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

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.