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4 Application to Two-Dimensional (2D) Fermi Surface System of Uranium Dipnictides

4 Application to Two-Dimensional (2D) Fermi Surface System of Uranium Dipnictides

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7.4 APPLICATION TO TWO-DIMENSIONAL (2D) FERMI SURFACE SYSTEM OF URANIUM DIPNICTIDES



135



In 238 U M€

ossbauer spectroscopy, isomer shift, which is one of the important parameters to discuss the hybridization

between 5f electrons at uranium atoms and electrons at the other atoms, is difficult to observe as mentioned above.

However, hyperfine coupling constant at 238 U nuclei is a complementary parameter to discuss the hybridization at the

uranium site. Typical coupling constants in uranium-based intermetallics are about 150 T/mB, to our best knowledge.

When coupling constants smaller than 150 T/mB are obtained in some compounds, it can be concluded that the nature of

the 5f electrons in them is more delocalized than that in typical uranium-based intermetallics because of the expansion of

the wave functions of 5f electrons.

7.4.2 Hyperfine Interactions Correlated with the Magnetic Structures in Uranium Dipnictides

238



U M€

ossbauer spectra previously reported in magnetically ordered compounds such as UO2, UGe2, and UPd2Al3

[2,3,6,7] are a pure magnetic pattern. In some of them, absence of the nuclear quadrupole interaction is caused by the

ossbauer spectra of uranium dipnictides differ from the

same reason as in UO2 as mentioned in Section 7.2. The 238 U M€

spectra of UO2 and UPd2Al3 as shown in Fig. 7.12 [11]. The spectra in uranium dipnictides are asymmetric except for UBi2

where the spectrum is symmetric. Since the spectral asymmetry is caused only by the presence of the nuclear quadrupole

ossbauer spectroscopy, the observation of the asymmetric spectra in UP2, UAs2, and

interaction in the case of 238 U M€

USb2 indicates the coexistence of magnetic dipole and electronic quadrupole interactions at the 238 U nuclei. On the other

hand, the interpretation of the spectrum of UBi2 is the observation of pure hyperfine field or pure nuclear quadrupole

interaction with the asymmetry parameter of h ¼ 1. Considering the Neel temperature of 181 K and the large ordered

magnetic moment of 2.1mB at the uranium site in UBi2 as shown in Table 7.3, the latter case was ruled out. The measured

temperatures in all the samples are 5.1 K, much lower than their Neel temperature. The spectra shown in Fig. 7.12 were

measured in the antiferromagnetic states, because all the uranium dipnictides exhibit an antiferromagnetic ordering at

about 200 K. Why is the spectrum of UBi2 different from those of the other dipnictides, although their crystal structure is

the same or similar among uranium dipnictides? One of possible reasons is the different magnetic sequence in UBi2. The

magnetic unit cell volume in UBi2 is a double of the unit cell in its crystal structure, while that in the other uranium

dipnictides is a quadruple of the unit cell. On the other hand, the two-dimensional Fermi surface formed by a flat magnetic

Brillouin zone was observed by de Haas–van Alphen measurements [68–71]. This is one of the evidence that the

electronic state of the ground state is correlated with the magnetic sequence (structure) in uranium dipnictides.

Therefore, the obtained M€

ossbauer spectra indicate the ground state of the electronic structure strongly connected with

magnetic sequences in these compounds.



1.00



UP2

0.95



Relative transmission



1.00



UAs2



0.95



1.00



USb2



0.95



1.00



UBi2



0.96

–100



–50



0



50



Velocity (mm s–1)



100



FIGURE 7.12

238

€ ssbauer spectra of

U Mo

uranium dipnictides at 5.1 K.



136





7 STUDY OF EXOTIC URANIUM COMPOUNDS USING 238U MOSSBAUER

SPECTROSCOPY



UX2



(a)



Hhf (T)



300



250



200

(b)



2



e qQ (mm s–1)



0



-50



FIGURE 7.13

238

€ ssbauer parameters of

U Mo

uranium dipnictides at 5.1 K.



