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Local structural changes in Tl2Ba2Ca2Cu3O10 single crystal between 90 K and 290 K
PHYSICA® ELSEVIER
Physica C 267 (1996) 31-44
Local structural changes in T12Ba2Ca2Cu3Olosingle crystal between 90 K and 290 K Masashi Hasegawa * ,a, Yoshitaka Matsushita a, Humihiko Takei h a Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo, Japan b Faculty of Science, Osaka University, Machikaneyama, Toyonaka-shi, Osaka 560, Japan Received 13 March 1996; revised manuscript received 30 April 1996
Abstract
The local structure changes of a Tl2BaeCa2Cu30~0 single crystal (Tc = 117 K) with good crystallinity and superconducting properties have been investigated between 290 K and 90 K using an X-ray diffraction technique. The linear thermal expansion coefficients are obtained to be a a = 1.0(1)X 10 -5 K -~ and c%= 1.9(2)× 10 -5 K -1 in the a- and c-axes, respectively. The oxygen atoms in the thallium layers are found to exist at the disordered two split sites above around Tc and become ordered at a fixed site below around T~. The mobile in-plane oxygen atom in the CuO 5 square pyramid induced the local structure changes in and around the pyramid. Besides, it should be noted that they are extremely correlated with the superconductivity of the cuprate superconductors.
1. Introduction
A great deal of interest has been focused on the relationship between local structure changes and superconductivity in the high-temperature superconducting cuprates [1-12]. The first attractive study was reported on the local structure in T12Ba 2CaCu208 investigated using a powder pulsed-neutron scattering by B.H. Toby et al. [2]. Their pair-distribution function analysis showed a clear change in the local structure at the onset of superconductivity, indicating the correlated displacements of oxygen and copper atoms perpendicular to the C u - O plane. They also mentioned the important role of the local shortening between the apical and the in-plane oxygen atoms in a CuO 5 square pyramid on the super-
* Corresponding author. Fax: +81 3 3401 5169.
conductivity. This was also suggested by a local-density approximation calculation on the electronic state of YBa2Cu307 [13,14]. Similar results on the local structure change in the powder pulsed-neutron scattering studies have been reported in other high temperature superconducting cuprates, such as N d 2 _ x C e x C u O 4 _ y [6] and YBa2Cu408 [11]. The local structure change in TI2Ba2CaCu208 above 60 K was also reported in the single crystal X-ray diffraction study using a four-circle automatic diffractometer by V.N. Molchanov et al. [8]. They also reported the anomaly in the temperature dependence of the distance between apical oxygen and copper in the CuO 5 square pyramid. The important roles of the local mobile oxygen have been also reported in high pressure effect studies [15], such as in TI2BazCuO6+ ~ [16-20], YBa2Cu307 [21], La2CuO4+ 8 [22]. The mobile oxygen behavior is thought to determine the pressure
0921-4534/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 2 8 - 0
32
M. Hasegawa et al. / Physica C 267 (1996) 31-44
dependence of the physical properties in these cuprates. In the case of the thallium-based cuprate superconductors, a rearrangement of the oxygens in the T1-O layers, which are located above or below the CuO 5 square pyramid, on applying high pressures was reported to play an important role for the superconductivity. In this study, T12Ba2Ca2Cu3Oto is selected as a research target to investigate the local structure changes above and below T~, T12Ba2Ca2Cu3Olo has the highest T~ in the Tl-based cuprate superconductors and non pyramidal CuO 4 planes in the unit cell, which is different from TI2Ba2CuO6+ 8 and T12Ba2CaCu208. Numbers of the X-ray and neutron diffraction studies on the crystal structure of Tl2Ba2Ca2Cu3Oto have been reported [23-31]. However, no one have investigated the temperature dependence of the local structure changes, especially around T~. In addition, few studies have used the specimens well-characterized on the chemicalphysical properties, although the relationship between the structure and chemical or physical properties is one of the important keypoints to elucidate the high temperature superconductivity. Accordingly, in this study, we have used exactly the same high quality single crystal as the sample which has been well-characterized on chemical features, crystallinity and superconducting properties. Then, the local structure changes, especially around the CuO 5 square pyramid in T12Ba2Ca2Cu30~o have been investigated on the single crystal X-ray diffraction analyses in the wide range of temperature above and below T~.
2. Experimental procedure 2.1. Sample preparation and characterization A mixture of 2BaCuO 2 + T1203 + 2CaO + 2CuO was used as a starting material. BaCuO 2 was presynthesized from BaCO 3 and CuO at 1073 K for 48 h. The mixture was closely packed in a gold tube and then soaked at 1203 K for 3 h. After soaking, it was cooled to 1143 K at a rate of 400 K / h and held for 20 h at 1143 K, and then quenched to room temperature. The quenched sample was pulverized, packed and reheated in a gold tube at 1173 K for 3 h, and
cooled to room temperature at a rate of 2 K / h . Details of the crystal growth was described elsewhere [12,31-33]. The as-grown single crystals with well-developed habits were obtained in the dimension of 300 X 300 X 100 p~m3, which corresponded to the axes al X a 2 X c, respectively. The sample was cut into an irregular planar shape with the dimension of 90 x 90 x 60 ~m 3 and annealed at 843 K for 24 h in a flow of oxygen gas in order to make disordered oxygen atoms rearranged and/or over-oxygenated. The cut and annealed sample was exactly used for all of characterization experiments mentioned below and in the local structure change investigation. The magnetic susceptibilities of the grown crystals were measured by a DC-SQUID magnetometer, MPMS of Quantum Design Co. Ltd. The crystallinity was checked by the Burger precession and four-circle diffraction methods using MoKtx-radiation. The crystal symmetry was initially estimated from the Burger precession analysis. The conditions limiting possible reflections corresponded to the I 4 / m or I 4 / m m m space group. Annealed crystal showed higher quality on the superconducting properties and crystallinities than the as-grown one, as respectively shown in Fig. 1 and Table 1. The magnetic susceptibilities after oxygen annealing show a sharp superconducting transition a t Tc°nset = 117 K, changing from a broad one at T~°nset = 105 K before annealing. The tetragonal cell dimensions of the I lattice and the full-width at half-maximum (FWHM) values of 220 and 019 reflections were also improved by annealing, accompanied by a unit cell change in the tetragonal I-lattice, owing to the oxygen ordering a n d / o r oxidation. The chemical composition was averaged in five different points estimated by a scanning electron microscope with an electron-prove microanalyser (SEM-EPMA), JSMT220 of JEOL Co. Ltd. using the atomic-number absorption and fluorescence (ZAF) corrections. The final formula was TIL9Ba2Cal.7Cu3.10x, where the atom content of oxygen was not determined.
2.2. Crystal structure analysis The intensity collection for the structure analysis was carried out at 290, 190, 130, 120, 115 and 90 K using a four-circle automatic diffractometer, AFC5S
33
M. Hasegawa et al./Physica C 267 (1996) 3 1 4 4
(a) 2
H = 50e//c-plane
0
S
Q~I~!
/ j
-2
T c = 105 K
o
-4 O •
-6 • -8
|
|
|
I
•
i
•
i
•
!
20
0
• •
•
•
•
i
40
0~
• .
.
i
I
60
•
•
•
80
i
.
100
.
,
120
Temperature (K)
(b) 2
•
..
i . , ,
I
q
.
.
I
.
,
l
l
J
l
,
I,
H = 50e//c-plane
0
/
I . | , ,
•
d ~
•
=. -2
/ T c = 117 K
o
-4
=o • • • , , •,•,...m,
-8
-10
i
0
i
i
I
20
I . ,
I
40
.
,
.
I
,
,
60
.
I
,
,
t
80
l
$' .
,
.
100
I . ,
120
140
Temperature (K)
Fig. 1. Temperature dependence of the magnetic susceptibility under the magnetic field of 5 0 e applied parallel to the c-plane: (a) before oxygen annealing, (b) after oxygen annealing.
of RIGAKU Co. Ltd. with a cold nitrogen gas blowing system, as well as analyses of the symmetry and cell dimensions. They were installed in a radiation-protected box covered with vinyl resin sheets inside and outside. Cold gas was blown to the sample through a Kapton foil nozzle attached on the transfer tube. Temperature at the sample position in the cold nitrogen gas was calibrated using a K-type thermocouple before and after every intensity collection. The temperature was varied in _ 1.5 K whose width depended on measuring time and three axial (co, X, ~b) angles of the diffractometer. The sample of which the c-axis was parallel to the @axis of the diffractometer was placed on top of the quartz glass fiber (°D~bl50 mm) by epoxy resin. The sample was cooled at a rate of 1 K / m i n between 290 K and 150 K and 0.5 K / m i n between 150 K down to 90 K, and
held for an hour at each temperature for the intensity collection. The cell dimensions were measured at an interval of 10 K between 290 K and 150 K and at an interval of 5 K between 150 K and 90 K, and then calculated using well-centered 24-25 reflections in the half-slit method in the range 2 0 ° < 2 0 < 33 ° using M o K ~ (0.71069 A,). Each reflection used for every cell dimension calculation was the same as the checking sample. Before all intensity data collections, the symmetry was checked using the reflections in the range 3 ° < 2 0 < 40 ° in no systematic absent setting mode and at a slow collecting speed (2 ° min-1 in co). All intensity data were only collected equivalently in the range of 2 0 < 155 ° in the co-scan mode. Values of an A term in the scan ranges formula for A co = A + B tan 0 were refined before each intensity collection, and B values were fixed at 0.0. The used slits were fixed to be 1.5 ° horizontal and vertical ones, respectively. The other collection conditions are summarized in Table 2. The centrosymmetric space group I 4 / m m m was chosen and finally confirmed by following the structure solution. After corrections for the Lorentz and polarization effects, the structure was solved by the Patterson synthesis following the successive Difference-Fourier synthesis, and refined on the structure factor F by the full-matrix least-squares methods. A weighting scheme (co) of 1 / o -2 IF01 was assigned to each reflection, where o-IF01 was the error derived from counting statistics. The function, E co([ F0 ] - I Fc [)2, was minimized. Before the refinement of the anisotropic temperature factors, the F 0 values were corrected for the absorption effect by the program DIFABS [34], in the mode utilizing 0-dependent systematic deviations IF01 - IF c I. The decay correction was applied to the data sets of which the monitorious reflection intensities changed over + 3 % . The isotropic secondary extinction was also corrected in the final stage of refinement. The
Table 1 Refined cell dimensions and FWHM values for the 109 and 220 reflections before and after •2-annealing, respectively a (,~) Before After
c (/~)
3.856(4) 35.761(8) 3.8603(6) 35.778(4)
FWHMIo9(°)
FWHM22o(°)
0.22 0.15
0.30 0.18
M. Hasegawa et al./ Physica C 267 (1996) 31-44
34
Table 2 Crystallographic data and intensity collection conditions
neutral atomic scattering factors were taken from the International Tables for X-ray Crystallography, Vol. IV [35]. All calculations were conducted using the structure determination package teXsan [36]. The results of each refinement are summarized in Table 3. The final reliability values, R and wR, were 3 - 5 % and 3 - 4 % for each data set, respectively.
