Acetophenone (4.68 mL, 40 mmol, 99.0% Fluka) was added to Pr(O iPr) (159.09 mg, 0.5 mmol, 99.9% Strem Chemicals) in a reaction vessel containing a stirring magnet. (7) All chemicals were stored and handled in an oxygen- and water-free atmosphere in a glovebox and used as purchased without further purification. The synthesis of Pr(OH) 3 nanoparticles is analogous to the previously reported synthesis of La(OH) 3. We can unambiguously assign these changes to the occupancy of the O 2 p–Pr 4 f band and show that they directly relate to changes in the electrical conductance. Upon repeated heating and cooling of the Pr 6O 11 nanoparticles, we observe a reversible variation of the spectral features related to the tetravalent Pr ions. Thus, we can exclude the chemical and structural transformation. Surprisingly, we observe strong discrepancies between spectra of Pr 6O 11 measured at room and at high temperatures, whereas the in situ PXRD and TGA remain unchanged. We calculate the RIXS maps of tri-, tetra-, and mixed-valent Pr and correlate them with the measured HERFD-XANES spectra during temperature-induced transformation of Pr(OH) 3 to Pr 6O 11. (13) The local coordination and hence the crystal structure determines the degree of hybridization and the filling level of this band. (12) The f-states of Pr and the O 2 p-states overlap, hybridize, and form a band. The conduction mechanism in Pr 6O 11 is described with the small polaron model, where the dimensions of the polarons are of the same size as the distance between two sites in the lattice. To this end, we focus on Pr 6O 11 because it is an n-type semiconductor exhibiting rich spectroscopic features related to f-electrons in the Pr L 3-edge HERFD-XANES and RIXS spectra.
Here, we demonstrate that hard X-ray photon-in photon-out spectroscopy can be used as a contactless probe to track the temperature-induced changes of the delocalization of f-electrons, which are taking part in the electrical conductivity. Our results demonstrate how HERFD-XANES can be used to particularly study in situ the electronic properties of f-electrons in a semiconductor and how this method can be further extended to other classes of semiconducting nanomaterials. We assign these changes to the occupancy of the O 2 p–Pr 4 f band and show that they directly relate to changes in the electrical conductance. We observe a reversible variation of the spectral features related to the tetravalent Pr ions upon heating and cooling, whereas structural and chemical transformations can be excluded. Here, we show, with the example of Pr 6O 11 nanoparticles, an n-type semiconductor, how high-energy resolution fluorescence detected (HERFD) X-ray absorption near edge structure (XANES) can be used to track the changes of partially filled f-bands.
Hard X-ray photon-in photon-out spectroscopy has so far mainly been applied to investigate fundamental physical phenomena in superconductors and chemical reactivity of bioinorganic, photocatalytic, and catalytic materials.
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