Point Defects and Photoluminescence of Green Phosphors Ca(1-1.5x)WO4:Tbx3+ and Ca(1-2x)WO4:Tbx3+ Nax+

Abstract

Two types of Tb- and Na-substituted green phosphors <TEX>$Ca_{(1-1.5x)}WO_4:Tb_x^{3+}$</TEX>: and <TEX>$Ca_{(1-2x)}WO_4:Tb_x^{3+},Na_x^+$</TEX> were synthesized with various x values, using a solid-state reaction. The former phosphors contained both substitutional and vacancy point defects, while the later had only substitutional defects. X-ray diffraction results showed that the main diffraction peak, (112), was centered at <TEX>$2{\theta}=28.72^{\circ}$</TEX> and indicated that there was no basic structural deformation caused by substitutions or vacancies. The photoluminescence emission and photoluminescence excitation spectra revealed the optical properties of trivalent terbium ions, <TEX>$Tb^{3+}$</TEX>. Typical transitions, <TEX>$^5D_3{\rightarrow}^7F_6,\;^7F_5,\;^7F_4$</TEX> and <TEX>$^5D_4{\rightarrow}^7F_6,\;^7F_5,\;^7F_4,\;^7F_3$</TEX>, and cross relaxations were observed. Subtle differences in the photoluminescence of green phosphors were observed as a result of the point defects. The FT-IR spectra indicated that some of the ungerade vibrational modes had shifted positions and changed shapes, spreading out over a wide range of frequencies. This change can be attributed to the different masses of <TEX>$Tb^{3+}$</TEX> and <TEX>$Na^+$</TEX> ions and <TEX>$V_{Ca}$</TEX>" vacancies compared to <TEX>$Ca^{2+}$</TEX> ions. The gerade normal modes of the Raman spectra exhibited subtle differences resulting from point defects in <TEX>$Ca_{(1-1.5x)}Tb_xWO_4$</TEX> and <TEX>$Ca_{(1-2x)}Tb_xNa_xWO_4$</TEX>.