Enhanced crystallinity and thermal stability of Ba2+and Al3+-O2-co-substituted Sr2Si5N8:Eu2+

By co-substituting a cation (M2+: Ba(2+)or Ca2+) and Al3+-O(2-)into Sr2Si5N8:Eu(2+)in a solid-state method, we successfully synthesized the nitrido-alumino-silicate phosphors Sr(2-y)MySi5-xAlxN8-xOx:Eu2+(Ba-AlO-258 (M-y= Ba-0.3,x> 0) and Ca-AlO-258 (M-y= Ca-0.3,x> 0)), which were compared with conventional AlO-258 (y= 0,x> 0), Ba-258 (M-y= Ba-0.3,x= 0), Ca-258 (M-y= Ca-0.3,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+-N(3-)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+(Ba(2+)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.