Crystal data T1 ,.91Ba2.o3Ca 1.70Cu 3.06Ox (from EPMA) 2, 447.93, 6.97, 1114.19 0.9 × 0.09 × c 0.06 mm 3
Formula: Z,/z, d c , FW: Sample size: Intensity data collection Sample setting: Used collimator: Scan technique: Scan width ( A-value): Used slits ( H and V ): Scan speed: Max. number of repetition: Repetition criterion:
c-axis parallel to ~b-axis "0.3 mm to 1.3° - 1.0° (B-value fixed at 0.1 ) 1.5°, 1.5° 6 ° min- l 10 o'(Fo)/[ Fo I < 0.01
Max. (sin 0 ) / a : Max. h, k, l, 20: Unobservation indices:
1.3737 ,~- i 10, 10, 97, 155° h + k + l = 2n - 1 and h > k
3. Results and discussion
3.1. Temperature dependence of cell dimensions The temperature dependence of the cell dimensions is shown in Fig. 2. The ratios of each cell dimension change between 90 K and 290 K were estimated to be 0.998 and 0.996 for the a- and c-axes, respectively. Linear thermal expansion coefficients of T12Ba2Ca2Cu3Oi0 in the measured temperature range were obtained to be a a = 1.0(1)x 10 -s K -1 and a c = 1 . 9 ( 2 ) × 10 -5 K -1 in the aand c-axes, respectively. The coefficient of the c-axis is larger than that of the a-axis. A similar tendency was also reported on the Y B a 2 C u 3 0 x phase [37], whose coefficients are a .YBCO c = 6 × 10 -6 K-1 and a~cBc° = 2 × 10 -5 K - ~ for the a- and c-axes, respectively. The thermal expansion coefficient in the
Intensity Monitorius Reference reflections: Interval:
220, 109 Every 50 reflections
Intensity data reduction Lp correction: Absorption correction type: Decay correction:
Yes DIFABS over + 3%
Table 3 Final refinement data Temperature T (K) a(A.)
T = 290
3.8603(6)
c(/~)
35.778(4)
R(%)
4.5 4.1 2.91 1535 603 0.2075(4) 0.48(2)E-6 0.06 0.981-0.999 3.047
wg(%) GOF No. of obsedved ref. No. of used ref. Scale factor Extinction factor Max. shift Absorption range Decay (%) Max. density (e,~,) Min. density (e.~) O4A occupancy (%) O4B occupancy (%)
T = 190
T = 130
T = 120
T = 115
T = 90
3.855(2)
3.854(1)
3.853(2)
3.853(2)
3.853(1)
35.716(7)
35.672(7)
35.655(7)
35.653(7)
35.640(5)
4.4 5 1.52 737 550 0.1498(4) 4.9(4)E-7 0.09 0.946-1.032 - 6.502
4.5 3.3 2.99 1142 712 0.2036(3) 3.4(1)E-7 0.04 0.822-1.006 -0.041
3.9 3.3 2.29 1250 757 0.1717(3) 0.6(1)E-7 0.02 0.968-1.003 - 1.122
4.6 4.5 1.88 1126 595 0.1384(3) 0.34(2)E-6 0.05 0.980-1.002 3.581
4.4 4.6 1.99 865 530 0.1447( 1) 0.66(3)E-7 0.03 0.969-1.004 2.658
3.885
3.749
4.156
5.145
4.214
4.493
- 3.130
- 4.362
- 3.338
- 3.700
- 4.717
- 3.727
82.4 17.6
52.0 48.0
52.8 47.2
96.0 4.0
99.2 (0.8)
100.0 0
M. Hasegawa et al. / Physica C 267 (1996) 31-44
35
cell volume of Tl2Ba2Ca2Cu30]0 was estimated to be av = 4.0(3) x 10 -5 K - 1 . These behaviors of the temperature dependence show no drastic change in this temperature range, indicating the maintenance of the crystal structure down to 90 K.
3.2. General features of the solved structure and chemical formula From the structure analysis, all atoms were located on the special positions of I 4 / m m m . Ogbome et ai. maintained that the oxygen site in the T1-O
(a)3.864 T
,.< 3.862
~
117
K
3.860 ell
" o
.t
3.858 3.856
!
T1-O4 Ba-O3
3.854
Cu2-O2 Ca Cul-O1
,.a 3.852 3.850
. . . .
:
.
[ . . .
100
50
:
. . . .
i
. . . .
150
200 Temperature (K)
:
. . . .
250
300
(b)35.8o r 'c- ,
117
K
..~
. ~ 35.77
i
= 35.74 e~
i~÷~f~÷~
35.71 '~ 35.68 ,..a 35.65
..
(C) 534
,
100
50
. . . .
i
533
200 Temperature (K) - :-
T
•
i= 117
",,
•
i
. . . .
|
. . . .
K
250
i
300
. . . .
,.~..~.....
A" 532
•
~..t.~..~ ~
531
~..~
530
::
529
~1,~'~I~
528
,
150
50
......
100
'. . . . . . . . . . . . . . . . . . 150 200 Temperature (K)
250
al Fig. 3. Crystal structure of T12Ba2Ca2Cu3Olo at 90 K. Each circle designates the following: closed: TI; dotted: Ba; shaded: Ca; and open: O atoms, respectively. Each copper atom is located at the centre of the square plane in the following manner: squareplane: Cul and square-pyramid: Cu2.
300
Fig. 2. Temperature dependence of the cell dimensions: (a) a-axis, (b) c-axis, (c) cell volume.
layer was located at the 16 m site [6] and Sinclair et al. maintained that the thallium and oxygen sites in the T I - O layer were also located on 16 m sites [7]. In this study, however, these atoms are found to be located on 4e sites and form an ideal perovskite-related structure. The refined structure at 90 K is shown in Fig. 3, which is essentially the same as that reported in the previous studies [1-8]. The estimated atomic parameters and bond distances are listed in Tables 4.1, 4.2 and 5.1, 5.2, 5.3, respectively, where the bond distances concerned with the oxygen atoms in the thallium-oxygen layers above 120 K are omitted because these atoms exist at disordered two split sites (O4A and O4B) above 120 K and an ordered one site (04) below 115 K, as mentioned in the next section. Refinements on the site occupancies (Table 3) indicate that there exist vacancies in the
36
M. Hasegawa et aL / Physica C 267 (1996) 31-44
Tl-site and partial substitution of thallium atom in the Ca site. The total oxygen content estimated from each oxygen occupancy refined from the 90 K data was almost 10 which is equal to the stoichiometric value in the Tl2Ba2CazCu3Olo phase. The chemical formula estimated from the refined site occupancies is T12.110Ba2Cal.696CU3Olo, which is almost the same as Tll.9Ba2Cal.7Cu3.10 x estimated from the EPMA data where the atom content of oxygen was not determined. The valences of thallium and copper were estimated from the crystallochemical requirements as follows. The thallium valence at the TI site is thought to be trivalent based on both of the coordination type and the average T1-O distance. The thallium substituted for the Ca site is also estimated to be trivalent on the grounds of the evidence that thallium actually has a tendency to substitute for the Ca site of Tl2Sro.2Ca2.806 in the trivalent state [38].
The copper valences at the Cul and Cu2 sites are estimated from the coordination types. Copper at the former site is in the square plane coordination, and this type of coordination was observed not only in the divalent copper compounds, SrCuO 2 [39] and YaCu205 [40], but also in the trivalent copper compounds, KCuO 2 [41], NaCuO 2 [41] and Ba 4MCuO4(CO3) 2 ( M = Li and Na) [42]. For these crystallochemical reasons, the divalent and trivalent copper ions are thought to have an ability to coexist together in the present Cul site. As the copper at the latter site in the monocapped pyramid type coordination has been reported to exist only in the compound containing a divalent copper ion, the valence state of the present Cu2 should be kept divalent. The final crystallochemical requirement, charge neutrality, leads to the chemical formula of this crystal to be 3+ 2+ 2+ 3+ 2+ 2+ 3+ T1Lso6Ba2 (Cao.848Tlo.152)2Cu2 (Cu0.722Cu0.278)-
o,20.
