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High temperature spin state transitions in misfit-layered Ca3Co4O9
Journal of Alloys and Compounds 587 (2014) 40–44
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
High temperature spin state transitions in misfit-layered Ca3Co4O9 S. Altin, M.A. Aksan ⇑, A. Bayri Inonu Universitesi, Fen Edebiyat Fakultesi, Fizik Bolumu, 44280 Malatya, Turkey
a r t i c l e
i n f o
Article history: Received 23 June 2013 Received in revised form 21 October 2013 Accepted 22 October 2013 Available online 30 October 2013 Keywords: Spin crossover Magnetic oxides Ferrimagnetics Magnetic ordering spin arrangements
a b s t r a c t This study reports high temperature magnetic properties of the unsubstituted one together with B and Sb-substituted Ca3Co4O9 system. The measured data indicated that there is an anomaly in the magnetic susceptibility, v, between 680 and 920 K. It is believed that this anomaly is related to a critical threshold number of the high spin Co-ions such that when this threshold number is achieved, some exchange interactions between Co3+ and Co4+ take place which causes an abrupt increase in the v–T curve. The anomaly was further investigated with B and Sb-substitutions. It is realized that both dopants promote more Coions in the rock salt unit cell to high spin state. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, thermoelectric oxide materials such as NaxCO2, Bi2Sr2Co2O9, Ca3Co4O9, have attracted considerable attention due to their good thermal and chemical stability at high temperatures (>300 K) compared to intermetallic thermoelectric compounds such as Bi2Te3 and Bi2Se3. [1]. However, the application of NaxCO2, Bi2Sr2Co2O9 to the power generation is limited because of the volatility of Na and Bi at high temperatures [2]. Therefore, Ca3Co4O9 is a better candidate at high temperatures than other thermoelectric oxide materials. Crystal structure of Ca3Co4O9 consists of misfit layered structure stacking along the c-axis and two monoclinic subsystems; one is CdI2-type CoO2 layer and the other is rock-salt type Ca2CoO3 layer [3]. These two subsystem have similar unit cell parameters (a = 4.83 Å and c = 10.83 Å) but b1 = 4.55 Å for Ca2CoO3 and b2 = 2.81 Å for CoO2 [4]. The CoO2 subsystem is mainly responsible for the electronic and magnetic properties of the system, whereas Ca2CoO3 layer acts as a charge reservoir which supply charge carriers into the CoO2 subsystem. Substitutions/dopings to Ca3Co4O9 have an important effect on transport and magnetic properties of the system. The reason of good transport properties of Ca3Co4O9 at high temperatures is not exactly clear but it is suggested that strong electron correlation and spin-entropy relation may be responsible for the transport properties. Ca3Co4O9 offers interesting magnetic properties as well as its good transport properties. The system displays three different magnetic transitions at 380 K, 27 K and 19 K: spin state transition of the Co-ions at 380 K, long-range incommensurate spin-density ⇑ Corresponding author. Tel.: +90 422 3773720; fax: +90 422 341 03 19. E-mail address: [email protected] (M.A. Aksan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.170
wave (IC-SDW) arrangement at 27 K and ferrimagnetic transition at 19 K, respectively [5,6]. A short-range IC-SDW arrangement appears with decreasing the temperature down to 100 K which is accompanied by a broad minimum near 80 K in q–T curve [7]. The structure of IC-SDW is not clear but it is assumed that ICSDW may originate from the half-spin (s = 1/2) in the 2D-triangular lattice of the Co-ions in the CoO2 layer. Long-range IC-SDW arrangement is completed at 27 K and the system is in ferrimagnetic state at 19 K. It is believed that the ferrimagnetism arises from interlayer coupling of the spins of the Co-ions between Ca2CoO3 and CoO2 subsystems [8]. The magnetic transition at 380 K is attributed to the spin-state transition of the Co-ions: the spin state of the Co3+ (3d6) and Co4+ (3d5) ions changes from low temperature-low spin state + intermediate spin state (LS + IS) below 380 K to high temperature-intermediate spin state + high spin state (IS + HS) above 380 K [9,10]. This spin-state transition corresponds to a broad maximum near 400 K in q–T curve [11]. Co2+ ions have always the low spin state configuration. It is well known that 3d4–3d7 ions may have low spin, intermediate and high spin configuration depending on both environment and crystal field splitting strength. l+SR measurements indicated that the spin-state gradually changes above 400 K [7,12]. It is more probable that if LS and IS exist in low temperatures, a high spin state should be observed in the high temperature regime. Any information on the magnetic behavior above 400 K in Ca3Co4O9 does not exist in literature. Aim of the study is to investigate the substitution effect (especially changes in the magnetic properties) on the Co sites by substituting different element for Ca in the Ca3Co4O9 system. In this study, the magnetic behavior in the unsubstituted Ca3Co4O9 and the B and Sb-substituted Ca3Co4O9 has been investigated up to 1000 K. In order to observe a possible HS for all the Co ions, we performed the DC magnetic
S. Altin et al. / Journal of Alloys and Compounds 587 (2014) 40–44
susceptibility, v, measurement up to 1000 K and investigated the magnetic properties of the samples fabricated. 2. Experimental (Ca3xMx)Co4O9 (where M = B and Sb and x = 0.0, 0.5, 1.0) samples were fabricated using conventional solid-state reaction technique. High purity powders of Ca3CO3, CoO, Sb2O3 and B2O3 were weighted in the appropriate amounts to give nominal compositions. The powders were mixed in an agate mortar and then sintered at 900 °C for 24 h with intermediate grinding and mixing. After sintering, the powders were pressed into pellets under pressure of 4 tons. The pellets were heat treated at 900 °C for 36 h under oxygen atmosphere. The structural characterization of the samples fabricated was investigated by Xray diffraction (XRD). Automated Rigaku RadB Dmax X-ray diffractometer having Cu Ka radiation was used. Scan speed was selected as 2° min1 in the range of 2h = 3–80°. The lattice parameters of the unsubstituted sample were calculated using Rietveld Refinement technique using Jade 5.0 Software. Temperature dependence of DC-magnetization between 2 and 1000 K under magnetic field H = 2 kOe were measured using Quantum Design PPMS system with VSM and oven attachments in vacuum atmosphere.
