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Platelet crystals of thermoelectric layered cobaltites; pure and Sr-doped [Ca2CoO3]0.62[CoO2]
ARTICLE IN PRESS
Journal of Crystal Growth 276 (2005) 519–524 www.elsevier.com/locate/jcrysgro
Platelet crystals of thermoelectric layered cobaltites; pure and Sr-doped ½Ca2CoO30:62½CoO2 Changtai Xia1, Jun Sugiyama, Hiroshi Itahara, Toshihiko Tani Toyota Central Research and Development Laboratories Inc., Nagakute, Aichi, 480-1192, Japan Received 7 October 2004; accepted 24 November 2004 Communicated by K. Sato Available online 28 January 2005
Abstract Single crystal plates ð 5 5 0:1 mm3 Þ of pure and Sr-doped ½Ca2 CoO3 0:62 ½CoO2 have been grown in a SrCl2 flux. The ratio between the flux and raw materials strongly affected the composition of the grown crystals; that is, Sr ions were doped into the Ca site in the lattice when the flux-rich melt was used. It is found that the Sr-doping suppresses the ferrimagnetic transition temperature and decreases the magnitude of thermoelectric power coefficient, while the valence of Sr2þ ions is equal to that of Ca2þ ions. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.h; 61.10.Nz; 72.20.Pa; 75.50.Gg Keywords: A1. Doping; A2. Growth from melt; B1. Oxides; B2. Thermoelectrics
1. Introduction The layered cobaltite, ½Ca2 CoO3 RS 0:62 ½CoO2 [1–3] (RS denotes a rocksalt-type subsystem) exhibits metallic conductivities and extraordinarily large thermoelectric power coefficients S (above þ150 mV=K at 300 K) simultaneously, for reasons Corresponding author. Tel.: +81 561 63 6196;
fax: +81 561 63 6156. E-mail address: [email protected] (J. Sugiyama). 1 Present address: Shanghai Institute of Optics and Fine Mechanics.
currently not fully understood. Because of its large thermoelectric figure of merit (ZT 1 at 1000 K), ½Ca2 CoO3 RS 0:62 ½CoO2 is considered to be a promising candidate for a p-type material component of thermoelectric power generation systems. The crystal structure of ½Ca2 CoO3 RS 0:62 ½CoO2 consists of alternating stacks of two monoclinic subsystems along the c-axis [2–5]. The two subsystems are (1) triple rocksalt-type ½Ca2 CoO3 subsystem; and (2) single CdI2-type [CoO2] subsystem. Both subsystems have identical a, c and b parameters but different b parameters. The monoclinic ½Ca2 CoO3 RS 0:62 ½CoO2 belongs to the space
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.415
ARTICLE IN PRESS C. Xia et al. / Journal of Crystal Growth 276 (2005) 519–524
520
( ( group C m (a ¼ 4:834 A; b1 ¼ 4:554 A; b2 ¼
( ( 2:798 A; c ¼ 10:832 A and b ¼ 98:13 ) [4]. There is an incommensurate spatial modulation along the b-axis caused by a misfit between the two subsystems. Because of its layered structure, the physical properties of ½Ca2 CoO3 RS 0:62 ½CoO2 are expected to be anisotropic. Indeed, several workers reported that both resistivity (r) and susceptibility ðwÞ exhibit a large anisotropy [1,2,6]. That is, rab 10rc at 300 K, where rab represents the resistivity in the ab plane and rc the resistivity along the caxis [2]. Such anisotropy leads to the problem how to obtain a large single crystal or a large aligned polycrystal for the use in thermoelectric power generation systems at high temperatures. In particular, platelet crystals are necessary for templates in the preparation of the aligned polycrystal. Furthermore, there are many reports on the doped ½Ca2 CoO3 RS 0:62 ½CoO2 to improve its thermoelectric properties [7–13], although, to authors’ knowledge, all efforts were failed to achieve a drastic increase in ZT. It is worth noting that almost all of the previous experiments for the doped cobaltite were carried out using polycrystalline samples. Hence, a possible segregation of the dopant at grain boundaries could increase a magnitude of r; even if the dopant looks to be soluble in the ½Ca2 CoO3 RS 0:62 ½CoO2 lattice. Therefore, in order to clarify the effect of dopants in ½Ca2 CoO3 RS 0:62 ½CoO2 ; we have performed the single crystal growth reported here. Also, we have measured the physical properties of the crystals obtained here to know the nature of the doping into the cobaltite.
