To study the valence and spin state of nickel ions, we measured the X-ray absorption near edge spectroscopy (XANES) at nickel L2,3 edge in powder Ni(NO3)2
sample and NiO single crystal as a reference.
4-1 Experimental Design
The XAS experiments were performed at the H-SGM beamline at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. Clean sample surface were obtained by cleaving samples in chamber with pressure lower than 10-7 mbar.
Room temperature Ni L-edge XAS was recorded in total electron yield (TEY) in ultra high vacuum (~10-10 mbar) chamber.
The crystal structure of nickel (II) nitrate, Ni(NO3)2 was solved to have a rhombohedral R
3
space group (a = 1.03569(1) nm, c = 1.26761(1) nm, Z = 12),which shows two different sites for Ni2+ ions with the ratio Ni-1 : Ni-2 = 3 : 1 [1]. Two nickel sites have different distorted NiO6 octahedral local environments as shown in Fig.4-1. In the NiO6 octahedral environment, the Ni 3d degenerate state was separated into eg and t2g state with 10Dq difference by the crystal field, and the eg state was separated into x2-y2 and 3z2-r2 with ∆eg difference by the distortion. Ni2+ ions (3d8) has six electrons in the t2g state and two in the eg state, depending on the ratio between Hund’s coupling JH and ∆eg splitting, it may exhibit either low-spin (LS,S=0) or high-spin (HS,S=1) states as shown in Fig. 4-2. An easy way to calculate the total energy of HS or LS state is to set the t2g energy level to be zero, gain one crystal field energy 10Dq for each electron which stays in eg state, and also consider about the exchange interaction (Hund’s coupling) between the 3d electrons.20
n is the number of spin up electrons, m is the number of spin down electrons, JH is the Hund’s coupling energy. For Ni2+, the total energy of HS and LS can be describe as :
J
HS state would be ground state in a small distorted case.
The Ni L2,3 (2p→3d) absorption spectra obeys the dipole selection rule, the transition can be describe as α|2p63dn> + β|2p63dn+1L ＞ →α’ |2p53dn+1> +β’
|2p53dn+2L>, where L denotes a charge transfer from ligand to nickel. The Ni 2p core-hole spin-orbit coupling splits the spectra into two parts, namely the L3 (hν～853 eV) and L2 (hν～871 eV) white lines regions. The line shape of spectrum depends on the atomic mutiplet effect including Ni 3d-3d and 2p-3d Coulomb and exchange interactions, hybridization of Ni 3d orbital with the O 2p ligands, and the local crystal field.
Fig. 4-1. Crystal structure of NiO and Ni(NO3)2.
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Fig. 4-2. High spin and low spin in Ni2+ (3d8) case.
4-2 Ni L-edge XANES spectra
Fig. 4-3 shows the Ni L2,3 XAS experiment spectra of NiO and Ni(NO3)2 taken at room temperature. NiO is generally accepted to have divalence nickel with a HS(S=1) state as a standard sample. In the Ni L3-edge (2p3/2→3d transition), We found the line shape in of Ni(NO3)2 is far away from the Ni3+ spectrum as shown in Fig. 4-4[10].
Both NiO and Ni(NO3)2 spectra have a mean peak at 853.35 eV in L3-edge, which can be used to determine the valence of nickel ions, means the nickel ions are also divalence in Ni(NO3)2[10,11]. At the second peak of L3-edge, Ni(NO3)2 has a higher intensity at 855.1 eV and more peak splitting from the mean peak than NiO. We also see a shoulder characteristic at 856.5 eV in NiO but it disappeared in Ni(NO3)2
spectrum. These two differences come from the ligand field multiplet effect, which includes the hybridization with O 2p ligands and the local crystal field from their different NiO6 environments. In the L2-edge (2p1/2→3d), both spectra split into two peaks, Ni(NO3)2 has a slightly lower intensity at 870.7eV but a visible higher intensity
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at 871.8 eV than NiO. Again, these intensity differences come from the ligand field multiplet effect. The broad peak at 859 eV is the |2p63d9L＞→|2p53d10L> transition from charge transfer effect, while a broad characteristic at 866.7 eV in NiO spectrum is the 2p→4s-like transition called as continue edge jump [9].
850 855 860 865 870 875
0.0
Since two nickel sites in Ni(NO3)2 have different distorted NiO6 octahedral local environment, and the distortion may cause the presence of low spin state, first, we suspect whether all divalence nickel stay in pure low spin state in Ni(NO3)2. The Ni2+
low spin state has six electrons in the t2g state and two electrons stay in one orbital of eg state, this cause the LS spectrum has only one single peak in both L2,3 edge[11].
Compare with the calculated XAS spectra of Ni2+ LS in Fig.4-5, the line shape and the intensity ratio in L3-edge of Ni(NO3)2 tells that it is not a pure low spin spectra[10,11]. Second, we think about an inhomogeneous mixed spin state in the Ni(NO3)2, which means the higher intensity at 855.1 eV and 871.8 eV may be the
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contribution from some LS population. Fig. 4-6 shows the Ni2+ HS-LS mixed spectra from Dr. S.M. Peng et al who studied one-dimensional nickel material [12]. The button spectrum shows a Ni2+ 2HS+7LS spectrum, the mean peak in L3-edge come from the LS contributions, the energy position is 1 eV higher than the mean peak of Ni2+ HS spectra (top one) and sits between the mean peak and the second peak of HS spectrum. Comparing with the energy position, we assume the higher intensity at 855.1 eV and 871.8 eV are not the contribution from LS spectra. At the end, we assume all divalence nichels stay in high spin state in Ni(NO3)2, and the spectra shape difference between NiO and Ni(NO3)2 may come from those different local environments, which means an undistorted NiO6 octahedral cluster in NiO and two closely distorted NiO6 octahedral in Ni(NO3)2 ( see Fig. 2-4).
