- Email: [email protected]
Structural study on high-pressure synthesized (Hg,Tl)2Ba4Cu2CO3O8 compound
___ k!EI
&
PHYSICA Ii
ELSEVIER
Physica C 292 ( 1997) 89-96
Structural study on high-pressure synthesized (Hg,Tlj ,Ba,Cu,CO,O, compound Xiao-Jing Wu
*,
T. Tatsuki, S. Adachi, K. Tanabe
Superconductivity Research Laboratory, International Superconductivity Technology Center, IO-13 Shinonome. I-Chome, Koto-Ku, Tokyo 135, Japan Received 21 August 1997; accepted 24 September
1997
Abstract A sample with a nominal composition of Hg:Tl:Ba:Cu:C = 1:1:4:2:1 was prepared by a, high-pressure synthesis technique. A new phase with a space group of 14/mmm and lattice parameters of a = 3.9256 A and c = 38.875 A was found in this sample. An energy dispersive X-ray spectrum analysis indicated that the composition of the new phase is not far from the nominal one. A transmission electron microscopic observation revealed that in this phase the stacking order is . CO-BaO-CuO,-BaO-(Hg,Tl)O-(Hg,Tl)O-BaO-CuO,-BaO . . along the c-axis, and there is no superstructure in the &plane. A crystal structural model was proposed based on the transmission electron microscopic observation. By using this model, the high resolution transmission electron microscopic images as well as powder X-ray diffraction pattern were simulated. It was found that the simulations are in good agreement with the experimental ones. 0 1997 Elsevier Science B.V. Keywwds:
(Hg,,,,Tl,,
,)zBa,Cu,CO,O,
compound;
TEM observation;
1. Introduction In recent years, many carbonate-contained superconductors have been reported. In some cases it was found that C-1201 could intergrow with another superconducting phase to form a new phase. So far, such intergrown phases have been reported for (Tl,,,,Pb,.,)Sr,Cu,CO,O, [ll, (Tl,_,,Bi,k@.~, CO,O, 121, (Pb,,,,Hg,.,)Sr,Cu,CO,O, [3], (Tl, _* Hg,)Sr,Ba,Cu,CO,O, [4], Bi,Sr,_,Cu,0,,(C03)2 [5], (Bi,Pb),Sr,Cu,Cd,O, and (Bi,Pb),Sr,(CO,),O,,
* Corresponding author. Tel.: 81 3 3536 5709; Fax: 81 3 3536 5717; E-mail: [email protected].
HRTEM simulation;
XRD pattern simulation;
Intergrowth
structure
[6], etc. The former four phases were formed by intergrowth between C-l 201 and M- 1201 (M: Tl, Pb, Hg and Bi), while the later three phases were intergrown between C-1201 and (Bi,_,,Pb,)-2201. An interesting thing here is that although both of (Pb,,, Hg,&r,CuO, [7] and Sr,CuO,CO, (i.e. C-1201) [8] are nonsuperconductors, the intergrowth phase between them, (Pb,,,,Hg,,,)Sr,Cu,CO,O,, shows superconductivity at about 70 K [3]. Another example is that the highest T, of (T1,,,,Pb,,j)Sr,CuO, [9] is about 60 K, but the intergrowth (T1,,,,Pb,,,)Sr,Cu2 CO,O, phase shows superconductivity at a higher temperature above 70 K [l]. These results indicated that under a certain condition the intergrowth structure is of benefit to improving superconductivity.
0921-4534/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO92 I-4534(97)01753-X
90
X-J. Wu c’t a/. / Ph~.sico C 2Y2 (IYY7J
Recently, several phases belonging to the (Hg.Tl)22(n - I >n family have been synthesized by Tokiwa-Yamamoto et al. [IO.1 I] and Tatsuki et al. [12- 141 by employing a high-pressure synthesis technique. Though most members in this family showed superconductivity. the first member, (Hg,Tl)-2201, d oes not become a superconductor yet. By transmission electron microscopic (TEM) observation, Wu et al. [ 151 found several new phases. which are formed by intergrowth between the members of the (Hg,Tll-22(N - 1)n family, and between the members of the (Hg,Tl)-22(n - l)rr as well as (Hg,Tl)-12(n - I>II f dmilies. It seems quite easy to form an intergrowth phase in the (Hg,Tl)-0 doublelayer system. In this paper. we report our recent results on a high-pressure phase of (Hg,Tll,Ba,Cu,CO,O,. which may be understood as an intergrowth phase between C- I201 and (Hg,Tl)-2201. A structural model for this phase is proposed based on TEM observation as well as the composition determined by energy dispersive X-ray (EDX) analysis. By using this structural model. the high resolution transmission electron microscopic (HRTEM) images as well as X-ray diffraction pattern (XDP) are simulated.