2. Experimental procedure Single crystals of ½Ca2 CoO3 RS 0:62 ½CoO2 were prepared using a modified SrCl2-flux technique as reported by Shikano et al. [14]. Reagent-grade CaCO3, Co3O4 and SrCl2 powders or calcined ½Ca2 CoO3 RS 0:62 ½CoO2 and SrCl2 powders were thoroughly mixed by a ball mill. The mixed powders were packed into a Pt crucible, the crucible was heated and kept at around 1200 K, and then furnace-cooled down to 400 K slowly. The conditions we employed are summarized in Table 1. The grown crystals were washed several times in hot water to remove the SrCl2 flux. The obtained crystals were thin platelets with typical dimensions of 5 5 0:1 mm3 : The platelets were then annealed at 723 K in an oxygen gas flow for 12 h and then furnace-cooled down to room temperature at a rate of 1 K/min to remove a possible oxygen deficiency [15]. A crystal structure of the sample was determined by an X-ray powder diffraction (XRD) with Cu-Ka radiation. For thin plate crystals, they were adhered to a glass sample holder with grease. The accurate lattice parameters were determined by pulverizing crystal to fine powders and then measure an XRD pattern. An X-ray diffraction study indicated that the growth surface of the platelets was the ab plane. Both optical microscope and SEM observations showed that the platelet was a stack of several pieces of small platelets. In other words, the platelets were a c-aligned stack along the c-axis of randomly rotated ab planes. The magnetic properties were measured using a SQUID magnetometer (mpms, Quantum Design)
Table 1 Crystal growth conditions employed in this report; P denotes precursor, F SrCl2 flux, T max the highest temperature hold the Pt crucible. Run No.
P
P:F (w:w)
T max (K)
Cooling rate (K/h)
1 2
CaCO3 þ Co3 O4 [Ca2 CoO3 RS 0:62 ½CoO2
1:2 1:4
1173 K for 3 h 1173 K for 0.1 h
3
CaCO3 þ Co3 O4
1:1
1173 K for 10 h
5.5 (to 633 K) 1 (to 1074 K) 6 (to 773 K) 1 (to 1074 K) 6 (to 773 K)
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in the temperature range between 400 and 5 K under magnetic field Hp55 kOe: In order to increase the magnetic signal, 5 platelets were stacked in a plastic sample holder. Also, to determine anisotropy, H was applied parallel or perpendicular to the ab plane. Hereby, we will abbreviate susceptibility obtained with H ? ab as wc and H==ab as wab ; respectively. Thermoelectric power coefficients (S) were measured using a parametric double reference measurement technique (SB-100, MMR Technologies) in the temperature range between 90 and 450 K. In this technique, two pairs of thermocouples are used; one pair is copper vs. constantan and the other copper vs. sample. The typical temperature gradient in the sample was kept 2:5 K in the whole temperature range.
3. Results and discussion 3.1. Structural and compositional analyses Table 2 gives the refined lattice parameters of polycrystalline precursor and pulverized crystals. As we can see from Table 1, crystal 1 has almost the same lattice parameters as those of the pure polycrystalline ½Ca2 CoO3 RS 0:62 ½CoO2 : On the other hand, the cell volume of crystal 2 was significantly enlarged, which is consistent with the fact that a significant amount of Sr was detected in crystal 2 by the ICP analysis. This is because the ionic ( is larger than that of radius of Sr2þ ions (1.13 A) 2þ ˚ Ca (0.99 A) [16]. Table 3 gives the ICP analysis result of grown crystals. The composition of the grown crystal was found to strongly depend on the ratio between the
521
SrCl2 flux and the precursor. Almost pure crystals were obtained from a melt mixture with a low flux to precursor ratio. On the other hand, Sr in the flux entered into the Ca site in ½Ca2 CoO3 RS 0:62 ½CoO2 ; when we used a high flux to precursor ratio. 3.2. Magnetic and transport properties Figs. 1(a)–(c) show the temperature dependences of w for the three single crystals grown in the present work. As temperature decreases from 80 K, both wab and wc increase for all samples, indicating a Curie–Weiss behavior of the Co ions in the lattice. Below 19 K, the wc ðTÞ curves for the No. 1 and 3 crystals exhibit a rapid increase with further decreasing T due to a ferrimagnetic transition (T FR ¼ 19 K) [6,7]. On the other hand, the wc ðTÞ curve for the No. 2 crystal lacks the divergence below 19 K, although the thermal hysteresis (i.e., the difference between the data obtained in the zero field-cooling (ZFC) mode and in the field-cooling (FC) mode) is observed below 14 K, suggesting the existence of a magnetic transition below 14 K. Our w measurements [7] on the polycrystalline samples indicated that the ferrimagnetic transition is sensitive to the substitu-
