Results and discussion on PrSrCoMnO 6

To study the valence and spin state of transition metal ions in polycrystal PrSrCoMnO6 sample, and expect to see the existence of Co³⁺ high spin state in perovskite structure, we measured the X-ray absorption near edge spectroscopy (XANES) on Co K-edge, Co L-edge, Mn K-edge and Mn L-edge in polycrystal PrSrCoMnO₆ sample.

5-1 Experimental Design

In one paper studied on LaSrCoMnO6 sample from J. Androulakis[3]. They assume the valence is +4 for manganese ion, and +3 for cobalt ions, and the trivalence cobalt may stay in intermediate-spin state (3d⁶, S=1) or high-spin state (3d⁶, S=2).

Since PrSrCoMnO₆ and LaSrCoMnO₆ belong to the same series, we wonder whether the valence and spin state of transition metal changed in different rare earth compound.

The Co K-edge and Mn K-edge XAS experiments were performed at the BL.17C at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The Co L-edge and Mn L-edge XAS experiments were performed at both H-SGM beam line

and Dragon beam line in NSRRC Taiwan. We also combine with the X-rat diffraction refinement and theoretical calculations to investigate the relationship between XAS spectra with crystal structure.

5-2 Mn XANES spectra

Room temperature Mn L-edge XAS data was recorded in total electron yield (TEY) mode (see Fig.5-1). In the previous work, which shows the PrSrCoMnO6

sample has a similar spectra shape and energy position with MnO2, we infer that the valence on manganese ion should be Mn⁴⁺.

29

Room temperature Mn K-edge XAS was recorded in the transmission mode. The edge position is sensitive to Mn valence in the K-edge spectra, which also represents the Mn⁴⁺ in PrSrCoMnO6 sample (see Fig.5-2), same as those Mn⁴⁺ in Eu₂CoMnO₆ and La2CoMnO6 double perovskite[18,19,20].

635 640 645 650 655 660

NCTU 高政男 Thesis (2012)

PrSrCoMnO

6

Mn-L₃

Mn-L₂

Mn-L2,3XAS spectra at room temperature

Total Electron Yield (arb. units)

6530 6535 6540 6545 6550 6555 6560 6565 6570 6575

0.0

Mn K-edge transmition mode at room temperature

Photon Energy (eV)

Normalized Absorption (arb. units)

Fig. 5-2 Mn K-edge XAS data in transimission mode

30 that the cobalt ions are also trivalence in PrSrCoMnO6 sample.

7700 7710 7720 7730 7740 7750 7760 7770 7780 0.0

Co K-edge XAS , Transmission mode

Fig.5-3. Co K-edge XAS spectra in transmission mode

In Co L-edge, room temperature XAS experiments were performed at the H-SGM beamline in NSRRC. Clean sample surface were obtained by cleaving samples in chamber with pressure lower than 10^-7 mbar. XAS data was recorded in total electron yield (TEY) in ultra high vacuum (~10^-10 mbar) chamber.

To estimate the valence and spin state of cobalt ions in PrSrCoMnO₆ sample, we compare the XAS data with some reference sample in Fig.5-4, such as CoO(Co²⁺), EuCoO3(Co³⁺-LS) and Sr2CoO3Cl2. If we compare with the divalence cobalt standard sample CoO, we find that the spectrum shape of PrSrCoMnO6 is far away from CoO,

31

and PrSrCoMnO₆ sits in a higher energy position, means the cobalt ions are not pure divalence. To think about the Co²⁺-Co³⁺ mixed valence state, C. F. Chang[21]

mentioned about the spectra shape of Co²⁺-Co³⁺ mixed valence state looks like Co³⁺

high spin state (see Fig. 5-5), but we still can distinguish them from the L3-edge mean peak energy position and a Co²⁺ characteristic at 777.7eV. Fig. 5-6 shows the Co³⁺-Co⁴⁺ mixed valence state studied by H.-J. Lin [22] . In Co⁴⁺ XAS spectra, the mean peak in L₃-edge is 1.5 eV higher than the mean peak of Co³⁺ spectra, and Co⁴⁺

spectra also present an obvious peak at 779.4 eV. Both the characteristics of Co⁴⁺

spectra were not displayed in PrSrCoMnO₆ spectrum. So far, we assume the cobalt ions are trivalence in PrSrCoMnO6 sample, consistent with the Co K-edge.

