, the Mn2+ ion sites may be ascribed to the Ca or Ga sites. To finally solve this ascription issue, ESR measurements of Eu2+ in CaGa2S4 are carried out, where the Eu2+ ions have been shown to exclusively substitute at Ca sites. From the similar angular variation of the ESR signals of Eu2+ and Mn2+, the Mn substitutional sites are finally determined to be the three Ca ones. A broad ESR signal appearing in the vicinity of the Lande g factor (g = 2.0) suggests the possible presence of Mn clusters. We have to investigate to see whether these may be the actual emission centers.