Table 3 ICP-AES results of elemental analysis (molar ratio) in the grown crystals Sample Precursor
Co 4
Ca 3.04
Sr 0
Crystal 1 Crystal 2 Crystal 3
4 4 4
2.84 2.84 2.96
0.08 0.21 0.06
Table 2 Refined lattice parameters of polycrystalline precursor and pulverized crystals
Precursor for 2nd run
( a ðAÞ 4.834(3)
( b ðAÞ 4.557(3)
( c ðAÞ 10.839(7)
98.14(4)
( Þ V ðA 236.466(2)
Crystal 1 Crystal 2 Crystal 3
4.832(1) 4.839(3) 4.837(2)
4.558(1) 4.582(4) 4.558(3)
10.846(3) 10.873(7) 10.855(4)
98.01(2) 98.18(3) 98.15(3)
236.593(1) 238.674(2) 236.903(1)
Sample
b ðo Þ
3
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χ (10-3 emu/g)
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No.1, H=10 kOe ZFC FC
0.2
0.1
χc
(a)
0
χ (10-3 emu/g)
χab
0.2
No.2, H=10 kOe ZFC FC
χc
0.1
(b)
0
χ (10-3 emu/g)
χab
0.2
No.3, H=10 kOe ZFC
χc
0.1 χab 0 0
(c)
20
40
60
80
TEMPERATURE (K)
Fig. 1. Temperature dependences of magnetic susceptibility w for single crystal plates obtained from (a) run No.1, (b) run No.2 and (c) run No.3. w was measured both in the zero fieldcooling (ZFC) mode and the field-cooling (FC) mode with H ¼ 10 kOe; here, H was applied parallel or perpendicular to the ab plane.
tion for Ca by Sr and T FR decreases below 5 K even for the ½Ca2x Srx CoO3 RS y ½CoO2 sample with x ¼ 0:067: We, therefore, conclude that the No. 2 crystal is the Sr-doped ½Ca2 CoO3 RSRS 0:62 ½CoO2 ; whereas the No. 1 and 3 are the pure phase. The
magnetic transition below 14 K in the Sr-doped sample is most likely to correspond to the ferrimagnetic transition, which is suppressed by the lightly Sr-doping. According to muon spin rotation and relaxation ðmSRÞ experiments on the pure and Sr-doped samples [7,17,18], a long-range incommensurate spin density wave (IC-SDW) order is observed below 27 K ð¼ T end SDW Þ for the pure sample and below 40 K for the ½Ca1:8 Sr0:2 CoO3 RS y ½CoO2 sample. Actually, a shoulder associated with the formation of the long-range IC-SDW order is clearly seen only in the wc ðTÞ curve at 27 K for the No. 1 and 3 crystals, whereas no shoulder around 27 K for the No. 2 crystal. This is also consistent with the above conclusion; that is, the No. 2 crystal is the Sr-doped phase, but No. 1 and 3 are the pure phase. Fig. 2 shows the temperature dependences of S for the No. 1 and 2 crystals, i.e., the pure and Srdoped crystals. Since the thermoelectric voltage, which is induced by the temperature gradient in the a2b plane, was measured in the a2b plane, the present S denotes Sab : As T increases from 90 K, S for the pure crystal increases with decreasing its slope (dS/dT), and roughly levels off to a constant value ( 200 mVK1 ) above 250 K: The SðTÞ curve for the Sr-doped crystal looks similar to that for the pure crystal, although the Sr-doping decreases the magnitude of S by 5–10 mVK1 in the whole temperature range measured. Since Miyazaki et al. [9] reported the decrease in S by the Sr-doping using the polycrystalline samples, the present S measurement also confirms the conclusion that the No. 2 crystal is the Sr-doped ½Ca2 CoO3 RS 0:62 ½CoO2 : Interestingly, the lightly Sr-doping suppresses both the ferrimagnetic transition temperature T FR and the magnitude of S, as reported for polycrystalline samples [7,9]. The structural analysis suggests that the doped Sr2þ ions locate at the Ca2þ site in the ½Ca2 CoO3 subsystem. Also, the substitution of Ca2þ by Sr2þ is found to increase mainly the volume of the rocksalt-type subsystem. The incommensurate modulation period y (¼ 0:62 for the pure compound) hence decreases to match the two subsystems in the lattice [19]. Assuming that such substitution does not alter the valence of
ARTICLE IN PRESS C. Xia et al. / Journal of Crystal Growth 276 (2005) 519–524
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discrepancy based only on the present result, further works, i.e., the single crystal growth with larger size, are necessary to clarify the anisotropy in S.