In the Co L-edge XANES spectra, PrSrCoMnO₆ shows a very different look with those ReCoO3 perovskite (Re=Eu, Sm). According to Z. Hu et al.(2004)[4], they mentioned about the cobalt ions shows trivalence in EuCoO3 single crystal sample.

The Co³⁺ ions formed an octahedral coordination with the six oxygen neighbours and Co³⁺ stays in the low spin state (LS, 3d⁶, S=0). In the Co³⁺ LS XAS spectrum, there shows a mean peak and a shoulder characteristic at higher energy range in both L2 and L₃-edge. Contrarily, Co L-edge XANES spectrum of PrSrCoMnO₆ is slightly similar to the spectrum of layered Sr2CoO3Cl compound, which was studied by Z. Hu et al.(2004)[4]. Z. Hu et al. demonstrate that the Co³⁺ ions with the CoO5 pyramidal coordination in the layered Sr2CoO3Cl compound shows an unambiguously high spin state (HS, 3d⁶, S=2). In the HS spectrum, there shows a mean peak and a shoulder characteristic in the lower energy range in L3-edge, while there are two shoulders in the L₂-edge, one stays in the higher energy range than mean peak and another one stays in the lower energy range.

If we compare the PrSrCoMnO6 XAS spectrum with other trivalence cobalt spectra more carefully, we can find that the L3-edge of PrSrCoMnO6 spectrum is very

32

similar to Sr₂CoO₃Cl₂ Co³⁺ HS state, but the L₂-edge looks like the combination of Co³⁺ HS and LS spectra. We are interested in such an unusual phenomenon.

775 780 785 790 795 800

PrSrCoMnO

6

Sr₂CoO₃Cl (Co³⁺ HS) EuCoO₃ (Co³⁺ LS)

CoO (Co

²⁺

)

Co-L₂ Co-L₃

T o ta l E le c tr o n Y ie ld ( a rb . u n it s)

Co-L

2,3

XAS spectra at room temperature

Photon Energy (eV) Fig. 5-4 Co L-edge XAS data

Fig. 5-5 Co L-edge spectra from ref.[21] Fig. 5-6 Co L-edge spectra [22]

33

In order to study the spin state of Co³⁺ ions in PrSrCoMnO₆, we also measured the temperature dependent Co L-edge XAS spectra at Dragon beam line in NSRRC.

The temperature dependent Co L-edge spectra is shown in Fig. 5-7, we find a shoulder characteristic presence at lower energy range of L3-edge in all temperature, and the shoulder intensity decrease with temperature decreasing. On the other hand, the intensity of peak at 795eV in L2-edge increase with temperature decreasing.

We focus on the L₃-edge first, there shows a shoulder characteristic in the lower energy range, which is the contribution from Co³⁺ HS state. As the temperature increasing, the shoulder intensity also increases. We infer that the HS population increases with temperature increasing. Now turn to the L2-edge, there shows a peak at 795eV which come from Co³⁺ LS spectra. As the temperature increasing, we saw the peak intensity become lower. We infer that the LS population decreases with the temperature increasing, this result consists with that in the L₃-edge. Up to now, we assume that the Co³⁺ ions in PrSrCoMnO6 sample is an HS-LS mixed-spin state system, and the HS state presence more at higher temperature.

