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Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation Masanori Koshimizu *, Hirotaka Tsukahara, Keisuke Asai Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, Japan
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 30 January 2009 Available online 19 March 2009 PACS: 61.80.Jh 78.70.g 71.30.+h Keywords: Ion irradiation Nanowire Thiospinel compound CuIr2S4 G-value
a b s t r a c t We observed an increase in the conductivity of a thiospinel compound, CuIr2S4, induced by H+ and He+ irradiation with energies of 1–2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron–lattice interaction of this compound. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Spinel compounds that consist of transition metal elements have been studied extensively because they have interesting physical properties due to strong electron correlation. CuIr2S4 is one such compound; it has a normal spinel structure with Cu in the tetrahedrally coordinated A site, and Ir in the octahedrally coordinated B site. This compound exhibits metallic electrical conductivity at room temperature and a sharp metal–insulator transition at 230 K [1]. The ionic configuration of this compound is expected to in a low-temperature insulating phase, as rebe Cu1þ Ir3þ Ir4þ S2 4 ported by a series of studies conducted using NMR [2], XPS [3,4] and band calculations [5]. Based on the discussion in these reports, it is accepted that CuIr2S4 is a mixed-valence compound where Ir has two valence states. It is reported that the transition exhibited by CuIr2S4 is accompanied by a change in the crystal structure and the formation of the Ir4+ dimer [1]. In fact, an intensive diffraction study has shown that this is indeed a simultaneous spin dimerization and charge-ordering transition [6]. Recently, it has been found that the conductivity of this compound increases by several orders of magnitude under X-ray irradiation at temperature lower than 10 K, accompanying a triclinic * Corresponding author. Tel./fax: +81 22 795 7219. E-mail address: [email protected] (M. Koshimizu). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.02.052
to tetragonal structural transition [7–9]. It was also reported that the initial state was thermally recovered at ca. 80 K [7]. Furubayashi et al. have proposed that the breaking of the Ir dimer during the change in the crystal structure is responsible for the increase in conductivity [7]. In this case, it is assumed that a bonding electron is removed from the Ir dimer due to ionization by X-rays, and the Ir dimer is destroyed. After the destruction of the Ir dimer, its recombination is inhibited by a potential barrier [7]. Thus, the metastability of this X-ray-induced phase can be explained by the activation energy of the Ir dimer recombination. On the other hand, Ishibashi et al. have reported that the Ir dimer remains, while the long-range order of the dimers is destroyed, as was confirmed by the Rietveld refinement [9]. In addition, it has been shown that a long-range incommensurate structure is formed while the Ir dimer is preserved after X-ray or electron irradiation [10]. Thus, it has been shown that the displacement of Ir dimers is profoundly related to the change in conductivity induced by X-ray irradiation. From the viewpoint of radiation research, this compound is very useful for analyzing the yield and/or the microstructure due to the ion-induced change without the post-irradiation treatment such as chemical etching; instead, the analysis can be performed by simply measuring the conductivity of the compound as a function of ion fluence. In this paper, we report that the conductivity of CuIr2S4 increases due to ion irradiation. It is well known that light ions with
M. Koshimizu et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
energies of the order of MeV deposit most of their energy by electronic excitation instead of elastic collision with constituent atoms. In comparison to X-rays or high-energy electrons, electronic excitation by the ion beam has the following characteristics: (1) energy is deposited in a cylindrical region around an ion trajectory at a much higher density than X-rays or high-energy electrons, and (2) the depth profile of the deposited energy for each ion is very similar, as opposed to that in the case of X-rays where the depth profile is scattered significantly for each X-ray photon and only the statistical average of the depth profile can be estimated. These two characteristics help us to obtain the yield of the irradiation-induced change in this compound as well as the microstructure of the change. In this paper, the ion-irradiation effect in this new class of material is discussed in comparison to those in other inorganic solids.
1000 100
Conductivity [S/cm]
1126
cooling
10
heating
1 0.1 0.01 0.001 0.0001 30
60
90
120 150 180 210 240 270 300
Temperature [K] Fig. 1. Conductivity of CuIr2S4 as a function of temperature.
2. Experimental
3. Results and discussion 3.1. Analysis of the conductivity change Fig. 1 shows the temperature dependence of the conductivity of CuIr2S4. As reported in a previous paper [1], we observed a sharp metal-to-insulator transition at approximately 230 K. It should be noted that this transition during cooling is not a reverse reaction of the X-ray-induced change. Fig. 2 shows the conductivity of CuIr2S4 as a function of ion fluence. Due to a small difference in temperature when the irradiation was started, the conductivity was slightly different before the irradiation of each ion; however, this small difference in temperature hardly affects the following discussion about the microscopic structure and the yield of the ion-induced change. We observed that the conductivity increased and became constant with increasing irradiation fluence for 2.0 MeV He+ and 1.0 MeV H+. For 2.0 MeV H+, the conductivity increased in a similar fashion. At a sufficiently large fluence where the conductivity became constant, the change in conductivity
0.001
Conductivity [S/cm]
Polycrystalline CuIr2S4 was obtained by heating a mixture of Cu, Ir, and S powders in an evacuated quartz tube at 850 °C for 10 days. The sample in pellet form was then prepared by sintering the powder at 850 °C for 48 h after molding. For the ion-irradiation experiment, the temperature of the sample was controlled by using a closed-cycle refrigerator (IWATANI CW503) and a temperature controller (IWATANI TCU-4 SU18-12P-1T). The sample was mounted on a Cu sample holder by using heat-conductive grease (Apiezon N). The conductivity of the sample was measured with using the DC 4-terminal method by applying the current vertically to the direction of the ion beam at a constant temperature in the range of 40–50 K for each ion. The sample was irradiated with 1.0 MeV H+, 2.0 MeV H+ and 2.0 MeV He+ from a Van de Graaff accelerator at the High Fluence Irradiation Facility, The University of Tokyo. The beam current density was (12.5) 1010 ions/cm2/s, and the temperature rise due to ion irradiation was less than 1 K. After the ion irradiation was terminated, the conductivity was measured point-by-point at each fluence. Immediately after stopping the irradiation, higher conductivity was obtained due to the small increase in temperature. Hence, the conductivity was measured after the temperature became constant. According to the previous study, the conductivity after X-ray irradiation gradually reduces to the value before irradiation [7]. In this study, the effect of the reduction in conductivity during the experiment was negligible. The LET (linear energy transfer) and the penetration depth of each ion were derived by the SRIM2003 code [11]. In this study, we used the average LET over a penetration depth for discussions.
