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Broadening of the orbitally-induced Peierls phase transition in Cu1−xNaxIr2S4
Journal of Alloys and Compounds 617 (2014) 774–778
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Broadening of the orbitally-induced Peierls phase transition in Cu1xNaxIr2S4 Hui Han a, Lei Zhang b,⇑, Hui Liu b, Langsheng Ling b, Ranran Zhang b, Changjin Zhang b, Li Pi b,c, Yuheng Zhang b,c a b c
School of Physics and Material Science, Anhui University, Hefei 230601, and High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031,People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, University of Science and Technology of China, Hefei 230026, People’s Republic of China
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Article history: Received 13 June 2014 Received in revised form 22 July 2014 Accepted 6 August 2014 Available online 19 August 2014 Keywords: Spinel CuIr2S4 The orbitally-induced phase transition Peierls instability
a b s t r a c t The orbitally-induced Peierls phase transition in the Na-doped Cu1xNaxIr2S4 has been investigated. The increases while that on cooling T CM decreases with x phase transition temperature on warming T W M increasing, which means that the phase transition is broadened by the doping of Na ions. However, the average phase transition temperature T AM is not changed. The Raman spectroscopy and X-ray diffraction studies demonstrate that no structural distortion is induced by the doping of Na in Cu1xNaxIr2S4, which excludes the lattice distortion effect. The tendency to metallic behavior with the doping of Na indicates a hybridization between Na+ and surrounding S2 ions via the outer shell p electrons, which enhances the itinerant characteristic of electrons. It is suggested that the scattering from itinerant electrons and lattice defects may be responsible for the broadening of the orbitally-induced Peierls phase transition. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The iridium-based compounds with 5d outer shell electrons have triggered great interest due to the fascinating physical phenomena, such as topological quantum phase transition [1,2], orbital magnetism [3], modulation-induced superconductivity [4–8], unique Jeff ¼ 1=2 state [9,10]. It has been demonstrated that the spin–orbital coupling, which exists widely in the iridium-based materials, plays an important role in these extraordinary physical properties [11]. Especially, the spinel sulphide CuIr2S4 is well known as an orbitally-induced Peierls phase transition, which provides a prototype to investigate the interplay between charge, spin, orbital and lattice degrees of freedom [12–15]. The spinel CuIr2S4 exhibits a first-order metallic-insulating phase transition at 230 K with an opening energy gap, which accompanies with a magnetic transition from Pauli paramagnetic to diamagnetic state [16–19]. Meanwhile, the structure changes from cubic (space group Fd3m) to triclinic ðP1Þ cell with lattice shrinkage of 0.7% [20,21]. The copper ions situated on A-sites (formed tetrahedrally by S ions) are in monovalent Cu+ state [22–24]. The iridium ions on B-sites (formed octahedrally by S ⇑ Corresponding author. E-mail address: zhanglei@hmfl.ac.cn (L. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.08.063 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
ions) present as average valance of +3.5 in metallic phase, which 6 5 are separated equally into Ir3+ ð3d ; S ¼ 0Þ and Ir4+ (3d , S ¼ 1=2) in insulating sate [25]. The iridium ions form isomorphic octamers 4þ of Ir3þ 8 S24 and Ir8 S24 respectively after the phase transition, where chemical dimerization occurs to Ir4+ along the h1 1 0i direction [25,26]. The motivation of the phase transition has been explained by an orbitally-induced Peierls phase transition mechanism, which suggested that the anisotropic coupling of 5dxy orbits of Ir4+ along the h1 1 0i direction due to the crystal field and Jahn– Teller distortion results into the orbital Peierls instability [12,27]. It has been demonstrated that the alkaline metal substitution yields some attractive effects, such as the enhancement of the phase transition in Cu1xLixIr2S4 [28], the metallic tendency of transport behavior in Cu1xKxIr2S4 [29]. In this work, the Na-doped Cu1xNaxIr2S4 system has been investigated. It is found the phase transition is broadened by the doping of Na. However, the average phase transition temperature is almost not changed. Moreover, the metallic tendency and Pauli paramagnetism are enhanced by the doping of Na in Cu1xNaxIr2S4 system. 2. Experiment Polycrystalline samples of Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) were synthesized by the solid-state reaction method. The starting materials, powder of Cu (purity 99.999%), Ir (99.95%), S (99.9999%) and Na (99.9%), were mixed thoroughly
H. Han et al. / Journal of Alloys and Compounds 617 (2014) 774–778 according to the stoichiometric ratio with 1 wt% excess S. The mixed powder samples sealed in vacuumed quartz tubes were sintered at 1123 K for 8 days. The samples were cooled down to room temperature slowly. The procedure was repeated after the powder samples pressed into pellets. The sample preparing process was done in a glove-box full of argon atmosphere (pO2 and pH2O < 0.1 ppm). The chemical compositions and distribution were carefully determined by the energy dispersive X-ray (EDX) spectrometry. The structure and phase purity were checked by the Rigaku-TTR3 X-ray diffractometer using high-intensity graphite monochromatized Cu Ka radiation. The Raman scattering measurements were performed using a Horiba Jobin Yvon T64000 Micro-Raman instrument with a Kr+–Ar+ mixed gas laser (k = 514.5 nm) as an excitation source in a backscattering geometry. The magnetization was measured by a Magnetic Property Measurement System (Quantum Design MPMS 7T-XL) with a superconductive quantum interference device (SQUID). The magnetization of the background was subtracted. The resistivity measurement was carried out by the conventional four-probe method. The X-ray diffraction (XRD) patterns at selected temperatures were registered to investigate the structural phase transition, where the variation temperature was realized by using a closed He-gas cycle refrigerator established on the X-ray diffractometer.