775 780 785 790 795 800

0.0

Fig. 5-7 Temperature dependent Co L-edge XAS spactra on PrSrCoMnO6 sample

34

5-4 Powder X-ray diffraction

The X-ray diffraction experiment of powder PrSrCoMnO6 sample is down by the Brucker D8 diffractometer with Cu Ka radiation. The Rietveld refinement is down by the Brucker DIFFRAC.TOPAS software. Fig.5-8,9(a~d) shows the room-temperature XRD results of PrSrCoMnO₆, blue curve represents the experiment data, red curve represents the simulation result from Rietveld refinement, and the green curve represents the difference between experiment data with simulation. The crystal structure is solved to have monoclinic P21n space group [a=5.401453(53) Å , b=5.427901(50) Å , c=7.663798(70) Å; α=γ=90.0 degree and β=89.55283(63) degree]

as shown in the Fig.5-10.

For room-temperature XRD data, Rietveld refinement gives two sites for transition metals in table 5-1, both sites have six oxygen around and form a TMO6

octahedral coordination. Since the XRD can only tell the distance around but not atoms, we tried to distinguish them from some reference. First we compare the average Mn-O bond length between PrSrCoMnO₆ to La_1-xCa_xMnO₃ perovskite, x=0, 0.3, 0.5, 0.9 and x=1 in table 5-2[23~27]. For a higher valence of Mn ion, the average bond length should become shorter, so the site-2 has more chance to be the Mn site.

Second, we compare with the La_2-xSr_xCoMnO₆(x=0 and 0.4) double perovskite[28,29]

in table 5-3, which also show site-2 <Mn-O>=1.9202 Å might be the Mn site. Now we compare the average Co-O bond length of CoO6 coordination with ReCoO3

(Re=Eu, Sm, La) perovskite[30~32] in table 5-4, which are all Co³⁺ case.

In the CoO6 octahedral environment, the Co 3d degenerate state was separated into e_g and t_2g state with 10Dq difference by the crystal field, and the e_g state was separated into x²-y² and 3z²-r² with ∆eg difference by the distortion. Depending on the ratio between Hund’s coupling 10Dq and JH splitting, Co³⁺ ions (3d⁶) may exhibit either low-spin (LS,S=0) or high-spin (HS,S=1) states as shown in Fig.5-11.

35

An easy way to calculate the total energy of HS or LS state is to set the t_2g energy level to be zero, gain one crystal field energy 10Dq for each electron which stays in the e_g state, and also consider about the exchange interaction (Hund’s coupling)

n is the number of spin up electrons, m is the number of spin down electrons, J_H is the Hund’s coupling energy. For Ni²⁺, the total energy of HS and LS can be describe as :

H

HS state would be ground state in a small crystal field case.

In the Co L-edge XAS data, PrSrCoMnO6 shows a high spin like spectrum shape and ReCoO₃ shows low spin spectra, which means the crystal field in PrSrCoMnO6 is smaller than those three else, so Co ions should stay in site-1 and have the longest average Co-O bond lengh 1.9342 Å .

To give a summary, at room temperature, Co ion has a average Co-O bond length of 1.937 Å in the CoO₆ coordination, and Mn ion has a average Mn-O bond length of 1.920 Å in the MnO6 coordination.

36

Fig.5-8 X-ray diffraction data of powder PrSrCoMnO6 sample

32.8 33.0 33.2 33.4 33.6

40.4 40.6 40.8 41.0 41.2 41.4

-10000

Fig.5-9(a,b,c,d) Details of X-ray diffraction data and simulations on PrSrCoMnO6.

37

Fig.5-10 Solved crystal structure of PrSrCoMnO₆ at T=300 K.

Site-1 Site-2

Co/Mn-O

1

(Å ) 1.87(11) 1.98(12) Co/Mn-O

2

(Å ) 1.97(12) 1.89(12) Co/Mn-O

3

(Å ) 1.970(73) 1.889(73)

d

_ave

(Å ) 1.937(101) 1.920(104)

Table 5-1. TM-O bond length of two TM sites

38

Table 5-2. Mn-O bond lengh in MnO₆ coordination

Table 5-3. Mn-O bond lengh in double perovskite samples

Table 5-4. Co-O bond lengh in CoO6 coordination

39

Fig. 5-11. High spin and low spin in Ni²⁺ (3d⁸) case.