Ahf (T / μB)



(c)



150



100

P



As



Sb



Bi



The spectral analyses were carried out by the Hamiltonian of the hyperfine interaction at 238 U nuclei without any

perturbation. Since the direction of hyperfine field is parallel to that of ordered magnetic moment, the direction of the

observed hyperfine field is fixed along the c-axis in all the compounds. On the other hand, the main axis of the electric

field gradient was fixed along the c-axis, considering their crystal structures. The spectral analyses are successful when

asymmetry parameter h is equal to zero. The obtained M€

ossbauer parameters in the series of uranium dipnictides are

shown in Fig. 7.13. Reflecting the spectral shapes, the nuclear quadrupole interaction shows significant pnictogen

dependence. The magnitude of the quadrupole interaction is zero in UBi2, whereas it is a finite value in the other

dipnictides. The magnitude of the observed hyperfine field is larger than $200 T.

Pnictogen dependence of the nuclear quadrupole interactions in the series of uranium dipnictides implies the

correlation between the electronic states and magnetic sequences, although the crystal structure is the same among

UAs2, USb2, and UBi2 except for the atomic position parameters. It is worth to discuss whether the quadrupole

interaction in UBi2 is caused by the absence of the EFG or by an accidental cancellation among the contributions of the 5f

quadrupole moments, the local atomic arrangement (including crystal electric field (CEF) contribution), and the

conduction electrons. Observed difference in the nuclear quadrupole interaction is correlated with the contribution

of the 5f quadrupole moments, corresponding to the orbital contribution in 5f electrons, to the electronic ground states

in the series of uranium dipnictides. Assuming that the contribution of the conduction electrons is small, the EFG tensor

depends on the contribution of the 5f quadrupole moments and the local atomic contribution. If the spectra can be

measured above the Neel temperatures in these compounds, the lattice contribution to the EFG can be estimated.

ossbauer

However, the Neel temperatures in the series of uranium dipnictides are too high to measure the 238 U M€

spectra. In uranium intermetallics, typical Debye temperature is about 200 K [7,10]. This means that it is difficult to

distinguish the contributions of the 5f quadrupole and the local atomic contributions. On the other hand, the

interpretation of the physical properties in the series of uranium dipnictides using a crystal electric field model was

proposed by Amoretti et al. [63]. The results calculated by this theory reasonably agree with most of the experimental

results in UP2, UAs2, and USb2, such as the magnitude of the ordered magnetic moments and their transition

temperatures, although the results in UBi2 were not reported. Assuming that the same CEF level schemes are realized

in the series of uranium dipnictides because of the same or similar crystal structure in these compounds, the nuclear

quadrupole interactions obtained in the present work are difficult to explain at all. Particularly, the difference of the EFG



137



7.5 SUMMARY



Ahf = – 12.16 meff + 219.89



Ahf (T / μB)



UAs2

150



USb2



FIGURE 7.14



UP2

UBi2



100



5



10



meff (m0)



Plots of the hyperfine coupling

constant at uranium nuclei by

238

€ ssbauer spectroscopy and

U Mo

the cyclotron masses obtained by

de Haas–van Alphen effect

measurements in the series of

uranium dipnictides.



values between UBi2 and the others is large to interpret by the same level schemes. Therefore, the pnictogen-dependent

EFG tensor in the uranium dipnictides is caused by the 5f quadrupole contributions.

The hyperfine field observed in the series of the uranium dipnictides is relatively large. The magnitude of the

hyperfine field is larger than 200 T in all the compounds, which corresponds to more than 1mB/U, as shown in Fig. 7.13b.

Unlike the nuclear quadrupole interactions, the hyperfine field seems not to be correlated with the magnetic structure.

On the other hand, magnetic moments in the uranium dipnictides determined by neutron scattering, which are shown in

Table 7.3, are not proportionate to the hyperfine field at uranium nuclei. This indicates that the magnetic hyperfine

coupling constant is not constant in this system. The hyperfine coupling constant obtained in the present work is shown in

Fig. 7.13c. The hyperfine coupling constant in UAs2 and USb2 is larger than that in UP2 and UBi2. The former value is close

to the hyperfine coupling constant reported in the other uranium-based intermetallic compounds such as those in UGe2

and UPd2Al3. This means that the nature of the 5f electrons in UP2 and UBi2 is more itinerant than that in UAs2 and USb2.

