Luminous properties of a red phosphor, Eu–Y2O3 (Eu-doped Y2O3) have been studied by 89Y (nuclear spin 1/2)-static NMR. Pure Y2O3 showed an anisotropic pattern with the peak centered at 299 ppm assigned to the Y atom in the site, where none of the 12 nearest neighbouring Y atoms are substituted by an Eu atom. The signal became broader with increasing doped Eu content. Additionally, a shoulder peak assigned to the Y atom in the site where one of the 12 nearest neighbouring Y atoms was replaced by one Eu atom appeared around 100—110 ppm in the samples with more than 1 mol/% Eu content. The spin–spin relaxation of the centered signal was mainly dominated by the dipole–dipole interaction mechanism between the 89Y nucleus and the paramagnetic electrons of Eu3+ with 4f6 electron configuration which substituted at the second nearest and further neighbouring Y. In conclusion, the line-broadening of 89Y signal at constant Eu content was correlated with the Eu distribution on the basis of the facts that the phosphor sample with higher brightness showed broader linewidth and the intensity of the ESR signal corresponding to the lattice defects has no relation to the brightness.
The chemical shift, the range of chemical shift anisotropy, and the spin-lattice relaxation times (T1) in Y2O3, Y3Al5O12, and Y2O2S have been measured by solid state 89Y (nuclear spin 1/2)-MAS and -static NMR. The static NMR data and T1 are reported for the first time. The range of chemical shift anisotropy was 1500—2400 Hz. This range was influenced more by the nature of the atom bound to Y than by the coordination number or the crystallographic symmetry of Y. Very long T1 values were obtained for Y2O2S (6.61 h) and Y2O3 (3.92 h at 24d site and 3.81 h at 8b site). On the other hand, the T1 value (1.10 h) of Y3Al5O12 is much shorter compared with those of Y2O2S and Y2O3. The next-nearest neighboring atom of Y in Y3Al5O12 is identified as Al, which has the nuclear spin 5/2 of 100% natural abundance. The most likely origin of significantly shorter T1 of Y3Al5O12 is a dipole–dipole interaction between 89Y and 27Al.
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.
11B- and 13C-solid state NMR of boron carbide with different isotope ratios, B4C(11B/10B = 80.42/19.58; natural abundance isotope) and 11B4C(11B/10B = 99.5/0.5; 11B enriched sample), has been investigated. The linewidth of the icosahedral B (6h1and 6h2) in 11B-static NMR signal in 11B4C was greater in 11B4C than in B4C, which indicates that the B–B dipole–dipole interaction is the main mechanism for the broadening of the 11B-static NMR signal. The quadrupole coupling constant calculated from the signal position of (±1/2 ↔ ±3/2) transition was about 0.1 MHz. 11B-MAS spectra revealed the presence of at least two additional B sites (37 ppm and near −60 ppm) besides the icosahedral B sites (−6 ppm).
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