Formal valence, 3d-electron occupation, and charge-order transitions
Yundi Quan1, Victor Pardo, Warren E Pickett
1Department of Physics, University of California, Davis, California 95616, USA.
Physical Review Letters
|December 11, 2012
Summary
Formal valence and charge state concepts in materials science are imprecise. This study shows 3d orbital occupation from first principles calculations offers a clearer view of transition metal oxides, aiding theoretical modeling.
Area of Science:
- Materials Physics and Chemistry
- Computational Materials Science
- Solid-State Chemistry
Background:
- Formal valence and charge state concepts are crucial in materials science but have loose connections to actual charge.
- This imprecision leads to uncertainties in modeling material behavior and interpreting experimental data.
- Existing models struggle with accurately representing charge distribution in complex materials.
Purpose of the Study:
- To investigate the utility of 3d orbital occupation, derived from first principles calculations, as a more accurate descriptor than formal valence.
- To analyze charge states and charge-ordering phenomena in selected transition metal oxides.
- To highlight the implications for theoretical modeling and understanding charge disproportionation.
Main Methods:
- Utilized first principles calculations to determine 3d orbital occupations for cations in transition metal oxides.
- Analyzed several examples including La(2)VCuO(6), YNiO(3), CaFeO(3), AgNiO(2), and V(4)O(7).
- Compared 3d orbital occupations with formal charge states, particularly in systems exhibiting charge order.
Main Results:
- Demonstrated that while dividing crystal charge into atomic contributions is ill-posed, 3d orbital occupation is readily calculable.
- Observed cases in transition metal oxides where distinct formal charge states exhibit identical 3d orbital occupations.
- Identified instances of charge order (disproportionation) where cation 'charge states' have the same 3d electron count.
Conclusions:
- 3d orbital occupation provides a more robust and quantitatively accessible metric than formal charge states for cations in transition metal oxides.
- The findings necessitate a re-evaluation of theoretical models that rely heavily on formal valence for describing charge states and ordering.
- Accurate modeling of charge states and charge-ordering mechanisms requires considering direct electronic structure information like 3d occupation.
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