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50 Handbook on the Physics and Chemistry of Rare Earths Although the structure types are different from each other, these materials have a similar layered structure, in which AE layers are separated by highly condensed (Si2O2N2)2 layers. The (Si2O2N2)2 layers are built up of corner-sharing SiON3 tetrahedra in which all nitrogen atoms are shared between three neighboring tetra- hedra, while each oxygen atom is terminally bonded to the tetrahedra. The SrSi2O2N2 and BaSi2O2N2 compounds have a similar an up/down sequence of the SiON3 motifs and, however, the silicate layers are shifted toward each other in SrSi2O2N2 (Kechele et al., 2009a,b; Seibald et al., 2011). In BaSi2O2N2, Ba2+ ions adopt eightfold oxygen coordination in the form of a cuboid that is additionally capped by two nitrogen ions. In SrSi2O2N2, Sr2+ ions adopt sixfold oxygen coordi- nation in the form of a distorted trigonal prism—leading to the shift of the silicate layer, as in BaSi2O2N2—and are additionally capped by one nitrogen atom. CaSi2O2N2 has similar crystallographic environments around Ca2+ ions to that of SrSi2O2N2, but the up/down sequence of SiON3 tetrahedra is different (Kechele et al., 2009a,b; Zeuner et al., 2011). As shown in Fig. 48, the excitation spectra of AESi2O2N2:Eu2+ (AE1⁄4Ca, Sr, and Ba) have strong broad bands attributed to the crystal-field splitting of 5d of the excited 4f65d1 configuration of Eu2+ (Bachmann et al., 2009b; Gu et al., 2010; Li et al., 2015b). These phosphors can be efficiently excited by near-UV or blue light. The emission bands show apparent red shifts in the order BaSi2O2N2:Eu2+ < SrSi2O2N2:Eu2+ < CaSi2O2N2:Eu2+, due to the increase in crystal-field strength around Eu2+. BaSi2O2N2:Eu2+ shows an extremely blue–green emission band peaking at 494 nm, suggesting a higher symmetry around Ba sites than those of Sr and Ca sites. SrSi2O2N2:Eu2+ exhi- bits a broad green emission peak at 537 nm. CaSi2O2N2:Eu2+ shows a broad 1 0.5 0 1 0.5 0 1 0.5 0 ex. em AE = Ba AE = Sr AE = Ca 300 500 600 700 Wavelength (nm) 400 Excitation and emission spectra of AESi2O2N2:Eu2+ (AE1⁄4Ca, Sr, Ba). Redrawn from Bachmann, V., Ronda, C., Oeckler, O., Schnick, W., Meijerink, A., 2009. Color point tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for white light LEDs. Chem. Mater. 21, 316–325. FIG. 48 Normalize intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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