Distinguishing between High- and Low-Spin States for Divalent Mn in

Letter pubs.acs.org/JPCL

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X‑ray Emission Spectroscopy Daisuke Asakura,*,† Yusuke Nanba,† Masashi Okubo,† Yoshifumi Mizuno,† Hideharu Niwa,‡,§ Masaharu Oshima,§ Haoshen Zhou,† Kozo Okada,∥ and Yoshihisa Harada‡,§ †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan ‡ Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan § Synchrotron Radiation Research Organization, The University of Tokyo, Bunkyo-ku, Tokyo 113-8586, Japan ∥ The Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan S Supporting Information *

ABSTRACT: We combine Mn L2,3-edge X-ray absorption, high resolution Mn 2p−3d−2p resonant X-ray emission, and configuration−interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn2+ low-spin state separately from that of the Mn2+ high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn2+ low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn2+ high spin state. The d− d-excitation peak nearest to the elastic scattering was ascribed to the Mn2+ LS state by the CIFM calculation, indicating that the Mn2+ LS state with a hole on the t2g orbital locates near the Fermi level. SECTION: Spectroscopy, Photochemistry, and Excited States

S

(RXES), or resonant inelastic X-ray scattering, is capable of analyzing the electronic structure with site, valence, and spin selectivity by choosing the excitation energy (incoming-photon energy) hνin from the XAS profile. In addition, the photon-in/ photon-out XES process provides bulk-sensitive information. In this paper, we demonstrate how the combined use of XAS and high energy-resolution RXES with theoretical analyses is powerful to distinguish the electronic structure of the same TM elements at different sites in Mn-based Prussian blue analogue: K1.72MnII[MnII(CN)6]0.93·0.65H2O (MnMn-PBA)9 with almost the same structure of K2MnII[MnII(CN)6].10−12 MnMn-PBA has two crystallographically independent C-coordinating and N-coordinating TM sites,9−21 where Mn2+ low-spin (LS) and high-spin (HS) states may coexist. Although the coexistence has been suggested by magnetic and infrared (IR) studies,10 local spin states of C-coordinated Mn (MnC) and Ncoordinated Mn (MnN) are still unclear; the site-selective and spin-selective information could not be obtained since magnetic susceptibility measurements provide average information on all

oft X-ray absorption spectroscopy (XAS) has played an important role in the fields of both physics and chemistry.1,2 Direct observation of 3d orbitals via transitionmetal (TM) L2,3-edge absorption allows us to clarify the electronic structure including spin states, crystal-field splitting, exchange splitting, intra-atomic Coulomb interaction, core− hole potential, and charge-transfer (CT) energy. Furthermore, one can select the probing depth by choosing the detection modes, for example, surface-sensitive total electron yield (TEY) and bulk-sensitive total fluorescence yield (TFY) detection modes. Thus, development of the better energy-resolution techniques as well as more precise theoretical calculation method is highly desired to analyze the multiplet structures of XAS spectra in detail. However, in spite of the element selectivity and selective probing depth, XAS could hardly distinguish between identical TMs at different sites or in different valence/spin states. For example, it is difficult to resolve the Fe L2,3-edge XAS spectrum of γ-Fe2O3 (Fe ions at the tetrahedral and octahedral sites3,4) into the respective contributions. Soft X-ray emission spectroscopy (XES) is complementary to XAS.5,6 The recent progress in the energy resolution of XES has contributed significantly to the full understanding of the 3d states in various compounds.7,8 In particular, resonant XES © XXXX American Chemical Society

Received: August 17, 2014 Accepted: October 30, 2014

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dx.doi.org/10.1021/jz501738m | J. Phys. Chem. Lett. 2014, 5, 4008−4013