300 pure & Sr-doped [Ca2CoO3]0.62[CoO2] S (µV/K)
200
4. Summary 100 No.1, TFR=19 K No.2, para above 5 K 0 0
100
200
300
400
500
TEMPERATURE (K)
Fig. 2. Temperature dependences of thermoelectric power coefficient S for single crystal plates obtained from (solid circles) run No.1 and (open circles) run No.2. The No.1 crystal exhibits a ferrimagnetic transition at 19 K, while the No.2 paramagnetic down to 5 K.
Co ions ( 3þ) in the rocksalt-type subsystem, the decrease in y could increase the valence of Co ions in the CoO2 plane, as well as the decrease in y for Nay CoO2 : This mechanism is considered to be reasonable for explaining why S decreases in the Sr-doped ½Ca2 CoO3 RS 0:62 ½CoO2 [9,19], in spite of the fact that the valence of Sr2þ ions is equal to Ca2þ ions. A lightly Y- and Bi-doping are found to enhance the magnitude of S indicating the decrease in the Co valence as expected [10], although the both doping suppress T FR as in the case of the Srdoping [7]. In other words, T FR is unlikely to depend on the Co valence but likely to be sensitive to the structure of the rocksalt-type subsystem. Furthermore, not only the Sr-doping but also the Y- and Bi-doping increase T end SDW [18]. In order to explain whole changes in electronic and magnetic properties of ½Ca2 CoO3 RS 0:62 ½CoO2 induced by doping, it is necessary to perform more precise investigations on the doped compounds. Also, It is worth to note that the magnitude of S at 300 K is 200 mVK1 for the pure crystal. This value is significantly higher than those in the previous reports; that is, 130 mVK1 at 300 K for both single-crystal and polycrystalline samples [2,3]. Since it is difficult to explain such a large
We investigated the single crystal growth of ½Ca2 CoO3 RS 0:62 ½CoO2 using a SrCl2 flux method. The composition of the grown crystal was found to strongly depend on the ratio between the flux and precursor. Almost pure crystals were obtained from a melt mixture with a low flux to precursor ratio. On the other hand, Sr in the flux entered into the Ca site in ½Ca2 CoO3 RS 0:62 ½CoO2 ; when we used a high flux to precursor ratio. The Sr-doping was found to suppress both the ferrimagnetic transition temperature and the magnitude of thermoelectric power coefficient, as reported for the polycrystalline samples. Thus, such suppressions were induced not by a possible impurity at grain boundaries but by an intrinsic change in the electronic structure, in spite of the same valence state of Sr2þ and Ca2þ ions.
Acknowledgements Thermoelectric power coefficient S measurements were carried out at Nagoya University. We appreciate Professor U. Mizutani, Professor H. Ikuta and Professor T. Takeuchi of Nagoya University for their permission and help to use the apparatus and their fruitful discussions. We also thank Mr. Y. Kawai of Toyota CRDL for his ICP analysis. This work was partially supported by joint research and development with International Center for Environmental Technology Transfer in 2002–2004, commissioned by the Ministry of Economy Trade and Industry of Japan.
References [1] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, Jpn. J. Appl. Phys. 39 (2000) L1127.
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[2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [3] Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Jpn. J. Appl. Phys. 39 (2000) L531. [4] Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani, J. Phys. Soc. Jpn. 71 (2002) 491. [5] S. Lambert, H. Leligny, D. Grebille, J. Solid State Chem. 160 (2001) 322. [6] J. Sugiyama, C. Xia, T. Tani, Phys. Rev. B 67 (2003) 104410. [7] J. Sugiyama, H. Itahara, T. Tani, J.H. Brewer, E.J. Ansaldo, Phys. Rev. B 66 (2002) 134413. [8] S. Li, R. Funahashi, I. Matsubara, H. Yamada, K. Ueno, S. Sodeoka, Ceram. Int. 27 (2001) 321. [9] Y. Miyazaki, T. Miura, Y. Ono, T. Akashi, T. Goto, T. Kajitani, in: Proceedings of the 21st International Conference on Thermoelectrics, Long Beach, 2002, Institute of Electrical and Electronics Engineers, Piscataway, 2002, pp. 226–229.
