89Y-Static and -MAS NMR, and 27Al-MAS NMR in Green Phosphor, Tb-Doped Y3Al5O12 and Luminous Characteristics

Abstract

Abstract Luminous properties of a green phosphor, Tb-doped Y3Al5O12, have been studied by 89Y (nuclear spin 1/2)-static and -magic angle spinning (MAS) NMR and 27Al (nuclear spin 5/2)-MAS NMR. In pure Y3Al5O12, a sharp signal at 239 ppm was observed in 89Y-MAS NMR spectrum, which was expected from a powder pattern due to 8O-coordinated Y in D2 symmetry in 89Y-static NMR spectrum. In the 27Al-MAS NMR spectrum of Y3Al5O12, two typical signals were observed, i.e., a sharp signal at 0 ppm due to 6O-coordinated Al in the octahedral site and a characteristic signal due to 4O-coordinated Al in the tetrahedral site. These 89Y and 27Al signals became broader in Tb-Y3Al5O12 with increasing in content of the doped Tb. In addition to the main peak at 239 ppm, furthermore, extra peaks appeared in the bottom of the main peak in 89Y-MAS NMR; these were assigned to the second nearest to the fifth nearest neighboring Y (Y2 to Y5) atoms to Tb atom substituted in the site of Y atom. Shifts of these Y atoms were reasonably explained by a pseudocontact shift via a paramagnetic Tb3+ ion. Incorporation of Tb3+ ions caused a large reduction in 89Y nuclear spin-lattice relaxation time from 4000s (pure Y3Al5O12) to several hundred seconds (3% Tb-Y3Al5O12) as well. The line-broadening of the signals was mainly dominated by the dipole–dipole interaction mechanism between the Al or Y resonating nucleus and a Tb3+ ion. The signals of Y atoms in the sites where either one of the nearest neighboring Y1 atoms was replaced by Tb atom were not detected because of the large line-broadening. The stronger brightness was obtained in the specimens which showed the much broader linewidth in 27Al-MAS signals and in 89Y-static NMR signals under the same Tb content. So we concluded that the homogeneous distribution of Tb3+ ions in Tb-Y3Al5O12 plays a key role for the higher brightness in the green phosphor as in the red phosphor.