ossbauer

In addition, the positive correlation is found between the hyperfine coupling constant obtained by 238 U M€

spectroscopy and the cyclotron masses obtained by de Haas–van Alphen effect measurements. It is believed that the

cyclotron masses obtained by the de Haas–van Alphen effect measurements reflect the hybridization between f electrons

and conduction electrons in rare-earth and actinide compounds. The typical masses in uranium dipnictides depend on the

compounds. When the hyperfine coupling constants at 238 U nucleus are plotted against the cyclotron masses, as shown in

Fig. 7.14, the linear correlation was found between them. This indicates that the dependence of the hyperfine coupling

constant on the uranium dipnictides is caused by the hybridization between the 5f electrons and conduction electrons.

€ ssbauer Spectroscopy of Uranium Dipnictides

7.4.3 Summary of 238 U Mo

Hyperfine interactions at 238 U nuclei in uranium dipnictides are correlated with the magnetic sequence in their

antiferromagnetic states. Because of high Neel temperature, the lattice contribution in the EFG tensor is difficult to

estimate. Therefore, it has not been clarified experimentally whether the nuclear quadrupole interaction is caused by the

cancellation between the 5f quadrupole moments and lattice contributions or not. Considering the crystal structure of the

series of the uranium dipnictides, the pnictogen dependence of the lattice contribution in the EFG tensor is small. The results

ossbauer spectroscopy imply that the electronic structure that affects the hyperfine interactions at 238 U nuclei

of the 238 U M€

is correlated with the magnetic sequences as the results of de Haas–van Alphen effect measurements pointed out.



7.5 SUMMARY

238



U M€

ossbauer measurements of exotic uranium compounds were carried out in the present work. Systematic

measurements of uranium intermetallics reveal that the hyperfine coupling constant is 150 T/mB at uranium nuclei. This

hyperfine coupling constant depends on the hybridization between 5f and conduction electrons as well as on the



138





7 STUDY OF EXOTIC URANIUM COMPOUNDS USING 238U MOSSBAUER

SPECTROSCOPY



magnitude of the magnetic moments. In uranium-based heavy fermion superconductors such as UPd2Al3, URu2Si2, and

ossbauer spectroscopy, was observed by 238 U

UPt3, paramagnetic relaxation, which was discussed previously in 237 Np M€

M€

ossbauer spectroscopy. The observation of the paramagnetic relaxation is correlated with the formation of the heavy

fermion behavior in these compounds. In uranium dipnictides, hyperfine interactions at uranium nuclei exhibit magnetic

sequence dependence. Since this is mainly found in the nuclear quadrupole interactions, the electronic quadrupole

moments are correlated with their magnetic sequence. In the present work, the observation of the isomer shifts was not

successful. However, the present work clarified that the hyperfine coupling constant in uranium intermetallics is a useful

tool to discuss the hybridization between 5f and conduction electrons. Although the lower limit to detect the hyperfine

ossbauer spectroscopy is a useful tool to investigate the

field is about 0.3mB/U, the present work has proved that 238 U M€

magnetic properties in uranium compounds.



ACKNOWLEDGMENTS

One of the authors (ST) would like to acknowledge for the experimental assistance of Drs A. Nakamura, N.M. Masaki,

and K. Ikushima; the sample preparation and fruitful discussion of Drs Y. Haga, E. Yamamoto, T. Honma, H. Ohkuni,



N. Kimura, D. Aoki, and P. Wisniewski; the stimulating discussion with Profs S. Nasu, G.M. Kalvius, and Y. Onuki;

and the

encouragement of Profs M. Date and H. Yasuoka, and Drs M. Saeki, T. Yamashita, and Z. Yoshida. ST is also thankful to

the Japan Atomic Energy Research Institute that gave the opportunity to carry out this work at the plutonium research

facility through their students’ program and post-doctoral research. This work was partially supported by the Grant-inAid for COE research (10CE2004) and for the Research on Innovative Area “Heavy Electrons” (20102004) from the

Ministry of Education, Culture, Sports, Science, and Technology, and REIMEI Research Resources of Japan Atomic Energy

Research Institute.