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the main phase was obtained to be Ca3Co4O9, but some impurity phases, such as Co3O4 and Co3(BO3)2, were also detected. In addition, the peak intensity of the main phase decreased compared to the unsubstituted sample. When the substitution level was increased to x = 1.0, the peaks of the main phase, Ca3Co4O9, suppressed and many impurity phases such as Co3O4, Co3(BO3)2, CaB2O4, were observed in the sample. When Sb was substituted into the system (x = 0.5 and 1.0), the crystal structure of Ca3Co4O9 was found to be highly deformed. The peak of Ca3Co4O9 almost disappeared with increasing the Sb-content and impurity phases, such as Ca3Sb2O4, Sb6O13 and Ca2Sb2O5, were detected. When the ionic radii of Sb3+ and B3+ and Ca2+ is considered (rSb3+ = 0.8 Å, rB3+ = 0.27 Å and rca2+ = 1.00 Å), even small amount of the substitution into the system results in a significant distortion in the main lattice. Therefore, we were not able to calculate the lattice parameters due to this distortion, so it is difficult to make a discussion on occupation of the Sb and B atoms to the atomic site of Ca and the lattice parameters of the substituted samples considering the XRD results.
3. Results and discussion 3.1. Crystal structure X-ray diffraction (XRD) patterns of the samples are shown in Fig. 1a. Unsubstituted sample consisted of pure Ca3Co4O9 phase without any impurity phases as seen the difference plot of Rietveld refinement and peak positions of sublattice-1 (Ca2CoO3) and sublattice-2 (CoO2), Fig. 1b. The unit cell parameters were calculated to be a = 4.812 Å, b1 = 4.56 Å, b2 = 2.812 Å c = 10.791 Å and b = 98° with monoclinic symmetry. In the case of x = 0.5-B substitution,
3.2. First spin state transition Fig. 2 shows temperature dependence of the magnetic susceptibility, v in the temperature range of 2–1000 K. Abrupt change in v was obtained at 380 K, which is attributed to the onset of the spin state transition of the Co-ions. The spin state and leff values of Co in the octahedral environment are given in Fig. 3. Co ions in Ca3Co4O9 can be in valance states; divalent (Co2+), trivalent (Co3+) and tetravalent (Co4+). Masset [10] reported that 32 Co2+ ions, 10 Co3+ ions and 16 Co4+ ions exist in the ideal chemical formula (Ca3Co4O9). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi The theoretical leff value using leff ¼ 4 SðS þ 1Þ=lB was calculated to be leff = 1.44 lB for LS state. The spin state of Co3+ and Co4+ ions changes from LS + IS to IS + HS at 380 K [9–11]. Using susceptibility data in this study, it was found that leff is around 1.38 lB which is compatible with mostly LS and a little bit of LS + IS for all the valence states (Co3+, Co4+) of the Co-ions in the unit cell below 380 K. It should be noted that the population of HS in the high temperature region (>300 K) is strongly related with the sample temperature. 3.3. Second spin state transition? The susceptibility measurements (v–T) in literature are generally performed in the temperature range of 2–400 K [9–11]. It is clear that if the sample is heated to high temperatures, the
Fig. 1. (a) XRD patterns of the samples, and (b) rietveld refinement of XRD pattern. Sublattice-1 and sublattice-2 show Ca2CoO3 and CoO2, respectively.
Fig. 2. Temperature dependence of v of Ca3Co4O9 between 4 and 1000 K. The insets show temperature dependence of v between 0 and 100 K and between 300 and 700 K.
