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

Abstract

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)M_ySi_5−xAl_xN_8−xO_x: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⁺–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.