Blind spheres of paramagnetic dopants in solid state NMR

Abstract

Solid-state NMR on paramagnetically doped crystal structures gives information about the spatial distribution of dopants in the host. Paramagnetic dopants may render NMR active nuclei virtually invisible by relaxation, paramagnetic broadening or shielding. In this contribution blind sphere radii r₀ have been reported, which could be extracted through fitting the NMR signal visibility function f(x) = exp(−ar₀³x) to experimental data obtained on several model compound series: La_1−xLn_xPO₄ (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb), Sr_1−xEu_xGa₂S₄ and (Zn_1−xMn_x)₃(PO₄)₂·4H₂O. Radii were extracted for ¹H, ³¹P and ⁷¹Ga, and dopants like Nd³⁺, Gd³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and Mn²⁺. The observed radii determined differed in all cases and covered a range from 5.5 to 13.5 Å. While these radii were obtained from the amount of invisible NMR signal, we also show how to link the visibility function to lineshape parameters. We show under which conditions empirical correlations of linewidth and doping concentration can be used to extract blind sphere radii from second moment or linewidth parameter data. From the second moment analysis of La_1−xSm_xPO₄³¹P MAS NMR spectra for example, a blind sphere size of Sm³⁺ can be determined, even though the visibility function remains close to 100% over the entire doping range. Dependence of the blind sphere radius r₀ on the NMR isotope and on the paramagnetic dopant could be suggested and verified: for different nuclei, r₀ shows a -dependence, γ being the gyromagnetic ratio. The blind sphere radii r₀ for different paramagnetic dopants in a lanthanide series could be predicted from the pseudo-contact term.