3. Results and discussion Fig. 1(a) shows the EDX spectrum for Cu1xNaxIr2S4 (only the typical spectrum for x ¼ 0:2 is given), where the contents of Na are listed in Table 1. The EDX spectra demonstrate that the compositions are in agreement with the designed contents. Moreover, the inset of Fig. 1(a) depicts the elements distribution of Na (red) and Cu (green), indicating well-distribution of the elements. Fig. 1(b) gives the powder XRD patterns at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2). The XRD patterns show that the structures of the Na-doped samples keep cubic cell belonging to the space group Fd3m, in agreement with the previous report [20]. The XRD patterns were refined by the Rietveld method using the Rietica program. The lattice constant a vs the Na-content x was plotted
Fig. 1. (a) The energy dispersive X-ray (EDX) spectrum for Cu1xNaxIr2S4 (x ¼ 0:2) (the inset gives the distribution of Na (red) and Cu (green); (b) powder X-ray diffraction (XRD) patterns at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) (the inset shows the lattice constant a vs the Na-content x). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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in the inset of Fig. 1(b). The ionic radius of Na+ ion is 0.99 Å (with outer shell 2p6 ), which is bigger than 0.60 Å of Cu+ (with outer 10 shell 3d ) [30]. However, the doping of Na has little effect on a, which is similar to that in the K-doped Cu1xKxIr2S4 system [29]. The lattice fluctuation of Na-doped system is 0.15%, even smaller than 0.3% of the K-doped system [29]. The slight doping effect on the lattice by Na or K is different from the Li-doped Cu1xLixIr2S4 system. Although the ionic radius of Li+ (0.59 Å, 1s2 ) matches that 10 of Cu+ (0.6 Å, 3d ) well, the lattice constant of Cu1xLixIr2S4 is expanded by the doping of Li [28]. The different effects between the Na(K)-doping and Li-doping systems can be attributed to the different electronic configurations of the outer shell. It should be noted that A-sites are surrounded tetrahedrally by four S2 ions with 2p6 outer shell electrons. The transport behaviors suggested that hybridization exists between 3p6 out shell of K+ ions with 3p6 electrons of S2 in the K-doped system [29]. Similar hybridization should exist in the Na-doped system due to the similar outer shells of Na+ and K+ ions. However, the 1s2 outer shell electrons of Li+ ions cannot hybridize with that of S2 ions due to the great difference in energy level between s and p levels. Therefore, the electronic repulsion in the Li-doped system expands the lattice. Instead, the lattice in the Na(K)-doped system is hardly influenced due to the electronic hybridization. In order to have further investigation on the effect of Na-doping on the crystal lattice, the Raman spectra for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) were measured at 300 K, as shown in Fig. 2(a). Each Raman spectrum exhibits four peaks at the wave numbers of 300 cm1 , 326 cm1 , 370 cm1 and 400 cm1. The peaks are corresponding to the active Raman modes, which ð2Þ ð1Þ are determined as Eg , F 2g ; F 2g , and A1g modes respectively [31,32]. Fig. 2(b) depicts the peaks of the active Raman modes as a function of x. It can be seen that Eg mode changes hardly with ð1Þ ð2Þ x. However, A1g , F 2g , and F 2g modes decrease with the increase of x, especially for A1g mode. As we know, the different ionic radius and mass will affect the bonds interactions, which in turn influence the phonon vibration. Therefore, the Raman modes, which are determined by the phonon vibration, can be changed by the dopants with different ionic radius and mass. It is noted that the Cu(Na)–Ir bonds affect hardly on the Eg mode, because Eg mode is mainly contributed by the Ir–S and S–S bonds [31]. On the other ð2Þ ð1Þ hand, F 2g ; F 2g , and A1g modes are mainly contributed by Ir–S and Cu(Na)–S bonds [31], which are distinctly shifted by the doping of Na. The Raman spectra study testifies that the Na ions occupy the A-sites in CuIr2S4 system. Moreover, it is noticed that no extra peaks are observed in Cu1xNaxIr2S4, which indicates that the doping of Na ions does not bring into localized distortion but only affects the phonon vibration slightly. Fig. 3(a) shows the temperature of magnetization [MðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2). Each MðTÞ curve exhibits a magnetic phase transition with a magnetic step (DM) on cooling and warming, as depicted in the inset of Fig. 3(a). With temperature decreasing, all MðTÞ curves rise at low temperature, which can be attributed to the so called Curie tail [33,34]. As can be seen, MðTÞ curve is elevated with the doping of Na. Previous investigations suggested a spin-dimerization occurs, after which the spins of Ir4+ form spin-singlet in the orbitally-induced Peierls state. However, recent lSR study suggested a novel paramagnetic state, showing a quasistatic spin glasslike magnetism below 100 K associating with the spin–orbital coupling and magnetic frustration [35]. However, the magnetic moment is extremely small, which makes it difficult to detect the local spins by the conventional macroscopic measurement [35]. Therefore, below the phase transition temperature T M , the macro-susceptibility (v ¼ M=H) can be still described as [33,36]:
v ¼ v0 þ C=T
ð1Þ
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H. Han et al. / Journal of Alloys and Compounds 617 (2014) 774–778
Table 1 EDX and fitting results of the magnetization for Cu1-xNaxIr2S4. Sample(x)
Na-content
v0 (104 emu/mol)
C(emu K/mol)
DM(104 emu/g)
DT (K)
0 0.05 0.1 0.15 0.2
0 0.05(7) 0.09(3) 0.14(3) 0.20(5)
1.4 0.2 1.7 2.6 3.8
0.0013 0.0046 0.0019 0.0035 0.0050
2.8967 3.0868 1.7493 2.0401 2.0275
11 15 17 18 18
should be attributed to the hybridization between 2p6 outer shell electrons of Na+ with the 3p6 electrons of S2 ions. The substitution of Cu by Na has distinct influence on the orbitally-induced phase transition. The magnetic transition tempera ture T M is given in Fig. 3(d). The T M on cooling T CM and warming T W are determined by the peak of dM=dT vs T respecM C tively, as shown in the inset of Fig. 3(d). It can be seen that T M M TM increases (decreases) with the doping of Na, which are fitted linearly as shown in Fig. 3(d). As can be seen, T W M increases with x as a W
speed of dT M =dT ¼ 24 K/x, while T CM
decreases with x as
C dT M =dT ¼ 20 K/x. However, the average phase transition temper ature T AM , which defined as T AM ¼ T CM þ T W M =2, changes hardly with
A the doping of Na jdT M =dxj < 2 K=x . The width of the phase tranC sition DT are listed in Table 1, which is defined as DT ¼ T W M T M . As can be seen, DT increases with x increasing, which indicates that the phase transition is broadened by the doping of Na. The broadening of the phase transition is also found in the Cu1xKxIr2S4 sys-
Fig. 2. (a) Raman spectra at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2); (b) the active Raman modes as a function of x with the dotted horizontal lines for reference.
where C is the Curie constant, v0 represents the temperature-independent susceptibility (including the Pauli paramagnetism and Larmor diamagnetism, etc). The MðTÞ curves below T M can be fitted well by Eq. (1) as shown in Fig. 3(b), where the fitting parameters are listed in Table 1. It can be seen that v0 increases with the increase of x from negative to positive. As we know, the positive v0 is mainly contributed by the Pauli paramagnetism, which is the characteristic of the metallic state. Thus, the increase of v0 indicates the enhancement of the metallic state. On the other hand, in the primitive CuIr2S4, the magnetic step DM is caused by the disappearance of the Pauli paramagnetism due to the phase transition from metallic to insulating state. Therefore, there is DM=H / 23 l0 lB NðEF Þ [20,37] (where l0 and lB represent the vacuum magnetoconductivity and Bohr magneton respectively, NðEF Þ is the density of states at Fermi surface, H is applied field). The magnetic steps DM of Cu1xNaxIr2S4 is also listed in Table 1, which shows that DM changes hardly with x. The little change of DM with x indicates that the Pauli paramagnetism corresponding to the phase transition is hardly affected by the doping of Na. In other word, the doping of Na enhances the Pauli paramagnetism, but not affect the phase transition. As mentioned above, Pauli paramagnetism originates from the itinerant electrons in the metallic state. Therefore, the increase of Pauli paramagnetism indicates the enhancement of the metallic behavior. On the other hand, the tendency of the metallic behavior is confirmed by the temperature resistivity [qðTÞ], as shown in Fig. 3(c). The qðTÞ curve at low temperature for x ¼ 0 exhibits an insulating behavior (dq=dT < 0), however, that for x ¼ 0:2 demonstrates a metallic-like behavior (dq=dT > 0). The metallic tendency of MðTÞ and qðTÞ with doping