0.0008
2.0 MeV H + 1.0 MeV H + 2.0 MeV He+
0.0006 0.0004 0.0002 0 1011
1013
1012
Fluence
1014
1015
[ions/cm2]
Fig. 2. Conductivity of CuIr2S4 as a function of ion fluence at 40–50 K. The solid lines are the fitting curves according to Eq. (1).
was significant in the order of 2.0 MeV He+, 1.0 MeV H+ and 2.0 MeV H+. This corresponds to the order of the penetration depth, i.e., the thickness of the layer where electronic energy is deposited by the ion beam. In addition, the electronic energy loss dominates the energy loss due to elastic collision in the case of the ion beams used in this study. Furthermore, if the elastic scattering plays a dominant role in this irradiation effect, the difference for the cases of 1.0 MeV and 2.0 MeV proton irradiations would be much smaller and the conductivity change during 2.0 MeV He irradiation would be much faster and larger than the result shown in Fig. 1. Thus, it can be denied that the defect formation due to the elastic scattering is the main cause of the observed conductivity change, and the change in conductivity is induced by the electronic excitation of the ion beam. Conductivity as a function of fluence of each ion is analyzed by using a simple model. When an ion impinges on the sample, electronic excitation is induced in a cylindrical region around the ion trajectory. Thus, the ion-induced change is assumed to occur in a cylinder whose length corresponds to the penetration depth of each ion. The cross-sectional area of the cylinder is a measure of the spatial extent of the ion-induced change, and is reserved for a fitting parameter. Another fitting parameter is the conductivity of the ion-induced phase. Here, we assume that when an ion impinges onto an area where the ion-induced change has already occurred, it does not induce a further change. Thus, the results can be fitted with the following equation:
rðxÞ ¼ r1 þ ðr0 r1 Þ expðSxÞ:
ð1Þ
Here, x is the fluence of the ion beam; r(x), the conductivity as a function of the fluence; r1, the conductivity of the ion-induced phase; r0, the conductivity before irradiation; and S, the
M. Koshimizu et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
cross-sectional area of the ion-induced change. Here, the fitting parameters were r1 and S, and the conductivity before irradiation was used for r0. This model is commonly used to analyze ion-irradiation effects in solids. As shown in Fig. 2, the result of the fitting is excellent. The parameters obtained by the fitting are listed in Table 1 together with the penetration depth and the LET calculated by the SRIM code [11]. By using r1, r0, and the penetration depth for each ion, the conductivities of the ion-induced phase were calculated to be 0.047, 0.057 and 0.031 S/cm for 2.0 MeV He+, 1.0 MeV H+, and 2.0 MeV H+, respectively. These values are comparable to the conductivity of the X-ray-induced phase at 40–50 K [7]. In addition, during the heating of the irradiated samples, we observed that the temperature dependence of the conductivity merged to that of the pre-irradiated state at ca. 100 K, which qualitatively agreed with the case of X-ray irradiation [7], and that the conductivity returns to the pre-irradiated value during subsequent cooling, as shown in Fig. 3. This result shows the metastable nature of the ion-induced phase, as reported for the conductive phase induced by X-ray irradiation [7]. Furthermore, as discussed above, the ion-induced change occurred due to the electronic excitation by each ion, rather than by defect formation due to the elastic collision between the impinging ions and the atoms comprising the material. From the above arguments, it is natural to consider that the conductive phase induced by the ion beam was identical to that induced by X-ray irradiation. Although the change in conductivity reported for X-ray irradiation was by several orders of magnitude [7], we observed the change in conductivity due to ion irradiation by several times in this study. This is because the conductivity before the irradiation differs between both the cases by several orders of magnitude due to the different irradiation temperatures. As shown in Table 1, the obtained cross-sectional area was of the order of square nanometers and was an increasing function of the LET. On the other hand, the length of the cylindrical volume corresponded to the penetration depth, which was comparable to or greater than several microns. The conductivity before irradiation was 15 104 S/cm, and those of the ion-induced phase (r1)
Table 1 Penetration depth, LET, cross-sectional area, and G-value for each ion. G-value is defined as the number of displaced Ir4+ dimers per 100 eV of the deposited energy. Ions
Penetration depth (lm)
LET (eV/nm)
Cross-sectional area (nm2)
G-value
2.0 MeV He+ 1.0 MeV H+ 2.0 MeV H+
4.2 9.2 25.5
470 110 77
5.4 2.1 1.4
4.8 8.2 7.7
Conductivity [S/cm ]
0.1
0.01