The Journal of Physical Chemistry Letters

Letter

peak at 640 eV should largely be suppressed by the selfabsorption. Owing to this distortion, for the TFY spectrum, it is difficult to extract exact parameter values for the Mn2+ LS and HS states by comparing with the full multiplet calculation. One of the solutions to the problem about the distorted peak profile in TM L-edge TFY XAS is recently established partialfluorescence-yield (PFY) mode or inverse-PFY (IPFY) mode in soft X-ray region which can suppress such self-absorption and saturation effects.24−27 However, in this study, we carried out high energy-resolution Mn 2p−3d−2p RXES measurements to more explicitly distinguish the mixed spin states rather than XAS (even for the PFY/IPFY mode) and to obtain more quantitative information about the 3d electronic states of MnC and MnN. In order to select the Mn2+ HS and Mn2+ LS states, the excitation energies were tuned to hνin = 640 and 643 eV for the L3 region of the XAS spectra where the Mn2+ HS and Mn2+ LS characters are dominant respectively according to the above discussion about the TFY XAS and to the Mn L2,3-edge TEY XAS for K4Mn(CN)6.23 Figure 2 shows the experimental RXES results for hνin = 640 eV (spectrum A) and 643 eV (spectrum B). The spectra are plotted as a function of energy loss (= hνin − hνout, where hνout is emitted photon energy), locating the elastic peaks to 0 eV. The peaks from 1.5 to 5.5 eV for both the spectra are attributed to the d−d excitation.1,5,6,22 The broad structures in the higher energy loss region than the d−d excitation peaks are attributed to the CT excitation between the Mn 3d and CN 2p orbitals. As displayed in Figure 2a, spectrum A shows a strong elastic peak and sharp d−d excitation peaks from 1.9 to 5 eV. The d−d excitation profile is similar to that for MnO,22 suggesting that the Mn2+ HS character is actually dominant at hνin = 640 eV for MnMn-PBA. In addition, the possibilities of higher valence states such as Mn3+ and Mn4+ are excluded because the RXES spectra for manganese oxides with Mn3+ and Mn4+ have exhibited considerably large CT excitations.28−30 The CT excitation in the RXES spectra for Mn2+ HS state should be less pronounced than those for Mn3+ and Mn4+ HS states because the Mn2+ HS state (S = 5/2: (t2g3, eg2)) with fully occupied majority-spin band is highly stable. In spectrum B (Figure 2b), the d−d excitation peaks were observed between 1.9 and 7 eV. Remarkably, the enhanced peaks from 3.8 to 6 eV are characteristic of MnMn-PBA. This is largely different from RXES results for MnO with the Mn2+ HS state22 and manganese oxides with Mn3+ and Mn4+ states.28−30 Figure 2 also indicates the best-fitted CIFM-calculated results using electronic structure parameter sets summarized in Table 1 that also explain the XAS spectra (see Figure S2 in Supporting Information). As expected from general consideration, the XAS and XES profiles are well explained by the HS and LS states for the N-coordinating (MnN) and Ccoordinating (MnC) metals of the PBA frameworks, respectively.19,20 Moreover, we investigated the effect of 10Dq (and also Δ) for both MnN and MnC on the RXES profile as shown in Supporting Information. Spectrum A can be explained by the Mn2+ HS state for the MnN, except for the peak at 1.9 eV (Figure 2a). In the calculation for the MnN site, the peaks at 2.5 and 3.2 eV should reflect the occupied t2g and eg orbitals, respectively. In contrast, as shown in Supporting Information, the calculated spectra for the MnC site using a small 10Dq representing the Mn2+ HS state considerably deviate from the experimental result (note that the other parameters such as Δ was fixed to the best result). Thus, the Mn2+ HS state on the