tem [29]. However, in Cu1xLixIr2S4 system, both T CM and T W M are increased by Li-doping [28]. The enhancement of the phase transition in Cu1xLixIr2S4 system is attributed to the localized lattice distortion caused by the doping of Li, which favors the tetragonal structure after the phase transition. In order to clarify whether the mechanism of the Na-doping system is similar to that of the Li-doped system, the XRD patterns at selected temperatures are measured. Fig. 4(a) shows XRD patterns from 2h ¼ 10 to 100° at selected temperatures for Cu0:8 Na0:2 Ir2S4. With temperature decreasing, changes happen to the XRD patterns, indicating a structure phase transition. The structure above T M is cubic belonging to space group Fd3m. Obviously, the XRD pattern at 220 K is composed by two sets of diffraction peaks. The XRD pattern at 200 K changes completely into the other structure which remains to low temperature. Fig. 4(b) gives the fitting results of the XRD pattern at 20 K by the Rietveld method. The XRD pattern at 20 K can be well fitted by the triclinic cell belonging to space group P1 [21]. For deep investigation on the structural phase transition, (4 0 0)C peak of the cubic structure for Cu0.8Na0.2Ir2S4 around T M was studied in detail, as shown in Fig. 5. Fig. 5(a) is measured on cooling, while Fig. 5(b) on warming. With temperature decreasing, the (4 0 0)C peak splits into (0 3 2)T and (2 3 0)T of the triclinic phase gradually. The XRD patterns are well fitted, which are shown as orange solid curves in Fig. 5 (only several typical fitting results are shown). The changes of the structural phase are plotted in Fig. 6, where Fig. 6(a) gives the cubic proportion and Fig. 6(b) depicts the triclinic proportion. The structural phase transition temperature determined from 50% of the proportion is 234 K for warming and 222 K for cooling, respectively. The width obtained from the structural transition is 16 K for Cu0.8Na0.2Ir2S4, which is broader than the primitive CuIr2S4 (DT ¼ 11 K obtained for MðTÞ curve). It should be stated that no extra diffraction peaks are detected like that in the Cu1xLixIr2S4 system, which indicates no lattice distortion in Cu1xNaxIr2S4. In addition, the refinement of the structure and Raman spectroscopy
H. Han et al. / Journal of Alloys and Compounds 617 (2014) 774–778
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Fig. 3. (a) The temperature dependence of magnetization [MðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) (the inset gives the magnification of the phase transition region); (b) the fitting of MðTÞ (x = 0.15) below T M by Eq. (1); (c) the temperature dependence of resistivity [qðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.1, and 0.2); (d) the transition temperature T M (T CM for cooling, T CM for warming, and T AM for the average) as a function of x with the solid linear fitting lines (the inset depicts the dMðTÞ=dT curves vs T around T M for x = 0.05).
Fig. 5. The XRD patterns of the (4 0 0) peak of the cubic structure for Cu0.8Na0.2Ir2S4 around the phase transition temperature with the fitting solid curves (only several typical fitting results are shown).
Fig. 4. (a) The XRD patterns from 2h ¼ 10 to 100° at selected temperatures for Cu0.8Na0.2Ir2S4; (b) the Rietveld refinement of the XRD pattern at 20 K.
show that the doping of Na does not bring into the lattice distortion. In addition, the average phase transition temperature T AM changes hardly with x. All these results indicate that the phase transition is just broadened, but not enhanced or weakened by the doping of Na. Apparently, the broadening mechanism of the Cu1xNaxIr2S4 is different from that of Cu1xLixIr2S4 system. The broadening of the phase transition in Cu1xNaxIr2S4 is similar to that in Cu1xKxIr2S4 system, which has been suggested to be attributed to the potential barriers caused by the dopant [29].
However, potential barriers alone can not explain the broadening of the phase transition because the broadening is not prominent in other doped systems. It should be noted that a common point in Cu1xNaxIr2S4 and Cu1xKxIr2S4 system is the hybridization between the Na or K ions with that of S via outer shell p electrons, which enhances the itinerant characteristic of electrons. Therefore, above T M , there are two kinds of itinerant electrons. The first kind of itinerant electrons are from metallic Ir ions, which will participate into the orbitally-induced phase transition and disappear below T M . The second kind of itinerant electrons are from the hybridization between Na(K) and S, which will persist through the phase transition but not participate into the phase transition. However, the second kind of itinerant electrons will hinder the
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2011CBA00111, the National Natural Science Foundation of China (Grant Nos. U1332140, U1232142, 11004196, 11174262 and 11174290), and the Hundred Talents Program of the Chinese Academy of Sciences. References
Fig. 6. The proportion of the cubic (a) and triclinic (b) phases around the phase transition temperature.