XY-Y6
Comparing the simulations with the experimental ones. we find that the agreement between them is quite well.
2. Experimental Starting materials were commercial powders of HgO, TlZ03, BaOz, Cu and BaCO,. These powders were mixed to the nominal composition of mixture was HgTlBa,Cu$G, ,.4. The powder pressed into a pellet and charged into a gold capsule. BaO, was used as both a Ba source and an oxidizing agent. The sample was heat-treated at 1000°C for 1 h under a pressure of 5 GPa using a cubic-anvil-type apparatus. X-ray diffraction data was collected by MX18 diffractometer using Cu K (Y radiation. A JEM-4000 FX analytic transmission electron microscope equipped with an EDX detector was utilized to perform composition analysis. HRTEM images were taken by JEM-4000 EX with a point resolution of about 1.7 A at an operating accelerating voltage of 400 kV. The specimen for TEM observation was prepared by conventional crush method. The sample
Fig. I A powder X-ray diffraction pattern for the sample 0 with a nominal composition of Hg:TI:Ba:Cu:C = 1: I :4:2: I, The reflections are indexed by using the lattice parameters of (I = 3.9256 A and L’= 38.875 A. The reflections from (Hg,TI)-2201 are labeled by the indexes with brackets. and the reflections from unknown phases are labeled hy the asterisks.
X.-J.
wu et 01. / P/?y.~icrr c 292 (IYY7)
XY-Y6
91
was ground in an agate mortar to give fine-fractured particles. The particles were dispersed in alcohol and a drop of the resultant suspension was placed on a microgrid. The golden film was evaporated on the microgrid as the internal standard.
and Tl may reside in the same site in the crystal. Taking the measurement error into account, we can say that the composition of this phase is not far from the nominal one, and the chemical formula of the phase then can be written as (Hg , , .TI ,),Ba,Cu, c ,o; (.r - 0.5).
3. Results
3.2. XDP and EDP
EDX composition analysis was performed for several grains. The average atomic ratio of Hg:Tl:Ba:Cu is 10.7:12.2:52.6:24.5. The ratio of Ba and Cu in each grain is rather stable, but the content of Hg and Tl is variable from grain to grain. The sum of Hg and Tl atomic ratio, however, is always in a range of 22.2 - 23.8%. This result implies that Hg
Fig. 1 shows an XDP for the sample. It is found that the major phase in the sample has a tetragonal symmetry, and the main reflections can be indexed by using the lattice parameters of (I = 3.92.56(4) A and c = 38.875(3) A. However, many reflections from other phases still can be seen in this pattern. Besides the reflections from (Hg,Tl)-2201 which are labeled by the indexes with brackets, the peaks from some unknown phases are labeled by asterisks. The
Fig. 2. EDPs
taken
along (a) [OIO],
(b) [I IO] and Cc) [OOI].
The diffraction
rings in (c) are from the gold film
92
X. -J. Wu et al. / Physica C 292 ( 19971 89-96
intensity of the strongest peak for such an unknown phase is above 15% of that of (HgO,,,T1&Ba,Cu, CO?. Fig. 2 shows the electron diffraction patterns (EDPs) taken along the (a) [OIOI, (b) [ 1101 and (c) [OOl] directions. The lattice is body-centered, and no satellites are observed in the patterns. This means that the distribution of Hg and Tl in the (Hg,Tl)-site is completely random.
3.3. HRTEM observation
and structural model
Fig. 3 shows an HRTEM image taken along the [OlOl direction, in which the rectangle indicates the unit cell. The crystal has a layered structure, and 9 sheets in each half unit cell along the c-direction. One white line at the center of the unit cell seems corresponding to the position of the C-O sheet. Such kind of contrast caused by the C-O sheet was re-
Fig. 3. An HRTEM micrograph taken along [OlO]. The rectangle shows an unit cell. The CuO,, CO, (Hg,TI)O and BaO sheets are indicated by the arrows with labels of GO,, CO, HTO and BaO, respectively.