On the other hand, we also did the XRD experiment on PrSrCoMnO₆ at T=12 K.

The XRD data and the Rietveld refinement results are shown in the Fig.5-12,13(a~d).

The low temperature crystal structure is solved to have monoclinic P21_n space group [a=5.438825(83) Å , b=5.395555(91) Å , c=7.62190(12) Å ; α=γ=90.0 degree and β=89.9572(19) degree] as shown in Fig.5-14. The solved crystal structure also shows two transition metal sites with different TM-O bond length. Table 5-5 shows the comparison of lattice parameter in PrSrCoMnO₆ between T=300 K and T=12 K.

PrSrCoMnO6 still keeps the space group P21n at low temperature, same as that in the room temperature. An interesting thing is that the lattice volume become smaller and the average Mn-O bond length also become shorter at low temperature, but the average Co-O bond length become 1.980(57) Å at low temperature, which is longer than the Co-O bond length at room temperature.

40

Fig.5-12 X-ray diffraction data of powder PrSrCoMnO6 sample at T=12K.

32.6 32.8 33.0 33.2 33.4 33.6

-100000

40.4 40.6 40.8 41.0 41.2 41.4

-20000

47.0 47.2 47.4 47.6 47.8 48.0 48.2

0

58.5 58.8 59.1 59.4 59.7 60.0

0

Fig.5-13 (a,b,c,d) Details of X-ray diffraction data and simulations on PrSrCoMnO6 at T=12 K

41

Fig.5-14 Solved crystal structure of PrSrCoMnO₆ at T=12 K.

Table.5-5 Lattice parameters of PrSrCoMnO6 in T=300 K and T=12 K.

42

5-5 XANES theoretical calculation by XTLS

To confirm the spin state of Co³⁺ in PrSrCoMnO6, we also discussed the theoretical calculations of L_2,3 XAS line shape by XTLS software from A. Tanaka [8]

using the full atomic multiplet theory, together with hybridization of Co 3d orbital with the O 2p ligands and the point charge crystal field in CoO₆ cluster. The simulation input files were provided by Dr. Z. Hu.

If compare the room temperature PrSrCoMnO₆ spectrum with the Co³⁺ HS and LS spectra more carefully, we find there shows a shoulder characteristic at 777.7 eV in PrSrCoMnO₆ spectrum, and this characteristic do not come from Co³⁺ HS spectra or Co³⁺ LS spectra. Compare the 777.7 eV characteristic with CoO spectrum, the shoulder characteristic match well with the first peak of L₃-edge in CoO, means there exist few oxygen absence at sample surface and cause few presence of Co²⁺ ions. To get a pure Co³⁺ spectrum, we subtract a suitable ratio of CoO spectrum as shown in Fig. 5-15.

In the simulation of room temperature data, we use the hybridization coefficient pdσ = -1.70 eV [4], set ∆eg = 0 eV (since distortion doesn’t change the Co³⁺ spin state).

We focus on the peak splitting and ratio of intensity in both L₂ and L₃ edge, a shoulder characteristic in L3-edge and a charge transfer characteristic at 788.5 eV. Finally we get a best fit to PrSrCoMnO₆ spectra in 10Dq=0.495 eV (see Fig.5-16).

In the theoretical calculation from XTLS software, the calculation result gives the ground state and first 15 exciting states. For a given temperature parameter, the calculation result spectrum shows the combination from the 16 states and obeys the Boltzmann rule. Table.5-6 shows the energy and total spin S(S+1) for each state, ∆E is the energy difference related to the ground state. The total spectrum considered the thermal excitation at finite temperature. In state-1, the total spin S(S+1) = 0.4339917 presents a low spin state (LS, S=0), while the state-2~4 show the S(S+1) = 5.983913

43

means the total spin is very close to the high spin state (HS, S=2).

