Homogeneity of doping with paramagnetic ions by NMR

Abstract

In NMR, paramagnetic dopants change the relaxation behavior and the chemical shift of the nuclei in their immediate environment. Based on the concept that the "immediate environment" in a diamagnetic host material can be described as a sphere with radius r0, we developed a function for the fraction of unperturbed nuclei (the fraction of nuclei outside the sphere) which gives a link between the effective radius and the doping concentration. In the case of a homogeneous doping scenario a characteristic dependence is observed in both theory and experiment. We validated the model on a sample series where paramagnetic Eu(II) ions are doped into crystalline SrH2. The fraction of unperturbed nuclei was determined from the (1)H NMR signal and follows the predicted curve for a homogeneous doping scenario where the radius r0 is 17 Å.

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

“…The resulting peak areas from one pulse experiments were calculated and tested by the visibility function . Deviations from the theoretical visibility function for homogeneous doping indicate a lower degree of NMR homogeneity.…”

“…The NMR visibility was calculated as observed peak area per mole of doped sample normalized by that of the non‐doped sample. The NMR visibility fitting function for homogeneously doped sample was shown to be , with a =4π N hostUC /3 V UC =0.055/Å 3 for monazite LaPO 4 , where N hostUC is the number of “dopable” sites in the unit cell and V UC is the volume of the unit cell. Herein N hostUC =4 and V UC =305.73 Å 3 .…”

“…In terms of luminescence the radii of these spheres (big circles) could be interpreted as the critical energy transfer distance R c . The radii r 0 and R c are not the same but typical values fall into a similar range: Å – few nm …”

“…A disadvantage of a lineshape analysis is that it implicitly only refers to the NMR visible part of the compound but not to the nuclei inside the blind sphere, i. e. in direct vicinity of paramagnetic centers, for which the signal may vanish within the dead‐time of spectrometer. An alternative to the lineshape analysis approach is the visibility function, i. e. the observed peak area as a function of doping concentration which was also shown to be related to doping homogeneity (Figure ) . It is interesting to note that for lanthanide ions the blind sphere radii and the critical energy transfer distances share a similar size range.…”

“… Typical NMR visibility curves f ( x ) as a function of the paramagnetic dopant concentration x . The NMR visibility is defined as the visible signal of the doped material normalized by the signal of the diamagnetic host . For a homogeneous doping scenario, the NMR signals vanish in the dead‐time of the spectrometer more efficiently thus f ( x ) approaches zero earlier as x increases.…”