was 36 102 S/cm. Thus, the conductivity of the ion-induced phase was higher than that of the non-irradiated phase by two orders of magnitude, although this difference is much smaller than the thermally-induced metal–insulator transition at ca. 230 K. It should be noted that the ion-irradiation-induced phase is not identical to the high-temperature phase (T > 230 K), and it is suggested that a single ion produces a nanowire of the conductive phase. Here, the cross-sectional area is the average value over the penetration depth of each ion because the LET changes significantly during the slowing down process in the sample. 3.2. Yield of the ion-induced change We will consider the yields of the ion-induced change by using ‘‘G-values,” which are frequently used in radiation chemistry. A G-value is defined as the yield of the irradiation-induced change per absorbed energy of 100 eV. The number of the displaced Ir dimers was calculated as follows: it is assumed that all the Ir4+ ions form dimers in the low-temperature phase and that the conductivity change is proportional to the number of displaced Ir dimers. In addition, the number of the displaced Ir dimers was calculated by assuming that all the Ir dimers were displaced in the cylindrical volume that has the cross-sectional area shown in Table 1 and length equal to the penetration depth. Subsequently, we should calculate the number of the Ir dimers in unit volume. The number of Ir atoms in the unit cell is eight, and the number of Ir4+ ions is four; therefore, there are two Ir dimers in the unit cell. In order to calculate the unit cell volume, we used a = b = 6.8645 Å and c = 10.0257 Å as the lattice parameters [1]. Thus, the number of the Ir dimers displaced by one impinged ion was calculated. Finally, G-values were obtained dividing the number of the displaced Ir dimers by the ion energy. The G-value for 2.0 MeV He+ irradiation was significantly lower than that for H+ irradiation. This difference can be explained as follows. In the case of the H+ irradiation, due to the low LET, all the Ir dimers are not displaced and some dimers are left intact in the volume where the electronic excitation is induced. In this case, the obtained cross-sectional area of the ion-induced phase is the effective one. On the other hand, in the case of the 2.0 MeV He+ irradiation, the density of the electronic excitation is so high that all the dimers are displaced in the energy-deposited volume. Thus, the energy fraction used for the displacement of the dimers is lowered. This is because there should be a fraction of electronic excitation that is not involved in the displacement of the dimers. Consequently, a low G-value is observed at a high LET. In this case, a dense nanowire is formed where all the dimers are displaced. It is reported that the crystal structure of the X-ray-induced phase is different from that of the non-irradiated phase. Considering the similarity between the X-ray-induced phase and the ion-induced phase, the 2.0 MeV He+-induced phase having a cylindrical shape should have a distinct crystal structure from the non-irradiated phase. This situation is similar to that of the crystalline track formed under heavy ion irradiation in semiconductors [12]. 3.3. Comparison of the yield with ion-induced change in other solids
Before irradiation
0.001
After irradiation (heating) After irradiation (cooling)
0.0001 40
1127
60
80
100
120
140
160
Temperature [K] Fig. 3. Conductivity of CuIr2S4 before and after irradiation. Conductivity was measured after the 2.0 MeV H+ irradiation at a fluence of 2.0 1014 ions/cm2.
We now compare the yield of the ion-induced change of CuIr2S4 with that of other inorganic crystals. To the best of our knowledge, for inorganic crystals there are few reports that apparently treat the yield of the defect formation or lattice deformation induced by ionizing radiation. Here, the F-center formation in alkali halide crystals is discussed in comparison to our data. This is because these crystals are rather sensitive to the ionizing radiation among inorganic crystals. The G-value for the F-center formation in LiF has been reported to be 0.2 or less [13]. In addition, the yield can be calculated from the data obtained in previous studies. For example,
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M. Koshimizu et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
by using an F-center concentration of 1.4 1016 cm2 in a cylinder around the ion trajectory and the cylinder of radius 30 nm obtained by the optical absorption measurement at various ion fluences [14], the G-value of the F-center formation in NaCl is calculated to be less than 0.01 for 197Au irradiation of the energy of 10.5 MeV/nucleon. These G-values are much smaller than those obtained in CuIr2S4. In addition, the G-values, which are defined as the deformed atoms in the formula unit of the latent track, can be obtained from reports on track formation under the heavy ion irradiation of inorganic crystals. In the previous reports on track formation, the LET and track radius are used for the calculation of the G-values. For example, in SiO2 where low threshold LET is determined for the track formation [15], the G-value for 238U irradiation is calculated to be as high as 20, and the G-values for the LET of several keV/nm are approximately 5–10 [16]. For Al2O3, where much higher threshold LET is reported [15], the G-value is calculated to be 11 for the LET of 44.2 keV/nm [17]. The Gvalues in CuIr2S4 are comparable to the G-values for track formation. Thus, the ion-induced change in CuIr2S4 and track formation in inorganic crystals have similarities in their high yield of the ion-induced change and the nanowire formation. However, we will point out the difference between the nanowire formation in CuIr2S4 and the track formation in inorganic materials. First, there is a large difference in the LET between both these cases. The track formation in many inorganic materials occurs for the LET higher than several keV/nm [15]. In contrast, the ion-induced phase is produced for the LET that is considerable less than keV/nm in CuIr2S4. Second, the LET-dependence of the G-value is complementary to these two