The Structural, Optical, Electronic, and Magnetic Properties Oxygen

The structural, optical, electronic, and magnetic properties were studied. XRD confirmed the cubic fluorite structure of all the samples. The nature of the variation in crystallite size and lattice parameter confirmed the incorporation of Pr ions in the CeO2 lattice. We find that oxygen vacancy creation occurs more easily under tensile strain than under compressive strain. This is in good agreement with the experimental results. There are more oxygen vacancies and associated Ce3+ with the expansion of the lattice parameter. The oxygen vacancy creation is closely related to the bonding length and strength between the surface O and Ce atoms.

This can also be seen in the pure and Pr-doped CeO2.

The oxygen vacancies were evident in the materials. After adding Pr through Raman spectroscopy, a broad defect band at 500–650 cm-1 was attributed to the oxygen vacancy defects. A lower shift in the F2g mode was also attributed to increasing oxygen vacancies. This shift can be related to a symptomatic increment in tensile stress and related defects, such as the generation of oxygen vacancies when Pr3+ cations substitute for the Ce4+ cations in the matrix of CeO2 nanoparticles to maintain the charge balance. The expansion of the Raman active mode (F2g) has been observed with the incorporation of Pr cations and was found to increase with Pr ion concentration increments, which may be due to the reduction in phonon lifetime in nanocrystalline samples. However, the red shift and the broadening of the F2g peak for Pr-doped CeO2 samples may also be ascribed to the decrease in crystallite size, which is confirmed by our XRD analysis (jalcom).

UV-vis spectroscopy spectra revealed the presence of Pr3+ cation. To maintain the charge balance of the lattice, Pr cations in the +3-oxidation state are required to convey the oxygen vacancies.

The analysis of the electronic structure spectra shows that, at tensile strain, the bandwidth of the O 2p orbitals decreases, and the overlap between O 2p and Ce 5d, as well as four f orbitals, also decreases, leading to a weaker Ce–O bond. Furthermore, a surface with larger lattice constant can better accommodate the larger Ce3+ and thus facilitate the structural relaxation of the reduced surface.

Doping with Pr can produce different magnetic and nonmagnetic complexes in the CeO2 matrix. We will now confine our attention only to the essential types of complexes that can be held accountable for the degradation of the ferromagnetic order. When oxygen vacancies are created in a CeO2 matrix consisting of two unpaired electrons, they are left behind. If one electron is restrained on a Ce4+ cation, creating a Ce4+–VO–Ce3+ complex, and the other electron is trapped in the hydrogenic orbital around the oxygen vacancy (VO), it results in an F+ center that establishes the ferromagnetic behavior. In our samples, the doping of Pr cations in the CeO2 matrix, similar to the Pr4+–VO–Ce3+ complex, can favor the ferromagnetic ordering. However, if both electrons are restrained on cations, then a significant portion of the Pr3+–VO–Ce3+ or Pr3+–VO–Pr3+ complexes will also be formed.

Raman and XPS analyses show that integrating Pr3+ cations in the CeO2 matrix increased the number of additional oxygen vacancies. If both electrons continue to localize on the oxygen vacancy, the oxygen vacancy becomes even more occupied (F0 centers). Contrary to the singly occupied oxygen vacancies, which mediate ferromagnetic exchange, the doubly occupied oxygen vacancies can only mediate weak antiferromagnetic exchange [76]. The formation of F2+ and F0 centers would reduce the number of F+ centers and, more importantly, decrease concentration below the separation threshold of the related magnetic polarons, avoiding the establishment of a long-range ferromagnetic ordering. Hence, the preventing effect of Pr3+ ions on the ferromagnetic order in a CeO2 matrix is shown through the presence of F2+ and F0 centers, which do not help ferromagnetic exchange.

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