[10] S. Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, H. Yamada, Chem. Mater. 12 (2000) 2424. [11] Y. Zhou, I. Matsubara, R. Funahashi, G. Xu, M. Shikano, Mat. Res. Bull. 37 (2002) 1123. [12] G. Xu, R. Funahashi, M. Shikano, Q. Pu, B. Liu, Solid State Commun. 124 (2002) 73. [13] G. Xu, R. Funahashi, M. Shikano, I. Matsubara, Y. Zhou, Appl. Phys. Lett. 80 (2002) 3760. [14] M. Shikano, R. Funahashi, Appl. Phys. Lett. 82 (2003) 1851. [15] J. Shimoyama, K. Otzschi, M. Suzuki, S. Horii, K. Kishio, in: Extended Abstracts of 48th Autumn Meeting of Japan Society Applied Physics, vol. 128, The Japan Society of Applied Physics, Tokyo, 2001 (in Japanese). [16] R.D. Shannon, Acta. Crystallogr. A 32 (1976) 751. [17] J. Sugiyama, J.H. Brewer, E.J. Ansaldo, M. Bayer, H. Itahara, T. Tani, Physica B 326 (2003) 518. [18] J. Sugiyama, J.H. Brewer, E.J. Ansaldo, H. Itahara, K. Dohmae, Y. Seno, C. Xia, T. Tani, Phys. Rev. B 68 (2003) 134423. [19] D. Pelloquin, S. He´bert, A. Maignan, B. Raveau, Solid State Sci. 6 (2004) 167.
Journal of Crystal Growth 276 (2005) 519–524 www.elsevier.com/locate/jcrysgro
Platelet crystals of thermoelectric layered cobaltites; pure and Sr-doped ½Ca2CoO30:62½CoO2 Changtai Xia1, Jun Sugiyama, Hiroshi Itahara, Toshihiko Tani Toyota Central Research and Development Laboratories Inc., Nagakute, Aichi, 480-1192, Japan Received 7 October 2004; accepted 24 November 2004 Communicated by K. Sato Available online 28 January 2005
Abstract Single crystal plates ð 5 5 0:1 mm3 Þ of pure and Sr-doped ½Ca2 CoO3 0:62 ½CoO2 have been grown in a SrCl2 flux. The ratio between the flux and raw materials strongly affected the composition of the grown crystals; that is, Sr ions were doped into the Ca site in the lattice when the flux-rich melt was used. It is found that the Sr-doping suppresses the ferrimagnetic transition temperature and decreases the magnitude of thermoelectric power coefficient, while the valence of Sr2þ ions is equal to that of Ca2þ ions. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.h; 61.10.Nz; 72.20.Pa; 75.50.Gg Keywords: A1. Doping; A2. Growth from melt; B1. Oxides; B2. Thermoelectrics
1. Introduction The layered cobaltite, ½Ca2 CoO3 RS 0:62 ½CoO2 [1–3] (RS denotes a rocksalt-type subsystem) exhibits metallic conductivities and extraordinarily large thermoelectric power coefficients S (above þ150 mV=K at 300 K) simultaneously, for reasons Corresponding author. Tel.: +81 561 63 6196;
fax: +81 561 63 6156. E-mail address: [email protected] (J. Sugiyama). 1 Present address: Shanghai Institute of Optics and Fine Mechanics.
currently not fully understood. Because of its large thermoelectric figure of merit (ZT 1 at 1000 K), ½Ca2 CoO3 RS 0:62 ½CoO2 is considered to be a promising candidate for a p-type material component of thermoelectric power generation systems. The crystal structure of ½Ca2 CoO3 RS 0:62 ½CoO2 consists of alternating stacks of two monoclinic subsystems along the c-axis [2–5]. The two subsystems are (1) triple rocksalt-type ½Ca2 CoO3 subsystem; and (2) single CdI2-type [CoO2] subsystem. Both subsystems have identical a, c and b parameters but different b parameters. The monoclinic ½Ca2 CoO3 RS 0:62 ½CoO2 belongs to the space
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.415
ARTICLE IN PRESS C. Xia et al. / Journal of Crystal Growth 276 (2005) 519–524
520
( ( group C m (a ¼ 4:834 A; b1 ¼ 4:554 A; b2 ¼
( ( 2:798 A; c ¼ 10:832 A and b ¼ 98:13 ) [4]. There is an incommensurate spatial modulation along the b-axis caused by a misfit between the two subsystems. Because of its layered structure, the physical properties of ½Ca2 CoO3 RS 0:62 ½CoO2 are expected to be anisotropic. Indeed, several workers reported that both resistivity (r) and susceptibility ðwÞ exhibit a large anisotropy [1,2,6]. That is, rab 10rc at 300 K, where rab represents the resistivity in the ab plane and rc the resistivity along the caxis [2]. Such anisotropy leads to the problem how to obtain a large single crystal or a large aligned polycrystal for the use in thermoelectric power generation systems at high temperatures. In particular, platelet crystals are necessary for templates in the preparation of the aligned polycrystal. Furthermore, there are many reports on the doped ½Ca2 CoO3 RS 0:62 ½CoO2 to improve its thermoelectric properties [7–13], although, to authors’ knowledge, all efforts were failed to achieve a drastic increase in ZT. It is worth noting that almost all of the previous experiments for the doped cobaltite were carried out using polycrystalline samples. Hence, a possible segregation of the dopant at grain boundaries could increase a magnitude of r; even if the dopant looks to be soluble in the ½Ca2 CoO3 RS 0:62 ½CoO2 lattice. Therefore, in order to clarify the effect of dopants in ½Ca2 CoO3 RS 0:62 ½CoO2 ; we have performed the single crystal growth reported here. Also, we have measured the physical properties of the crystals obtained here to know the nature of the doping into the cobaltite.