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SPECTROSCOPY



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PA R T I I I



SPIN DYNAMICS



C H A P T E R 8



REVERSIBLE SPIN-STATE

SWITCHING INVOLVING A

STRUCTURAL CHANGE



SATORU NAKASHIMA

Natural Science Center for Basic Research and Development, Hiroshima University, Higashi-Hiroshima, Japan



8.1 INTRODUCTION

Self-assembled coordination polymers containing transition metal ions and organic bridging ligands have attracted intensive interest because of their potential abilities for selective inclusion and transformation of ions and molecules [1–4].

Construction of a variety of assembled structures plays an important role in designing the size and shape of vacancy. Both

the variety of assembled structures and sorption of guest molecules can affect the properties of assembled complexes.

When the bridging ligand is flexible, a variety of assembled structures can be expected, depending on the conformer

of the ligand. 1,2-Bis(4-pyridyl)ethane (bpa, Scheme 8.1) molecule has two methylene groups, and forms anti and gauche

conformers. 1,3-Bis(4-pyridyl)propane (bpp, Scheme 8.1) has three methylene groups, forming anti–anti, anti–gauche,

anti–gauche0 , gauche–gauche, gauche–gauche0 , and gauche0 –gauche conformers. The conformer of the bridging ligand will

determine the type of assembled structure. The obtained assembled structure gives rise to a vacancy with different size

and shape. This soft character of the bridging ligand and the formed vacancy are expected to bring about a novel property

of the assembled complexes.

Spin-crossover phenomenon is one of the hot topics in the assembled complexes [5–20]. In the study of spincrossover complexes, intermolecular interaction (cooperativity) is important. High-spin and low-spin states can coexist

at the same temperature (bistability), depending on the cooperativity. The bistability will be stabilized by the

cooperativity. In this situation, two states of bistability are interconverted by outer stimuli, bringing a function of

“molecular switch” to the sample. In order to achieve the bistability, assembly of complexes becomes important. In this

context, it is important to investigate the assembled complexes, especially from the viewpoint of spin-crossover

phenomenon, that is, whether the presence of a guest molecule has a great influence on the construction of assembled

structures and the spin state of the metal ions [21,22].

In this chapter, it is shown that anti and gauche conformers of bpa or bpp are related to the type of assembled

structures. The assembled structure will change itself in order to fit the guest molecules or to avoid vacancy. By changing

the assembled structure and the type of guest molecules, we can design the local symmetry around the iron. In some

cases, reversible structural change and spin-state switching of iron can be achieved by the sorption or desorption of

solvent molecules such as benzene.



M€

ossbauer Spectroscopy: Applications in Chemistry, Biology, and Nanotechnology, First Edition.

Edited by Virender K. Sharma, G€

ostar Klingelh€

ofer, and Tetsuaki Nishida.

Ó 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.



143



144



8 REVERSIBLE SPIN-STATE SWITCHING INVOLVING A STRUCTURAL CHANGE



N



N



bpa: 1,2-bis(4-pyridyl)ethane



SCHEME 8.1

Bridging ligand.



N



N



bpp: 1,3-bis(4-pyridyl)propane



8.2 THREE ASSEMBLED STRUCTURES OF Fe(NCX)2(bpa)2 (X ¼ S, Se) AND THEIR

STRUCTURAL CHANGE BY DESORPTION OF PROPANOL MOLECULES [23]

One-dimensional (1D) structure has already been reported for Fe(NCS)2(bpa)2 [24]. With careful inspection, three

types of pale yellow crystals were obtained by the diffusion method. One type of crystals had a 1D structure that has

already been reported. The other types of crystals had two-dimensional (2D) grid and interpenetrated structures

(Scheme 8.2) . Both 2D grid and interpenetrated structures enclathrated ethanol or propanol molecules during the

sample preparation.

57

Fe M€

ossbauer spectroscopy revealed that iron atoms in 1D, 2D grid, and interpenetrated crystals are in high-spin

II

Fe state. It was shown that the quadrupole splitting (QS) value changed drastically, depending on the crystal structure

and the type of anion. Different structure dependences of the QS value were observed between NCSÀ and NCSeÀ

complexes. In the former, the QS value (2.17 mm sÀ1 at 78 K and 1.60 mm sÀ1 at 298 K) of the 1D structure was twice of

that (1.02 mm sÀ1 at 78 K) obtained for the interpenetrated structure. In the latter, the QS value (0.74 mm sÀ1 at 78 K) for

the 1D structure was half of that (1.53 mm sÀ1 at 78 K) obtained for the interpenetrated structure.