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S. Altin et al. / Journal of Alloys and Compounds 587 (2014) 40–44
Fig. 3. The spin state and leff values of Co in the octahedral environment.
population of HS would increase and so all the HS states for the Co-ions should be populated in a temperature region. In order to obtain this temperature range, the susceptibility measurements (v–T) were carried out at high temperatures (>400 K). As the temperature was increased, the magnetization increased very slowly towards HS magnetization. But, a sudden anomaly is observed between 680 and 920 K. The measurements were repeated three times and the same anomaly in all the measurements was observed. In order to get more information about the spin states of the Co-ions in high temperature region, the magnetization data were fitted to Curie–Weiss equation given by 2 v1 ¼ 3K B ðT hÞN1 0 leff
ð1Þ
where kB is the Boltzmann constant, h the Curie–Weiss constant, N0 Avagadro number and leff the effective magnetic moment [10,13]. If we accept that the Curie Weiss law is valid at the anomaly temperature, the leff value calculated by using Eq. (1) is found to be around 11 lB in this anomaly region which is much larger than that of high spin state of all the Co-ions. It is worth noting that if all the Co ions are considered in the high spin state, the leff value would be around 4.9 lB. Such a large magnetization and thus the leff value are not obtained even by adding some orbital contribution (It should be noted that although the orbital angular momentum is partially quenched in a cubic field, complete quenching occurs very rarely). We believe that this anomaly in the vT curve is not directly related to the spin states of the Co-ions. However, some possible reasons for the magnetic anomaly between 680 and 920 K can be derived: First possible reason can be related to the number of the populated HS states. It seems that there is a critical threshold number of the Co-ions in the high spin state such that when this number is achieved, the magnetic moment, at least, in one Co-site would favor parallel alignment. This magnetic ordering causes a weak spin–spin interaction which leads to an increase on the susceptibility after HS transition. The ordering began at 680 K and reached the maximum at the peak temperature. Another possibility is that if the threshold number is achieved, the system will be in ferrimagnetic state with high spin states, similar to the ferrimagnetic regime having low spin states below 19 K. Although it is expected that the susceptibility value should be constant after the peak value, the structural deformations will occur above the peak temperature as seen in the section
of irreversibility. Since temperature is so high, a complete ordered state cannot be obtained because thermal vibrations break it up. Probably, this is the reason behind the decrease in magnetization around 850 K. Some structural deformations with thermal vibration probably break a complete ordering of the magnetic moments. As can be seen in Fig. 2, this small ordering destroyed. As explained earlier, this tiny structural deformation with thermal vibration becomes dominant at about 900 K and destroyed complete ordering of magnetic moments. It is well known that the magnetic properties of metal–oxides strongly depend on the spin states of metal ions. Therefore, we have also investigated the effects of the substitution of B and Sb into the Co sites on the high temperature magnetic properties. While the starting temperature of the magnetic anomaly for the B-substitution slightly decreased, it did not change by the Sb-substitution. But, it should be noted that there is an increase in magnitude of the susceptibility by the B and Sb substitution at the anomaly temperature, compared to the unsubtituted sample. Since neither B nor Sb is magnetic elements, it is expected that there is no exchange interactions between these dopants and the Co-ions. In both cases, the magnetic spin state transition at 380 K disappeared with increasing the substitution level, inset of Fig. 4a and b, namely, in the B- and Sb-substituted samples, the much weaker spin-state transition near 380 K is concealed by the magnetism of the magnetic impurities in the sample. However, it was seen that this anomaly did not disappear, which is depicted in Fig. 4. In addition, the calculated leff with the substitution of B and Sb increased, compared to that of the unsubstituted Ca3Co4O9. It was found that the Sb-substituted Ca3Co4O9 sample showed the highest leff value. The results clearly suggest that both B and Sb-substitutions promote more Co ions to high spin states. It is clear that since this weak ordering occurs in high temperature region, this behavior would find some technological usages. More detailed studies with other dopants should be done for future work. Moubah et al. reported that CaCo2O4impurity phase which is not detectable with XRD analysis can present in the Ca3Co4O9 system [15]. Due to small amount of CaCo2O4, we believe that its contribution to the magnetic susceptibility can be negligible. The samples with B and Sb-substitutions consisted of impurity phases such as Co3O4. The spin state of the Co ions in Co3O4 is expected to be IS + LS state at low temperatures and changes slowly from IS + HS to HS with increasing the temperature. It is thought that
S. Altin et al. / Journal of Alloys and Compounds 587 (2014) 40–44
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Fig. 5. Temperature dependence of v of Ca3Co4O9 between 300 and 1000 K while heating and cooling.