movement of the first kind of itinerant electrons due to the electron–electron scattering. Therefore, the phase transition is hindered but not prohibited. When temperature decreasing, the dimerization is delayed due to the electron–electron scattering. When temperature increasing, the de-dimerization is also postponed. This hindrance results into the broadening of the phase transition. However, intrinsically, the phase transition is not enhanced or suppressed because the phase transition is only determined by the first kind of itinerant electrons whose number is not changed by the doping. Therefore, T AM is not changed by the doping. This also gives an explanation to the increase of v0 and the little change of DM. The second kind of itinerant electrons yield extra Pauli paramagnetism, which enhances the Pauli paramagnetism. However, the DM is only determined by the first kind of itinerant electrons, the number of which is not changed by the doping effect. 4. Conclusion In summary, the orbitally-induced Peierls phase transition in Cu1xNaxIr2S4 has been investigated. It is found T CM decreases while TW M increases with the increase of x. However, the average phase transition temperature T AM is hardly influenced by the doping of Na. These results indicate that the phase transition is broadened while the transition temperature is not changed by the doping of Na. Moreover, the Pauli paramagnetism is increased, while DM changes hardly with the doping of Na. It is suggested that the hybridization between Na and S enhances the itinerant characteristic of electrons. The broadening of the phase transition may be due to the scattering from the itinerant electrons and the lattice defects. In addition, the enhanced itinerant characteristic of electrons increases the Pauli paramagnetism, but not changes DM. Acknowledgements This work was supported by the State Key Project of Fundamental Research of China through Grant Nos. 2010CB923403 and
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Broadening of the orbitally-induced Peierls phase transition in Cu1xNaxIr2S4 Hui Han a, Lei Zhang b,⇑, Hui Liu b, Langsheng Ling b, Ranran Zhang b, Changjin Zhang b, Li Pi b,c, Yuheng Zhang b,c a b c
School of Physics and Material Science, Anhui University, Hefei 230601, and High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031,People’s Republic of China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China High Magnetic Field Laboratory, University of Science and Technology of China, Hefei 230026, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 13 June 2014 Received in revised form 22 July 2014 Accepted 6 August 2014 Available online 19 August 2014 Keywords: Spinel CuIr2S4 The orbitally-induced phase transition Peierls instability
a b s t r a c t The orbitally-induced Peierls phase transition in the Na-doped Cu1xNaxIr2S4 has been investigated. The increases while that on cooling T CM decreases with x phase transition temperature on warming T W M increasing, which means that the phase transition is broadened by the doping of Na ions. However, the average phase transition temperature T AM is not changed. The Raman spectroscopy and X-ray diffraction studies demonstrate that no structural distortion is induced by the doping of Na in Cu1xNaxIr2S4, which excludes the lattice distortion effect. The tendency to metallic behavior with the doping of Na indicates a hybridization between Na+ and surrounding S2 ions via the outer shell p electrons, which enhances the itinerant characteristic of electrons. It is suggested that the scattering from itinerant electrons and lattice defects may be responsible for the broadening of the orbitally-induced Peierls phase transition. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The iridium-based compounds with 5d outer shell electrons have triggered great interest due to the fascinating physical phenomena, such as topological quantum phase transition [1,2], orbital magnetism [3], modulation-induced superconductivity [4–8], unique Jeff ¼ 1=2 state [9,10]. It has been demonstrated that the spin–orbital coupling, which exists widely in the iridium-based materials, plays an important role in these extraordinary physical properties [11]. Especially, the spinel sulphide CuIr2S4 is well known as an orbitally-induced Peierls phase transition, which provides a prototype to investigate the interplay between charge, spin, orbital and lattice degrees of freedom [12–15]. The spinel CuIr2S4 exhibits a first-order metallic-insulating phase transition at 230 K with an opening energy gap, which accompanies with a magnetic transition from Pauli paramagnetic to diamagnetic state [16–19]. Meanwhile, the structure changes from cubic (space group Fd3m) to triclinic ðP1Þ cell with lattice shrinkage of 0.7% [20,21]. The copper ions situated on A-sites (formed tetrahedrally by S ions) are in monovalent Cu+ state [22–24]. The iridium ions on B-sites (formed octahedrally by S ⇑ Corresponding author. E-mail address: zhanglei@hmfl.ac.cn (L. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.08.063 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
ions) present as average valance of +3.5 in metallic phase, which 6 5 are separated equally into Ir3+ ð3d ; S ¼ 0Þ and Ir4+ (3d , S ¼ 1=2) in insulating sate [25]. The iridium ions form isomorphic octamers 4þ of Ir3þ 8 S24 and Ir8 S24 respectively after the phase transition, where chemical dimerization occurs to Ir4+ along the h1 1 0i direction [25,26]. The motivation of the phase transition has been explained by an orbitally-induced Peierls phase transition mechanism, which suggested that the anisotropic coupling of 5dxy orbits of Ir4+ along the h1 1 0i direction due to the crystal field and Jahn– Teller distortion results into the orbital Peierls instability [12,27]. It has been demonstrated that the alkaline metal substitution yields some attractive effects, such as the enhancement of the phase transition in Cu1xLixIr2S4 [28], the metallic tendency of transport behavior in Cu1xKxIr2S4 [29]. In this work, the Na-doped Cu1xNaxIr2S4 system has been investigated. It is found the phase transition is broadened by the doping of Na. However, the average phase transition temperature is almost not changed. Moreover, the metallic tendency and Pauli paramagnetism are enhanced by the doping of Na in Cu1xNaxIr2S4 system. 2. Experiment Polycrystalline samples of Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) were synthesized by the solid-state reaction method. The starting materials, powder of Cu (purity 99.999%), Ir (99.95%), S (99.9999%) and Na (99.9%), were mixed thoroughly
H. Han et al. / Journal of Alloys and Compounds 617 (2014) 774–778 according to the stoichiometric ratio with 1 wt% excess S. The mixed powder samples sealed in vacuumed quartz tubes were sintered at 1123 K for 8 days. The samples were cooled down to room temperature slowly. The procedure was repeated after the powder samples pressed into pellets. The sample preparing process was done in a glove-box full of argon atmosphere (pO2 and pH2O < 0.1 ppm). The chemical compositions and distribution were carefully determined by the energy dispersive X-ray (EDX) spectrometry. The structure and phase purity were checked by the Rigaku-TTR3 X-ray diffractometer using high-intensity graphite monochromatized Cu Ka radiation. The Raman scattering measurements were performed using a Horiba Jobin Yvon T64000 Micro-Raman instrument with a Kr+–Ar+ mixed gas laser (k = 514.5 nm) as an excitation source in a backscattering geometry. The magnetization was measured by a Magnetic Property Measurement System (Quantum Design MPMS 7T-XL) with a superconductive quantum interference device (SQUID). The magnetization of the background was subtracted. The resistivity measurement was carried out by the conventional four-probe method. The X-ray diffraction (XRD) patterns at selected temperatures were registered to investigate the structural phase transition, where the variation temperature was realized by using a closed He-gas cycle refrigerator established on the X-ray diffractometer.