Table 4.1 Final metal-atomic parameters Temperature T (K)
T= 290
T= 190
T= 130
T= 120
T= 115
T= 90
TI Oct. Z Beq (~,) U11 033
0.1142(6) 0.22010(2) 1.972(5) 0.0310(2) 0.0130(2)
0.0935(8) 0.22009(2) 1.303(6) 0.0207(3) 0.0082(2)
0.1107(8) 0.22003(2) 1.172(5) 0.0186(2) 0.0073(2)
0.0979(9) 0.22004(2) 1.112(5) 0.0190(2) 0.0043(2)
0.1125(6) 0.22007(1) 1.154(4) 0.0194(2) 0.0050(1)
0.1129(5) 0.21999(1) 0.987(3) 0.0162(1) 0.00516(9)
Ba O c c . Z Beq (,~) U 11 U33
0.125(Fix) 0.14488(2) 1.009(5) 0.0106(2) 0.0172(2)
0.125(Fix) 0.1 4475(2) 0.541(5) 0.0038( 1) 0.0130(3)
0.125(Fix) 0.1 4470(2) 0.383(5) 0.0019( 1) 0.0107(3)
0.125(Fix) 0.1 4474(2) 0.364(3) 0.0025(2) 0.0088(3)
0.125(Fix) 0.1 4473(2) 0.335(4) 0.0023( 1) 0.0081(2)
0.125(Fix) 0.14472(1) 0.245(3) 0.00088(9) 0.0076(I)
Ca/TI O c c . Z Beq (,~) UI 1 U33
0.096(3)/0.029 0.0459(1) 0.50(2) 0.0074(8) 0.004(1)
0.099(3)/0.026 0.046(1) 0.6(1) 0.005(1) 0.0014(2)
0.118(3)/0.007 0.04601(8) 0.49(2) 0.0033(5) 0.012(1)
0.109(4)/0.016 0.0462(1) 0.42(2) 0.0024(9) 0.011(1)
0.112(3)/0.013 0.04613(7) 0.35(2) 0.0022(1) 0.0009(2)
0.106(3)/0.019 0.04600(8) 0.49(2) 0.0005(5) 0.018(1)
Cu I Occ. Beq (.~,) U 11 U33
0.0625(Fix) 0.64(1) 0.0061(4) 0.0123(6)
0.0625(Fix) 0.26(1) 0.0009(4) 0.0080(6)
0.0625(Fix) 0.36(9) 0.0040(2) 0.0039(7)
0.0625(Fix) 0.221(8) 0.0030(8) 0.0024(6)
0.0625(Fix) 0.075(6) 0.0031(2) 0.0022(4)
0.0625(Fix) 0.015(8) 0.001 4(2) 0.0034(3)
Cu20cc. Z Beq (,~) U11 U33
0.125(Fix) 0.08921(4) 0.83(1) 0.0077(3) 0.0163(5)
0.125(Fix) 0.08926(5) 0.47(1) 0.0029(3) 0.011 9(5)
0.125(Fix) 0.08905(5) 0.30(1) 0.001 9(3) 0.0086(6)
0.125(Fix) 0.08910(4) 0.281(6) 0.0018(4) 0.0072(5)
0.125(Fix) 0.08919(3) 0.250(8) 0.13015(2) 0.0066(4)
0.125(Fix) 0.08924(3) 0.120(6) 0.0007(2) 0.0060(3)
37
M. Hasegawa et a l . / Physica C 267 (1996) 31-44
Table 4.2 Final oxygen atomic parameters Temperature r (K)
T= 290
T = 190
T = 130
T = 120
T= 115
T= 90
O10cc. Beq (A) U 11 U22 U33
0.125(Fix) 1.0(1) 0.013(3) 0.003(3) 0.022(3)
0.125(Fix) 0.6(1) 0.007(3) - 0.000(2) 0.001 6(3)
0.125(Fix) 0.53(9) 0.004(3) 0.003(3) 0.01 4(3)
0.125(Fix) 0.54(9) 0.010(3) 0.000(2) 0.010(3)
0.125(Fix) 0.37(6) 0.004(2) 0.000(2) 0.010(2)
0.125(Fix) 0.34(8) 0.005(2) 0.003(2) 0.006( 1)
02 Occ. Z Beq (,~,) U11 U22 U33
0.25(Fix) 0.0878(2) 1.06(6) 0.017(3) 0.001(2) 0.021(2)
0.25(Fix) 0.0879(2) 0.49(5) 0.005(2) 0.001(2) 0.012(2)
0.25(Fix) 0.0874(2) 0.53(6) 0.006(2) 0.001(2) 0.013(2)
0.25(Fix) 0.0876(2) 0.57(8) 0.005(2) 0.005(2) 0.012(2)
0.25(Fix) 0.0877(1) 0.46(7) 0.006(2) 0.003(2) 0.008(1)
0.25(Fix) 0.0879(1 ) 0.32(5) 0.003(1) 0.000( 1) 0.009(1)
03 Occ. Z Beq (,~) U 11 U33
0.125(Fix) 0.1646(3) 2.2( 1) 0.034(4) 0.01 6(3)
0.125(Fix) 0.1642(3) 1.4( 1) 0.017(3) 0.017(4)
0.125(Fix) 0.1634(4) 1.0( 1) 0.007(2) 0.024(5)
0.125(Fix) 0.1632(3) 1.1( 1) 0.012(3) 0.020(5)
0.125(Fix) 0.1633(2) 1.34(7) 0.022(3) 0.006(2)
0.125(Fix) 0.1641 (2) 0.85(5) 0.009(2) 0.01 4(2)
O4A(O4) Occ. Z Beq (,~)
0.103(6) 0.2740(6) 4.0(6)
0.065(18) 0.2756(12) 2.4( 1)
0.066(7) 0.275(1) 1.4(5)
0.12(2) 0.2755(5) 3.3(4)
0.124(1) 0.2754(4) 4.3(4)
0.125(1) 0.2762(3) 2.9(2)
O4B Occ. Z Beq (,~)
0.022 0.306(2) 1(1)
0.060 0.2763(15) 2.3(Fix)
0.059 0.278(2) 4( 1)
0.01 0.309(2) 2.3(Fix)
lyzed to be located on one site. However, they showed large anisotropic temperature factors and low oxygen occupancy especially above 130 K. In particular, the U33 value of the anisotropic temperature factor was grossly large. This suggests a possi-
3.4. Order-disorder of oxygen atom in the TI-O layer
In the refinement process at the first stage, oxygen atoms in the double thallium layers were anaTable 5.1 Metal-metal bond distances M-M
T1-TI TI-Ba Ba-Ba Ba-Ca Ba-Cu2 Ca-Ca Ca-Ca Ca-Cul Ca-Cu2 Cu 1-Cu 1 Cul-Cu2 Cu2-Cu2
Mult. 4 4 4 4 1 4 1 4 4 4 4 2 4
290 K
190 K
130 K
120 K
115 K
90 K
(~.)
(~)
(~.)
(~.)
~.)
(~.)
3.4682(7) 3.8603(6) 3.8332(8) 3.8603(6) 3.541(4) 3.379(1) 3.284(5) 3.8603(6) 3.186(2) 3.139(2) 3.8603(6) 3.192(2) 3.8603(6)
3.764(1)
3.4633(8) 3.854(1) 3.8269(9) 3.854(1) 3.518(3) 3.372(1) 3.289(4) 3.854(1) 3.183(2) 3.126(2) 3.854( 1) 3.177(2) 3.854(1)
3.462(1) 3.853(2) 3.825(1) 3.853(2) 3.513(4) 3.370(1) 3.295(5) 3.853(2) 3.184(2) 3.125(2) 3.853(2) 3.177(2) 3.853(2)
3.4609(6) 3.853(1) 3.8260(8) 3.853(1) 3.515(3) 3.36681(9) 3.289(4) 3.853(1) 3.183(1) 3.127( 1) 3.853( 1) 3.180(1) 3.853(1)
3.4637(5) 3.853(1) 3.8233(6) 3.853(1) 3.518(3) 3.3662(8) 3.279(4) 3.853(1) 3.180(2) 3.130(2) 3.853( 1) 3.181(1) 3.853(1)
3.831(I) 3.527(4) 3.370(1) 3.286(5) 3.183(2) 3.134(2) 3.188(2) 3.855(2)
M. Hasegawa et al./ Physica C 267 (1996) 31-44
38
bility of two occupied sites split along the c-axis. From the results of the difference Fourier synthesis, the splitting could not be confirmed, because this oxygen atom was surrounded by two heavy atoms, thallium and barium. Accordingly, another trial examination by the split model was carried out on the degree of freedom of the atomic position with respect to the z-parameter. The result converged to the model of two independent split sites (Table 3). Fig. 4 shows schematic views of the structure change of T12Ba2Ca2Cu30 m between 290 K and 90 K. The oxygen atoms in the thallium layers exist at the disordered two split sites O4A and O4B above 120 K and the ordered one site (04) below 115 K. It is interesting that the disorder-order behavior of the 0 4 atoms is similar to the results of the high pressure studies on T12Ba2CuO6+ ~, which have suggested that a rearrangement of the oxygens in the TI-O layers on applying high pressures is important for superconductivity [15-20]. For example, Klehe et al. recently investigated the high pressure effects on the electrical resistivity and Hall coefficient for three TI2Ba2CuO6+ 8 samples with different oxygen con-
tent [20], and reported a local oxygen ordering of interstitial oxygen atoms between equivalent sites in the TI-O double layer of T12Ba2CuO6+ ~ below 75-105 K. Since the TI-O layers are located above or below the CuO 5 square pyramid which is directly concerned with the electrical conduction, it is noteworthy that the local oxygen ordering in the TI-O double layer is observed below around T~ in the two different thallium-based cuprate superconductors. 3.5. Local structure changes around CuO 5 pyramid and CuO4 plane and correlation with the superconductivity
As described in the introduction, the local structure changes around the copper atoms are thought to play an important part in the superconductivity of the cuprate superconductors. The TI2Ba2Ca2Cu30 m structure has two independent copper sites, Cu 1 and Cu2. Cul is in the plane coordination, with four oxygen atoms, of CuO2 (O1) which forms an infinite two-dimensional layer spreaded along the a Itetra-a2tetra plane. This plane coordination does not exist in the
Ti 04
O4A O4B Ba-O3
O4Ao4B
04
Cu2-O2 Ca Cul-O1
90 K
115 K
130 K
290 K
Fig. 4. Projection of the TIzBazCazCu30 m structure on the (010) plane. Splitting of the O4-site to O4A and O4B sites in the TI-O layer is observed at 290 K and 130 K.
M. Hasegawa et al./Physica C 267 (1996) 31-44
structure of TI2Ba2CuO6+ d and Tl2Ba2CaCu208. Cu2 is in the CuO 5 square-pyramid coordination where the basal CuO 4 (02) plane also forms an infinite layer. In this section, the local structure changes, especially around the CuO 5 square pyramid, are described and discussed. Figs. 5 and 6 show the domain of the temperature dependence of each bond distance forming the respective coordination of the non-pyramidal CuO 4 square plane and the CuO 5 square pyramid. All distances except the Cu2-O3 (apical oxygen) distance in the CuO 5 square pyramid decrease with decreasing temperature. It should be noted that there is a slight but distinct difference between in the non-pyramidal CuO 4 square plane and the pyramidal CuO 4 basal plane. The temperature dependence of the Cu2-O2 distance shows a decline of the decrease above Tc (around 190 K) in contrast to the almost linear temperature dependence of the C u l - O 1 distance; the 0 2 - 0 2 and O1-O1 distances change in the same way. Besides, the temperature dependence of the 0 3 - 0 2 distance shows an uptrend of the decrease below around T~. It is noteworthy that only the Cu2-O3 distance decreases on cooling down to
a)
39
2.731 . . . . .