Fig. 4. Temperature dependence of v of (a) the B-substituted Ca3Co4O9 and (b) Sbsubstituted Ca3Co4O9 between 4 and 1000 K. The insets show temperature dependence of v between 300 and 500 K for both samples.
when the number of HS is reached a threshold number, a weak spin–spin interaction takes place and causes a weak ordering. Therefore, Co3O4 may contribute to the susceptibility but since any impurity phases were not observed in the unsubstituted sample, it can be concluded that this contribution cannot be as much as one observed in the anomaly region. This may be one of the reasons behind high magnitude of magnetic anomaly in the B and Sb-substituted samples, compared to the magnetic anomaly in the unsubstituted sample. We are not ruling out the other probabilities here, such as contributions of other impurity phases. But, we believe that a big increase in the susceptibility may not be due to the impurity phases. 3.4. Irreversibility The magnetic data measured during heating and cooling is given in Fig. 5. It is obvious that the ordered phase is not observed while cooling the sample. This is not a surprised observation: When the sample is heated up to 1000 K in vacuum for the magnetic measurements, the structural deformation occurred considerably, as seen in Fig. 6. We believe that the structural deformation may be due to oxygen loss in the system. It is reported previously that the oxygen content in the Ca3Co4O9 system decreases at temperatures above 570 K in the nitrogen atmosphere [14]. Possibly, the exchange path for Co(III) and Co(IV) due to the structural deformation disappeared during the cooling, Fig. 5. The leff calculated from the d v1/dT data is consistent with high-spin states of Co(III) and Co(IV) in the temperature range of 600–1000 K.
Fig. 6. XRD pattern of the Ca3Co4O9 sample before and after high temperature magnetic measurement.
There is a gradual decrease in the magnetization between 580 and 550 K, which may be attributed to a spin-crossover for both Co(III) and Co(IV) ions from high-spin to intermediate-spin. We assume that the calculated leff fluctuates around the same value till 300 K. Since there is a structural deformation considerably, it is not worthy to give up the interpretation further.
4. Conclusion In this study, the high temperature magnetic properties of misfit layered Ca3Co4O9 has been investigated. The detailed analysis on v (T) and leff exhibited that as the temperature increased, the magnetization suddenly increased at around 680 K. This anomaly was also investigated for the B and Sb-substitutions for Ca in Ca3Co4O9. It is believed that this anomaly is somehow related to a threshold number of the high spin Co-ions. If the population of the Co-ions is achieved to this threshold number, an exchange interaction between Co3+ and Co4+ ions takes place. It is believed that although the population of HS states increases with increasing temperature, the thermal vibrations together with the structural deformation break a complete ordering of magnetic moments. This can originate from the reason behind the decrease in magnetization at about 850 K. The system behaves as paramagnetic above 900 °C. It should be noted that since the sample used in the present study is polycrystalline, its response to the external magnetic field would be different than the single crystalline materials. It is believed that
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S. Altin et al. / Journal of Alloys and Compounds 587 (2014) 40–44
the increase in magnetization in the anomaly region for the single crystalline materials would be much larger than that of our results. Effect of both B and Sb-substitutions for Ca on the high temperature magnetic properties in Ca3Co4O9 has also been investigated. The magnetic results of both B and Sb-substituted samples showed that the magnetic anomaly obtained at high temperatures did not disappear. It was found that the Sb-substituted Ca3Co4O9 sample gives the highest leff value. It is obvious from the results that both B and Sb-substitutions promote more Co-ions to high spin. The ordered phase was not obtained during cooling of the sample from 1000 K to 300 K. When the sample is heated to 1000 K for the magnetic measurement in vacuum atmosphere, a structural deformation in the Ca3Co4O9 system takes place and therefore the oxygen loss. References [1] S. Pinitsoontorn, N. Lerssongkram, A. Harnwunggmoung, K. Kurosaki, S. Yamanaka, J. Alloys Comp. 503 (2010) 431–435. [2] F. Delorme, C.F. Martin, P. Marudhachalam, D.O. Ovono, G. Guzman, J. Alloys Comp. 509 (2011) 2311–2315.