X.-J. Wuet al./Physica C 292 (1997) 89-96 Table 1 Proposed
93
atomic positions of (Hg,,,Tl,,,),Ba,Cu2C030s Site
x
Y
Z
0
0
Hg,Tl O(l)
2a 4c 4e 4e 4e 2b
‘/2 ‘/2 0 0
‘/2 l/2 0 0
O(2)
4e
O(3) O(4) O(5)
4e
l/2 0 0 0
l/2 0 0
0 0.055 0.155 0.107 0.222 0 0.055 0.155 0.107 0.222
C Ba(l) Ba(2) CU
gg 4e
l/2
t/2 l/2
Space group: I4/mmm (139); lattice parameters: a = 3.9256(4) A and c = 38.875(3) A. The oxygen atoms are assumed to be in the same planes as the company metal or carbon ones.
cuo*
ported previously, for example, in Ref. [3]. Combining this image with the EDX analysis result, we suggest that the stacking order in the crystal is:
BaO
. . . CO-Ba0-Cu0,-Ba0-(Hg,Tl)O-(Hg,Tl)O -BaO-CuO,-BaO
*. .
If the oxygen sites were fully occupied in each sheet, the chemical formula of the crystal should be written as (Hgo.S,Tl,.,),Ba4Cu2C0308. Based on the HRTEM observation, we can construct a structural model for this phase. Fig. 4(a) is a three dimensional view of the structure, and (b) is a projection of the structure along the [OlO] direction. This model belongs to a tetragonal system, and has a space group of 14/mmm (139). The proposed atomic positions are listed in Table 1, in which all oxygen atoms are assumed to be exactly in the same planes as the company metal or carbon atoms. 3.4. Simulations for HRTEM images and XDP Using the crystallographic data listed in Table 1, we performed HRTEM image simulations. The observed and simulated images along the [OlO] direction are shown in Fig. 5. The simulated micrographs are obtained under the conditions of t (thickness) = 30 A, Af (defocus) = - 100 i for Fig. 5 (a,), and
l : Hg, Tl
:Ba
l :cu
l :c
o:o
Fig. 4. A three-dimensional schematic view of the proposed structural model (a) and the structural projection along [OlO] (b).
94
X.-J. WU et (11./Pl~wictr
C 2Y2 (IYY7I XYPYC,
Fig. 5. Observed HRTEM
micrographs ((I~,) and (/I<,) taken along [OIO] together with their corresponding simulated ones (a,)
simulation conditions are:
t
120,
(thickneaa) = 30 A, and A/’ (defocu\) = -
I
IO0i for ((I,) I
I
- 750 i
and (h,). The
for (I?, ).
I
-
I
: Measured ._
: Simulated .
Fig. 6. Measured (solid line) and simulated (dashed line) XDP\.
and A,f’=
: Peaks from impurities
Several stronger extra peaks labeled by the dots are from the impurities.
The thinner vertical lines near to the bottom indicate the simulated peak positions of the (HE,) r.Tl,, ,),Ba,CuzCO,O,
phase.
X.-J. Wu et al./ Phwicn C 292 (IYY71 89-96
t = 30 A, A f = - 750 A for Fig. 5 (6,). Comparing the simulated micrographs with the corresponding observed ones, we find that the agreement between (a,) and (a,), (b,) and (b,,) is rather well. Since our present sample still contains an amount of unknown impurity phases, we can not expect to determine the crystal structure by using X-ray powder diffraction method. Instead, we performed XDP simulation using the proposed model. Fig. 6 shows the measured (upper solid line) and the simulated (lower dash line) XDPs. Some stronger peaks from impurities are labeled by the dots. The relative intensities of the simulated pattern are quite similar to those of the measurement. Both the simulations of HRTEM and XDP show a good agreement with the experimental ones, indicating that the proposed structural mode1 in Table 1 is acceptable, and can be taken as a starting model for further structural refinement.
0
0
0 0
0
t (Hg, T1):2201 h
0
I
4. Discussion Soon after the Hg- 12(n - 1>n family was discovered, Schilling et al. [ 161 reported the presence of the long-periodicity intergrowth phases in this family. Later, Van Tendeloo et al. [17], Cantoni et al. [I81 and Antipov et al. [ 191 confirmed that such intergrowth phases are common in this family. Recently, Wu et al. [15] found several new intergrowth phases formed between the members of (Hg,Tl)-22(n - 1)n family, and the members of (Hg,Tl)-22( n - 1)n and (Hg,Tl)-l’(n - 1) n f amilies. The present new phase actually can be also considered as an intergrowth phase formed by a half (Hg,Tl)-2201 unit cell and a C- 1201 unit cell. From this point of view, we can illustrate the structural mode1 as shown in Fig. 7, and express the structure of the phase by (2201 h/1201) (h: half unit cell). As aforementioned, the similar intergrowth phases between C-l 201 and other systems have been reported by several groups, though most of them only contain units with a single atomic layer charge reservoir [l-6]. In this sense, we can say that such structural phenomena is quite common, and thus we may expect to synthesize more such new intergrowth phases in the future. We performed the susceptibility vs. temperature measurement for the present sample, and found a
BaO Cu-0 octahedron
co3
Fig. 7. A schematic illustration of the intergrowth structure between (Hg,Tl)-2201h and C-1201. In each half unit cell of (Hg, ,,T1,s)~Ba,CuzCO,O,, there are a half unit cell of(Hg,TI)2201 and an unit cell of C-1201.