3. Results and discussion Fig. 1(a) shows the EDX spectrum for Cu1xNaxIr2S4 (only the typical spectrum for x ¼ 0:2 is given), where the contents of Na are listed in Table 1. The EDX spectra demonstrate that the compositions are in agreement with the designed contents. Moreover, the inset of Fig. 1(a) depicts the elements distribution of Na (red) and Cu (green), indicating well-distribution of the elements. Fig. 1(b) gives the powder XRD patterns at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2). The XRD patterns show that the structures of the Na-doped samples keep cubic cell belonging to the space group Fd3m, in agreement with the previous report [20]. The XRD patterns were refined by the Rietveld method using the Rietica program. The lattice constant a vs the Na-content x was plotted
Fig. 1. (a) The energy dispersive X-ray (EDX) spectrum for Cu1xNaxIr2S4 (x ¼ 0:2) (the inset gives the distribution of Na (red) and Cu (green); (b) powder X-ray diffraction (XRD) patterns at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) (the inset shows the lattice constant a vs the Na-content x). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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in the inset of Fig. 1(b). The ionic radius of Na+ ion is 0.99 Å (with outer shell 2p6 ), which is bigger than 0.60 Å of Cu+ (with outer 10 shell 3d ) [30]. However, the doping of Na has little effect on a, which is similar to that in the K-doped Cu1xKxIr2S4 system [29]. The lattice fluctuation of Na-doped system is 0.15%, even smaller than 0.3% of the K-doped system [29]. The slight doping effect on the lattice by Na or K is different from the Li-doped Cu1xLixIr2S4 system. Although the ionic radius of Li+ (0.59 Å, 1s2 ) matches that 10 of Cu+ (0.6 Å, 3d ) well, the lattice constant of Cu1xLixIr2S4 is expanded by the doping of Li [28]. The different effects between the Na(K)-doping and Li-doping systems can be attributed to the different electronic configurations of the outer shell. It should be noted that A-sites are surrounded tetrahedrally by four S2 ions with 2p6 outer shell electrons. The transport behaviors suggested that hybridization exists between 3p6 out shell of K+ ions with 3p6 electrons of S2 in the K-doped system [29]. Similar hybridization should exist in the Na-doped system due to the similar outer shells of Na+ and K+ ions. However, the 1s2 outer shell electrons of Li+ ions cannot hybridize with that of S2 ions due to the great difference in energy level between s and p levels. Therefore, the electronic repulsion in the Li-doped system expands the lattice. Instead, the lattice in the Na(K)-doped system is hardly influenced due to the electronic hybridization. In order to have further investigation on the effect of Na-doping on the crystal lattice, the Raman spectra for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) were measured at 300 K, as shown in Fig. 2(a). Each Raman spectrum exhibits four peaks at the wave numbers of 300 cm1 , 326 cm1 , 370 cm1 and 400 cm1. The peaks are corresponding to the active Raman modes, which ð2Þ ð1Þ are determined as Eg , F 2g ; F 2g , and A1g modes respectively [31,32]. Fig. 2(b) depicts the peaks of the active Raman modes as a function of x. It can be seen that Eg mode changes hardly with ð1Þ ð2Þ x. However, A1g , F 2g , and F 2g modes decrease with the increase of x, especially for A1g mode. As we know, the different ionic radius and mass will affect the bonds interactions, which in turn influence the phonon vibration. Therefore, the Raman modes, which are determined by the phonon vibration, can be changed by the dopants with different ionic radius and mass. It is noted that the Cu(Na)–Ir bonds affect hardly on the Eg mode, because Eg mode is mainly contributed by the Ir–S and S–S bonds [31]. On the other ð2Þ ð1Þ hand, F 2g ; F 2g , and A1g modes are mainly contributed by Ir–S and Cu(Na)–S bonds [31], which are distinctly shifted by the doping of Na. The Raman spectra study testifies that the Na ions occupy the A-sites in CuIr2S4 system. Moreover, it is noticed that no extra peaks are observed in Cu1xNaxIr2S4, which indicates that the doping of Na ions does not bring into localized distortion but only affects the phonon vibration slightly. Fig. 3(a) shows the temperature of magnetization [MðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2). Each MðTÞ curve exhibits a magnetic phase transition with a magnetic step (DM) on cooling and warming, as depicted in the inset of Fig. 3(a). With temperature decreasing, all MðTÞ curves rise at low temperature, which can be attributed to the so called Curie tail [33,34]. As can be seen, MðTÞ curve is elevated with the doping of Na. Previous investigations suggested a spin-dimerization occurs, after which the spins of Ir4+ form spin-singlet in the orbitally-induced Peierls state. However, recent lSR study suggested a novel paramagnetic state, showing a quasistatic spin glasslike magnetism below 100 K associating with the spin–orbital coupling and magnetic frustration [35]. However, the magnetic moment is extremely small, which makes it difficult to detect the local spins by the conventional macroscopic measurement [35]. Therefore, below the phase transition temperature T M , the macro-susceptibility (v ¼ M=H) can be still described as [33,36]:
v ¼ v0 þ C=T
ð1Þ
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Table 1 EDX and fitting results of the magnetization for Cu1-xNaxIr2S4. Sample(x)
Na-content
v0 (104 emu/mol)
C(emu K/mol)
DM(104 emu/g)
DT (K)
0 0.05 0.1 0.15 0.2
0 0.05(7) 0.09(3) 0.14(3) 0.20(5)
1.4 0.2 1.7 2.6 3.8
0.0013 0.0046 0.0019 0.0035 0.0050
2.8967 3.0868 1.7493 2.0401 2.0275
11 15 17 18 18
should be attributed to the hybridization between 2p6 outer shell electrons of Na+ with the 3p6 electrons of S2 ions. The substitution of Cu by Na has distinct influence on the orbitally-induced phase transition. The magnetic transition tempera ture T M is given in Fig. 3(d). The T M on cooling T CM and warming T W are determined by the peak of dM=dT vs T respecM C tively, as shown in the inset of Fig. 3(d). It can be seen that T M M TM increases (decreases) with the doping of Na, which are fitted linearly as shown in Fig. 3(d). As can be seen, T W M increases with x as a W
speed of dT M =dT ¼ 24 K/x, while T CM
decreases with x as
C dT M =dT ¼ 20 K/x. However, the average phase transition temper ature T AM , which defined as T AM ¼ T CM þ T W M =2, changes hardly with
A the doping of Na jdT M =dxj < 2 K=x . The width of the phase tranC sition DT are listed in Table 1, which is defined as DT ¼ T W M T M . As can be seen, DT increases with x increasing, which indicates that the phase transition is broadened by the doping of Na. The broadening of the phase transition is also found in the Cu1xKxIr2S4 sys-
Fig. 2. (a) Raman spectra at 300 K for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2); (b) the active Raman modes as a function of x with the dotted horizontal lines for reference.
where C is the Curie constant, v0 represents the temperature-independent susceptibility (including the Pauli paramagnetism and Larmor diamagnetism, etc). The MðTÞ curves below T M can be fitted well by Eq. (1) as shown in Fig. 3(b), where the fitting parameters are listed in Table 1. It can be seen that v0 increases with the increase of x from negative to positive. As we know, the positive v0 is mainly contributed by the Pauli paramagnetism, which is the characteristic of the metallic state. Thus, the increase of v0 indicates the enhancement of the metallic state. On the other hand, in the primitive CuIr2S4, the magnetic step DM is caused by the disappearance of the Pauli paramagnetism due to the phase transition from metallic to insulating state. Therefore, there is DM=H / 23 l0 lB NðEF Þ [20,37] (where l0 and lB represent the vacuum magnetoconductivity and Bohr magneton respectively, NðEF Þ is the density of states at Fermi surface, H is applied field). The magnetic steps DM of Cu1xNaxIr2S4 is also listed in Table 1, which shows that DM changes hardly with x. The little change of DM with x indicates that the Pauli paramagnetism corresponding to the phase transition is hardly affected by the doping of Na. In other word, the doping of Na enhances the Pauli paramagnetism, but not affect the phase transition. As mentioned above, Pauli paramagnetism originates from the itinerant electrons in the metallic state. Therefore, the increase of Pauli paramagnetism indicates the enhancement of the metallic behavior. On the other hand, the tendency of the metallic behavior is confirmed by the temperature resistivity [qðTÞ], as shown in Fig. 3(c). The qðTÞ curve at low temperature for x ¼ 0 exhibits an insulating behavior (dq=dT < 0), however, that for x ¼ 0:2 demonstrates a metallic-like behavior (dq=dT > 0). The metallic tendency of MðTÞ and qðTÞ with doping
tem [29]. However, in Cu1xLixIr2S4 system, both T CM and T W M are increased by Li-doping [28]. The enhancement of the phase transition in Cu1xLixIr2S4 system is attributed to the localized lattice distortion caused by the doping of Li, which favors the tetragonal structure after the phase transition. In order to clarify whether the mechanism of the Na-doping system is similar to that of the Li-doped system, the XRD patterns at selected temperatures are measured. Fig. 4(a) shows XRD patterns from 2h ¼ 10 to 100° at selected temperatures for Cu0:8 Na0:2 Ir2S4. With temperature decreasing, changes happen to the XRD patterns, indicating a structure phase transition. The structure above T M is cubic belonging to space group Fd3m. Obviously, the XRD pattern at 220 K is composed by two sets of diffraction peaks. The XRD pattern at 200 K changes completely into the other structure which remains to low temperature. Fig. 4(b) gives the fitting results of the XRD pattern at 20 K by the Rietveld method. The XRD pattern at 20 K can be well fitted by the triclinic cell belonging to space group P1 [21]. For deep investigation on the structural phase transition, (4 0 0)C peak of the cubic structure for Cu0.8Na0.2Ir2S4 around T M was studied in detail, as shown in Fig. 5. Fig. 5(a) is measured on cooling, while Fig. 5(b) on warming. With temperature decreasing, the (4 0 0)C peak splits into (0 3 2)T and (2 3 0)T of the triclinic phase gradually. The XRD patterns are well fitted, which are shown as orange solid curves in Fig. 5 (only several typical fitting results are shown). The changes of the structural phase are plotted in Fig. 6, where Fig. 6(a) gives the cubic proportion and Fig. 6(b) depicts the triclinic proportion. The structural phase transition temperature determined from 50% of the proportion is 234 K for warming and 222 K for cooling, respectively. The width obtained from the structural transition is 16 K for Cu0.8Na0.2Ir2S4, which is broader than the primitive CuIr2S4 (DT ¼ 11 K obtained for MðTÞ curve). It should be stated that no extra diffraction peaks are detected like that in the Cu1xLixIr2S4 system, which indicates no lattice distortion in Cu1xNaxIr2S4. In addition, the refinement of the structure and Raman spectroscopy