[3] Y. Wang, Y. Sui, J. Cheng, X. Wang, W. Su, J. Alloys Comp. 477 (2009) 817–821. [4] R. Moubah, S. Colis, C. Leuvrey, G. Schmerber, M. Drillon, A. Dinia, Thin Solid Films 518 (2010) 4546–4548. [5] Y. Huang, B. Zhao, R. Ang, S. Lin, Z. Huang, L. Yin, S. Tan, Y. Liu, W. Song, Y. Sun, J. Amer. Ceram. Soc. 1–7 (2012). [6] Y. Wang, Y. Sui, P. Ren, L. Wang, X. Wang, W. Su, H. Fan, Chem. Mater. 22 (2010) 1155–1163. [7] J. Sugiyama, J.H. Brewer, E.J. Ansaldo, H. Itahara, K. Dohmae, Y. Seno, C. Xia, T. Tani, Phys. Rev. B 68 (2003) 134423. [8] Y. Wang, Y. Sui, J. Cheng, X. Wang, W. Su, X. Liu, H.J. Fan, J. Phys. Chem. C 114 (2010) 5174–5181. [9] Y. Huang, B. Zhao, X. Hu, S. Lin, R. Ang, W. Song, Y. Sun, Dalton Trans. 41 (2012) 11176–11186. [10] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166–174. [11] R. Moubah, S. Colis, C. Ulhaq-Bouillet, G. Schmerber, N. Viart, M. Dirillon, A. Dinia, D. Muller, J.J. Grob, Eur. Phys. J. B 66 (2008) 315–319. [12] J. Sugiyama, J.H. Brewer, E.J. Ansaldo, H. Itahara, M. Bayer, T. Tani, Physica B 326 (2003) 518–521. [13] B.C. Zhao, Y.P. Sun, W.J. Lu, X.B. Zhu, W.H. Song, Phys. Rev. B 74 (2006) 144417. [14] Y. Morita, J. Poulsen, K. Sakai, T. Motohashi, T. Fujii, I. Terasaki, H. Yamauchi, M. Karppinen, J. Solid State. Chem. 177 (2004) 3149–3155. [15] R. Moubah, S. Colis, C. Ulhaq-Bouillet, A. Dinia, Appl. Phys. Lett. 96 (2010) 041902.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
High temperature spin state transitions in misfit-layered Ca3Co4O9 S. Altin, M.A. Aksan ⇑, A. Bayri Inonu Universitesi, Fen Edebiyat Fakultesi, Fizik Bolumu, 44280 Malatya, Turkey
a r t i c l e
i n f o
Article history: Received 23 June 2013 Received in revised form 21 October 2013 Accepted 22 October 2013 Available online 30 October 2013 Keywords: Spin crossover Magnetic oxides Ferrimagnetics Magnetic ordering spin arrangements
a b s t r a c t This study reports high temperature magnetic properties of the unsubstituted one together with B and Sb-substituted Ca3Co4O9 system. The measured data indicated that there is an anomaly in the magnetic susceptibility, v, between 680 and 920 K. It is believed that this anomaly is related to a critical threshold number of the high spin Co-ions such that when this threshold number is achieved, some exchange interactions between Co3+ and Co4+ take place which causes an abrupt increase in the v–T curve. The anomaly was further investigated with B and Sb-substitutions. It is realized that both dopants promote more Coions in the rock salt unit cell to high spin state. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, thermoelectric oxide materials such as NaxCO2, Bi2Sr2Co2O9, Ca3Co4O9, have attracted considerable attention due to their good thermal and chemical stability at high temperatures (>300 K) compared to intermetallic thermoelectric compounds such as Bi2Te3 and Bi2Se3. [1]. However, the application of NaxCO2, Bi2Sr2Co2O9 to the power generation is limited because of the volatility of Na and Bi at high temperatures [2]. Therefore, Ca3Co4O9 is a better candidate at high temperatures than other thermoelectric oxide materials. Crystal structure of Ca3Co4O9 consists of misfit layered structure stacking along the c-axis and two monoclinic subsystems; one is CdI2-type CoO2 layer and the other is rock-salt type Ca2CoO3 layer [3]. These two subsystem have similar unit cell parameters (a = 4.83 Å and c = 10.83 Å) but b1 = 4.55 Å for Ca2CoO3 and b2 = 2.81 Å for CoO2 [4]. The CoO2 subsystem is mainly responsible for the electronic and magnetic properties of the system, whereas Ca2CoO3 layer acts as a charge reservoir which supply charge carriers into the CoO2 subsystem. Substitutions/dopings to Ca3Co4O9 have an important effect on transport and magnetic properties of the system. The reason of good transport properties of Ca3Co4O9 at high temperatures is not exactly clear but it is suggested that strong electron correlation and spin-entropy relation may be responsible for the transport properties. Ca3Co4O9 offers interesting magnetic properties as well as its good transport properties. The system displays three different magnetic transitions at 380 K, 27 K and 19 K: spin state transition of the Co-ions at 380 K, long-range incommensurate spin-density ⇑ Corresponding author. Tel.: +90 422 3773720; fax: +90 422 341 03 19. E-mail address: [email protected] (M.A. Aksan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.170