the elements in the unit cell and IR spectra around the CN stretching frequency are sensitive to only the valence of MnC.9−12 In contrast, a combined use of XAS and RXES with theoretical analyses can easily distinguish the spin states of MnC and MnN. In addition to the spin states, the combined approach can also extract detailed information on the Mn 3d states. In general, PBAs show strong CT interaction between the TMs and CN ligands, that is, ligand-to-metal CT (LMCT) and metal-to-ligand CT (MLCT). Thus, the theoretical calculation of both XAS and RXES should include the electron configurations such as 3dn and 3dn+1L for the LMCT, 3dn−1L for the MLCT, and higher order CTs (3dn+2LL, 3dn−2LL, and 3dnLL). Here, L and L are ligand electron and hole, respectively. In this work, we applied configuration-interaction full-multiplet (CIFM) calculation in which the molecular orbitals (MOs) of CN and NC for [MnII(CN)6]4− and [MnII(NC)6]4− octahedrons are precisely treated by considering the transfer integrals of (ppσ) and (ppπ) between the ligand 2p orbitals, leading to distinguishing the electronic structures of MnC and MnN in MnMn-PBA. This method was also successful for analyzing TM L2,3-edge XAS of other PBAs.20,21 In the CIFM calculation, the electronic structure parameters of CT energy (Δ), intra-atomic Coulomb interaction (U), exchange splitting (J), core−hole potential (Q), crystal-field splitting (10Dq) under the Oh symmetry and transfer integrals are taken into account. These parameters are mutually applied to both XAS and RXES spectra. Definition of each parameter and more detail of the calculation are described in Methods. Figure 1 shows the experimental Mn L2,3-edge XAS (TEY and TFY) spectra. The TEY spectrum consists of four peaks for

Figure 1. Mn L2,3-edge TEY and TFY XAS for MnMn-PBA. The arrows indicate the excitation energies used for RXES.

the L3 region and two broad peaks for the L2 one, which is very similar to the Mn2+ HS multiplet structure under Oh symmetry as seen for MnO,22 whereas the result is clearly different from Mn L2,3-edge TEY XAS for K4Mn(CN)6, which was attributed to Mn2+ LS state by Cramer et al.23 On the other hand, the TFY mode is efficient in terms of the bulk sensitivity and the Mn L2,3-edge TFY XAS is quite different from the TEY result although the peak positions are similar. In contrast to the TEY spectrum, the peaks at 644 and 652.5 eV are enhanced and new shoulder structures appear around 645−646 and 655 eV in the TFY spectrum. These features would imply the contribution of Mn2+ LS for the MnC site. However, the TFY spectrum should be distorted by selfabsorption and saturation effects;24 particularly, the highest 4009



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Figure 2. Mn 2p−3d−2p RXES spectra and the best calculated results for (a) hνin = 640 and (b) 643 eV. The Mn2+ HS and Mn2+ LS spectra were calculated for [Mn(NC)6]4− and [Mn(CN)6]4− octahedrons (MnN and MnC), respectively. The 10Dqs for the calculated spectra for the MnN and MnC sites were set to 0.4 and 4.0 eV for both (a) and (b). For the calculated spectra, the width for Lorentzian function is set to 0.2 eV.

Recent ab initio calculation will also be powerful for such a system with lower symmetry.32−35 Both spectra A and B have a peak at 1.9 eV, which can be assigned to the excitation within the Mn2+ LS state. Thus, the top of valence orbitals could be the t2g orbital of the Mn2+ LS state in an ionic picture. This consideration is consistent with that the t2g orbital has a hole (S = 1/2: (t2g5)), whereas the Mn2+ HS state has a large exchange splitting between the fully occupied majority-spin orbitals and completely unoccupied minority-spin orbitals (Scheme 1). Most likely, because of the exchange splitting and the partially unoccupied t2g orbital, the gap between the lowest d−d peak and the elastic peak would become smaller than the 10Dq of 4.0 eV for the Mn2+ LS state. The occupied orbitals for the Mn2+ HS state should be located below the top of the valence orbitals originated from

Table 1. Electronic Structure Parameters Used in the CIFM Calculationsa 10Dq Δ (pdσ) U J Q κ (ppσ)CC (ppσ)NN (ppσ)CN εC εN

[Mn(NC)6]4− (HS)

[Mn(CN)6]4− (LS)

0.4 3.2 −1.0 (for Mn−N bond) 5.3 0.6 5.0 0.7

4.0 −1.2 −1.8 (for Mn−C bond) 5.6 0.72 5.0 0.85 1.0

0.8 8.0 0.0 −2.5

8.0 0.0 −2.5

In eV. εC and εN are the one-electron C and N 2p levels used to define the MO energy levels. κ is a coefficient to reduce the Slater integrals.1 Details of these parameters are described in Methods.

a

Scheme 1. Schematic MnN-NC-MnC Framework and Electron Configurations of MnN and MnCa