Fig. 5-17 shows the ground state of several simulations in different fitting parameter 10Dq, and find the ground changes from LS (10Dq=0.47259 eV) to HS (10Dq=0.47258). From the Co L2-edge XAS spectra line shape, we obviously see that PrSrCoMnO₆ is neither pure high spin nor pure low spin state

So far, from the list of each state, we assume the cobalt ion is a HS-LS mixed system and can be described as the combination of the ground state (state-1) and the triply degenerate 1^st exciting state (state-2~4).

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0.0

PrSrCoMnO₆ T=300 K,raw data CoO (Co²⁺)

PrSrCoMnO₆ T=300 K,pure Co⁺³

Fig. 5-15 Co L-edge XAS spectra with Co²⁺ impurities

775 780 785 790 795 800

Normalized intensity (arb. units)

Fig. 5-16 Co L-edge XAS spectra with theoretical calculation

44

state Energy (eV) S(S+1) ∆E (eV)

State-1 -7.8035207919660046 0.4339917 0

State-2 -7.7709324835719755 5.983913 0.032588 State-3 -7.7709324835719746 5.983913 0.032588 State-4 -7.7709324835719746 5.983913 0.032588 State-5 -7.7484215104846887 5.910728 0.055099 State-6 -7.7484215104846879 5.910728 0.055099 State-7 -7.7442833378101614 5.946827 0.059237 State-8 -7.7442833378101605 5.946827 0.059237 State-9 -7.7442833378101597 5.946827 0.059237 State-10 -7.7089677557410514 5.530704 0.094553 State-11 -7.7078332127177269 5.872373 0.095688 State-12 -7.7078332127177269 5.872373 0.095688 State-13 -7.7078332127177269 5.872373 0.095688 State-14 -7.7037092828197791 5.875087 0.099812 State-15 -7.7037092828197808 5.875087 0.099812 State-16 -7.7037092828197800 5.875087 0.099812

Table 5-6 Energy and total spin S(S+1) for each state in 10Dq=0.495eV by XTLS

775 780 785 790 795 800

0.0

Fig. 5-17 The ground state of simulations in different 10Dq

45

M.W.Haverkort et al.[33] measured Co L-edge XAS on LaCoO3 sample in a wide temperature range and saw the spectra shape changes with temperature in Fug.

5-18. They reveal that LaCoO₃ at finite temperatures is an inhomogeneous mixed-spin state system, and the spin state transition in LaCoO3 can be well described as a low-spin ground state and a triply degenerate high-spin first excited state. There exist around 25% HS state in room temperature LaCoO3 spectrum, and the HS population increase with temperature increasing.

Fig. 5-18 Co L-edge XAS in LaCoO3 from M.W.Haverkort et al.[17]

46

In the temperature dependent Co L-edge XAS experiment of PrSrCoMnO₆, we also subtract a suitable ratio of CoO spectrum for each spectrum to get a pure Co³⁺

spectrum as shown in Fig. 5-19. In the fitting of temperature dependent Co L-edge XAS data, we first set the temperature parameter equals to the real experiment condition. We focus on the peak splitting and ratio of intensity in both L₂ and L₃ edge, a shoulder characteristic in L3-edge and a charge transfer characteristic at 788.5 eV, finally we found that the 10Dq=0.495 eV calculation results can fit both T=300 K and T=400 K data but not for T=18.3 K case. This problem was also mentioned by M.W.