wave (IC-SDW) arrangement at 27 K and ferrimagnetic transition at 19 K, respectively [5,6]. A short-range IC-SDW arrangement appears with decreasing the temperature down to 100 K which is accompanied by a broad minimum near 80 K in q–T curve [7]. The structure of IC-SDW is not clear but it is assumed that ICSDW may originate from the half-spin (s = 1/2) in the 2D-triangular lattice of the Co-ions in the CoO2 layer. Long-range IC-SDW arrangement is completed at 27 K and the system is in ferrimagnetic state at 19 K. It is believed that the ferrimagnetism arises from interlayer coupling of the spins of the Co-ions between Ca2CoO3 and CoO2 subsystems [8]. The magnetic transition at 380 K is attributed to the spin-state transition of the Co-ions: the spin state of the Co3+ (3d6) and Co4+ (3d5) ions changes from low temperature-low spin state + intermediate spin state (LS + IS) below 380 K to high temperature-intermediate spin state + high spin state (IS + HS) above 380 K [9,10]. This spin-state transition corresponds to a broad maximum near 400 K in q–T curve [11]. Co2+ ions have always the low spin state configuration. It is well known that 3d4–3d7 ions may have low spin, intermediate and high spin configuration depending on both environment and crystal field splitting strength. l+SR measurements indicated that the spin-state gradually changes above 400 K [7,12]. It is more probable that if LS and IS exist in low temperatures, a high spin state should be observed in the high temperature regime. Any information on the magnetic behavior above 400 K in Ca3Co4O9 does not exist in literature. Aim of the study is to investigate the substitution effect (especially changes in the magnetic properties) on the Co sites by substituting different element for Ca in the Ca3Co4O9 system. In this study, the magnetic behavior in the unsubstituted Ca3Co4O9 and the B and Sb-substituted Ca3Co4O9 has been investigated up to 1000 K. In order to observe a possible HS for all the Co ions, we performed the DC magnetic
S. Altin et al. / Journal of Alloys and Compounds 587 (2014) 40–44
susceptibility, v, measurement up to 1000 K and investigated the magnetic properties of the samples fabricated. 2. Experimental (Ca3xMx)Co4O9 (where M = B and Sb and x = 0.0, 0.5, 1.0) samples were fabricated using conventional solid-state reaction technique. High purity powders of Ca3CO3, CoO, Sb2O3 and B2O3 were weighted in the appropriate amounts to give nominal compositions. The powders were mixed in an agate mortar and then sintered at 900 °C for 24 h with intermediate grinding and mixing. After sintering, the powders were pressed into pellets under pressure of 4 tons. The pellets were heat treated at 900 °C for 36 h under oxygen atmosphere. The structural characterization of the samples fabricated was investigated by Xray diffraction (XRD). Automated Rigaku RadB Dmax X-ray diffractometer having Cu Ka radiation was used. Scan speed was selected as 2° min1 in the range of 2h = 3–80°. The lattice parameters of the unsubstituted sample were calculated using Rietveld Refinement technique using Jade 5.0 Software. Temperature dependence of DC-magnetization between 2 and 1000 K under magnetic field H = 2 kOe were measured using Quantum Design PPMS system with VSM and oven attachments in vacuum atmosphere.
41
the main phase was obtained to be Ca3Co4O9, but some impurity phases, such as Co3O4 and Co3(BO3)2, were also detected. In addition, the peak intensity of the main phase decreased compared to the unsubstituted sample. When the substitution level was increased to x = 1.0, the peaks of the main phase, Ca3Co4O9, suppressed and many impurity phases such as Co3O4, Co3(BO3)2, CaB2O4, were observed in the sample. When Sb was substituted into the system (x = 0.5 and 1.0), the crystal structure of Ca3Co4O9 was found to be highly deformed. The peak of Ca3Co4O9 almost disappeared with increasing the Sb-content and impurity phases, such as Ca3Sb2O4, Sb6O13 and Ca2Sb2O5, were detected. When the ionic radii of Sb3+ and B3+ and Ca2+ is considered (rSb3+ = 0.8 Å, rB3+ = 0.27 Å and rca2+ = 1.00 Å), even small amount of the substitution into the system results in a significant distortion in the main lattice. Therefore, we were not able to calculate the lattice parameters due to this distortion, so it is difficult to make a discussion on occupation of the Sb and B atoms to the atomic site of Ca and the lattice parameters of the substituted samples considering the XRD results.