Fig. 4-4. Ni 2p XAS spectra[10] Fig. 4-5 Ni2+ XAS simulation from [11]
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Fig. 4-6 Ni2+ HS-LS mixed XAS spectra[12]
4-3 XANES theoretical calculation by XTLS
To confirm the spin state of divalence nickel in Ni(NO3)2, we also discussed the theoretical calculations of L2,3 XAS line shape using the full atomic multiplet theory, together with hybridization of Ni 3d orbital with the O 2p ligands and the point charge crystal field in NiO6 cluster [8].
For NiO case, the NiO6 octahedral is almost undistorted and nickel ion has same distance between all six O neighbors. In our calculations, we use the hybridization coefficient pdσ = -1.29 eV [13], set ∆eg = 0 eV (undistorted octahedral). We focus on the peak splitting and ratio of intensity in both L2 and L3 edge, a shoulder at 856.5 eV and a charge transfer characteristic at 859 eV. Finally we get a best fit to NiO spectra in 10Dq=1.0 eV.
For Ni(NO3)2 case, the crystal structure shows the averagy Ni－O bond length is 2.071Å , this value is very close to the bond length of 2.08(1) Å in NiO [14,15]. Since
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NiO and Ni(NO3)2 have a closely average Ni－O bond length, their 10Dq should also be closed. We focus on the peak splitting and intensity ratio in both L2 and L3 edge, our calculations shows 10Dq=1.0 eV,∆eg=0.8 eV and 10Dq=0.9 eV,∆eg=0.7 eV can fit the Ni(NO3)2 spectra, and 10Dq=0.9 eV,∆eg=0.7 eV is the best fit to Ni(NO3)2 spectra while the L3 edge looks too broad in ∆eg=0.8 eV calculation. Fig. 4-7 shows the best fitting on Ni(NO3)2, ∆eg=0.7 eV comes from the distorted NiO6 cluster.
The calculation result of 10Dq=0.9 eV, ∆eg=0.7 eV shows triply degenerate ground state with total spin S(S+1)=1.9988(1), and S(S+1)=1.9746 in the first excited state. The energy of first excited state is 0.98 eV higher than the triply degenerate ground state, the energy difference is too high for the thermal excitation at room temperature.
In our calculations we consider the point charge crystal field and hybridization with ligands separately, we called as ligand field model(LFM). Given the fact that various X-ray absorption studies use 10Dq around 1.6~1.8 eV to fit NiO spectra [11,16,17], that’s because the octahedral crystal and ligand field splitting are included
in one single effective parameter 10Dq, we called as a free ion model(FIM) approach.
10Dq is the t2g/eg orbital splitting in octahedral symmetry. We also tried the FIM calculations (pdσ = 0 eV), and shows 10Dq=1.6 eV, ∆eg=0 eV can fit NiO spectra well, while 10Dq=1.5 eV, ∆eg=0.6 eV result can fit Ni(NO3)2 well(see Fig.4-8). The 10Dq=1.5, ∆eg=0.6 eV calculation shows S(S+1)=1.9984(2), also a HS state. Either LFM or FIM calculations represent the nickel ions is divalence and still with a high spin state S=1 in Ni(NO3)2, like those nickel ions in NiO. Compare the FIM and LFM calculations, the LFM can get more splitting and fit beter in L2 edge. The different spectra shape come from their different ligand field multiplet effect, which means an undistorted NiO6 octahedral in NiO and two closely distorted octahedral in Ni(NO3)2.
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850 855 860 865 870 875
0.0
Ni L-edge XAS data with LFM calculation
Photon Energy (eV)
Fig. 4-7. Nickel L2,3 edge XAS data with ligand field model calculations.
850 855 860 865 870 875
0.0
Ni L-edge XAS data with FIM calculation
Ni-L3
Fig. 4-8. Nickel L2,3 edge XAS data with free ion model calculations.
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4-4 Conclusions of Ni(NO
3)
2First, the XAS spectra shows the divalence nickel in Ni(NO3)2 compound.
Combine with the theoretical calculation, we assume that the divalence nickel shows a high spin state S=1 in Ni(NO3)2, same as NiO. The spectrum shape differences between NiO and Ni(NO3)2 come from the different local environments.
On the other hand, very recently, O. Volkova et al. [2] measured magnetization measurement under magnetic field as shown in Fig. 4-9, the obtained value of saturation moment amounts about 2.03 B/f.u. which is very close to the theoretical one MS = ngSB = 2.15 B/f.u. for Ni2+, S=1, which is consistent to our simulations.
0 2 4 6 8
0.0 0.5 1.0 1.5 2.0
theoretical : MS = 2.15 B/f.u. for Ni2+, S=1
Ni(NO
3)
2M ( B/f.u.)
B (Tesla)
M
S~ 2.03
B
measured at T=2 K
Fig. 4-9. Magnetization measurement of Ni(NO3)2 At 2 K under magnetic field.[2]
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