superconductive signal at 56 K. However. the amount of superconducting volume fraction is lower than 1% at 5 K. Since the sample contains several impurity phases, we can not isolate the superconducting phase in the sample at this moment. Actually, we think that the a-axis of the phase seems too large to make it a p-type superconductor. Noticing that all the intergrowth phases of (T1,S,Pb,,,,)Sr,Cu,CO,O, [I], (Tl, ~, ,Bi.,)Sr,Cu,CO,O, 121, (Pb,,.,,Hg,,3)Sr,Cu2
96
X.-J. Wu et al./Physicu
CO& [3], (Tl, ~.,Hgx)Sr,Ba?Cu2C030~ ]4], Bi,Sr,_,~Cu,O,,(CO,), [51, (Bi,Pb),Sr,Cu,CO,O, and (Bi,Pb),Sr,(CO,),O,, [6] showed superconductivity, it is considered that the present phase may become a superconductor, too, if we can reduce the length of a-axis by element substitution, for instance, substituting Sr for Ba and induce proper carrier doping. Based on the HRTEM observation we proposed the structural model and determined the atomic positions listed in Table 1. The HRTEM image as well as XDP simulations showed that the model is essentially correct. However, the simulations are not so sensitive to the atomic positions, and the crystallographic data in Table 1 are only a first order approximation. In order to determine the atomic positions with a higher accuracy, it is necessary to synthesize a purer sample which enables any further structural refinement.
5. Summary with a main phase of (a) A sample (Hg,Tl),Ba,Cu,CO,O, has been prepared by a high pressure synthesis technique. (b) The crystal structure of (Hg,Tl),Ba,Cu,CO,, belongs to the tetragonal system with a space group of 14/mmm and Lattice parameters of a = 3.9256 A and c = 38.875 A. No superstructure is found by EDP analysis. (c) This phase has a layered structure with a . . . CO-BaO-Cu02-BaOstacking order of (Hg,Tl)O-(Hg,Tl)O-BaO-CuO2-BaO . . . . This structure could be considered as an intergrowth between (Hg,Tl)-2201 and C- 120 1. A structural model is proposed. The simulations of HRTEM images and XDP are in good agreement with the experimental ones. (d) A weak superconducting signal was detected, but it likely comes from the impurities in the sample. The u-axis of this phase seems too large to make it a p-type superconductor. It is considered that this phase may become a superconductor, if the u-axis could be reduced by element substitution and sufficient hole doping could be induced.
C 292 (1997) 89-96
Acknowledgements This work was supported by the NED0 for the R&D of Industrial Science and Technology Frontier Program. The authors would like to thank Drs. Y. Ikuhara, T. Hirayama and Mr. T. Suzuki of JFCC for their help for doing the ATEM experiment.
References Ill M. Huve, C. Michel, A. Maignan, M. Hervieu, C. Martin, B. Raveau, Physica C 205 (1993) 219.
l21 A. Maignan, M. Huve, C. Michel. M. Hervieu. C. Martin, B. Raveau, Physica C 208 (1993) 149. 131 C. Martin, M. Hervieu, M. Huve, C. Michel, A. Maignan, G. Van Tendeloo, B. Raveau, Physica C 222 (1994) 19. 141 T. Noda, M. Ogawa, J. Akimitsu, M. Kikuchi, E. Ohshima, Y. Syono, Physica C 242 (1995) 12. l51 D. Pelloquin, A. Maignan, M. Caldes, M. Hervieu, C. Michel, B. Raveau, Physica C 212 (1993) 199. l61 M. Uehara, H. Nakata, J. Akimitsu, T. Den, T. Kobayashi, Y. Matsui, Physica C 2 13 (I 993) 5 I. [71 F. Goutenoire, P. Daniel, M. Hervieu, G. van Tendeloo, C. Michel, A. Maignan, B. Raveau, Physica C 216 (1993) 243. RI Y. Miyazaki, H. Yamane, T. Kajitani, Y. Morii, S. Funahashi, K. Hiraga, T. Hirai, Physica C 215 (1993) 159. [91 M.-H. Pan, M. Greenblatt, Physica C 176 (1991) 80. T. Tatsuki, X.-J. Wu, S. Adachi, K. [lOI A. Tokiwa-Yamamoto, Tanabe, Physica C 257 (1996) 36. T. Tatsuki, Y. Moriwaki, T. Tamura, 1111 A. Tokiwa-Yamamoto, X.