2,730
T I
IIT'K
. . . . . . . . . . . . . . .
./
2.729 2.720
.,,-"
2.727 ¢~1
C3
2,726 2.725 2.724 . . . .
2,723
|
.'...
a . . . .
100
30
100
i . . . .
i . . . .
200
250
300
Temperature (K)
(b) 1.932 . . . . .
T" -~ l'lT"K . . . . . . . . . . . . . . .
1.931 ,~
i
1.930
. ../"
i
1.929 ~
/
1.920 1.927 1.926
1oo
50
130
2o0
23o
300
Temperature (K) C)
2.710
.....
2.700
• "J ll,'K ..............
0~
~
•
2.690
.........s"'""" i
2.680 2.670 2.660 2.6$0
2.640 2,630 50 (a)
2.733 L . . . . .
T" " I 1 7 " K . . . . . . . . . . . . . . .
2.730 /
*
~
2.728
......
2.727
~+ ....................
3.724
so
i
-
-
-
i
.
' . . .
i
IOO
. . . .
10o
,
-
-
.
•
2oo
I
,
,
,
3.300
+'"'"'
= 117 K ,,
........,'"
-
1.926 . . . . 50
. . . .
t . '~. . 100
.
t . . . . 150
' 100
-
-
•
.
i 150
. . . .
I
.
,
200
.
.
I 250
. . . . 300
Temperature (K) ...'"""
1.925
..,'"""
i"
3.310
50
1,930
.
300
(b) 1.931 T
i
~.................... i
3.290
t . , 200
, .
I • , 250
o , 300
Temperature (K) Fig. 5. Temperature dependence of each bond distance forming the coordination of the CuO~ square plate. (a) O 1 - O 1 , (b)
CuI-O1.
300
.....
C~ ~ 3.320
,
20o
Temperature (K)
1.927
250
..........
3.330
,,
2.723
~
•
.g 3340
......
1.929
200
3.350
/'"'/'"
2,,2,2,,2,
.~
150
Temperature (K)
(d) 3.370
2.729 0~
I00
...+"
'
Fig. 6. Temperature dependence of each bond distance forming the coordination of the CuO 5 square pyramid. (a) 0 2 - 0 2 , (b) C u 2 - O 2 , (c) C u 2 - O 3 , (d) 0 2 - 0 3 .
Tc and then abruptly increases below Tc (Fig. 6c). These results suggest that the superconductivity is correlated with the local structure changes around Cu2. The abrupt increase similar to our data around Tc in the bond distance from copper to apical oxygen
40
M. Hasegawa et a l . / Physica C 267 (1996) 31-44
oO o
the perpendicular axes in the figures are set at 0.08 ~,. Fig. 8a shows the temperature dependence of the distance between Cul and T1 along the c-axis which
a) o~
0.02
T'"l17
. . . . .
0.01
K . . . . . . . . . . . . . . .
:: eel
c-axis/4.5392
i
TT ~.~.::+'+~ •
-0+01 -0.02 -0.03
~'~
-0.04
<~
-0.05
....
-0.06
.
.
.
.
i
Fig. 7. Part of the unit cell around the CuO 5 square pyramid of Tl2Ba2Ca2CU3Olo. Symbol notation of circles is the same as in Fig. 3; small closed circles indicate copper atoms.
.
.
.
]oo
30
i
.
.
.
.
|
.
.
.
.
i
1so 300 Temperature (K)
.
•
i
i
zso
3oo
(b) o.o7 T = 117 K
0.06
0.03
° ~
0.04
m
atom in the CuO 5 square pyramid has also been observed in T12Ba2CaCu208 by V.N. Molchanov et al. [8]. In their paper the abrupt increase was 0.3%, which is much smaller than the present data of 0.7%. They explained the abrupt increase in the CuO 5 square pyramid in TI2Ba2CaCu208 by the JahnTeller effect where the population of local holes in the C u - O bonds changed with temperature. However, it is hard to explain the changes of the C u - O distances in both Tl2Ba2Ca2Cu3Ol0 and Tl2Ba 2 CaCu208 structures by this effect, because the Cu20 2 distance forming a basal plane in the square pyramid shows no significant change around T~, as shown in Fig. 6b as well as in their result of Fig. 4 in Ref. [8]. If the Jahn-Teller effect influences these distance changes, both the Cu2-O2 distance in the TlEBaECa2Cu3Ol0 structure and the Cu-O1 in TlEBaECaCu2Os, forming the basal plane in the CuO 5 square pyramid coordination, have to show a more considerable change than observed in the Molchanov's and our data around Tc. In order to understand the anomalous behavior of the Cu2-O3 bond distance, we, first, pay attention to the distance changes around each Tl and Cul atom along the c-axis, that is, C u l - C u 2 , Cu2-O3 and O3-Tl. Fig. 7 shows a local structure around Cul, Cu2, 0 3 and T1. The temperature dependence of the bond distances is presented in Fig. 8, where the perpendicular axes show differences in the bond distances from those at 290 K. The entire lengths of
A 0
..............
0.03
0.02 0.01 0
(1.99) •
.
.0.0150 • . .
.
i
.
.
.
,
100
.
.
.
.
,
150
.
,
,
,
200
!
,
,
,
300
250
Temperature (K) (C)
o4
o.ol
.....
o (2.70) -0.01
T:
+........... +............. ,++...........
-0.03 -0.04
<3
-0.05
+,L.,,"
-0+02
¢~
.............
'",
,
-0.06 .0,07
.
.
.
.
50
i
.'
.
.
I
100
.
.
.
.
150
i
o
,
,
300
,
I
,
•
,
j
250
300
Temperature (K)
(d)
oo,
o~.
0.02
T c = 117 K
0.03
0.01 0
,,,.q (3,102)
...O ................................ •
-0.0l <~
O l h ,~ ip...+,, -.......... ""
.o.o2 -0.03 -0.04
.
50
.
.
.
'
tO0
•
'"
•
"
+
150
.
.
.
.
'
.
.
200
.
.
a
250
.... 300
Temperature (K) Fig. 8. Temperature dependence of the bond distance along the c-axis (see Fig. 7). The perpendicular axis shows the diffence from the bond distance at 290 K (the value is written in parentheses). All lengths of the perpendicular axes in the figures are 8 ~. (a) total distance of C u l - C u 2 - O 3 - T 1 . The open circles are plots of about one fourth of the c-axis. (b) O3-TI, (c) Cu2-O3, (d) Cu I -Cu2.
M. Hasegawa et al./Physica C 267 (1996) 31-44
is equal to the total distance of Cul-Cu2-O3-T1, and to about one fourth of the c-axis length. It is found that these two data items from the local structure analysis and from the lattice constant determination (Fig. 2b) behave in almost the same way. Figs. 8b, c and d show the temperature dependence of each two-atom bond distance between Cul and TI. It is found that both the Cu2-O3 and O3-TI distances change drastically below T~ in the opposite directions. On the other hand, the C u l - C u 2 distance shows no drastic change below Tc. From these resuits, it is concluded that the local movement of 03 except Cu2 occurs during cooling and the anomalous behavior of the Cu2-O3 distance as shown in Fig. 6b is attributable to this anomalous movement of the 03 atom. In order to understand the anomalous behavior of the 0 3 atom, we pay attention to the local behavior of the in-plane 0 2 oxygen atom in the basal plane of the CuO 5 square pyramid, and try to visualize the relative changes in the 0 2 atom with temperature
41
towards 03 and Cu2 on the basis of the experimental results on bond distances in Tables 5.2, 5.3. Since from the structure analysis the structure of the T12Ba2Ca2Cu3O10 single crystal in this study was found to keep the I 4 / m m m symmetry down to 90 K, the 03, the Cu2 and 0 2 atoms in the CuO 5 square pyramid form a triangle and are located at the two-dimensional rectangular coordinates as shown in Fig. 9. There the Cu2 atom is fixed at (0, 0) because of the conclusion on the mobile 0 3 atom in the above paragraph, and because the 03 and 0 2 atoms are located at (0, Y3) and ( x z, Y2), respectively. One can obtain the relative position of each atom and angles in the triangle at each temperature by the following calculations based on the geometric relations of the O3-Cu2-O2 triangle. If a = 03-02,
(1)
b = Cu2-02,
(2)
c = O3-Cu2,
(3)
we have the following equations from the geometric relations:
(a)
03
a2 = x2 + (Y3 - Y2) 2,
(4)
b 2 = x~ +y22,
(5)
c=Y3
(6)
(Y3 > 0 ) -
From Eqs. (4)-(6), we can obtain each position and angle as follows: 02 02 02 02'
Cu2 = (0, 0),
(7)
03 = (0, c),
(8)
0 2 = (¢b 2 - ( c 2 - 02 + b2)2/4c 2 ,
(b)
( c 2 - a 2 + b2)/2c), 03(0, Ys)
(9)
/_ O3-Cu2-O2 = cos[(b 2 + c 2 - a 2 ) / 2 / O / c ] .
(m)
Cu2(O, O) ~
02(xv Y~)
Fig. 9. (a) CuO5 square pyramid, (b) O3, Cu2 and 02 atoms in the CuO5 square pyramid forming a triangle in the two-dimensional rectangular coordinates.
The temperature dependence of the relative behavior of each atom, Cu2-O2 bond, and /_ 0 3 Cu2-O2 angle is shown in Fig. 10, where (a) and (b) show the relative atom positions in two-dimensional rectangular coordinates in units of A. It is found from Fig. 10a that the local structure changes at each temperature are far smaller than the bond distances. Fig. 10b indicates that the in-plane oxygen 0 2 atom
M. Hasegawa et al./Physica C 267 (1996) 31-44
42
(a) o<~
atom position in O3-(Cu2-O2) pyramid 3.0
.
, . . .
,
.
.
.
,
.
.
.
~ .
T 2.5
I
r"2
2.0
[o3~ I -I
~
1.O
"~
0.5
,
.