2. Experimental procedure Single crystals of ½Ca2 CoO3 RS 0:62 ½CoO2 were prepared using a modified SrCl2-flux technique as reported by Shikano et al. [14]. Reagent-grade CaCO3, Co3O4 and SrCl2 powders or calcined ½Ca2 CoO3 RS 0:62 ½CoO2 and SrCl2 powders were thoroughly mixed by a ball mill. The mixed powders were packed into a Pt crucible, the crucible was heated and kept at around 1200 K, and then furnace-cooled down to 400 K slowly. The conditions we employed are summarized in Table 1. The grown crystals were washed several times in hot water to remove the SrCl2 flux. The obtained crystals were thin platelets with typical dimensions of 5 5 0:1 mm3 : The platelets were then annealed at 723 K in an oxygen gas flow for 12 h and then furnace-cooled down to room temperature at a rate of 1 K/min to remove a possible oxygen deficiency [15]. A crystal structure of the sample was determined by an X-ray powder diffraction (XRD) with Cu-Ka radiation. For thin plate crystals, they were adhered to a glass sample holder with grease. The accurate lattice parameters were determined by pulverizing crystal to fine powders and then measure an XRD pattern. An X-ray diffraction study indicated that the growth surface of the platelets was the ab plane. Both optical microscope and SEM observations showed that the platelet was a stack of several pieces of small platelets. In other words, the platelets were a c-aligned stack along the c-axis of randomly rotated ab planes. The magnetic properties were measured using a SQUID magnetometer (mpms, Quantum Design)
Table 1 Crystal growth conditions employed in this report; P denotes precursor, F SrCl2 flux, T max the highest temperature hold the Pt crucible. Run No.
P
P:F (w:w)
T max (K)
Cooling rate (K/h)
1 2
CaCO3 þ Co3 O4 [Ca2 CoO3 RS 0:62 ½CoO2
1:2 1:4
1173 K for 3 h 1173 K for 0.1 h
3
CaCO3 þ Co3 O4
1:1
1173 K for 10 h
5.5 (to 633 K) 1 (to 1074 K) 6 (to 773 K) 1 (to 1074 K) 6 (to 773 K)
ARTICLE IN PRESS C. Xia et al. / Journal of Crystal Growth 276 (2005) 519–524
in the temperature range between 400 and 5 K under magnetic field Hp55 kOe: In order to increase the magnetic signal, 5 platelets were stacked in a plastic sample holder. Also, to determine anisotropy, H was applied parallel or perpendicular to the ab plane. Hereby, we will abbreviate susceptibility obtained with H ? ab as wc and H==ab as wab ; respectively. Thermoelectric power coefficients (S) were measured using a parametric double reference measurement technique (SB-100, MMR Technologies) in the temperature range between 90 and 450 K. In this technique, two pairs of thermocouples are used; one pair is copper vs. constantan and the other copper vs. sample. The typical temperature gradient in the sample was kept 2:5 K in the whole temperature range.