cases. For track formation in inorganic crystals, the ‘‘G-value” increases with the LET in a nonlinear fashion above the threshold LET. Below the threshold, track formation is not observed and/or the ‘‘G-value” of the ion-induced change is very small. In contrast, the G-value is significantly low for the high LET in CuIr2S4. These differences can be explained as follows. In inorganic crystals, the track is formed after the complete destruction of the crystal structure in the cylindrical region. Regardless of the basic process involved, the track formation is brought about by the electronic excitation at an extremely high density that triggers the destruction of the crystal structure. On the other hand, the ion-induced change in CuIr2S4 occurs even for low-LET radiations such as X-rays or electrons, and it is not peculiar to the dense electronic excitation or high LET. Thus, the ion-induced change in CuIr2S4 is apparently not a collective phenomenon but an individual phenomenon located closely to each other. Nevertheless, the observed G-value for the ion-induced change in CuIr2S4 is much higher than that of inorganic crystals at low LET. The possible reasons for obtaining this high G-value are the low band-gap energy and the strong electron–lattice interaction. According to the result of XPS measurement, the band-gap energy of CuIr2S4 in the insulating phase is estimated to be approximately 0.15–0.20 eV [3,4], which is much smaller than that of inorganic crystals where a low G-value is reported for low LET. The lower band-gap energy leads to the formation of more electron–hole pairs or ionized atoms per unit energy of the ionizing radiation. In CuIr2S4, the irradiation-induced change is caused by the e-h pairs and/or ionized atoms. Therefore, the low band-gap energy
is one of the reasons for the high G-value in this compound. The strong electron–lattice interaction is another factor responsible for the high G-value in CuIr2S4 where lattice deformation due to electronic excitation is observed. As opposed to the case of CuIr2S4, most inorganic crystals having a strong electron–lattice interaction have a large band-gap energy that leads to a low G-value. Thus, these two factors should be responsible for the high G-value in this compound. In addition, some process analogous to chain reactions may be responsible for this high G-value, and further research is necessary for analyzing the yield and the microscopic structural change of this irradiation-induced change. 4. Conclusion We observed an increase in the conductivity of a thiospinel compound, CuIr2S4, induced by ion beam irradiation. The metastable conductive phase was produced by electronic excitation caused by the ion beam, and this phase was similar to the X-ray-induced phase. From the obtained cross-sectional area, it was found that an impinging ion produced a nanowire of the conductive phase. In addition, the G-value of the displacement of the Ir dimer was rather high, probably due to the low band-gap energy and strong electron–lattice interaction. The ion-irradiation effect reported in this paper is unique in view of the high G-value and the low LET in the formation of the conductive-phase nanowire. Acknowledgement This research was financially supported by the Asahi Glass Foundation and Research Foundation for Materials Science. References [1] T. Furubayashi, T. Matsumoto, T. Hagino, S. Nagata, J. Phys. Soc. Jpn. 63 (1994) 3333. [2] S. Tsuji, K. Kumagai, N. Matsumoto, S. Nagata, Physica C 282–287 (1997) 1107. [3] J. Matsuno, T. Mizokawa, A. Fujimori, D.A. Zatsepin, V.R. Galakhov, E.Z. Kurmaev, Y. Kato, S. Nagata, Phys. Rev. B 55 (1997) R15979. [4] K. Takubo, S. Hirata, J.Y. Son, J.W. Quilty, T. Mizokawa, N. Matsumoto, S. Nagata, Phys. Rev. Lett. 95 (2005) 246401. [5] T. Oda, M. Shirai, N. Suzuki, K. Motizuki, J. Phys.: Condens. Matter 7 (1995) 4433. [6] P.G. Radaelli, Y. Horibe, M.J. Gutmann, H. Ishibashi, C.H. Chen, R.M. Ibberson, Y. Koyama, Y.S. Hor, V. Kiryukhin, S.W. Cheong, Nature (London) 416 (2002) 155. [7] T. Furubayashi, H. Suzuki, T. Matsumoto, S. Nagata, Solid State Commun. 126 (2003) 617. [8] T. Furubayashi, H. Suzuki, T. Matsumoto, S. Nagata, J. Magn. Magn. Mater. 272– 276 (2004) 446. [9] H. Ishibashi, T.Y. Koo, Y.S. Hor, A. Borissov, P.G. Radaelli, Y. Horibe, S.W. Cheong, V. Kiryukhin, Phys. Rev. B 66 (2002) 144424. [10] V. Kiryukhin, Y. Horibe, Y.S. Hor, H.J. Noh, S.W. Cheong, C.H. Chen, Phys. Rev. Lett. 97 (2006) 225503. [11] J.F. Ziegler, Nucl. Instr. and Meth. B 219&220 (2004) 1027. [12] G. Szenes, Z.E. Horvath, B. Pecz, F. Paszti, T. Toth, Phys. Rev. B 65 (2002) 045206. [13] K. Schwartz, C. Trautmann, A.S. El-Said, R. Neumann, M. Toulemonde, W. Knolle, Phys. Rev. B 70 (2004) 184104. [14] M. Enculescu, K. Schwartz, C. Trautmann, M. Toulemonde, Nucl. Instr. and Meth. B 229 (2005) 397. [15] N. Itoh, A.M. Stoneham, Nucl. Instr. and Meth. B 146 (1998) 362. [16] A. Meftah, F. Brisard, J.M. Constantini, E. Dooryhee, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [17] B. Canut, A. Benyagoub, G. Marest, A. Meftah, N. Monocoffre, S.M.M. Ramos, F. Studer, P. Thevenard, M. Toulemonde, Phys. Rev. B 51 (1995) 12194.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Formation of metastable conductive nanowire in a thiospinel compound CuIr2S4 induced by ion irradiation Masanori Koshimizu *, Hirotaka Tsukahara, Keisuke Asai Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, Japan
a r t i c l e
i n f o