O : A:
~
~
.
.
,
.
290 K 190 K
[3: 13OK O : 120 K • : 11s K
~
Co2 0
•~ ,~
.
= 117 K
~
~"
.
lm
-O.S
am
. . . . .
'
0
. . . . . . .
0.4
'
0.8
.
.
.
.
.
1.2
.
.
'
1.6
-
2.0
a-axis position from Cu2 (~)
(b) 0
~
.o.o2
~
-0.04
~x.
~
"~
• ! 11s K
290 0 2 behavior
190
-0.07 . . . . * . . . . ' . . . . ' . . . . ' . . . . ' . . . . 1.925 1.926 1.927 1.928 1.929 1.93 1.931
a-axis position from Cu2 (A)
(c) 0.01
-O.Ol
90
-0,02
-0.03 ~, ~,
-0.04
/ A: 190 K ]- [] : 130 K / © : 12o~
-0.05
•
.
* .
.
.
0
r,.)
.
.
l
|
~ . .
.... .
,.~ .
..I
|
/
115
b
-0.07
C
|
~
: 115 K
.~ -o.o6
(d)
~"-
~
.
.
.
~
.
.
.
t
.
.
.
,f._.f
.
.
.
,4 O
.
.
J
2.0
. . . .. . . . . . . . . . .
.•,
~, o .............. • ...... . . . . . . .
9t.s
9Lo
t
0.4 o.s 1.2 1.6 a-axis position from Cu2 (A)
C
O "
i,
179
90.0
. . . . 50
* . ,...
1O0
L~
,4 O
i
i
lS0
. . . .
i
. . . .
200
i . . . . 250
180 300
Temperature (K) Fig. 10. T e m p e r a t u r e dependence o f the atom positions in the O 3 - C u 2 - O 2 triangle a n d angle / O 3 - C u 2 - O 2 in the C u O 5 square pyramid. (a), (b) and (c) are in two-dimensional rectangular coordinates in ,~ units. (a) O 3 - C u 2 - O 2 triangle, (b) in-plane o x y g e n 0 2 , (d) C u 2 - O 2 b o n d , (d) angle / O 3 - C u 2 - O 2 in the C u O 5 square pyramid.
moves in two steps. At the first step, the 0 2 atom approaches toward the Cu2 atom above Tc up to within about 1.927 ~,, as is well shown in Fig. 6b,
which is almost equal to the sum of the ionic radii of Cu 2+ and 0 2 - . Then, it is noteworthy that at the second step the 0 2 atom moves up largely and abruptly around Tc. Accordingly, it is concluded that this abrupt movement of the in-plane oxygen 0 2 atom results in the anomalous upward movement of the apical oxygen 03 atom around Tc. As mentioned above, the mobile in-plane oxygen 0 2 atom is found to hold the key to the local structure change around the CuO 5 square pyramid. From the viewpoint of the correlation of the local structure change with the superconductivity, it should be noted that the most attractive point is shown in Figs. 10c, d. That is, the abrupt upward movement of the 0 2 atom results subsequently in making the "exactly planar" CuO 4 basal plane in the CuO s square pyramid just below Tc, as shown in Fig. 10c. This scheme is also shown in Fig. 10d. The angle/_ O 3 - C u 2 - O 2 suddenly tends to become 90 ° just below Tc which also means that the angle Z_ 0 2 Cu2-O2' (opposite side) becomes 180°. This movement of the 0 2 atom results in changing the electronic structure of the basal CuO 4 plane in the CuO 5 square pyramid and is thought to be advantageous for the electric conduction. Another notable point on the correlation with the superconductivity is that the mobile 0 2 atom induces the continuous decrease of the 0 3 - 0 2 distance down to 90 K and the further decrease below Tc, as shown in Fig. 6c, even though the Cu2-O3 distance increases below T~. As mentioned in the introduction, the local proximity between the apical oxygen atom and in-plane oxygen atom in the CuO 5 square pyramid was reported to play an important role on the superconductivity, experimentally by Egami et al. [2,6,10]; and theoretically by Andersen et ai. [ 13,14]. They pointed out that the shortening of the apical and in-plane oxygen atoms in the CuO 5 square pyramid could make the oxygen band in higher energylevel over the Fermi level. As a result, this scheme could produce holes as carriers and then stabilize such a short distance of the apical oxygen atom and in-plane oxygen one in the superconducting state. The energy gap due to the superconducting state makes its electronic energy and also its total energy lower, and subsequently keeps such a local structure more stable. Consequently, the mobile in-plane oxygen 0 2 atom, which induces the exactly planar CuO 4
M. Hasegawa et aL /Physica C 267 (1996) 31-44
basal plane and the continuous decrease of the 0 3 0 2 distance, can stabilize the changed local structure around the CuO 5 square pyramid. From this consideration, it is concluded that the mobile in-plane oxygen atoms in the CuO 5 square pyramid lead to the local structure changes in and around the pyramid and induce the change of the electronic structure. Besides, they would be exactly correlated with the superconductivity in all of the cuprate superconductors.
4. Conclusions The local structure changes between 290 K and 90 K of a Tl2Ba2Ca2Cu30~0 single crystal have been investigated using the X-ray diffraction technique. The linear thermal expansion coefficients were obtained to be ot~= 1.0(1)x 10 -5 K -~ and a , . = 1.9(2) × 10 -5 K-~ in the a- and c-axes, respectively. The final crystallochemical requirement, charge neutrality, leads to the chemical formula of this crystal to be TI 3+ 2+ (Cao.848T10.152)2Cu: 2+ 3+ 2+ 1.8o6Ba2 2+ 3+ 2(Cuo.722Cu0.278)O~0 . The oxygen atoms in the thallium-oxygen layers were found to exist at the disordered two split sites above 120 K and the ordered one site below 1 15 K. It was also found that the mobile in-plane oxygen atoms in the CuO s square pyramid induced the local structure changes in and around the pyramid. Besides, it should be noted that they were extremely correlated with the superconductivity of the cuprate superconductors.
Acknowledgments The authors are indebted to M. Tamura, Institute for Solid State Physics, the University of Tokyo, for his supports on employing SQUID magnetometer systems. They also thank to F. Sakai and M. Koike for their assistance throughout this work.
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43
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44
M. Hasegawa et aL / Physica C 267 (1996) 31-44
[29] S.D.C. Sinclair, M.A.G. Aranda, P. Attfield and J.R-Carvajal, Physica C 225 (1994) 307. [30] M. Nunez-Regueiro, J.-L. Tholence, E.V. Antipov, J.-J. Capponi and M. Marezio, Science 262 (1993) 97. [31] M. Hasegawa, Y. Matsushita, Y. lye and H. Takei, Physica C 231 (1994) 161. [32] M. Hasegawa, Y. Matsushita and H. Takei, Advances in Superconductivity VII, eds. K. Yamafuji and T. Morishita (Springer, 1995) pp. 723-728. [33] Y. Matsushita, M. Hasegawa and H. Takei, Jpn. J. Appl. Phys. 34 (1995) L1263. [34] N. Wacker and D. Stuart, Acta Crystallogr. A 39 (1983) 158. [35] International Tables for X-ray Crystallography, Vol. IV (Kynoch Press, Birmingham, 1974). [36] teXsan: Molecular Structure Corporation, Arizona, USA (1992).
[37] H. You, J.D. Axe, X.B. Kan, S. Hashimoto, S.C. Moss, J.Z. Liu, G.W. Crabtree and D.J. Lam, Phys. Rev. B 38 (1988) 9213. [38] K. Ruck, H. Bormann, and A. Simon, Z. Natuffosch. 49B (1994) 635. [39] Y. Matsushita, Y. Oyama, M. Hasegawa and H. Takei, J. Solid State Chem. 114 (1995) 289. [40] R.D. Adam, J.A. Estrada and T. Datta, J. Supercond. 5 (1992) 33. [41] N.E. Brese, M. O'Keeffe, R.B. yon Dreele and V.G. Young, Jr., J. Solid State Chem. 83 (1989) 1. [42] P.D. VerNooy and A.M. Stacy, J. Solid State Chem. 95 (1991) 270.
Physica C 267 (1996) 31-44
Local structural changes in T12Ba2Ca2Cu3Olosingle crystal between 90 K and 290 K Masashi Hasegawa * ,a, Yoshitaka Matsushita a, Humihiko Takei h a Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo, Japan b Faculty of Science, Osaka University, Machikaneyama, Toyonaka-shi, Osaka 560, Japan Received 13 March 1996; revised manuscript received 30 April 1996
Abstract
The local structure changes of a Tl2BaeCa2Cu30~0 single crystal (Tc = 117 K) with good crystallinity and superconducting properties have been investigated between 290 K and 90 K using an X-ray diffraction technique. The linear thermal expansion coefficients are obtained to be a a = 1.0(1)X 10 -5 K -~ and c%= 1.9(2)× 10 -5 K -1 in the a- and c-axes, respectively. The oxygen atoms in the thallium layers are found to exist at the disordered two split sites above around Tc and become ordered at a fixed site below around T~. The mobile in-plane oxygen atom in the CuO 5 square pyramid induced the local structure changes in and around the pyramid. Besides, it should be noted that they are extremely correlated with the superconductivity of the cuprate superconductors.