MnN site was confirmed, whereas the peak at 1.9 eV would be of the Mn2+ LS state on the MnC site. Spectrum B is mostly ascribed to the Mn2+ LS state on the MnC site (Figure 2b), whereas a small fraction of the Mn2+ HS state on the MnN site might be present below in the strong Mn2+ LS signal. In contrast to the d5 HS state, the d5 LS configuration (t2g5) should have many channels for the 2p−3d− 2p resonance because of the exchange splitting within the t2g orbital, resulting in the many peaks from 1.9 to 7 eV. Moreover, the CIFM calculation revealed that the Mn2+ LS state on the MnC site has negative Δ which causes a strong LMCT effect.31 The structure above 8 eV can be ascribed to the CT excitation mostly due to the d6L configuration. In the CIFM calculation, the Oh symmetry was considered for both MnC and MnN, whereas Her et al. had reported a slight tetragonal distortion of the [Mn(CN)6] octahedron (one Mn−C bond is longer than the other two directions by 5%).10 If the slight tetragonal distortion can be considered exactly in the CIFM calculation for the Mn2+ LS state and the experimental energy resolution is further improved, spectrum B will be reproduced much better.

a

The green arrows indicate the degrees of LMCT. The LMCT effect for MnC-CN bond is stronger than that for MnN-NC. An electron transferred from CN can be settled on the t2g hole of MnC (white circle in the energy level diagram); the charge-transferred d6L state is also displayed. For simplicity, the unoccupied d orbitals are not shown. 4010



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the Mn2+ LS state. Nevertheless, spectrum B shows a significant LS weight up to 12 eV including the d−d excitations from 1.9 to 7 eV and CT excitations above 7 eV, suggesting that the occupied orbitals of the Mn2+ LS state would extend from near the Fermi level (EF) to the lower energy region. This lower energy level should be of the charge-transferred d 6L configuration. Indeed, the CT excitation in spectrum B is large, suggesting a strong hybridization. Then, the strong CT excitation is consistent with the negative Δ for the Mn2+ LS state, leading to LMCT.31,36,37 The LMCT energy ΔLMLS for the Mn2+ LS state was calculated to be 1.61 eV by using the determined parameters while ΔLMHS was calculated to be 7.45 eV (see eqs 2 and 4 in Methods). The small ΔLMLS indicates that the LMCT effect between the Mn2+ LS state and CN should be much stronger than the LMCT between the Mn2+ HS state and NC. The calculated weights of the electron configurations and the average 3d electron numbers ⟨nd⟩ for the Mn2+ HS and LS states are summarized in Table 2. For the Mn2+ HS state, the d5

distinguishing mixed states such as spin states and charge states for a specific element in various materials. For example, a Mn cluster incorporated in photosystem II in the field of photochemistry38 has four inequivalent Mn sites whose charge valence is still under intense debate,39 which should be greatly benefited if the detailed 3d energy diagram for each Mn site could be elucidated using the present method. Furthermore, the method can also be useful to site-dependently clarify the Mn 3d states of perovskite manganites with charge, spin, or orbital ordering.40 The Mn−O−Mn framework is analogous to the Mn−CN−Mn in MnMn-PBA while the O 2p orbital should be simpler than the MO of CN.

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METHODS Synthesis. MnMn-PBA was synthesized by a precipitation method.9 The X-ray diffraction pattern confirmed successful formation of MnMn-PBA as reported in ref 9. (See Figure S1 in Supporting Information.) XAS and RXES. XAS and RXES measurements were performed at BL07LSU in SPring-8.41 Both TEY and TFY modes were employed for the Mn L2,3-edge XAS. The RXES measurements were carried out using ultrahigh resolution XES spectrometer, HORNET.8 The energy resolution of the incident beam was around 100 meV, and the total energy resolution for RXES was set to ΔE = 200 meV at hνin = 642 eV. All the XAS and RXES measurements were performed at room temperature. Theory. In the CIFM calculation, the electronic structure parameters of U, J, and Q are defined as same as ref 42. We used the transfer integrals for C−C, C−N, and N−N {(ppσ)CC, (ppσ)CN, and (ppσ)NN} to describe the MOs. The hybridization strength between the Mn 3d and MOs can be derived from combining the MO wave functions with the transfer integrals for (pdσ). (pdπ) and (ppπ) are fixed to (pdπ)/(pdσ) = −1.0/2.2 and (ppπ)/(ppσ) = −0.25.43 Δ for the Mn2+ HS state (t2g3, eg2) was given by