3. Results and discussion 3.1. Crystal structure X-ray diffraction (XRD) patterns of the samples are shown in Fig. 1a. Unsubstituted sample consisted of pure Ca3Co4O9 phase without any impurity phases as seen the difference plot of Rietveld refinement and peak positions of sublattice-1 (Ca2CoO3) and sublattice-2 (CoO2), Fig. 1b. The unit cell parameters were calculated to be a = 4.812 Å, b1 = 4.56 Å, b2 = 2.812 Å c = 10.791 Å and b = 98° with monoclinic symmetry. In the case of x = 0.5-B substitution,
3.2. First spin state transition Fig. 2 shows temperature dependence of the magnetic susceptibility, v in the temperature range of 2–1000 K. Abrupt change in v was obtained at 380 K, which is attributed to the onset of the spin state transition of the Co-ions. The spin state and leff values of Co in the octahedral environment are given in Fig. 3. Co ions in Ca3Co4O9 can be in valance states; divalent (Co2+), trivalent (Co3+) and tetravalent (Co4+). Masset [10] reported that 32 Co2+ ions, 10 Co3+ ions and 16 Co4+ ions exist in the ideal chemical formula (Ca3Co4O9). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi The theoretical leff value using leff ¼ 4 SðS þ 1Þ=lB was calculated to be leff = 1.44 lB for LS state. The spin state of Co3+ and Co4+ ions changes from LS + IS to IS + HS at 380 K [9–11]. Using susceptibility data in this study, it was found that leff is around 1.38 lB which is compatible with mostly LS and a little bit of LS + IS for all the valence states (Co3+, Co4+) of the Co-ions in the unit cell below 380 K. It should be noted that the population of HS in the high temperature region (>300 K) is strongly related with the sample temperature. 3.3. Second spin state transition? The susceptibility measurements (v–T) in literature are generally performed in the temperature range of 2–400 K [9–11]. It is clear that if the sample is heated to high temperatures, the
Fig. 1. (a) XRD patterns of the samples, and (b) rietveld refinement of XRD pattern. Sublattice-1 and sublattice-2 show Ca2CoO3 and CoO2, respectively.
Fig. 2. Temperature dependence of v of Ca3Co4O9 between 4 and 1000 K. The insets show temperature dependence of v between 0 and 100 K and between 300 and 700 K.
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Fig. 3. The spin state and leff values of Co in the octahedral environment.
population of HS would increase and so all the HS states for the Co-ions should be populated in a temperature region. In order to obtain this temperature range, the susceptibility measurements (v–T) were carried out at high temperatures (>400 K). As the temperature was increased, the magnetization increased very slowly towards HS magnetization. But, a sudden anomaly is observed between 680 and 920 K. The measurements were repeated three times and the same anomaly in all the measurements was observed. In order to get more information about the spin states of the Co-ions in high temperature region, the magnetization data were fitted to Curie–Weiss equation given by 2 v1 ¼ 3K B ðT hÞN1 0 leff
ð1Þ
where kB is the Boltzmann constant, h the Curie–Weiss constant, N0 Avagadro number and leff the effective magnetic moment [10,13]. If we accept that the Curie Weiss law is valid at the anomaly temperature, the leff value calculated by using Eq. (1) is found to be around 11 lB in this anomaly region which is much larger than that of high spin state of all the Co-ions. It is worth noting that if all the Co ions are considered in the high spin state, the leff value would be around 4.9 lB. Such a large magnetization and thus the leff value are not obtained even by adding some orbital contribution (It should be noted that although the orbital angular momentum is partially quenched in a cubic field, complete quenching occurs very rarely). We believe that this anomaly in the vT curve is not directly related to the spin states of the Co-ions. However, some possible reasons for the magnetic anomaly between 680 and 920 K can be derived: First possible reason can be related to the number of the populated HS states. It seems that there is a critical threshold number of the Co-ions in the high spin state such that when this number is achieved, the magnetic moment, at least, in one Co-site would favor parallel alignment. This magnetic ordering causes a weak spin–spin interaction which leads to an increase on the susceptibility after HS transition. The ordering began at 680 K and reached the maximum at the peak temperature. Another possibility is that if the threshold number is achieved, the system will be in ferrimagnetic state with high spin states, similar to the ferrimagnetic regime having low spin states below 19 K. Although it is expected that the susceptibility value should be constant after the peak value, the structural deformations will occur above the peak temperature as seen in the section
of irreversibility. Since temperature is so high, a complete ordered state cannot be obtained because thermal vibrations break it up. Probably, this is the reason behind the decrease in magnetization around 850 K. Some structural deformations with thermal vibration probably break a complete ordering of the magnetic moments. As can be seen in Fig. 2, this small ordering destroyed. As explained earlier, this tiny structural deformation with thermal vibration becomes dominant at about 900 K and destroyed complete ordering of magnetic moments. It is well known that the magnetic properties of metal–oxides strongly depend on the spin states of metal ions. Therefore, we have also investigated the effects of the substitution of B and Sb into the Co sites on the high temperature magnetic properties. While the starting temperature of the magnetic anomaly for the B-substitution slightly decreased, it did not change by the Sb-substitution. But, it should be noted that there is an increase in magnitude of the susceptibility by the B and Sb substitution at the anomaly temperature, compared to the unsubtituted sample. Since neither B nor Sb is magnetic elements, it is expected that there is no exchange interactions between these dopants and the Co-ions. In both cases, the magnetic spin state transition at 380 K disappeared with increasing the substitution level, inset of Fig. 4a and b, namely, in the B- and Sb-substituted samples, the much weaker spin-state transition near 380 K is concealed by the magnetism of the magnetic impurities in the sample. However, it was seen that this anomaly did not disappear, which is depicted in Fig. 4. In addition, the calculated leff with the substitution of B and Sb increased, compared to that of the unsubstituted Ca3Co4O9. It was found that the Sb-substituted Ca3Co4O9 sample showed the highest leff value. The results clearly suggest that both B and Sb-substitutions promote more Co ions to high spin states. It is clear that since this weak ordering occurs in high temperature region, this behavior would find some technological usages. More detailed studies with other dopants should be done for future work. Moubah et al. reported that CaCo2O4impurity phase which is not detectable with XRD analysis can present in the Ca3Co4O9 system [15]. Due to small amount of CaCo2O4, we believe that its contribution to the magnetic susceptibility can be negligible. The samples with B and Sb-substitutions consisted of impurity phases such as Co3O4. The spin state of the Co ions in Co3O4 is expected to be IS + LS state at low temperatures and changes slowly from IS + HS to HS with increasing the temperature. It is thought that
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Fig. 5. Temperature dependence of v of Ca3Co4O9 between 300 and 1000 K while heating and cooling.