1. Introduction
A great deal of interest has been focused on the relationship between local structure changes and superconductivity in the high-temperature superconducting cuprates [1-12]. The first attractive study was reported on the local structure in T12Ba 2CaCu208 investigated using a powder pulsed-neutron scattering by B.H. Toby et al. [2]. Their pair-distribution function analysis showed a clear change in the local structure at the onset of superconductivity, indicating the correlated displacements of oxygen and copper atoms perpendicular to the C u - O plane. They also mentioned the important role of the local shortening between the apical and the in-plane oxygen atoms in a CuO 5 square pyramid on the super-
* Corresponding author. Fax: +81 3 3401 5169.
conductivity. This was also suggested by a local-density approximation calculation on the electronic state of YBa2Cu307 [13,14]. Similar results on the local structure change in the powder pulsed-neutron scattering studies have been reported in other high temperature superconducting cuprates, such as N d 2 _ x C e x C u O 4 _ y [6] and YBa2Cu408 [11]. The local structure change in TI2Ba2CaCu208 above 60 K was also reported in the single crystal X-ray diffraction study using a four-circle automatic diffractometer by V.N. Molchanov et al. [8]. They also reported the anomaly in the temperature dependence of the distance between apical oxygen and copper in the CuO 5 square pyramid. The important roles of the local mobile oxygen have been also reported in high pressure effect studies [15], such as in TI2BazCuO6+ ~ [16-20], YBa2Cu307 [21], La2CuO4+ 8 [22]. The mobile oxygen behavior is thought to determine the pressure
0921-4534/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 3 4 ( 9 6 ) 0 0 3 2 8 - 0
32
M. Hasegawa et al. / Physica C 267 (1996) 31-44
dependence of the physical properties in these cuprates. In the case of the thallium-based cuprate superconductors, a rearrangement of the oxygens in the T1-O layers, which are located above or below the CuO 5 square pyramid, on applying high pressures was reported to play an important role for the superconductivity. In this study, T12Ba2Ca2Cu3Oto is selected as a research target to investigate the local structure changes above and below T~, T12Ba2Ca2Cu3Olo has the highest T~ in the Tl-based cuprate superconductors and non pyramidal CuO 4 planes in the unit cell, which is different from TI2Ba2CuO6+ 8 and T12Ba2CaCu208. Numbers of the X-ray and neutron diffraction studies on the crystal structure of Tl2Ba2Ca2Cu3Oto have been reported [23-31]. However, no one have investigated the temperature dependence of the local structure changes, especially around T~. In addition, few studies have used the specimens well-characterized on the chemicalphysical properties, although the relationship between the structure and chemical or physical properties is one of the important keypoints to elucidate the high temperature superconductivity. Accordingly, in this study, we have used exactly the same high quality single crystal as the sample which has been well-characterized on chemical features, crystallinity and superconducting properties. Then, the local structure changes, especially around the CuO 5 square pyramid in T12Ba2Ca2Cu30~o have been investigated on the single crystal X-ray diffraction analyses in the wide range of temperature above and below T~.
2. Experimental procedure 2.1. Sample preparation and characterization A mixture of 2BaCuO 2 + T1203 + 2CaO + 2CuO was used as a starting material. BaCuO 2 was presynthesized from BaCO 3 and CuO at 1073 K for 48 h. The mixture was closely packed in a gold tube and then soaked at 1203 K for 3 h. After soaking, it was cooled to 1143 K at a rate of 400 K / h and held for 20 h at 1143 K, and then quenched to room temperature. The quenched sample was pulverized, packed and reheated in a gold tube at 1173 K for 3 h, and
cooled to room temperature at a rate of 2 K / h . Details of the crystal growth was described elsewhere [12,31-33]. The as-grown single crystals with well-developed habits were obtained in the dimension of 300 X 300 X 100 p~m3, which corresponded to the axes al X a 2 X c, respectively. The sample was cut into an irregular planar shape with the dimension of 90 x 90 x 60 ~m 3 and annealed at 843 K for 24 h in a flow of oxygen gas in order to make disordered oxygen atoms rearranged and/or over-oxygenated. The cut and annealed sample was exactly used for all of characterization experiments mentioned below and in the local structure change investigation. The magnetic susceptibilities of the grown crystals were measured by a DC-SQUID magnetometer, MPMS of Quantum Design Co. Ltd. The crystallinity was checked by the Burger precession and four-circle diffraction methods using MoKtx-radiation. The crystal symmetry was initially estimated from the Burger precession analysis. The conditions limiting possible reflections corresponded to the I 4 / m or I 4 / m m m space group. Annealed crystal showed higher quality on the superconducting properties and crystallinities than the as-grown one, as respectively shown in Fig. 1 and Table 1. The magnetic susceptibilities after oxygen annealing show a sharp superconducting transition a t Tc°nset = 117 K, changing from a broad one at T~°nset = 105 K before annealing. The tetragonal cell dimensions of the I lattice and the full-width at half-maximum (FWHM) values of 220 and 019 reflections were also improved by annealing, accompanied by a unit cell change in the tetragonal I-lattice, owing to the oxygen ordering a n d / o r oxidation. The chemical composition was averaged in five different points estimated by a scanning electron microscope with an electron-prove microanalyser (SEM-EPMA), JSMT220 of JEOL Co. Ltd. using the atomic-number absorption and fluorescence (ZAF) corrections. The final formula was TIL9Ba2Cal.7Cu3.10x, where the atom content of oxygen was not determined.
2.2. Crystal structure analysis The intensity collection for the structure analysis was carried out at 290, 190, 130, 120, 115 and 90 K using a four-circle automatic diffractometer, AFC5S
33
M. Hasegawa et al./Physica C 267 (1996) 3 1 4 4
(a) 2
H = 50e//c-plane
0
S
Q~I~!
/ j
-2
T c = 105 K
o
-4 O •
-6 • -8
|
|
|
I
•
i
•
i
•
!
20
0
• •
•
•
•
i
40
0~
• .
.
i
I
60
•
•
•
80
i
.
100
.
,
120
Temperature (K)
(b) 2
•
..
i . , ,
I
q
.
.
I
.
,
l
l
J
l
,
I,
H = 50e//c-plane
0
/
I . | , ,
•
d ~
•
=. -2
/ T c = 117 K
o
-4
=o • • • , , •,•,...m,
-8
-10
i
0
i
i
I
20
I . ,
I
40
.
,
.
I
,
,
60
.
I
,
,
t
80
l
$' .
,
.
100
I . ,
120
140
Temperature (K)
Fig. 1. Temperature dependence of the magnetic susceptibility under the magnetic field of 5 0 e applied parallel to the c-plane: (a) before oxygen annealing, (b) after oxygen annealing.
of RIGAKU Co. Ltd. with a cold nitrogen gas blowing system, as well as analyses of the symmetry and cell dimensions. They were installed in a radiation-protected box covered with vinyl resin sheets inside and outside. Cold gas was blown to the sample through a Kapton foil nozzle attached on the transfer tube. Temperature at the sample position in the cold nitrogen gas was calibrated using a K-type thermocouple before and after every intensity collection. The temperature was varied in _ 1.5 K whose width depended on measuring time and three axial (co, X, ~b) angles of the diffractometer. The sample of which the c-axis was parallel to the @axis of the diffractometer was placed on top of the quartz glass fiber (°D~bl50 mm) by epoxy resin. The sample was cooled at a rate of 1 K / m i n between 290 K and 150 K and 0.5 K / m i n between 150 K down to 90 K, and
held for an hour at each temperature for the intensity collection. The cell dimensions were measured at an interval of 10 K between 290 K and 150 K and at an interval of 5 K between 150 K and 90 K, and then calculated using well-centered 24-25 reflections in the half-slit method in the range 2 0 ° < 2 0 < 33 ° using M o K ~ (0.71069 A,). Each reflection used for every cell dimension calculation was the same as the checking sample. Before all intensity data collections, the symmetry was checked using the reflections in the range 3 ° < 2 0 < 40 ° in no systematic absent setting mode and at a slow collecting speed (2 ° min-1 in co). All intensity data were only collected equivalently in the range of 2 0 < 155 ° in the co-scan mode. Values of an A term in the scan ranges formula for A co = A + B tan 0 were refined before each intensity collection, and B values were fixed at 0.0. The used slits were fixed to be 1.5 ° horizontal and vertical ones, respectively. The other collection conditions are summarized in Table 2. The centrosymmetric space group I 4 / m m m was chosen and finally confirmed by following the structure solution. After corrections for the Lorentz and polarization effects, the structure was solved by the Patterson synthesis following the successive Difference-Fourier synthesis, and refined on the structure factor F by the full-matrix least-squares methods. A weighting scheme (co) of 1 / o -2 IF01 was assigned to each reflection, where o-IF01 was the error derived from counting statistics. The function, E co([ F0 ] - I Fc [)2, was minimized. Before the refinement of the anisotropic temperature factors, the F 0 values were corrected for the absorption effect by the program DIFABS [34], in the mode utilizing 0-dependent systematic deviations IF01 - IF c I. The decay correction was applied to the data sets of which the monitorious reflection intensities changed over + 3 % . The isotropic secondary extinction was also corrected in the final stage of refinement. The
Table 1 Refined cell dimensions and FWHM values for the 109 and 220 reflections before and after •2-annealing, respectively a (,~) Before After
c (/~)
3.856(4) 35.761(8) 3.8603(6) 35.778(4)
FWHMIo9(°)
FWHM22o(°)
0.22 0.15
0.30 0.18
M. Hasegawa et al./ Physica C 267 (1996) 31-44
34
Table 2 Crystallographic data and intensity collection conditions
neutral atomic scattering factors were taken from the International Tables for X-ray Crystallography, Vol. IV [35]. All calculations were conducted using the structure determination package teXsan [36]. The results of each refinement are summarized in Table 3. The final reliability values, R and wR, were 3 - 5 % and 3 - 4 % for each data set, respectively.
Crystal data T1 ,.91Ba2.o3Ca 1.70Cu 3.06Ox (from EPMA) 2, 447.93, 6.97, 1114.19 0.9 × 0.09 × c 0.06 mm 3
Formula: Z,/z, d c , FW: Sample size: Intensity data collection Sample setting: Used collimator: Scan technique: Scan width ( A-value): Used slits ( H and V ): Scan speed: Max. number of repetition: Repetition criterion:
c-axis parallel to ~b-axis "0.3 mm to 1.3° - 1.0° (B-value fixed at 0.1 ) 1.5°, 1.5° 6 ° min- l 10 o'(Fo)/[ Fo I < 0.01
Max. (sin 0 ) / a : Max. h, k, l, 20: Unobservation indices:
1.3737 ,~- i 10, 10, 97, 155° h + k + l = 2n - 1 and h > k
3. Results and discussion
3.1. Temperature dependence of cell dimensions The temperature dependence of the cell dimensions is shown in Fig. 2. The ratios of each cell dimension change between 90 K and 290 K were estimated to be 0.998 and 0.996 for the a- and c-axes, respectively. Linear thermal expansion coefficients of T12Ba2Ca2Cu3Oi0 in the measured temperature range were obtained to be a a = 1.0(1)x 10 -s K -1 and a c = 1 . 9 ( 2 ) × 10 -5 K -1 in the aand c-axes, respectively. The coefficient of the c-axis is larger than that of the a-axis. A similar tendency was also reported on the Y B a 2 C u 3 0 x phase [37], whose coefficients are a .YBCO c = 6 × 10 -6 K-1 and a~cBc° = 2 × 10 -5 K - ~ for the a- and c-axes, respectively. The thermal expansion coefficient in the
Intensity Monitorius Reference reflections: Interval:
220, 109 Every 50 reflections
Intensity data reduction Lp correction: Absorption correction type: Decay correction:
Yes DIFABS over + 3%
Table 3 Final refinement data Temperature T (K) a(A.)