Table 2. Ratios for Each Electron Configurationa and Average 3d Electron Numbers ⟨nd⟩ Determined from CIFM Calculations d5 d6L d4L d7L2 d5LL d3L2 ⟨nd⟩ a

[Mn(NC)6]4− (HS)

[Mn(CN)6]4− (LS)

91.1 5.3 3.2 0.1 0.4 0.0 5.02

51.2 31.9 7.5 1.9 7.3 0.2 5.28

In percent.

configuration is dominant: 91.1%. Both the LMCT (d6L: 5.3%) and MLCT (d4L: 3.2%) weights are small, resulting in that ⟨nd⟩ is calculated to be 5.02, very close to the nominal valency. On the other hand, for the Mn2+ LS state, the weight of d6L configuration reaches 31.9% while the d5 configuration is suppressed to 51.2%. Thus, the LMCT is strong in the Mn2+ LS state. The weight of d4L configuration is 7.5%, suggesting that the MLCT is not so strong compared to the Fe3+ LS states in PBAs with C-coordinated Fe,19,20 having the same configuration t2g5 as the Mn2+ LS state. Consequently, ⟨nd⟩ is calculated to be 5.28 for the Mn2+ LS state. The strong LMCT and the enhanced ⟨nd⟩ are consistent with the large CT excitation observed for spectrum B. In conclusion, we distinguished between the Mn2+ LS and Mn2+ HS states in MnMn-PBA using XAS, high energyresolution RXES, and CIFM calculation. The d−d-excitation peaks at the energy loss of 1.9 eV for both RXES spectra with hνin = 640 and 643 eV should originate from the Mn2+ LS state. The d−d-excitation peaks of the Mn2+ HS state appear in the region higher than 1.9 eV for the RXES spectrum with hνin = 640 eV. These results indicate that the Mn2+ LS state with a hole on the t2g orbital locates near the EF. CIFM calculation for the Mn2+ LS state with strong LMCT effects between C and MnC well reproduces the RXES spectrum for hνin = 643 eV. The enhanced structure from 3.8 to 6 eV characteristic of MnMn-PBA is ascribed to the bonding states between the Mn2+ LS t2g orbital and π orbital of CN due to the strong LMCT effects. Here, we emphasize that a combined use of XAS, high energy-resolution RXES, and CIFM calculations sheds light on

Δ = εd + 5U − 8J

(1)

where εd is the center of gravity of the d band. The LMCT for the HS state energy ΔLMHS was expressed as ΔLM HS = E{d6 L̲ (t 2g)} − E{d5(HS)} = Δ − 4Dq − ε B p(t 2g)

(2)

where ε p(t2g) is the energy level of the bonding-type MO. Similarly, for the LS state (t2g5), Δ was given by B

Δ = εd + 5U − 10J

(3,)

and the LMCT energy (ΔLMLS) was expressed as ΔLM LS = E{d6 L̲ (t 2g)} − E{d5(LS)} = Δ − 4Dq − ε B p(t 2g)

(4)

The MLCT energies are also given by a similar manner. The details of the equations are described in the refs 20, 21, and 42. To compare the calculated results with the experimental spectra for the powdered sample, we averaged the polarization dependence.42

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ASSOCIATED CONTENT

S Supporting Information *

Description of the material included. This material is available free of charge via the Internet at http://pubs.acs.org. 4011



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The XAS and RXES measurements were carried out by the joint research in the Synchrotron Radiation Research Organization and the Institute for Solid State Physics, the University of Tokyo (Proposal Nos. 2011A7414, 2011B7417, and 2012A7430).

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REFERENCES

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Distinguishing between High- and Low-Spin States for Divalent Mn in

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