-J. Wu, S. Adachi, K. Tanabc, in: Advances in Superconductivity IX (ISS’96) in press. A. Fukuoka, T. Tamura, [I21 T. Tatsuki, A. Tokiwa-Yamamoto, X.-J. Wu, Y. Moriwaki, R. Usami, S. Adachi, K. Tanabe, S. Tanaka, Jpn. J. Appl. Phys. 35 (1996) L205. T. Tamura, X.-J. Wu, Y. [I31 T. Tatsuki, A. Tokiwa-Yamamoto, Moriwaki, S. Adachi, K. Tanabe, Physica C 273 (1996) 65. T. Tamura, S. Adachi, K. [141 T. Tatsuki. A. Tokiwa-Yamamoto, Tanabe, Physica C, in press. T. Tatsuki, S. Adachi, K. [I51 X.-J. Wu, A. Tokiwa-Yamamoto, Tanabe, Physica C 275 (1997) 179. ll61 A. Schilling, M. Cantoni, J.D. Guo, H.R. Ott, Nature 363 (1993) 56. [I71 G. Van Tendeloo, C. Chaillout, J.J. Capponi, M. Marezio, E.V. Antipov, Physica C 223 (1994) 219. 1181 M. Cantoni, A. Schilling, H.-U. Nissen, H.R. Ott, Physica C 215 (1993) Il. [I91 E.V. Antipov, S.M. Loureiro, C. Chaillout, J.J. Capponi, P. Bordet, J.L. Tholence, S.N. Putilin, M. Marezio, Physica C 215 (1993) I.
&
PHYSICA Ii
ELSEVIER
Physica C 292 ( 1997) 89-96
Structural study on high-pressure synthesized (Hg,Tlj ,Ba,Cu,CO,O, compound Xiao-Jing Wu
*,
T. Tatsuki, S. Adachi, K. Tanabe
Superconductivity Research Laboratory, International Superconductivity Technology Center, IO-13 Shinonome. I-Chome, Koto-Ku, Tokyo 135, Japan Received 21 August 1997; accepted 24 September
1997
Abstract A sample with a nominal composition of Hg:Tl:Ba:Cu:C = 1:1:4:2:1 was prepared by a, high-pressure synthesis technique. A new phase with a space group of 14/mmm and lattice parameters of a = 3.9256 A and c = 38.875 A was found in this sample. An energy dispersive X-ray spectrum analysis indicated that the composition of the new phase is not far from the nominal one. A transmission electron microscopic observation revealed that in this phase the stacking order is . CO-BaO-CuO,-BaO-(Hg,Tl)O-(Hg,Tl)O-BaO-CuO,-BaO . . along the c-axis, and there is no superstructure in the &plane. A crystal structural model was proposed based on the transmission electron microscopic observation. By using this model, the high resolution transmission electron microscopic images as well as powder X-ray diffraction pattern were simulated. It was found that the simulations are in good agreement with the experimental ones. 0 1997 Elsevier Science B.V. Keywwds:
(Hg,,,,Tl,,
,)zBa,Cu,CO,O,
compound;
TEM observation;
1. Introduction In recent years, many carbonate-contained superconductors have been reported. In some cases it was found that C-1201 could intergrow with another superconducting phase to form a new phase. So far, such intergrown phases have been reported for (Tl,,,,Pb,.,)Sr,Cu,CO,O, [ll, (Tl,_,,Bi,k@.~, CO,O, 121, (Pb,,,,Hg,.,)Sr,Cu,CO,O, [3], (Tl, _* Hg,)Sr,Ba,Cu,CO,O, [4], Bi,Sr,_,Cu,0,,(C03)2 [5], (Bi,Pb),Sr,Cu,Cd,O, and (Bi,Pb),Sr,(CO,),O,,
* Corresponding author. Tel.: 81 3 3536 5709; Fax: 81 3 3536 5717; E-mail: [email protected].
HRTEM simulation;
XRD pattern simulation;
Intergrowth
structure
[6], etc. The former four phases were formed by intergrowth between C-l 201 and M- 1201 (M: Tl, Pb, Hg and Bi), while the later three phases were intergrown between C-1201 and (Bi,_,,Pb,)-2201. An interesting thing here is that although both of (Pb,,, Hg,&r,CuO, [7] and Sr,CuO,CO, (i.e. C-1201) [8] are nonsuperconductors, the intergrowth phase between them, (Pb,,,,Hg,,,)Sr,Cu,CO,O,, shows superconductivity at about 70 K [3]. Another example is that the highest T, of (T1,,,,Pb,,j)Sr,CuO, [9] is about 60 K, but the intergrowth (T1,,,,Pb,,,)Sr,Cu2 CO,O, phase shows superconductivity at a higher temperature above 70 K [l]. These results indicated that under a certain condition the intergrowth structure is of benefit to improving superconductivity.