3. Results and discussion 3.1. Structural and compositional analyses Table 2 gives the refined lattice parameters of polycrystalline precursor and pulverized crystals. As we can see from Table 1, crystal 1 has almost the same lattice parameters as those of the pure polycrystalline ½Ca2 CoO3 RS 0:62 ½CoO2 : On the other hand, the cell volume of crystal 2 was significantly enlarged, which is consistent with the fact that a significant amount of Sr was detected in crystal 2 by the ICP analysis. This is because the ionic ( is larger than that of radius of Sr2þ ions (1.13 A) 2þ ˚ Ca (0.99 A) [16]. Table 3 gives the ICP analysis result of grown crystals. The composition of the grown crystal was found to strongly depend on the ratio between the
521
SrCl2 flux and the precursor. Almost pure crystals were obtained from a melt mixture with a low flux to precursor ratio. On the other hand, Sr in the flux entered into the Ca site in ½Ca2 CoO3 RS 0:62 ½CoO2 ; when we used a high flux to precursor ratio. 3.2. Magnetic and transport properties Figs. 1(a)–(c) show the temperature dependences of w for the three single crystals grown in the present work. As temperature decreases from 80 K, both wab and wc increase for all samples, indicating a Curie–Weiss behavior of the Co ions in the lattice. Below 19 K, the wc ðTÞ curves for the No. 1 and 3 crystals exhibit a rapid increase with further decreasing T due to a ferrimagnetic transition (T FR ¼ 19 K) [6,7]. On the other hand, the wc ðTÞ curve for the No. 2 crystal lacks the divergence below 19 K, although the thermal hysteresis (i.e., the difference between the data obtained in the zero field-cooling (ZFC) mode and in the field-cooling (FC) mode) is observed below 14 K, suggesting the existence of a magnetic transition below 14 K. Our w measurements [7] on the polycrystalline samples indicated that the ferrimagnetic transition is sensitive to the substitu-
Table 3 ICP-AES results of elemental analysis (molar ratio) in the grown crystals Sample Precursor
Co 4
Ca 3.04
Sr 0
Crystal 1 Crystal 2 Crystal 3
4 4 4
2.84 2.84 2.96
0.08 0.21 0.06
Table 2 Refined lattice parameters of polycrystalline precursor and pulverized crystals
Precursor for 2nd run
( a ðAÞ 4.834(3)
( b ðAÞ 4.557(3)
( c ðAÞ 10.839(7)
98.14(4)
( Þ V ðA 236.466(2)
Crystal 1 Crystal 2 Crystal 3
4.832(1) 4.839(3) 4.837(2)
4.558(1) 4.582(4) 4.558(3)
10.846(3) 10.873(7) 10.855(4)
98.01(2) 98.18(3) 98.15(3)
236.593(1) 238.674(2) 236.903(1)
Sample
b ðo Þ
3
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χ (10-3 emu/g)
522
No.1, H=10 kOe ZFC FC
0.2
0.1
χc
(a)
0
χ (10-3 emu/g)
χab
0.2
No.2, H=10 kOe ZFC FC
χc
0.1
(b)
0
χ (10-3 emu/g)
χab
0.2
No.3, H=10 kOe ZFC
χc
0.1 χab 0 0
(c)
20
40
60
80
TEMPERATURE (K)
Fig. 1. Temperature dependences of magnetic susceptibility w for single crystal plates obtained from (a) run No.1, (b) run No.2 and (c) run No.3. w was measured both in the zero fieldcooling (ZFC) mode and the field-cooling (FC) mode with H ¼ 10 kOe; here, H was applied parallel or perpendicular to the ab plane.