Article history: Received 1 August 2008 Received in revised form 30 January 2009 Available online 19 March 2009 PACS: 61.80.Jh 78.70.g 71.30.+h Keywords: Ion irradiation Nanowire Thiospinel compound CuIr2S4 G-value
a b s t r a c t We observed an increase in the conductivity of a thiospinel compound, CuIr2S4, induced by H+ and He+ irradiation with energies of 1–2 MeV. It was indicated that the metastable conductive phase was produced by electronic excitation due to the ion beam and this phase was similar to the X-ray-induced phase. Conductivity as a function of ion fluence was analyzed by a simple model where the ion-induced change occurred in a cylindrical region around an ion trajectory. The cross-sectional area of the cylinder was obtained by analyzing the conductivity as a function of ion fluence for each ion, and it was found that an impinging ion produced a nanowire in the conductive phase. In addition, the yield of the Ir dimer displacement, which was related to the increase in conductivity, was considerably high. The ion irradiation effect reported in this paper is unique with regard to the high yield and low linear energy transfer (LET) in the formation of the conductive-phase nanowire. Both these unique aspects could be ascribed to the low band-gap energy and strong electron–lattice interaction of this compound. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Spinel compounds that consist of transition metal elements have been studied extensively because they have interesting physical properties due to strong electron correlation. CuIr2S4 is one such compound; it has a normal spinel structure with Cu in the tetrahedrally coordinated A site, and Ir in the octahedrally coordinated B site. This compound exhibits metallic electrical conductivity at room temperature and a sharp metal–insulator transition at 230 K [1]. The ionic configuration of this compound is expected to in a low-temperature insulating phase, as rebe Cu1þ Ir3þ Ir4þ S2 4 ported by a series of studies conducted using NMR [2], XPS [3,4] and band calculations [5]. Based on the discussion in these reports, it is accepted that CuIr2S4 is a mixed-valence compound where Ir has two valence states. It is reported that the transition exhibited by CuIr2S4 is accompanied by a change in the crystal structure and the formation of the Ir4+ dimer [1]. In fact, an intensive diffraction study has shown that this is indeed a simultaneous spin dimerization and charge-ordering transition [6]. Recently, it has been found that the conductivity of this compound increases by several orders of magnitude under X-ray irradiation at temperature lower than 10 K, accompanying a triclinic * Corresponding author. Tel./fax: +81 22 795 7219. E-mail address: [email protected] (M. Koshimizu). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.02.052
to tetragonal structural transition [7–9]. It was also reported that the initial state was thermally recovered at ca. 80 K [7]. Furubayashi et al. have proposed that the breaking of the Ir dimer during the change in the crystal structure is responsible for the increase in conductivity [7]. In this case, it is assumed that a bonding electron is removed from the Ir dimer due to ionization by X-rays, and the Ir dimer is destroyed. After the destruction of the Ir dimer, its recombination is inhibited by a potential barrier [7]. Thus, the metastability of this X-ray-induced phase can be explained by the activation energy of the Ir dimer recombination. On the other hand, Ishibashi et al. have reported that the Ir dimer remains, while the long-range order of the dimers is destroyed, as was confirmed by the Rietveld refinement [9]. In addition, it has been shown that a long-range incommensurate structure is formed while the Ir dimer is preserved after X-ray or electron irradiation [10]. Thus, it has been shown that the displacement of Ir dimers is profoundly related to the change in conductivity induced by X-ray irradiation. From the viewpoint of radiation research, this compound is very useful for analyzing the yield and/or the microstructure due to the ion-induced change without the post-irradiation treatment such as chemical etching; instead, the analysis can be performed by simply measuring the conductivity of the compound as a function of ion fluence. In this paper, we report that the conductivity of CuIr2S4 increases due to ion irradiation. It is well known that light ions with
M. Koshimizu et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
energies of the order of MeV deposit most of their energy by electronic excitation instead of elastic collision with constituent atoms. In comparison to X-rays or high-energy electrons, electronic excitation by the ion beam has the following characteristics: (1) energy is deposited in a cylindrical region around an ion trajectory at a much higher density than X-rays or high-energy electrons, and (2) the depth profile of the deposited energy for each ion is very similar, as opposed to that in the case of X-rays where the depth profile is scattered significantly for each X-ray photon and only the statistical average of the depth profile can be estimated. These two characteristics help us to obtain the yield of the irradiation-induced change in this compound as well as the microstructure of the change. In this paper, the ion-irradiation effect in this new class of material is discussed in comparison to those in other inorganic solids.
1000 100
Conductivity [S/cm]
1126
cooling
10
heating
1 0.1 0.01 0.001 0.0001 30
60
90
120 150 180 210 240 270 300
Temperature [K] Fig. 1. Conductivity of CuIr2S4 as a function of temperature.