0921-4534/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO92 I-4534(97)01753-X
90
X-J. Wu c’t a/. / Ph~.sico C 2Y2 (IYY7J
Recently, several phases belonging to the (Hg.Tl)22(n - I >n family have been synthesized by Tokiwa-Yamamoto et al. [IO.1 I] and Tatsuki et al. [12- 141 by employing a high-pressure synthesis technique. Though most members in this family showed superconductivity. the first member, (Hg,Tl)-2201, d oes not become a superconductor yet. By transmission electron microscopic (TEM) observation, Wu et al. [ 151 found several new phases. which are formed by intergrowth between the members of the (Hg,Tll-22(N - 1)n family, and between the members of the (Hg,Tl)-22(n - l)rr as well as (Hg,Tl)-12(n - I>II f dmilies. It seems quite easy to form an intergrowth phase in the (Hg,Tl)-0 doublelayer system. In this paper. we report our recent results on a high-pressure phase of (Hg,Tll,Ba,Cu,CO,O,. which may be understood as an intergrowth phase between C- I201 and (Hg,Tl)-2201. A structural model for this phase is proposed based on TEM observation as well as the composition determined by energy dispersive X-ray (EDX) analysis. By using this structural model. the high resolution transmission electron microscopic (HRTEM) images as well as X-ray diffraction pattern (XDP) are simulated.
XY-Y6
Comparing the simulations with the experimental ones. we find that the agreement between them is quite well.
2. Experimental Starting materials were commercial powders of HgO, TlZ03, BaOz, Cu and BaCO,. These powders were mixed to the nominal composition of mixture was HgTlBa,Cu$G, ,.4. The powder pressed into a pellet and charged into a gold capsule. BaO, was used as both a Ba source and an oxidizing agent. The sample was heat-treated at 1000°C for 1 h under a pressure of 5 GPa using a cubic-anvil-type apparatus. X-ray diffraction data was collected by MX18 diffractometer using Cu K (Y radiation. A JEM-4000 FX analytic transmission electron microscope equipped with an EDX detector was utilized to perform composition analysis. HRTEM images were taken by JEM-4000 EX with a point resolution of about 1.7 A at an operating accelerating voltage of 400 kV. The specimen for TEM observation was prepared by conventional crush method. The sample
Fig. I A powder X-ray diffraction pattern for the sample 0 with a nominal composition of Hg:TI:Ba:Cu:C = 1: I :4:2: I, The reflections are indexed by using the lattice parameters of (I = 3.9256 A and L’= 38.875 A. The reflections from (Hg,TI)-2201 are labeled by the indexes with brackets. and the reflections from unknown phases are labeled hy the asterisks.
X.-J.
wu et 01. / P/?y.~icrr c 292 (IYY7)
XY-Y6
91
was ground in an agate mortar to give fine-fractured particles. The particles were dispersed in alcohol and a drop of the resultant suspension was placed on a microgrid. The golden film was evaporated on the microgrid as the internal standard.
and Tl may reside in the same site in the crystal. Taking the measurement error into account, we can say that the composition of this phase is not far from the nominal one, and the chemical formula of the phase then can be written as (Hg , , .TI ,),Ba,Cu, c ,o; (.r - 0.5).
3. Results
3.2. XDP and EDP
EDX composition analysis was performed for several grains. The average atomic ratio of Hg:Tl:Ba:Cu is 10.7:12.2:52.6:24.5. The ratio of Ba and Cu in each grain is rather stable, but the content of Hg and Tl is variable from grain to grain. The sum of Hg and Tl atomic ratio, however, is always in a range of 22.2 - 23.8%. This result implies that Hg
Fig. 1 shows an XDP for the sample. It is found that the major phase in the sample has a tetragonal symmetry, and the main reflections can be indexed by using the lattice parameters of (I = 3.92.56(4) A and c = 38.875(3) A. However, many reflections from other phases still can be seen in this pattern. Besides the reflections from (Hg,Tl)-2201 which are labeled by the indexes with brackets, the peaks from some unknown phases are labeled by asterisks. The
Fig. 2. EDPs
taken
along (a) [OIO],
(b) [I IO] and Cc) [OOI].
