Praseodymium #
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Orbitals and Energies #
Note – these are listed in BINDING ENERGY
Pr 2p ≈ 1244 eV
Pr 3d ≈ 930 eV
Pr 4s ≈ 305 eV
Pr 4p ≈ 220 eV
Pr 4d ≈ 115 eV
Pr 5s ≈ 38 eV
Pr 5p ≈ 32 eV
Pr 4f ≈ 2 eV
Common Overlaps for Pr 3d #
I 3p – Cu 2p – Bi 4s – Sb 3s – Mn LMM (Al Ka X-rays)
Theory and Background #
Praseodymium metal comprises of a mostly singular asymmetric 3d5/2 peak with a spin-orbit splitting to the 3d3/2 of 20.2 eV.(1,4) There are, however, small satellite peaks to lower binding energy due to the screened 3d9 4f3 final state.(1) Additionally, the Pr 3d3/2 photoemission does undergo splitting, thought to be a shake-up satellite in which an electron from the sd band during photoemission is excited into an empty 4f state (pulled below the Fermi level following photoemission) with the absence of this transition from the 3d5/2 peak attributed to a lower ratio of this transition between 3d5/2:3d3/2.(1)
The oxides of Pr are complicated by the presence of intense satellite features. Praseodymiuin ions in Pr2O3 are in a 4f2 configuration in the ground state, however twin final states of 4f2 and 4f3L (L = hole in O 2p valence band) cause each 3d photoemission to split into two, induced by a core-hole potential acting on the 4f electrons.(2) PrO2 in the ground state is a mixture of 4f1 and 4f2L.(3)
The complex Pr 4+ spectra are described well by Schaefer et al,[5] where even in it’s highest oxidation state, Pr presents itself as 3 peaks. The high binding energy peak (at around 946 eV, from the Pr 3d5/2 – which sits just below the Pr3d3/2) results from a fully unscreened 3d9 4f1 state, whereas the two lower states are the product of core-hole screening (through O 2p – Pr 4f hybridisation and charge transfer). The state at ~ 932 eV is from a screened 3d94f2 configuration, while the state at ~928 eV from a 4f3 occupancy.
Experimental Advice #
Praseodymium XPS is typically performed on the Pr 3d orbitals. It will strongly overlap with the Cu 2p and I 3p photoemission, as well as weakly overlap with the Bi 4s emission, and augers from Cs, Mn and Ba when using Al X-rays (1486.7 eV).
Ensure collection of both the Pr 3d 5/2 and 3/2 for a complete appreciation of the Pr state.
Extend the region to > 980 eV to include O KLL Auger feature.
A fully oxidised Pr(IV) surface is very difficult to obtain, and Pr(III / IV) mixtures form commonly in the absence of treatments such as cold oxygen plasma.
Data Analysis Guidance #
Praseodymium metal comprises of a mostly singular asymmetric 3d5/2 peak with a spin-orbit splitting to the 3d3/2 of 20.2 eV.(1,4) There are, however, small satellite peaks to lower binding energy due to the screened 3d9 4f3 final state.(1) Additionally, the Pr 3d3/2 photoemission does undergo splitting, thought to be a shake-up satellite in which an electron from the sd band during photoemission is excited into an empty 4f state (pulled below the Fermi level following photoemission) with the absence of this transition from the 3d5/2 peak attributed to a lower ratio of this transition between 3d5/2:3d3/2.(1)
Note for below: WordPress does not support underlining characters – so core holes will be described using a bold text-type rather than an underlined text-type. Apologies for the confusion.
Distinguishing between Pr3+ and Pr4+ by XPS can be challenging, due to the complex nature of final state effects in such systems, however, careful analysis of the Pr 3d5/2 lineshape and high energy final state effects affords an appropriate methodology. Analysis of the 3d5/2 photoemission reveals the presence of one photoemission peak (m) and one shake-down satellite (s) at lower binding energy arising from a well screened 4f3 final state.
The ratio between these two peaks may be used as a semi-quantitative probe since it varies between Pr3+ and Pr4+, with Pr2O3 reporting a higher ratio of m:s than PrO2 (table 2).(4)
| Species | m:s ratio |
| Pr2O3 | ~ 3 |
| PrO2 | ~ 1.5 |
Further support for the assignment of Pr4+ can be found in the unscreened Pr 3d 3/2 state, not observed in Pr or Pr2O3, at ~967 eV arising from 3d4f1 final states, although this photoemission is relatively weak and may be hard to justify in materials with a lower praesodymium content, especially since it may overlap with the O KLL auger (note, the Pr 3d5/2 emission does also produce this peak in PrO2, however it overlaps with the 3d4f2L, and 3d4f3L2 peaks).(3)
References #
References
- Crecelius, G., et al. (1978). “Core-hole screening in lanthanide metals.” Physical Review B 18(12): 6519. Read it online here.
- Ogasawara, H., et al. (1991). “Praseodymium 3d-and 4d-core photoemission spectra of Pr2O3.” Physical Review B 44(11): 5465. Read it online here.
- Bianconi, A., et al. (1988). “Many-body effects in praesodymium core-level spectroscopies of PrO 2.” Physical Review B 38(5): 3433. Read it online here.
- Lütkehoff, S., et al. (1995). “3d and 4d x-ray-photoelectron spectra of Pr under gradual oxidation.” Physical Review B 52(19): 13808. Read it online here.
- Schaefer, A., et al. “Photoemission study of praseodymia in its highest oxidation state: The necessity of in situ plasma treatment.” The Journal of chemical physics 134.5 (2011). Read it online here.
