It has been well-known that silicon-based nitride phosphors such as CaAlSiN₃:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺ possess excellent red-light emission when excited by blue-light region. However, preparation of such silicon-based nitride phosphors in pure form should be needed to sinter precursor powders under high pressure and temperature conditions. On the contrary, Eu²⁺-activated silicates with alkaline earth ions have been reported as high efficient phosphors. Such silicon-based oxide phosphors in pure form can be easily obtained by simple preparation techniques without any critical conditions mentioned above. Therefore, productivity and production cost for silicate phosphors are advantageous for the commercial production of red phosphors. We have reported red-emitting Ca_1.2Eu_0.8SiO₄ phosphors excited by blue light region.¹⁾ The wavelength at the maximum emission of Ca_1.2Eu_0.8SiO₄ phosphors was 650nm.¹⁾ We have recently discovered new red-emitting Ca₃ZrSi₂O₉:Eu²⁺ phosphors with high Eu²⁺ concentration excited by near-UV and blue-light region. Ca₃ZrSi₂O₉ activated by low Eu²⁺ concentrations (Eu²⁺: 1 mol%) exhibited a green emission at 530 nm. However, with increasing Eu²⁺ concentration to 10 mol%, Ca₃ZrSi₂O₉:Eu²⁺ phosphors exhibited red emission peaking at 650 nm while the green emission disappeared. Based on the crystal structure data of Ca₃ZrSi₂O₉, there were three Ca sites in Ca₃ZrSi₂O₉, where Eu²⁺ ions were occupied.²⁾ Two of the Ca sites were smaller than another Ca site. We presume that emission at 530 nm from Eu²⁺ occupying the large Ca site was not observed owing to Eu²⁺ concentration quenching, as well as reabsorption of the green emission light from Eu²⁺ at the large Ca site by Eu²⁺ at the two small Ca sites. Thus, the deep-red emission of Ca₃ZrSi₂O₉:Eu²⁺ phosphors observed is considered to be emission from Eu²⁺occupying these small Ca sites.

¹⁾ Y. Sato, H. Kato M. Kobayashi, T Masaki, D. H. Yoon and M. Kakihana, Angew. Chem. Int. Ed. 53(2014) 7756.

²⁾ J. R. Plaister, J. Jansen, R. G. de Graaff, D. J. W. IJdo, J. Solid State Chem. 115 (1995) 464.