2. Experimental
3. Results and discussion 3.1. Analysis of the conductivity change Fig. 1 shows the temperature dependence of the conductivity of CuIr2S4. As reported in a previous paper [1], we observed a sharp metal-to-insulator transition at approximately 230 K. It should be noted that this transition during cooling is not a reverse reaction of the X-ray-induced change. Fig. 2 shows the conductivity of CuIr2S4 as a function of ion fluence. Due to a small difference in temperature when the irradiation was started, the conductivity was slightly different before the irradiation of each ion; however, this small difference in temperature hardly affects the following discussion about the microscopic structure and the yield of the ion-induced change. We observed that the conductivity increased and became constant with increasing irradiation fluence for 2.0 MeV He+ and 1.0 MeV H+. For 2.0 MeV H+, the conductivity increased in a similar fashion. At a sufficiently large fluence where the conductivity became constant, the change in conductivity
0.001
Conductivity [S/cm]
Polycrystalline CuIr2S4 was obtained by heating a mixture of Cu, Ir, and S powders in an evacuated quartz tube at 850 °C for 10 days. The sample in pellet form was then prepared by sintering the powder at 850 °C for 48 h after molding. For the ion-irradiation experiment, the temperature of the sample was controlled by using a closed-cycle refrigerator (IWATANI CW503) and a temperature controller (IWATANI TCU-4 SU18-12P-1T). The sample was mounted on a Cu sample holder by using heat-conductive grease (Apiezon N). The conductivity of the sample was measured with using the DC 4-terminal method by applying the current vertically to the direction of the ion beam at a constant temperature in the range of 40–50 K for each ion. The sample was irradiated with 1.0 MeV H+, 2.0 MeV H+ and 2.0 MeV He+ from a Van de Graaff accelerator at the High Fluence Irradiation Facility, The University of Tokyo. The beam current density was (12.5) 1010 ions/cm2/s, and the temperature rise due to ion irradiation was less than 1 K. After the ion irradiation was terminated, the conductivity was measured point-by-point at each fluence. Immediately after stopping the irradiation, higher conductivity was obtained due to the small increase in temperature. Hence, the conductivity was measured after the temperature became constant. According to the previous study, the conductivity after X-ray irradiation gradually reduces to the value before irradiation [7]. In this study, the effect of the reduction in conductivity during the experiment was negligible. The LET (linear energy transfer) and the penetration depth of each ion were derived by the SRIM2003 code [11]. In this study, we used the average LET over a penetration depth for discussions.
0.0008
2.0 MeV H + 1.0 MeV H + 2.0 MeV He+
0.0006 0.0004 0.0002 0 1011
1013
1012
Fluence
1014
1015
[ions/cm2]
Fig. 2. Conductivity of CuIr2S4 as a function of ion fluence at 40–50 K. The solid lines are the fitting curves according to Eq. (1).
was significant in the order of 2.0 MeV He+, 1.0 MeV H+ and 2.0 MeV H+. This corresponds to the order of the penetration depth, i.e., the thickness of the layer where electronic energy is deposited by the ion beam. In addition, the electronic energy loss dominates the energy loss due to elastic collision in the case of the ion beams used in this study. Furthermore, if the elastic scattering plays a dominant role in this irradiation effect, the difference for the cases of 1.0 MeV and 2.0 MeV proton irradiations would be much smaller and the conductivity change during 2.0 MeV He irradiation would be much faster and larger than the result shown in Fig. 1. Thus, it can be denied that the defect formation due to the elastic scattering is the main cause of the observed conductivity change, and the change in conductivity is induced by the electronic excitation of the ion beam. Conductivity as a function of fluence of each ion is analyzed by using a simple model. When an ion impinges on the sample, electronic excitation is induced in a cylindrical region around the ion trajectory. Thus, the ion-induced change is assumed to occur in a cylinder whose length corresponds to the penetration depth of each ion. The cross-sectional area of the cylinder is a measure of the spatial extent of the ion-induced change, and is reserved for a fitting parameter. Another fitting parameter is the conductivity of the ion-induced phase. Here, we assume that when an ion impinges onto an area where the ion-induced change has already occurred, it does not induce a further change. Thus, the results can be fitted with the following equation:
rðxÞ ¼ r1 þ ðr0 r1 Þ expðSxÞ:
ð1Þ
Here, x is the fluence of the ion beam; r(x), the conductivity as a function of the fluence; r1, the conductivity of the ion-induced phase; r0, the conductivity before irradiation; and S, the
M. Koshimizu et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1125–1128
cross-sectional area of the ion-induced change. Here, the fitting parameters were r1 and S, and the conductivity before irradiation was used for r0. This model is commonly used to analyze ion-irradiation effects in solids. As shown in Fig. 2, the result of the fitting is excellent. The parameters obtained by the fitting are listed in Table 1 together with the penetration depth and the LET calculated by the SRIM code [11]. By using r1, r0, and the penetration depth for each ion, the conductivities of the ion-induced phase were calculated to be 0.047, 0.057 and 0.031 S/cm for 2.0 MeV He+, 1.0 MeV H+, and 2.0 MeV H+, respectively. These values are comparable to the conductivity of the X-ray-induced phase at 40–50 K [7]. In addition, during the heating of the irradiated samples, we observed that the temperature dependence of the conductivity merged to that of the pre-irradiated state at ca. 100 K, which qualitatively agreed with the case of X-ray irradiation [7], and that the conductivity returns to the pre-irradiated value during subsequent cooling, as shown in Fig. 3. This result shows the metastable nature of the ion-induced phase, as reported for the conductive phase induced by X-ray irradiation [7]. Furthermore, as discussed above, the ion-induced change occurred due to the electronic excitation by each ion, rather than by defect