Enhanced crystallinity and thermal stability of Ba2+ and Al3+–O2− co-substituted Sr2Si5N8:Eu2+†
By co-substituting a cation (M2+: Ba2+ or Ca2+) and Al3+–O2− into Sr2Si5N8:Eu2+ in a solid-state method, we successfully synthesized the nitrido-alumino-silicate phosphors Sr(2−y)MySi5−xAlxN8−xOx:Eu2+ (Ba-AlO-258 (My = Ba0.3, x > 0) and Ca-AlO-258 (My = Ca0.3, x > 0)), which were compared with conventional AlO-258 (y = 0, x > 0), Ba-258 (My = Ba0.3, x = 0), Ca-258 (My = Ca0.3, x = 0), and 258 (y = 0, x = 0). SEM exhibited that the morphology and particle size were very different between Ba-AlO-258 and the Ca-AlO-258 and AlO-258. The crystallinity was evaluated by determining the lattice strain from the peak-width of the XRD pattern. It was found that the crystallinity of Ba-AlO-258 was much higher than that of Ca-AlO-258 and AlO-258 under the same synthesis conditions. We proposed the significance of a size balance between the ionic part, (Ba, Sr, Ca)–(N, O)n, and covalent part, (Si, Al)–(N, O)4 tetrahedron, where the larger Ba2+ (than Sr2+) matches the larger Al3+ (than Si4+) in crystal 258. The emission peaks of the three phosphors shifted to longer wavelengths with the substitution of Si4+–N3− with Al3+–O2−. The temperature dependency of our new Ba-AlO-258 was the best in all phosphors. We found that the co-substitution of M2+ (Ba2+ or Ca2+) and (AlO)+ improved the temperature dependency, although the conventional single substitution of (AlO)+ made it worse. From the advanced thermo-luminescence measurement (excitation: blue light), the significant improvement of the temperature dependency of the emission at blue excitation was strongly considered due to a continuous supply of the red emission from the electron at the trap center. The new Ba-AlO-258 is believed to be the best blue LED phosphor in the well-known industrial 258 series because of its superior thermal property and crystallinity.