Haverkort et al.[33], who studied the spin state transition on LaCoO3 sample in finite temperature. The best fit to the PrSrCoMnO6 Co L-edge XAS data at T=18.3 K was found at parameter 10Dq equals to 0.4743 eV (see Fig. 5-20). The simulation result of 10Dq=0.4743 eV also shows a low-spin ground state with the triply degenerate high-spin exciting states. From the calculated energy difference between the LS ground and the HS 1^st exciting states, together with the Boltzmann rule in finite temperature, our simulations show 39.79% HS population for T=18.3 K, 10Dq=0.4743 eV case; 55.70% HS population for T=300 K, 10Dq=0.495 eV case;

and 67.98% HS for T=400 K, 10Dq=0.495 eV case. We obviously see the HS population increases with temperature.

In general case, the crystal size would decrease at low temperature, means the size of CoO6 octahedral would decrease and the crystal field parameter 10Dq should become higher at low temperature, which may not consistent with our calculations.

Contrary to the general cases, the powder XRD data of PrSrCoMnO6 shows an unusual change at low-temperature, the lattice volume and the average Mn-O bond length decrease but the average Co-O bond length increase at low temperature. The longer Co-O bond length may cause a smaller crystal field 10Dq in CoO6 octahedral, which is consistent with our XAS simulation results.

47

So far, we infer that the cobalt ion in PrSrCoMnO₆ sample is a HS-LS mixed system, which has a LS ground state and the triply degenerate HS 1^st exciting states, and the higher HS population, which come from the thermal excitation, presence more at high temperature.

775 780 785 790 795 800

0.0

Fig 5-19 Temperature dependent Co³⁺ L-edge XAS spectra.

775 780 785 790 795 800

Normalized intensity (arb. units)

Co L-edge XAS data(TEY) on PrSrCoMnO₆

Co-L3

Fig 5-20 Co L-edge XAS experiment data with theoretical calculation (setting the fitting temperature parameter equals to real experiment conditoin).

48

At the end, we also fit the room temperature Co L-edge XAS spectra in different samples. The fitting parameter 10Dq=0.495 eV shows the best fit to PrSrCoMnO6, while the 10Dq=0.52 eV can fit LaCoO₃ well and 10Dq=0.54 eV can fit EuCoO₃ at room temperature (see Fig.5-21). Compare the average Co-O bond length with the fitting parameter 10Dq in each sample, we obviously see the fitting parameter 10Dq (point charge crystal field in octahedral symmetry) decreasing with the increasing of average Co-O bond length as shown in Fig.5-22. The gray dash curve shows the linear fit of room temperature cases, and the light blue curve shows the Co³⁺ ground state HS-LS transition boundary 10Dq=0.47258 eV calculated from XTLS software.

Compare with EuCoO3 and LaCoO3, PrSrCoMnO6 has the largest average Co-O bond length and the smallest crystal field on Co³⁺ ions in the CoO₆ coordination, and the crystal field in PrSrCoMnO6 is much closer to the HS-LS transition boundary, especially in the low temperature.

775 780 785 790 795 800

N o rm al iz ed i n te n si ty ( ar b . u n it s)

Co-L₂ Co-L₃

Co L-edge XAS data at room temperature

PrSrCoMnO₆ 10Dq=0.495 eV

LaCoO₃ 10Dq=0.52 eV

EuCoO₃ 10Dq=0.54 eV

Photon Energy (eV)

Fig. 5-21 Co L-edge XAS spectra with theoretical calculation

49

Table 5-7 The comparison of average Co-O bond length with the fitting parameter 10Dq in different Co³⁺ samples.

1.93 1.94 1.95 1.96 1.97 1.98 0.46

0.48 0.50 0.52 0.54

PrSrCoMnO

6

T=18 K PrSrCoMnO

6

T=300 K LaCoO

₃

fi tti ng paramet er 10 Dq (eV)

Co-O bond length (Angstrom)

data

fitting curve HS-LS transition boundary

EuCoO

3

Fig. 5-22 The relationship between the fitting parameter 10Dq with the average Co-O bond length in different Co³⁺ samples.

50

在文檔中 以吸收光譜研究八面體結構中過渡金屬價態與自旋態 (頁 38-60)