Ethanol- or propanol-enclathrated crystals are crystallographically unstable. A large difference in the QS values in

57

Fe M€

ossbauer spectra, depending on the structure, gives us useful information when investigating solid-to-solid

phase transformation. For M€

ossbauer measurement, each sample is wrapped up with hydrocarbon when measured at

room temperature lest the crystal structure should be destroyed. Figure 8.1 shows the change of M€

ossbauer spectra

for interpenetrated Fe(NCS)2(bpa)2 Á (propanol) measured at 298 K. The isomer shift (IS) value of about 1 mm sÀ1

reflects that all the iron atoms are in high-spin FeII state. New doublets are observed in the spectra measured after

18 and 26 days of storage. The small QS value observed for fresh Fe(NCS)2(bpa)2 Á (propanol) is typical of the

interpenetrated structure. The QS value for the doublet (1.69 mm sÀ1 at 298 K), which appeared after 18 days of

storage at room temperature, is comparable to that of the doublet obtained for the 1D structure. After 26 days of

storage at room temperature, original doublet with smaller QS doublet disappeared and the larger QS doublet

became predominant; it showed a maximal value of 2.15 mm sÀ1. Although this species has not been understood



SCHEME 8.2

Assembled structures (interpenetrated, 2D grid, and 1D structures) of Fe(NCX)2(bpa)2 (X ¼ S, Se).

(Reprodeuced from Ref. 26 with permission of Springer.)



145



8.3 OCCURRENCE OF SPIN-CROSSOVER PHENOMENON IN ASSEMBLED COMPLEXES



FIGURE 8.1

Time dependence of the 57 Fe

€ ssbauer spectra measured at

Mo

298 K for the sample obtained as

the interpenetrated structure of

Fe(NCS)2(bpa)2 Á (propanol).

(Reproduced from Ref. 23 with

permission of the Chemical Society of Japan.)



well, all the experimental results suggest a structural change of the host framework from the interpenetrated to 1D

structure by desorbing propanol molecules. This structural change was confirmed by the powder X-ray diffraction

study.



8.3 OCCURRENCE OF SPIN-CROSSOVER PHENOMENON IN ASSEMBLED COMPLEXES

Fe(NCX)2(bpa)2 (X ¼ S, Se, BH3) BY ENCLATHRATING GUEST MOLECULES [25–27]

Propanol in Fe(NCS)2(bpa)2 Á (propanol) is easily desorbed. If some bigger organic molecules are enclathrated, the guest

molecules are not easily desorbed. In the complexes bridged by trans-1,2-bis(4-pyridyl)ethylene, an assembled complex

was obtained by enclathrating ligand [18]. This suggests that larger organic compounds can be enclathrated in these

assembled complexes. Actually, inclusion compounds were obtained by mixing organic molecules such as biphenyl with

bpa of bridging ligand in the preparation process of assembled iron complexes; Fe(NCX)2(bpa)2 Á (guest) complexes

(X ¼ S, Se, BH3; guest ¼ biphenyl, 2-nitrobiphenyl, diphenylmethane, or 1,4-dichlorobenzene) were formed by the

diffusion method.

X-ray structural analysis revealed the inclusion of organic compounds in the assembled complexes Fe(NCX)2(bpa)2

(X ¼ S, Se, BH3). The frame of Fe(NCX)2(bpa)2 (X ¼ S, Se, BH3) becomes 1D, 2D grid, or interpenetrated structure,

depending on the type of guest molecule and the type of anion. These structures are summarized in Table 8.1, together with

TABLE 8.1 Summary of the Crystal Structure

Anion

Guest

1,4-Dichlorobenzene

Biphenyl

Diphenylmethane

2-Nitrobiphenyl



NCSÀ



NCSeÀ



1D

2D grid

Interpenetrated

Interpenetrated



1D

1D

Interpenetrated

Interpenetrated



Source: Reproduced from Ref. 27 with permission of Springer.

“Underline” represents the occurrence of spin-crossover phenomenon.



NCBH3À



1D

2D grid

2D grid



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