Fig. 4. Temperature dependence of v of (a) the B-substituted Ca3Co4O9 and (b) Sbsubstituted Ca3Co4O9 between 4 and 1000 K. The insets show temperature dependence of v between 300 and 500 K for both samples.
when the number of HS is reached a threshold number, a weak spin–spin interaction takes place and causes a weak ordering. Therefore, Co3O4 may contribute to the susceptibility but since any impurity phases were not observed in the unsubstituted sample, it can be concluded that this contribution cannot be as much as one observed in the anomaly region. This may be one of the reasons behind high magnitude of magnetic anomaly in the B and Sb-substituted samples, compared to the magnetic anomaly in the unsubstituted sample. We are not ruling out the other probabilities here, such as contributions of other impurity phases. But, we believe that a big increase in the susceptibility may not be due to the impurity phases. 3.4. Irreversibility The magnetic data measured during heating and cooling is given in Fig. 5. It is obvious that the ordered phase is not observed while cooling the sample. This is not a surprised observation: When the sample is heated up to 1000 K in vacuum for the magnetic measurements, the structural deformation occurred considerably, as seen in Fig. 6. We believe that the structural deformation may be due to oxygen loss in the system. It is reported previously that the oxygen content in the Ca3Co4O9 system decreases at temperatures above 570 K in the nitrogen atmosphere [14]. Possibly, the exchange path for Co(III) and Co(IV) due to the structural deformation disappeared during the cooling, Fig. 5. The leff calculated from the d v1/dT data is consistent with high-spin states of Co(III) and Co(IV) in the temperature range of 600–1000 K.
Fig. 6. XRD pattern of the Ca3Co4O9 sample before and after high temperature magnetic measurement.
There is a gradual decrease in the magnetization between 580 and 550 K, which may be attributed to a spin-crossover for both Co(III) and Co(IV) ions from high-spin to intermediate-spin. We assume that the calculated leff fluctuates around the same value till 300 K. Since there is a structural deformation considerably, it is not worthy to give up the interpretation further.
4. Conclusion In this study, the high temperature magnetic properties of misfit layered Ca3Co4O9 has been investigated. The detailed analysis on v (T) and leff exhibited that as the temperature increased, the magnetization suddenly increased at around 680 K. This anomaly was also investigated for the B and Sb-substitutions for Ca in Ca3Co4O9. It is believed that this anomaly is somehow related to a threshold number of the high spin Co-ions. If the population of the Co-ions is achieved to this threshold number, an exchange interaction between Co3+ and Co4+ ions takes place. It is believed that although the population of HS states increases with increasing temperature, the thermal vibrations together with the structural deformation break a complete ordering of magnetic moments. This can originate from the reason behind the decrease in magnetization at about 850 K. The system behaves as paramagnetic above 900 °C. It should be noted that since the sample used in the present study is polycrystalline, its response to the external magnetic field would be different than the single crystalline materials. It is believed that
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the increase in magnetization in the anomaly region for the single crystalline materials would be much larger than that of our results. Effect of both B and Sb-substitutions for Ca on the high temperature magnetic properties in Ca3Co4O9 has also been investigated. The magnetic results of both B and Sb-substituted samples showed that the magnetic anomaly obtained at high temperatures did not disappear. It was found that the Sb-substituted Ca3Co4O9 sample gives the highest leff value. It is obvious from the results that both B and Sb-substitutions promote more Co-ions to high spin. The ordered phase was not obtained during cooling of the sample from 1000 K to 300 K. When the sample is heated to 1000 K for the magnetic measurement in vacuum atmosphere, a structural deformation in the Ca3Co4O9 system takes place and therefore the oxygen loss. References [1] S. Pinitsoontorn, N. Lerssongkram, A. Harnwunggmoung, K. Kurosaki, S. Yamanaka, J. Alloys Comp. 503 (2010) 431–435. [2] F. Delorme, C.F. Martin, P. Marudhachalam, D.O. Ovono, G. Guzman, J. Alloys Comp. 509 (2011) 2311–2315.
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