formation due to the elastic collision between the impinging ions and the atoms comprising the material. From the above arguments, it is natural to consider that the conductive phase induced by the ion beam was identical to that induced by X-ray irradiation. Although the change in conductivity reported for X-ray irradiation was by several orders of magnitude [7], we observed the change in conductivity due to ion irradiation by several times in this study. This is because the conductivity before the irradiation differs between both the cases by several orders of magnitude due to the different irradiation temperatures. As shown in Table 1, the obtained cross-sectional area was of the order of square nanometers and was an increasing function of the LET. On the other hand, the length of the cylindrical volume corresponded to the penetration depth, which was comparable to or greater than several microns. The conductivity before irradiation was 15 104 S/cm, and those of the ion-induced phase (r1)
Table 1 Penetration depth, LET, cross-sectional area, and G-value for each ion. G-value is defined as the number of displaced Ir4+ dimers per 100 eV of the deposited energy. Ions
Penetration depth (lm)
LET (eV/nm)
Cross-sectional area (nm2)
G-value
2.0 MeV He+ 1.0 MeV H+ 2.0 MeV H+
4.2 9.2 25.5
470 110 77
5.4 2.1 1.4
4.8 8.2 7.7
Conductivity [S/cm ]
0.1
0.01
was 36 102 S/cm. Thus, the conductivity of the ion-induced phase was higher than that of the non-irradiated phase by two orders of magnitude, although this difference is much smaller than the thermally-induced metal–insulator transition at ca. 230 K. It should be noted that the ion-irradiation-induced phase is not identical to the high-temperature phase (T > 230 K), and it is suggested that a single ion produces a nanowire of the conductive phase. Here, the cross-sectional area is the average value over the penetration depth of each ion because the LET changes significantly during the slowing down process in the sample. 3.2. Yield of the ion-induced change We will consider the yields of the ion-induced change by using ‘‘G-values,” which are frequently used in radiation chemistry. A G-value is defined as the yield of the irradiation-induced change per absorbed energy of 100 eV. The number of the displaced Ir dimers was calculated as follows: it is assumed that all the Ir4+ ions form dimers in the low-temperature phase and that the conductivity change is proportional to the number of displaced Ir dimers. In addition, the number of the displaced Ir dimers was calculated by assuming that all the Ir dimers were displaced in the cylindrical volume that has the cross-sectional area shown in Table 1 and length equal to the penetration depth. Subsequently, we should calculate the number of the Ir dimers in unit volume. The number of Ir atoms in the unit cell is eight, and the number of Ir4+ ions is four; therefore, there are two Ir dimers in the unit cell. In order to calculate the unit cell volume, we used a = b = 6.8645 Å and c = 10.0257 Å as the lattice parameters [1]. Thus, the number of the Ir dimers displaced by one impinged ion was calculated. Finally, G-values were obtained dividing the number of the displaced Ir dimers by the ion energy. The G-value for 2.0 MeV He+ irradiation was significantly lower than that for H+ irradiation. This difference can be explained as follows. In the case of the H+ irradiation, due to the low LET, all the Ir dimers are not displaced and some dimers are left intact in the volume where the electronic excitation is induced. In this case, the obtained cross-sectional area of the ion-induced phase is the effective one. On the other hand, in the case of the 2.0 MeV He+ irradiation, the density of the electronic excitation is so high that all the dimers are displaced in the energy-deposited volume. Thus, the energy fraction used for the displacement of the dimers is lowered. This is because there should be a fraction of electronic excitation that is not involved in the displacement of the dimers. Consequently, a low G-value is observed at a high LET. In this case, a dense nanowire is formed where all the dimers are displaced. It is reported that the crystal structure of the X-ray-induced phase is different from that of the non-irradiated phase. Considering the similarity between the X-ray-induced phase and the ion-induced phase, the 2.0 MeV He+-induced phase having a cylindrical shape should have a distinct crystal structure from the non-irradiated phase. This situation is similar to that of the crystalline track formed under heavy ion irradiation in semiconductors [12]. 3.3. Comparison of the yield with ion-induced change in other solids
Before irradiation
0.001
After irradiation (heating) After irradiation (cooling)
0.0001 40
1127
60
80
100
120
140
160
Temperature [K] Fig. 3. Conductivity of CuIr2S4 before and after irradiation. Conductivity was measured after the 2.0 MeV H+ irradiation at a fluence of 2.0 1014 ions/cm2.
We now compare the yield of the ion-induced change of CuIr2S4 with that of other inorganic crystals. To the best of our knowledge, for inorganic crystals there are few reports that apparently treat the yield of the defect formation or lattice deformation induced by ionizing radiation. Here, the F-center formation in alkali halide crystals is discussed in comparison to our data. This is because these crystals are rather sensitive to the ionizing radiation among inorganic crystals. The G-value for the F-center formation in LiF has been reported to be 0.2 or less [13]. In addition, the yield can be calculated from the data obtained in previous studies. For example,
