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