Multiplet XAS study of 3d transition metal Cobalt compounds
Xiao Cheng
Jinghua Guo*
Yi Luo*
Motivation for the work
Cobalt related compounds have vast applications in various research fields, especially in renewable energy science.
Co(II/III)-based redox electrolyte: solar cell exceeds 12% efficiency1.
[1] Yella, A. etc., Science 334, 629(2011). [2] J. Phys. Chem. C 113(48), 20689(2009). [3] Int. J. Electrochem. Sci. 2 285(2007). [4] J. Porphyrins and Phthalocyanines 16, 1-7(2012) [5] Nature Physics 7, 303(2011).
proton exchange membrane fuel cell technology
CoPc, Co2(L)2 electrocatalysis as to catalyze cathodic oxygen reduction reaction2,3,4.
Imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) disc5.
Magnetic property research
High-energy spectroscopies have played an important role in identifying and describing the bonding character of highly oxidized metals. Characterize atoms, molecules, adsorbates, surface, liquids and solids[1].
The use of X-ray absorption spectrum
Electronic structure of the central sited metal ion
Local geometrical structure around the metal center.
conduction band/ empty orbital information local electronic structure
[1] Coordin. Chem. Rev. 249, 35(2005).
element specificity: study structure of a constituent element in a composite material. no long-range interaction: good for amorphous materials.
Physics meaning of each parameter, and their intrinsic effects on X-ray absorption spectrum.
Take CoO and CoCl2 as reference samples.
Co2+
Ligand
Oh symmetry
10Dq
e-
Δ charge transfer energy=Δ
Crystal field effect
Charge transfer effect
Upd
Hubbard Udd value Ligand 2p orbitals
Eg
T2g
Hopping parameters
T(eg)
T(t2g)
e- 2p 3d
First: atomic model calculation, Cowan’s method. Single impurity Anderson model. Hartree Fock method.
Second: crystal field effect, group theory.
Third: charge transfer effect, multiple configuration. 𝑖 (2𝑝63𝑑7 + 2𝑝63𝑑8𝐿) → 𝑓 (2𝑝53𝑑8 + 2𝑝53𝑑9𝐿)
Charge transfer energy: Δ Hubbard U value: 𝑈𝑑𝑑, Coulomb interaction: 𝑈𝑝𝑑
Hybridization energy: T(𝑡2𝑔), 𝑇(𝑒𝑔)
𝑑𝑥𝑦 𝑑𝑥𝑧 𝑑𝑦𝑧
𝑑𝑥2−𝑦2 𝑑𝑧2 Eg
T2g
3d orbital shell
CoO calculation: 10Dq=0.5 eV, Δ=2.4 eV, T(eg)=3.9 eV, T(t2g)=3.1 eV, Udd=Upd=6.0 eV. CoCl2 calculation: 10Dq=0.4 eV, Δ=1.2 eV, T(eg)=3.7 eV, T(t2g)=2.9 eV, Udd=Upd=6.0 eV.
As the crystal field strength 10Dq value increasing, we can expect a mutation on XAS feature when the spin reversal happens at certain 10Dq value, the ground state change from high spin state to low spin state due the increasing energy splitting in 3d orbital shell. This phenomenon is useful to identify spin state change when doped elements replace original ligand atoms causing crystal field strength change.
Co2+
Ligand
10Dq
Crystal field effect
Eg
T2g
Charge transfer energy for initial state: Δ𝑖 = 𝐸 2𝑝63𝑑7 − 𝐸(2𝑝63𝑑8𝐿)
Hubbard U value and core-hole Coulomb interaction: 𝑈𝑑𝑑 = 𝐸 3𝑑6 + 𝐸 3𝑑8 − 2𝐸 3𝑑7
Δ𝑓 = Δ𝑖 + (𝑈𝑑𝑑 − 𝑈𝑝𝑑)
Co2+
Ligand
e-
Δ
Co2+
e- Udd
Multiplet structure in L3 and L2 edges change. Interesting satellite peaks appear which confirms the metal to ligand charge transfer mechanism proposed before by Prof. Frank de Groot.