The diffraction
rings in (c) are from the gold film
92
X. -J. Wu et al. / Physica C 292 ( 19971 89-96
intensity of the strongest peak for such an unknown phase is above 15% of that of (HgO,,,T1&Ba,Cu, CO?. Fig. 2 shows the electron diffraction patterns (EDPs) taken along the (a) [OIOI, (b) [ 1101 and (c) [OOl] directions. The lattice is body-centered, and no satellites are observed in the patterns. This means that the distribution of Hg and Tl in the (Hg,Tl)-site is completely random.
3.3. HRTEM observation
and structural model
Fig. 3 shows an HRTEM image taken along the [OlOl direction, in which the rectangle indicates the unit cell. The crystal has a layered structure, and 9 sheets in each half unit cell along the c-direction. One white line at the center of the unit cell seems corresponding to the position of the C-O sheet. Such kind of contrast caused by the C-O sheet was re-
Fig. 3. An HRTEM micrograph taken along [OlO]. The rectangle shows an unit cell. The CuO,, CO, (Hg,TI)O and BaO sheets are indicated by the arrows with labels of GO,, CO, HTO and BaO, respectively.
X.-J. Wuet al./Physica C 292 (1997) 89-96 Table 1 Proposed
93
atomic positions of (Hg,,,Tl,,,),Ba,Cu2C030s Site
x
Y
Z
0
0
Hg,Tl O(l)
2a 4c 4e 4e 4e 2b
‘/2 ‘/2 0 0
‘/2 l/2 0 0
O(2)
4e
O(3) O(4) O(5)
4e
l/2 0 0 0
l/2 0 0
0 0.055 0.155 0.107 0.222 0 0.055 0.155 0.107 0.222
C Ba(l) Ba(2) CU
gg 4e
l/2
t/2 l/2
Space group: I4/mmm (139); lattice parameters: a = 3.9256(4) A and c = 38.875(3) A. The oxygen atoms are assumed to be in the same planes as the company metal or carbon ones.
cuo*
ported previously, for example, in Ref. [3]. Combining this image with the EDX analysis result, we suggest that the stacking order in the crystal is:
BaO
. . . CO-Ba0-Cu0,-Ba0-(Hg,Tl)O-(Hg,Tl)O -BaO-CuO,-BaO
*. .
If the oxygen sites were fully occupied in each sheet, the chemical formula of the crystal should be written as (Hgo.S,Tl,.,),Ba4Cu2C0308. Based on the HRTEM observation, we can construct a structural model for this phase. Fig. 4(a) is a three dimensional view of the structure, and (b) is a projection of the structure along the [OlO] direction. This model belongs to a tetragonal system, and has a space group of 14/mmm (139). The proposed atomic positions are listed in Table 1, in which all oxygen atoms are assumed to be exactly in the same planes as the company metal or carbon atoms. 3.4. Simulations for HRTEM images and XDP Using the crystallographic data listed in Table 1, we performed HRTEM image simulations. The observed and simulated images along the [OlO] direction are shown in Fig. 5. The simulated micrographs are obtained under the conditions of t (thickness) = 30 A, Af (defocus) = - 100 i for Fig. 5 (a,), and
l : Hg, Tl
:Ba
l :cu
l :c
o:o
Fig. 4. A three-dimensional schematic view of the proposed structural model (a) and the structural projection along [OlO] (b).
94
X.-J. WU et (11./Pl~wictr
C 2Y2 (IYY7I XYPYC,
Fig. 5. Observed HRTEM
micrographs ((I~,) and (/I<,) taken along [OIO] together with their corresponding simulated ones (a,)
simulation conditions are:
t
120,
(thickneaa) = 30 A, and A/’ (defocu\) = -
I
IO0i for ((I,) I
I
- 750 i
and (h,). The
for (I?, ).
I
-
I
: Measured ._
: Simulated .
Fig. 6. Measured (solid line) and simulated (dashed line) XDP\.
and A,f’=
: Peaks from impurities
Several stronger extra peaks labeled by the dots are from the impurities.
The thinner vertical lines near to the bottom indicate the simulated peak positions of the (HE,) r.Tl,, ,),Ba,CuzCO,O,
phase.
X.-J. Wu et al./ Phwicn C 292 (IYY71 89-96
t = 30 A, A f = - 750 A for Fig. 5 (6,). Comparing the simulated micrographs with the corresponding observed ones, we find that the agreement between (a,) and (a,), (b,) and (b,,) is rather well. Since our present sample still contains an amount of unknown impurity phases, we can not expect to determine the crystal structure by using X-ray powder diffraction method. Instead, we performed XDP simulation using the proposed model. Fig. 6 shows the measured (upper solid line) and the simulated (lower dash line) XDPs. Some stronger peaks from impurities are labeled by the dots. The relative intensities of the simulated pattern are quite similar to those of the measurement. Both the simulations of HRTEM and XDP show a good agreement with the experimental ones, indicating that the proposed structural mode1 in Table 1 is acceptable, and can be taken as a starting model for further structural refinement.