H. Han et al. / Journal of Alloys and Compounds 617 (2014) 774–778
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Fig. 3. (a) The temperature dependence of magnetization [MðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.05, 0.1, 0.15 and 0.2) (the inset gives the magnification of the phase transition region); (b) the fitting of MðTÞ (x = 0.15) below T M by Eq. (1); (c) the temperature dependence of resistivity [qðTÞ] for Cu1xNaxIr2S4 (x = 0, 0.1, and 0.2); (d) the transition temperature T M (T CM for cooling, T CM for warming, and T AM for the average) as a function of x with the solid linear fitting lines (the inset depicts the dMðTÞ=dT curves vs T around T M for x = 0.05).
Fig. 5. The XRD patterns of the (4 0 0) peak of the cubic structure for Cu0.8Na0.2Ir2S4 around the phase transition temperature with the fitting solid curves (only several typical fitting results are shown).
Fig. 4. (a) The XRD patterns from 2h ¼ 10 to 100° at selected temperatures for Cu0.8Na0.2Ir2S4; (b) the Rietveld refinement of the XRD pattern at 20 K.
show that the doping of Na does not bring into the lattice distortion. In addition, the average phase transition temperature T AM changes hardly with x. All these results indicate that the phase transition is just broadened, but not enhanced or weakened by the doping of Na. Apparently, the broadening mechanism of the Cu1xNaxIr2S4 is different from that of Cu1xLixIr2S4 system. The broadening of the phase transition in Cu1xNaxIr2S4 is similar to that in Cu1xKxIr2S4 system, which has been suggested to be attributed to the potential barriers caused by the dopant [29].
However, potential barriers alone can not explain the broadening of the phase transition because the broadening is not prominent in other doped systems. It should be noted that a common point in Cu1xNaxIr2S4 and Cu1xKxIr2S4 system is the hybridization between the Na or K ions with that of S via outer shell p electrons, which enhances the itinerant characteristic of electrons. Therefore, above T M , there are two kinds of itinerant electrons. The first kind of itinerant electrons are from metallic Ir ions, which will participate into the orbitally-induced phase transition and disappear below T M . The second kind of itinerant electrons are from the hybridization between Na(K) and S, which will persist through the phase transition but not participate into the phase transition. However, the second kind of itinerant electrons will hinder the
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2011CBA00111, the National Natural Science Foundation of China (Grant Nos. U1332140, U1232142, 11004196, 11174262 and 11174290), and the Hundred Talents Program of the Chinese Academy of Sciences. References
Fig. 6. The proportion of the cubic (a) and triclinic (b) phases around the phase transition temperature.
movement of the first kind of itinerant electrons due to the electron–electron scattering. Therefore, the phase transition is hindered but not prohibited. When temperature decreasing, the dimerization is delayed due to the electron–electron scattering. When temperature increasing, the de-dimerization is also postponed. This hindrance results into the broadening of the phase transition. However, intrinsically, the phase transition is not enhanced or suppressed because the phase transition is only determined by the first kind of itinerant electrons whose number is not changed by the doping. Therefore, T AM is not changed by the doping. This also gives an explanation to the increase of v0 and the little change of DM. The second kind of itinerant electrons yield extra Pauli paramagnetism, which enhances the Pauli paramagnetism. However, the DM is only determined by the first kind of itinerant electrons, the number of which is not changed by the doping effect. 4. Conclusion In summary, the orbitally-induced Peierls phase transition in Cu1xNaxIr2S4 has been investigated. It is found T CM decreases while TW M increases with the increase of x. However, the average phase transition temperature T AM is hardly influenced by the doping of Na. These results indicate that the phase transition is broadened while the transition temperature is not changed by the doping of Na. Moreover, the Pauli paramagnetism is increased, while DM changes hardly with the doping of Na. It is suggested that the hybridization between Na and S enhances the itinerant characteristic of electrons. The broadening of the phase transition may be due to the scattering from the itinerant electrons and the lattice defects. In addition, the enhanced itinerant characteristic of electrons increases the Pauli paramagnetism, but not changes DM. Acknowledgements This work was supported by the State Key Project of Fundamental Research of China through Grant Nos. 2010CB923403 and
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