NMR and ESR relaxation in Nd- and Gd-doped LaPO₄: towards the accurate determination of the doping concentration

Abstract

We present an original method based on the (31)P solid-state NMR relaxation to determine low concentrations (<1 at%) of paramagnetic ions in monazite LaPO4 crystals with a high accuracy (∼0.1 at%). NMR experiments under static and MAS (15 kHz) conditions show that the (31)P relaxation time T1 is strongly affected by the presence of paramagnetic ions in the vicinity of the phosphorus nuclei. A linear variation of 1/T1 as a function of Nd(3+) or Gd(3+) concentration is shown in the 0-10 at% range for a homogeneous distribution of the doping ions in the matrix, which is the case when doped LaPO4 is synthesized by a soft chemistry route, i.e. by aqueous co-precipitation followed by thermal annealing. As a proof of concept for the use of this tool to study dopant homogeneity, we show that in the case of a solid-state synthesis at 1350 °C, relaxation measurements show that the homogeneous distribution of the doping ions is ensured when the mixing of the oxide precursors is performed mechanically, but not in the case of manual grinding. The electronic relaxation times of Gd(3+) and Nd(3+) ions are evaluated by ESR measurements under saturation conditions. This allows us to provide a semi-quantitative interpretation of the nuclear (31)P relaxation measurements both in Nd and Gd doped LaPO4. In addition, the comparison between nuclear and electronic relaxation suggests that Nd-Gd codoping may improve the efficiency of the Gd(3+) ion as a relaxing agent in MRI (magnetic resonance imaging) techniques.

Local structure and magnetism of LaxEu1−xPO4Martel1, Rakhmatullin2, Baldoví3 et al. 2019Phys. Rev. B6030View full textAdd to dashboardCite

“…A very long T 1 is observed for LaPO 4 (1714 s, see Table I) as expected for this diamagnetic compound [14]. Our value differs from the one of 600 s published previously [21,83], probably due to the presence of different small quantities of impurity. EuPO 4 possesses a short T 1 of 210 ms being close to that of the paramagnetic GdPO 4 (460 ms) [21].…”

“…Previous studies on solid solutions containing rare-earth elements have shown that finding similar NMR shifts is an evidence for the presence of a similar environment around the nucleus detected by NMR [50][51][52]. It should also be noted that the line broadening is more important for the Eu-rich solid solutions (Table I), as expected for solid solutions with higher paramagnetic cation content (structural local disorder and/or anisotropic bulk magnetic susceptibility effects) [14,15,21,49].…”

“…In order to have a better understanding of the local environment around the 31 P atoms in these solid solutions, magic angle spinning (MAS) NMR has been proven as an efficient analytical tool. Indeed, several authors published data on related crystalline rare-earth orthophosphates, like the LnPO 4 series (Ln 3+ = La 3+ to Eu 3+ [19] and Ln 3+ = La 3+ to Yb 3+ [20]) and some of their solid solutions (Ln 3+ = Ce 3+ , Nd 3+ , and Gd 3+ ) [14,15,21]. It was shown that NMR allows to detect, in numerous types of materials, very low paramagnetic content in a unique manner [22].…”

A Guide to Brighter Phosphors‐Linking Luminescence Properties to Doping Homogeneity Probed by NMR

“…[17][18][19][20] Solid state NMR has been reported to be helpful for studying paramagnetic systems. [21][22][23][24][25][26] Especially, the distribution of paramagnetic dopants can be investigated by the spin-lattice relaxation time, [27][28][29][30] hyperfine shifts [31,32] and the line-broadening effect. [27,33,34] Homogeneous distributions of Tb 3 + and Eu 3 + were shown to be correlated to NMR line broadening and positively related to the brightness of phosphors.…”

Doping homogeneity in co-doped materials investigated at different length scales

“…20 In this contribution solid-state NMR is used to investigate co-doping heterogeneity. In case of a diamagnetic solid host with NMR-active nuclei solid-state NMR has been used to investigate the dopant distribution of a single kind of paramagnetic dopant (''mono-doped'') in different ways: spin-lattice T 1 relaxation, [21][22][23] line width [24][25][26] and visibility function (see below). 27,28 A T 1 -T 2 correlation map was also shown to be useful in studying a paramagnetic site distribution at the surface of porous materials.…”