0
0
0 0
0
t (Hg, T1):2201 h
0
I
4. Discussion Soon after the Hg- 12(n - 1>n family was discovered, Schilling et al. [ 161 reported the presence of the long-periodicity intergrowth phases in this family. Later, Van Tendeloo et al. [17], Cantoni et al. [I81 and Antipov et al. [ 191 confirmed that such intergrowth phases are common in this family. Recently, Wu et al. [15] found several new intergrowth phases formed between the members of (Hg,Tl)-22(n - 1)n family, and the members of (Hg,Tl)-22( n - 1)n and (Hg,Tl)-l’(n - 1) n f amilies. The present new phase actually can be also considered as an intergrowth phase formed by a half (Hg,Tl)-2201 unit cell and a C- 1201 unit cell. From this point of view, we can illustrate the structural mode1 as shown in Fig. 7, and express the structure of the phase by (2201 h/1201) (h: half unit cell). As aforementioned, the similar intergrowth phases between C-l 201 and other systems have been reported by several groups, though most of them only contain units with a single atomic layer charge reservoir [l-6]. In this sense, we can say that such structural phenomena is quite common, and thus we may expect to synthesize more such new intergrowth phases in the future. We performed the susceptibility vs. temperature measurement for the present sample, and found a
BaO Cu-0 octahedron
co3
Fig. 7. A schematic illustration of the intergrowth structure between (Hg,Tl)-2201h and C-1201. In each half unit cell of (Hg, ,,T1,s)~Ba,CuzCO,O,, there are a half unit cell of(Hg,TI)2201 and an unit cell of C-1201.
superconductive signal at 56 K. However. the amount of superconducting volume fraction is lower than 1% at 5 K. Since the sample contains several impurity phases, we can not isolate the superconducting phase in the sample at this moment. Actually, we think that the a-axis of the phase seems too large to make it a p-type superconductor. Noticing that all the intergrowth phases of (T1,S,Pb,,,,)Sr,Cu,CO,O, [I], (Tl, ~, ,Bi.,)Sr,Cu,CO,O, 121, (Pb,,.,,Hg,,3)Sr,Cu2
96
X.-J. Wu et al./Physicu
CO& [3], (Tl, ~.,Hgx)Sr,Ba?Cu2C030~ ]4], Bi,Sr,_,~Cu,O,,(CO,), [51, (Bi,Pb),Sr,Cu,CO,O, and (Bi,Pb),Sr,(CO,),O,, [6] showed superconductivity, it is considered that the present phase may become a superconductor, too, if we can reduce the length of a-axis by element substitution, for instance, substituting Sr for Ba and induce proper carrier doping. Based on the HRTEM observation we proposed the structural model and determined the atomic positions listed in Table 1. The HRTEM image as well as XDP simulations showed that the model is essentially correct. However, the simulations are not so sensitive to the atomic positions, and the crystallographic data in Table 1 are only a first order approximation. In order to determine the atomic positions with a higher accuracy, it is necessary to synthesize a purer sample which enables any further structural refinement.
5. Summary with a main phase of (a) A sample (Hg,Tl),Ba,Cu,CO,O, has been prepared by a high pressure synthesis technique. (b) The crystal structure of (Hg,Tl),Ba,Cu,CO,, belongs to the tetragonal system with a space group of 14/mmm and Lattice parameters of a = 3.9256 A and c = 38.875 A. No superstructure is found by EDP analysis. (c) This phase has a layered structure with a . . . CO-BaO-Cu02-BaOstacking order of (Hg,Tl)O-(Hg,Tl)O-BaO-CuO2-BaO . . . . This structure could be considered as an intergrowth between (Hg,Tl)-2201 and C- 120 1. A structural model is proposed. The simulations of HRTEM images and XDP are in good agreement with the experimental ones. (d) A weak superconducting signal was detected, but it likely comes from the impurities in the sample. The u-axis of this phase seems too large to make it a p-type superconductor. It is considered that this phase may become a superconductor, if the u-axis could be reduced by element substitution and sufficient hole doping could be induced.
C 292 (1997) 89-96
Acknowledgements This work was supported by the NED0 for the R&D of Industrial Science and Technology Frontier Program. The authors would like to thank Drs. Y. Ikuhara, T. Hirayama and Mr. T. Suzuki of JFCC for their help for doing the ATEM experiment.
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