T = 290
3.8603(6)
c(/~)
35.778(4)
R(%)
4.5 4.1 2.91 1535 603 0.2075(4) 0.48(2)E-6 0.06 0.981-0.999 3.047
wg(%) GOF No. of obsedved ref. No. of used ref. Scale factor Extinction factor Max. shift Absorption range Decay (%) Max. density (e,~,) Min. density (e.~) O4A occupancy (%) O4B occupancy (%)
T = 190
T = 130
T = 120
T = 115
T = 90
3.855(2)
3.854(1)
3.853(2)
3.853(2)
3.853(1)
35.716(7)
35.672(7)
35.655(7)
35.653(7)
35.640(5)
4.4 5 1.52 737 550 0.1498(4) 4.9(4)E-7 0.09 0.946-1.032 - 6.502
4.5 3.3 2.99 1142 712 0.2036(3) 3.4(1)E-7 0.04 0.822-1.006 -0.041
3.9 3.3 2.29 1250 757 0.1717(3) 0.6(1)E-7 0.02 0.968-1.003 - 1.122
4.6 4.5 1.88 1126 595 0.1384(3) 0.34(2)E-6 0.05 0.980-1.002 3.581
4.4 4.6 1.99 865 530 0.1447( 1) 0.66(3)E-7 0.03 0.969-1.004 2.658
3.885
3.749
4.156
5.145
4.214
4.493
- 3.130
- 4.362
- 3.338
- 3.700
- 4.717
- 3.727
82.4 17.6
52.0 48.0
52.8 47.2
96.0 4.0
99.2 (0.8)
100.0 0
M. Hasegawa et al. / Physica C 267 (1996) 31-44
35
cell volume of Tl2Ba2Ca2Cu30]0 was estimated to be av = 4.0(3) x 10 -5 K - 1 . These behaviors of the temperature dependence show no drastic change in this temperature range, indicating the maintenance of the crystal structure down to 90 K.
3.2. General features of the solved structure and chemical formula From the structure analysis, all atoms were located on the special positions of I 4 / m m m . Ogbome et ai. maintained that the oxygen site in the T1-O
(a)3.864 T
,.< 3.862
~
117
K
3.860 ell
" o
.t
3.858 3.856
!
T1-O4 Ba-O3
3.854
Cu2-O2 Ca Cul-O1
,.a 3.852 3.850
. . . .
:
.
[ . . .
100
50
:
. . . .
i
. . . .
150
200 Temperature (K)
:
. . . .
250
300
(b)35.8o r 'c- ,
117
K
..~
. ~ 35.77
i
= 35.74 e~
i~÷~f~÷~
35.71 '~ 35.68 ,..a 35.65
..
(C) 534
,
100
50
. . . .
i
533
200 Temperature (K) - :-
T
•
i= 117
",,
•
i
. . . .
|
. . . .
K
250
i
300
. . . .
,.~..~.....
A" 532
•
~..t.~..~ ~
531
~..~
530
::
529
~1,~'~I~
528
,
150
50
......
100
'. . . . . . . . . . . . . . . . . . 150 200 Temperature (K)
250
al Fig. 3. Crystal structure of T12Ba2Ca2Cu3Olo at 90 K. Each circle designates the following: closed: TI; dotted: Ba; shaded: Ca; and open: O atoms, respectively. Each copper atom is located at the centre of the square plane in the following manner: squareplane: Cul and square-pyramid: Cu2.
300
Fig. 2. Temperature dependence of the cell dimensions: (a) a-axis, (b) c-axis, (c) cell volume.
layer was located at the 16 m site [6] and Sinclair et al. maintained that the thallium and oxygen sites in the T I - O layer were also located on 16 m sites [7]. In this study, however, these atoms are found to be located on 4e sites and form an ideal perovskite-related structure. The refined structure at 90 K is shown in Fig. 3, which is essentially the same as that reported in the previous studies [1-8]. The estimated atomic parameters and bond distances are listed in Tables 4.1, 4.2 and 5.1, 5.2, 5.3, respectively, where the bond distances concerned with the oxygen atoms in the thallium-oxygen layers above 120 K are omitted because these atoms exist at disordered two split sites (O4A and O4B) above 120 K and an ordered one site (04) below 115 K, as mentioned in the next section. Refinements on the site occupancies (Table 3) indicate that there exist vacancies in the
36
M. Hasegawa et aL / Physica C 267 (1996) 31-44
Tl-site and partial substitution of thallium atom in the Ca site. The total oxygen content estimated from each oxygen occupancy refined from the 90 K data was almost 10 which is equal to the stoichiometric value in the Tl2Ba2CazCu3Olo phase. The chemical formula estimated from the refined site occupancies is T12.110Ba2Cal.696CU3Olo, which is almost the same as Tll.9Ba2Cal.7Cu3.10 x estimated from the EPMA data where the atom content of oxygen was not determined. The valences of thallium and copper were estimated from the crystallochemical requirements as follows. The thallium valence at the TI site is thought to be trivalent based on both of the coordination type and the average T1-O distance. The thallium substituted for the Ca site is also estimated to be trivalent on the grounds of the evidence that thallium actually has a tendency to substitute for the Ca site of Tl2Sro.2Ca2.806 in the trivalent state [38].
The copper valences at the Cul and Cu2 sites are estimated from the coordination types. Copper at the former site is in the square plane coordination, and this type of coordination was observed not only in the divalent copper compounds, SrCuO 2 [39] and YaCu205 [40], but also in the trivalent copper compounds, KCuO 2 [41], NaCuO 2 [41] and Ba 4MCuO4(CO3) 2 ( M = Li and Na) [42]. For these crystallochemical reasons, the divalent and trivalent copper ions are thought to have an ability to coexist together in the present Cul site. As the copper at the latter site in the monocapped pyramid type coordination has been reported to exist only in the compound containing a divalent copper ion, the valence state of the present Cu2 should be kept divalent. The final crystallochemical requirement, charge neutrality, leads to the chemical formula of this crystal to be 3+ 2+ 2+ 3+ 2+ 2+ 3+ T1Lso6Ba2 (Cao.848Tlo.152)2Cu2 (Cu0.722Cu0.278)-
o,20.
Table 4.1 Final metal-atomic parameters Temperature T (K)
T= 290
T= 190
T= 130
T= 120
T= 115
T= 90
TI Oct. Z Beq (~,) U11 033
0.1142(6) 0.22010(2) 1.972(5) 0.0310(2) 0.0130(2)
0.0935(8) 0.22009(2) 1.303(6) 0.0207(3) 0.0082(2)
0.1107(8) 0.22003(2) 1.172(5) 0.0186(2) 0.0073(2)
0.0979(9) 0.22004(2) 1.112(5) 0.0190(2) 0.0043(2)
0.1125(6) 0.22007(1) 1.154(4) 0.0194(2) 0.0050(1)
0.1129(5) 0.21999(1) 0.987(3) 0.0162(1) 0.00516(9)
Ba O c c . Z Beq (,~) U 11 U33
0.125(Fix) 0.14488(2) 1.009(5) 0.0106(2) 0.0172(2)
0.125(Fix) 0.1 4475(2) 0.541(5) 0.0038( 1) 0.0130(3)
0.125(Fix) 0.1 4470(2) 0.383(5) 0.0019( 1) 0.0107(3)
0.125(Fix) 0.1 4474(2) 0.364(3) 0.0025(2) 0.0088(3)
0.125(Fix) 0.1 4473(2) 0.335(4) 0.0023( 1) 0.0081(2)
0.125(Fix) 0.14472(1) 0.245(3) 0.00088(9) 0.0076(I)
Ca/TI O c c . Z Beq (,~) UI 1 U33
0.096(3)/0.029 0.0459(1) 0.50(2) 0.0074(8) 0.004(1)
0.099(3)/0.026 0.046(1) 0.6(1) 0.005(1) 0.0014(2)
0.118(3)/0.007 0.04601(8) 0.49(2) 0.0033(5) 0.012(1)
0.109(4)/0.016 0.0462(1) 0.42(2) 0.0024(9) 0.011(1)
0.112(3)/0.013 0.04613(7) 0.35(2) 0.0022(1) 0.0009(2)
0.106(3)/0.019 0.04600(8) 0.49(2) 0.0005(5) 0.018(1)
Cu I Occ. Beq (.~,) U 11 U33
0.0625(Fix) 0.64(1) 0.0061(4) 0.0123(6)
0.0625(Fix) 0.26(1) 0.0009(4) 0.0080(6)
0.0625(Fix) 0.36(9) 0.0040(2) 0.0039(7)
0.0625(Fix) 0.221(8) 0.0030(8) 0.0024(6)
0.0625(Fix) 0.075(6) 0.0031(2) 0.0022(4)
0.0625(Fix) 0.015(8) 0.001 4(2) 0.0034(3)
Cu20cc. Z Beq (,~) U11 U33
0.125(Fix) 0.08921(4) 0.83(1) 0.0077(3) 0.0163(5)
0.125(Fix) 0.08926(5) 0.47(1) 0.0029(3) 0.011 9(5)
0.125(Fix) 0.08905(5) 0.30(1) 0.001 9(3) 0.0086(6)
0.125(Fix) 0.08910(4) 0.281(6) 0.0018(4) 0.0072(5)
0.125(Fix) 0.08919(3) 0.250(8) 0.13015(2) 0.0066(4)
0.125(Fix) 0.08924(3) 0.120(6) 0.0007(2) 0.0060(3)
37
M. Hasegawa et a l . / Physica C 267 (1996) 31-44
Table 4.2 Final oxygen atomic parameters Temperature r (K)
T= 290
T = 190
T = 130
T = 120
T= 115
T= 90
O10cc. Beq (A) U 11 U22 U33
0.125(Fix) 1.0(1) 0.013(3) 0.003(3) 0.022(3)
0.125(Fix) 0.6(1) 0.007(3) - 0.000(2) 0.001 6(3)
0.125(Fix) 0.53(9) 0.004(3) 0.003(3) 0.01 4(3)
0.125(Fix) 0.54(9) 0.010(3) 0.000(2) 0.010(3)
0.125(Fix) 0.37(6) 0.004(2) 0.000(2) 0.010(2)
