Role of interface states associated with transitional nanophase precipitates in the photoluminescence enhancement of SrTiO3∶Pr3+,Al3+

Abstract

SrTiO₃∶Pr³⁺,Al³⁺ phosphor samples with varying ratios of Sr/Ti/Al were prepared by the gel-carbonate method and the mechanism of enhancement of the red photoluminescence intensity therein was investigated. The photoluminescence (PL) spectra of SrTiO₃∶Pr³⁺ show both ¹D₂ → ³H₄ and ³P₀ → ³H₄ emission in the red and blue spectral regions, respectively, with comparable intensity. The emission intensity of ¹D₂ → ³H₄ is drastically enhanced by the incorporation of Al³⁺ and excess Ti⁴⁺ in the compositional range Sr(Ti,Al_y)O_3+3y/2∶Pr³⁺ (0.2 ≤ y ≤ 0.4) and SrTi_1+xAl_yO_3+z∶Pr³⁺ (0.2 ≤ x ≤ 0.5; 0.05 ≤ y ≤ 0.1; z = 2x + 3y/2) with the complete disappearance of the blue band. This cannot be explained by the simple point defect model as the EPR studies do not show any evidence for the presence of electron or hole centers. TEM investigations show the presence of exsolved nanophases of SrAl₁₂O₁₉ and/or TiO₂ in the grain boundary region as well as grain interiors as lamellae which, in turn, form the solid-state defects, namely, dislocation networks, stacking faults and crystallographic shear planes whereby the framework of corner shared TiO₆ octehedra changes over to edge-sharing TiO₅–AlO₅ strands as indicated from the ²⁷Al MAS NMR studies. The presence of transitional nanophases and the associated defects modify the excitation–emission processes by way of formation of electronic sub-levels at 3.40 and 4.43 eV, leading to magnetic-dipole related red emission with enhanced intensity. This is evidenced by the fact that SrAl₁₂O₁₉∶Pr³⁺,Ti⁴⁺ shows bright red emission whereas SrAl₁₂O₁₉∶Pr³⁺ does not show red photoluminescence.