Charge transfer hopping parameters control possibility for ligand charge transferring to certain empty 3d orbital. Multiplet structure changes are found in the XAS features when T(eg), T(t2g) varies.
10Dq
Ligand 2p orbitals
Hopping parameters
T(eg)
T(t2g)
e-
Structure distortion study
The structure distortion on Co ion’s local structure can be demonstrated as a step by step symmetry branching chain as below, and their XAS are calculated.
bonds prolong angles expand angel distortion
Oh
CoO CoCl2
D4h
CoPc D2h
CoF2
C2v
Co2(L)2
Octahedral symmetry
Oh
Example system: CoO
basic symmetry
𝑑𝑥𝑦 𝑑𝑥𝑧 𝑑𝑦𝑧
𝑑𝑥2−𝑦2 𝑑𝑧2 Eg
T2g
multiplet structure
10Dq=1.0 eV
10Dq
Tetragonal symmetry
bonds prolong
Oh D4h
D4h structure distortion from Oh symmetry. The crystal field splitting will become:
𝑑𝑥𝑦 𝑑𝑥𝑧 𝑑𝑦𝑧
𝑑𝑥2−𝑦2 𝑑𝑧2
𝑑𝑥2−𝑦2
𝑑𝑧2
𝑑𝑥𝑦
𝑑𝑥𝑧 𝑑𝑦𝑧
D4h symmetry
Eg
T2g
B1g
B2g
A1g
Eg
Dihedral symmetry
angles expand
D4h D2h
D2h symmetry
𝑑𝑥2−𝑦2
𝑑𝑧2
𝑑𝑥𝑦
𝑑𝑥𝑧 𝑑𝑦𝑧
D2h structure distortion from D4h symmetry. The crystal field splitting will become:
𝑑𝑥2−𝑦2
𝑑𝑧2
𝑑𝑥𝑦
𝑑𝑥𝑧
𝑑𝑦𝑧
B1g
B2g
A1g
Eg
Ag
Ag
B1g
B2g
B3g
angel distortion
D2h C2v
Rotation-reflection symmetry C2v symmetry
𝑑𝑥2−𝑦2
𝑑𝑧2
𝑑𝑥𝑦
𝑑𝑥𝑧
𝑑𝑦𝑧
C2v structure is isomorphic to pure rotation D2h group. The crystal field splitting keeps five:
𝑑𝑥2−𝑦2
𝑑𝑧2
𝑑𝑥𝑦
𝑑𝑥𝑧
𝑑𝑦𝑧
Ag
Ag
B1g
B2g
B3g
A1
A1
A2
B1
B2
Besides the basic application on CoO and CoCl2 systems, multiplet calculation is also applied on various Cobalt compounds XAS experiments which performed in Advanced Light Source in Lawrence Berkeley Lab.
CoF2 CoF2 belongs to D2h symmetry, but it can be considered as a slightly distortion from CoCl2 Oh symmetry. The resulting crystal field splitting is assumed to be:
𝑑𝑥𝑦 𝑑𝑥𝑧 𝑑𝑦𝑧
𝑑𝑥2−𝑦2 𝑑𝑧2
XAS study on catalysts CoPc and Co2L2
CoPc
D4h
C2v
Co2L2
Theoretical calculation results effectively help us to figure out the local structure of the catalytic active center.
Acknowledgement:
Prof. Yi Luo of Royal Institute of Sweden
Dr. Jinghua Guo of Lawrence Berkeley Lab
Prof. Frank de Groot of Utrecht University
Dr. David Prendergast of Lawrence Berkeley Lab
Debajeet Bora, Qinggang He in Lawrence Berkeley Lab
Thank you for your attention