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by using an F-center concentration of 1.4 1016 cm2 in a cylinder around the ion trajectory and the cylinder of radius 30 nm obtained by the optical absorption measurement at various ion fluences [14], the G-value of the F-center formation in NaCl is calculated to be less than 0.01 for 197Au irradiation of the energy of 10.5 MeV/nucleon. These G-values are much smaller than those obtained in CuIr2S4. In addition, the G-values, which are defined as the deformed atoms in the formula unit of the latent track, can be obtained from reports on track formation under the heavy ion irradiation of inorganic crystals. In the previous reports on track formation, the LET and track radius are used for the calculation of the G-values. For example, in SiO2 where low threshold LET is determined for the track formation [15], the G-value for 238U irradiation is calculated to be as high as 20, and the G-values for the LET of several keV/nm are approximately 5–10 [16]. For Al2O3, where much higher threshold LET is reported [15], the G-value is calculated to be 11 for the LET of 44.2 keV/nm [17]. The Gvalues in CuIr2S4 are comparable to the G-values for track formation. Thus, the ion-induced change in CuIr2S4 and track formation in inorganic crystals have similarities in their high yield of the ion-induced change and the nanowire formation. However, we will point out the difference between the nanowire formation in CuIr2S4 and the track formation in inorganic materials. First, there is a large difference in the LET between both these cases. The track formation in many inorganic materials occurs for the LET higher than several keV/nm [15]. In contrast, the ion-induced phase is produced for the LET that is considerable less than keV/nm in CuIr2S4. Second, the LET-dependence of the G-value is complementary to these two cases. For track formation in inorganic crystals, the ‘‘G-value” increases with the LET in a nonlinear fashion above the threshold LET. Below the threshold, track formation is not observed and/or the ‘‘G-value” of the ion-induced change is very small. In contrast, the G-value is significantly low for the high LET in CuIr2S4. These differences can be explained as follows. In inorganic crystals, the track is formed after the complete destruction of the crystal structure in the cylindrical region. Regardless of the basic process involved, the track formation is brought about by the electronic excitation at an extremely high density that triggers the destruction of the crystal structure. On the other hand, the ion-induced change in CuIr2S4 occurs even for low-LET radiations such as X-rays or electrons, and it is not peculiar to the dense electronic excitation or high LET. Thus, the ion-induced change in CuIr2S4 is apparently not a collective phenomenon but an individual phenomenon located closely to each other. Nevertheless, the observed G-value for the ion-induced change in CuIr2S4 is much higher than that of inorganic crystals at low LET. The possible reasons for obtaining this high G-value are the low band-gap energy and the strong electron–lattice interaction. According to the result of XPS measurement, the band-gap energy of CuIr2S4 in the insulating phase is estimated to be approximately 0.15–0.20 eV [3,4], which is much smaller than that of inorganic crystals where a low G-value is reported for low LET. The lower band-gap energy leads to the formation of more electron–hole pairs or ionized atoms per unit energy of the ionizing radiation. In CuIr2S4, the irradiation-induced change is caused by the e-h pairs and/or ionized atoms. Therefore, the low band-gap energy
is one of the reasons for the high G-value in this compound. The strong electron–lattice interaction is another factor responsible for the high G-value in CuIr2S4 where lattice deformation due to electronic excitation is observed. As opposed to the case of CuIr2S4, most inorganic crystals having a strong electron–lattice interaction have a large band-gap energy that leads to a low G-value. Thus, these two factors should be responsible for the high G-value in this compound. In addition, some process analogous to chain reactions may be responsible for this high G-value, and further research is necessary for analyzing the yield and the microscopic structural change of this irradiation-induced change. 4. Conclusion We observed an increase in the conductivity of a thiospinel compound, CuIr2S4, induced by ion beam irradiation. The metastable conductive phase was produced by electronic excitation caused by the ion beam, and this phase was similar to the X-ray-induced phase. From the obtained cross-sectional area, it was found that an impinging ion produced a nanowire of the conductive phase. In addition, the G-value of the displacement of the Ir dimer was rather high, probably due to the low band-gap energy and strong electron–lattice interaction. The ion-irradiation effect reported in this paper is unique in view of the high G-value and the low LET in the formation of the conductive-phase nanowire. Acknowledgement This research was financially supported by the Asahi Glass Foundation and Research Foundation for Materials Science. References [1] T. Furubayashi, T. Matsumoto, T. Hagino, S. Nagata, J. Phys. Soc. Jpn. 63 (1994) 3333. [2] S. Tsuji, K. Kumagai, N. Matsumoto, S. Nagata, Physica C 282–287 (1997) 1107. [3] J. Matsuno, T. Mizokawa, A. Fujimori, D.A. Zatsepin, V.R. Galakhov, E.Z. Kurmaev, Y. Kato, S. Nagata, Phys. Rev. B 55 (1997) R15979. [4] K. Takubo, S. Hirata, J.Y. Son, J.W. Quilty, T. Mizokawa, N. Matsumoto, S. Nagata, Phys. Rev. Lett. 95 (2005) 246401. [5] T. Oda, M. Shirai, N. Suzuki, K. Motizuki, J. Phys.: Condens. Matter 7 (1995) 4433. [6] P.G. Radaelli, Y. Horibe, M.J. Gutmann, H. Ishibashi, C.H. Chen, R.M. Ibberson, Y. Koyama, Y.S. Hor, V. Kiryukhin, S.W. Cheong, Nature (London) 416 (2002) 155. [7] T. Furubayashi, H. Suzuki, T. Matsumoto, S. Nagata, Solid State Commun. 126 (2003) 617. [8] T. Furubayashi, H. Suzuki, T. Matsumoto, S. Nagata, J. Magn. Magn. Mater. 272– 276 (2004) 446. [9] H. Ishibashi, T.Y. Koo, Y.S. Hor, A. Borissov, P.G. Radaelli, Y. Horibe, S.W. Cheong, V. Kiryukhin, Phys. Rev. B 66 (2002) 144424. [10] V. Kiryukhin, Y. Horibe, Y.S. Hor, H.J. Noh, S.W. Cheong, C.H. Chen, Phys. Rev. Lett. 97 (2006) 225503. [11] J.F. Ziegler, Nucl. Instr. and Meth. B 219&220 (2004) 1027. [12] G. Szenes, Z.E. Horvath, B. Pecz, F. Paszti, T. Toth, Phys. Rev. B 65 (2002) 045206. [13] K. Schwartz, C. Trautmann, A.S. El-Said, R. Neumann, M. Toulemonde, W. Knolle, Phys. Rev. B 70 (2004) 184104. [14] M. Enculescu, K. Schwartz, C. Trautmann, M. Toulemonde, Nucl. Instr. and Meth. B 229 (2005) 397. [15] N. Itoh, A.M. Stoneham, Nucl. Instr. and Meth. B 146 (1998) 362. [16] A. Meftah, F. Brisard, J.M. Constantini, E. Dooryhee, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [17] B. Canut, A. Benyagoub, G. Marest, A. Meftah, N. Monocoffre, S.M.M. Ramos, F. Studer, P. Thevenard, M. Toulemonde, Phys. Rev. B 51 (1995) 12194.