tion for Ca by Sr and T FR decreases below 5 K even for the ½Ca2x Srx CoO3 RS y ½CoO2 sample with x ¼ 0:067: We, therefore, conclude that the No. 2 crystal is the Sr-doped ½Ca2 CoO3 RSRS 0:62 ½CoO2 ; whereas the No. 1 and 3 are the pure phase. The
magnetic transition below 14 K in the Sr-doped sample is most likely to correspond to the ferrimagnetic transition, which is suppressed by the lightly Sr-doping. According to muon spin rotation and relaxation ðmSRÞ experiments on the pure and Sr-doped samples [7,17,18], a long-range incommensurate spin density wave (IC-SDW) order is observed below 27 K ð¼ T end SDW Þ for the pure sample and below 40 K for the ½Ca1:8 Sr0:2 CoO3 RS y ½CoO2 sample. Actually, a shoulder associated with the formation of the long-range IC-SDW order is clearly seen only in the wc ðTÞ curve at 27 K for the No. 1 and 3 crystals, whereas no shoulder around 27 K for the No. 2 crystal. This is also consistent with the above conclusion; that is, the No. 2 crystal is the Sr-doped phase, but No. 1 and 3 are the pure phase. Fig. 2 shows the temperature dependences of S for the No. 1 and 2 crystals, i.e., the pure and Srdoped crystals. Since the thermoelectric voltage, which is induced by the temperature gradient in the a2b plane, was measured in the a2b plane, the present S denotes Sab : As T increases from 90 K, S for the pure crystal increases with decreasing its slope (dS/dT), and roughly levels off to a constant value ( 200 mVK1 ) above 250 K: The SðTÞ curve for the Sr-doped crystal looks similar to that for the pure crystal, although the Sr-doping decreases the magnitude of S by 5–10 mVK1 in the whole temperature range measured. Since Miyazaki et al. [9] reported the decrease in S by the Sr-doping using the polycrystalline samples, the present S measurement also confirms the conclusion that the No. 2 crystal is the Sr-doped ½Ca2 CoO3 RS 0:62 ½CoO2 : Interestingly, the lightly Sr-doping suppresses both the ferrimagnetic transition temperature T FR and the magnitude of S, as reported for polycrystalline samples [7,9]. The structural analysis suggests that the doped Sr2þ ions locate at the Ca2þ site in the ½Ca2 CoO3 subsystem. Also, the substitution of Ca2þ by Sr2þ is found to increase mainly the volume of the rocksalt-type subsystem. The incommensurate modulation period y (¼ 0:62 for the pure compound) hence decreases to match the two subsystems in the lattice [19]. Assuming that such substitution does not alter the valence of
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discrepancy based only on the present result, further works, i.e., the single crystal growth with larger size, are necessary to clarify the anisotropy in S.
300 pure & Sr-doped [Ca2CoO3]0.62[CoO2] S (µV/K)
200
4. Summary 100 No.1, TFR=19 K No.2, para above 5 K 0 0
100
200
300
400
500
TEMPERATURE (K)
Fig. 2. Temperature dependences of thermoelectric power coefficient S for single crystal plates obtained from (solid circles) run No.1 and (open circles) run No.2. The No.1 crystal exhibits a ferrimagnetic transition at 19 K, while the No.2 paramagnetic down to 5 K.
Co ions ( 3þ) in the rocksalt-type subsystem, the decrease in y could increase the valence of Co ions in the CoO2 plane, as well as the decrease in y for Nay CoO2 : This mechanism is considered to be reasonable for explaining why S decreases in the Sr-doped ½Ca2 CoO3 RS 0:62 ½CoO2 [9,19], in spite of the fact that the valence of Sr2þ ions is equal to Ca2þ ions. A lightly Y- and Bi-doping are found to enhance the magnitude of S indicating the decrease in the Co valence as expected [10], although the both doping suppress T FR as in the case of the Srdoping [7]. In other words, T FR is unlikely to depend on the Co valence but likely to be sensitive to the structure of the rocksalt-type subsystem. Furthermore, not only the Sr-doping but also the Y- and Bi-doping increase T end SDW [18]. In order to explain whole changes in electronic and magnetic properties of ½Ca2 CoO3 RS 0:62 ½CoO2 induced by doping, it is necessary to perform more precise investigations on the doped compounds. Also, It is worth to note that the magnitude of S at 300 K is 200 mVK1 for the pure crystal. This value is significantly higher than those in the previous reports; that is, 130 mVK1 at 300 K for both single-crystal and polycrystalline samples [2,3]. Since it is difficult to explain such a large
We investigated the single crystal growth of ½Ca2 CoO3 RS 0:62 ½CoO2 using a SrCl2 flux method. The composition of the grown crystal was found to strongly depend on the ratio between the flux and precursor. Almost pure crystals were obtained from a melt mixture with a low flux to precursor ratio. On the other hand, Sr in the flux entered into the Ca site in ½Ca2 CoO3 RS 0:62 ½CoO2 ; when we used a high flux to precursor ratio. The Sr-doping was found to suppress both the ferrimagnetic transition temperature and the magnitude of thermoelectric power coefficient, as reported for the polycrystalline samples. Thus, such suppressions were induced not by a possible impurity at grain boundaries but by an intrinsic change in the electronic structure, in spite of the same valence state of Sr2þ and Ca2þ ions.
Acknowledgements Thermoelectric power coefficient S measurements were carried out at Nagoya University. We appreciate Professor U. Mizutani, Professor H. Ikuta and Professor T. Takeuchi of Nagoya University for their permission and help to use the apparatus and their fruitful discussions. We also thank Mr. Y. Kawai of Toyota CRDL for his ICP analysis. This work was partially supported by joint research and development with International Center for Environmental Technology Transfer in 2002–2004, commissioned by the Ministry of Economy Trade and Industry of Japan.
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