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

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.

Paramagnetic interactions in the 31P NMR spectroscopy of rare earth element orthophosphate (REPO4, monazite/xenotime) solid solutions

“…It is a well-known phenomenon that the presence of paramagnetic species can dramatically increase nuclear spin relaxation rates and NMR peak widths (cf. Grey et al 1990;Harazono et al 1998aHarazono et al , 1998bBakhmutov 2004;Palke et al 2009;Palke and Stebbins 2011). In fact, synthetic materials used in NMR studies of spin-1/2 nuclei are often doped with small amounts of a paramagnetic material, for example, 0.1 to 0.2 wt% Co 3 O 4 , Fe 2 O 3 , or Gd 2 O 3 for 29 Si NMR of aluminosilicate glasses (cf.…”

“…However, the Fermi contact shift can be much more modest as for the lithium manganate rock salt structured materials studied by Lee et al (1998), in which each Li-O-Mn bond caused a shift of either 150 ppm or -75 ppm depending on the bond angle. In contrast, when the pseudocontact mechanism is the main paramagnetic interaction the shifts are typically smaller, usually on the order of tens to ∼100 ppm (Grey et al 1990;Harazono et al 1998aHarazono et al , 1999.…”

“…Such information is not readily available, although progress has been made in using DFT calculations to calculate electron spin density transfer (Carlier et al 2003;Chazel et al 2007). On the other hand, for the pseudocontact interaction, if there is an axis of symmetry through the paramagnetic center, then the geometric terms in Equation 4 can be calculated from the established crystal structure and the susceptibility terms adjusted to obtain a match to the observed paramagnetic shifts (see below) (Grey et al 1990;Harazono et al 1998aHarazono et al , 1999.…”

“…These are caused by an interaction between unpaired electron spins on the paramagnetic center (typically ions of most rare earths and transition metals) and the spin states of the NMR-observed nucleus. Paramagnetic shifts caused by rare earths have been well studied in various inorganic crystalline materials and have provided important information about the extent of solid solutions, cation order/disorder, site occupancy, and clustering, including a series of rare earth stannate and titanate pyrochlores (Grey et al 1989(Grey et al , 1990, Tb-doped YAG (yttrium aluminum garnet) (Harazono et al 1998b), and various other phosphors (Harazono et al 1998a(Harazono et al , 2001. * E-mail: apalke@stanford.edu Recently, Palke and Stebbins (2011) reported an analysis of "anomalous" resonances and the effects of temperature on them, in the 27 Al and 29 Si spectra of two natural (Mg,Fe) 3 Si 2 Al 3 O 12 pyrope garnets first observed by Stebbins and Kelsey (2009).…”

“…In this study, we have extended the methods of analysis used by Palke and Stebbins (2011) and previous researchers (cf. Grey et al 1989Grey et al , 1990Harazono et al 1998aHarazono et al , 1998bHarazono et al , 2001Dajda et al 2003;Lee et al 1998Lee et al , 2002 to a series of dilute La 1-x RE x PO 4 and Y 1-x RE x PO 4 (RE = paramagnetic lanthanide) solid solutions taking the monazite and xenotime crystal structures, respectively. This is an excellent system for a study of paramagnetic NMR shifts for several reasons.…”