NMR as a Probe of the Homogeneity of Paramagnetic Doping/Substitution

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Defects in crystalline structures may be used to change, adjust and control the materials properties of the host. Among the properties which can be influenced by doping is luminescence efficiency. Quenching is more efficient if the excited state is transported radiationless over a chain of doped paramagnetic atoms to so-called killer sites, which convert the excited state to the ground state and heat. Obviously the efficiency of quenching depends on the spatial distribution of dopands in the host structure. Thus a homogeneously doped sample should have better luminescence properties than a heterogeneously doped sample at a lower doping concentration. As shown in recent publications doping homogeneity can be evaluated on a scale of approx. 0.5 nm to 2 nm using the relative visibility of the NMR-active nuclei. The method is based on the decay of the contribution of the “NMR-visible” sample with rising doping concentration.The target of this project is to test the hypothesis that doping homogeneity as probed by solid-state NMR has a positive influence on luminescence properties. The planned experiments were designed, to answer the following questions.How does the “NMR-visible” sample volume relate to the electronic state of the dopant, the degree of homogeneity and temperature?How do luminescent properties depend on doping homogeneity?An important insight from prior work was the insight, that NMR probes doping homogeneity on the same length scale as the one required for radiationless transport.A requirement for this project is the phase-pure synthesis of doping series of crystalline host compounds. Lanthanides were chosen as the sole dopands deliberately, in order to reduce the influence of the chemical surrounding and to ease finding explanations for the results. For the successful application of the method the only requirement is the availability of an NMR signal of an arbitrary nucleus. Thus the method may be used for the characterization of a number of substance classes, e.g. of silicates (Si-29), halogenides (F-19, Cl-35/37, Br-79/81), nitrides (N-14/15), phosphates (P-31), borates (B-10/11) or aluminates (Al-27).