0.125(Fix) 0.34(8) 0.005(2) 0.003(2) 0.006( 1)
02 Occ. Z Beq (,~,) U11 U22 U33
0.25(Fix) 0.0878(2) 1.06(6) 0.017(3) 0.001(2) 0.021(2)
0.25(Fix) 0.0879(2) 0.49(5) 0.005(2) 0.001(2) 0.012(2)
0.25(Fix) 0.0874(2) 0.53(6) 0.006(2) 0.001(2) 0.013(2)
0.25(Fix) 0.0876(2) 0.57(8) 0.005(2) 0.005(2) 0.012(2)
0.25(Fix) 0.0877(1) 0.46(7) 0.006(2) 0.003(2) 0.008(1)
0.25(Fix) 0.0879(1 ) 0.32(5) 0.003(1) 0.000( 1) 0.009(1)
03 Occ. Z Beq (,~) U 11 U33
0.125(Fix) 0.1646(3) 2.2( 1) 0.034(4) 0.01 6(3)
0.125(Fix) 0.1642(3) 1.4( 1) 0.017(3) 0.017(4)
0.125(Fix) 0.1634(4) 1.0( 1) 0.007(2) 0.024(5)
0.125(Fix) 0.1632(3) 1.1( 1) 0.012(3) 0.020(5)
0.125(Fix) 0.1633(2) 1.34(7) 0.022(3) 0.006(2)
0.125(Fix) 0.1641 (2) 0.85(5) 0.009(2) 0.01 4(2)
O4A(O4) Occ. Z Beq (,~)
0.103(6) 0.2740(6) 4.0(6)
0.065(18) 0.2756(12) 2.4( 1)
0.066(7) 0.275(1) 1.4(5)
0.12(2) 0.2755(5) 3.3(4)
0.124(1) 0.2754(4) 4.3(4)
0.125(1) 0.2762(3) 2.9(2)
O4B Occ. Z Beq (,~)
0.022 0.306(2) 1(1)
0.060 0.2763(15) 2.3(Fix)
0.059 0.278(2) 4( 1)
0.01 0.309(2) 2.3(Fix)
lyzed to be located on one site. However, they showed large anisotropic temperature factors and low oxygen occupancy especially above 130 K. In particular, the U33 value of the anisotropic temperature factor was grossly large. This suggests a possi-
3.4. Order-disorder of oxygen atom in the TI-O layer
In the refinement process at the first stage, oxygen atoms in the double thallium layers were anaTable 5.1 Metal-metal bond distances M-M
T1-TI TI-Ba Ba-Ba Ba-Ca Ba-Cu2 Ca-Ca Ca-Ca Ca-Cul Ca-Cu2 Cu 1-Cu 1 Cul-Cu2 Cu2-Cu2
Mult. 4 4 4 4 1 4 1 4 4 4 4 2 4
290 K
190 K
130 K
120 K
115 K
90 K
(~.)
(~)
(~.)
(~.)
~.)
(~.)
3.4682(7) 3.8603(6) 3.8332(8) 3.8603(6) 3.541(4) 3.379(1) 3.284(5) 3.8603(6) 3.186(2) 3.139(2) 3.8603(6) 3.192(2) 3.8603(6)
3.764(1)
3.4633(8) 3.854(1) 3.8269(9) 3.854(1) 3.518(3) 3.372(1) 3.289(4) 3.854(1) 3.183(2) 3.126(2) 3.854( 1) 3.177(2) 3.854(1)
3.462(1) 3.853(2) 3.825(1) 3.853(2) 3.513(4) 3.370(1) 3.295(5) 3.853(2) 3.184(2) 3.125(2) 3.853(2) 3.177(2) 3.853(2)
3.4609(6) 3.853(1) 3.8260(8) 3.853(1) 3.515(3) 3.36681(9) 3.289(4) 3.853(1) 3.183(1) 3.127( 1) 3.853( 1) 3.180(1) 3.853(1)
3.4637(5) 3.853(1) 3.8233(6) 3.853(1) 3.518(3) 3.3662(8) 3.279(4) 3.853(1) 3.180(2) 3.130(2) 3.853( 1) 3.181(1) 3.853(1)
3.831(I) 3.527(4) 3.370(1) 3.286(5) 3.183(2) 3.134(2) 3.188(2) 3.855(2)
M. Hasegawa et al./ Physica C 267 (1996) 31-44
38
bility of two occupied sites split along the c-axis. From the results of the difference Fourier synthesis, the splitting could not be confirmed, because this oxygen atom was surrounded by two heavy atoms, thallium and barium. Accordingly, another trial examination by the split model was carried out on the degree of freedom of the atomic position with respect to the z-parameter. The result converged to the model of two independent split sites (Table 3). Fig. 4 shows schematic views of the structure change of T12Ba2Ca2Cu30 m between 290 K and 90 K. The oxygen atoms in the thallium layers exist at the disordered two split sites O4A and O4B above 120 K and the ordered one site (04) below 115 K. It is interesting that the disorder-order behavior of the 0 4 atoms is similar to the results of the high pressure studies on T12Ba2CuO6+ ~, which have suggested that a rearrangement of the oxygens in the TI-O layers on applying high pressures is important for superconductivity [15-20]. For example, Klehe et al. recently investigated the high pressure effects on the electrical resistivity and Hall coefficient for three TI2Ba2CuO6+ 8 samples with different oxygen con-
tent [20], and reported a local oxygen ordering of interstitial oxygen atoms between equivalent sites in the TI-O double layer of T12Ba2CuO6+ ~ below 75-105 K. Since the TI-O layers are located above or below the CuO 5 square pyramid which is directly concerned with the electrical conduction, it is noteworthy that the local oxygen ordering in the TI-O double layer is observed below around T~ in the two different thallium-based cuprate superconductors. 3.5. Local structure changes around CuO 5 pyramid and CuO4 plane and correlation with the superconductivity
As described in the introduction, the local structure changes around the copper atoms are thought to play an important part in the superconductivity of the cuprate superconductors. The TI2Ba2Ca2Cu30 m structure has two independent copper sites, Cu 1 and Cu2. Cul is in the plane coordination, with four oxygen atoms, of CuO2 (O1) which forms an infinite two-dimensional layer spreaded along the a Itetra-a2tetra plane. This plane coordination does not exist in the
Ti 04
O4A O4B Ba-O3
O4Ao4B
04
Cu2-O2 Ca Cul-O1
90 K
115 K
130 K
290 K
Fig. 4. Projection of the TIzBazCazCu30 m structure on the (010) plane. Splitting of the O4-site to O4A and O4B sites in the TI-O layer is observed at 290 K and 130 K.
M. Hasegawa et al./Physica C 267 (1996) 31-44
structure of TI2Ba2CuO6+ d and Tl2Ba2CaCu208. Cu2 is in the CuO 5 square-pyramid coordination where the basal CuO 4 (02) plane also forms an infinite layer. In this section, the local structure changes, especially around the CuO 5 square pyramid, are described and discussed. Figs. 5 and 6 show the domain of the temperature dependence of each bond distance forming the respective coordination of the non-pyramidal CuO 4 square plane and the CuO 5 square pyramid. All distances except the Cu2-O3 (apical oxygen) distance in the CuO 5 square pyramid decrease with decreasing temperature. It should be noted that there is a slight but distinct difference between in the non-pyramidal CuO 4 square plane and the pyramidal CuO 4 basal plane. The temperature dependence of the Cu2-O2 distance shows a decline of the decrease above Tc (around 190 K) in contrast to the almost linear temperature dependence of the C u l - O 1 distance; the 0 2 - 0 2 and O1-O1 distances change in the same way. Besides, the temperature dependence of the 0 3 - 0 2 distance shows an uptrend of the decrease below around T~. It is noteworthy that only the Cu2-O3 distance decreases on cooling down to
a)
39
2.731 . . . . .
2,730
T I
IIT'K
. . . . . . . . . . . . . . .
./
2.729 2.720
.,,-"
2.727 ¢~1
C3
2,726 2.725 2.724 . . . .
2,723
|
.'...
a . . . .
100
30
100
i . . . .
i . . . .
200
250
300
Temperature (K)
(b) 1.932 . . . . .
T" -~ l'lT"K . . . . . . . . . . . . . . .
1.931 ,~
i
1.930
. ../"
i
1.929 ~
/
1.920 1.927 1.926
1oo
50
130
2o0
23o
300
Temperature (K) C)
2.710
.....
2.700
• "J ll,'K ..............
0~
~
•
2.690
.........s"'""" i
2.680 2.670 2.660 2.6$0
2.640 2,630 50 (a)
2.733 L . . . . .
T" " I 1 7 " K . . . . . . . . . . . . . . .
2.730 /
*
~
2.728
......
2.727
~+ ....................
3.724
so
i
-
-
-
i
.
' . . .
i
IOO
. . . .
10o
,
-
-
.
•
2oo
I
,
,
,
3.300
+'"'"'
= 117 K ,,
........,'"
-
1.926 . . . . 50
. . . .
t . '~. . 100
.
t . . . . 150
' 100
-
-
•
.
i 150
. . . .
I
.
,
200
.
.
I 250
. . . . 300
Temperature (K) ...'"""
1.925
..,'"""
i"
3.310
50
1,930
.
300
(b) 1.931 T
i
~.................... i
3.290
t . , 200
, .
I • , 250
o , 300
Temperature (K) Fig. 5. Temperature dependence of each bond distance forming the coordination of the CuO~ square plate. (a) O 1 - O 1 , (b)
CuI-O1.
300
.....
C~ ~ 3.320
,
20o
Temperature (K)
1.927
250
..........
3.330
,,
2.723
~
•
.g 3340
......
1.929
200
3.350
/'"'/'"
2,,2,2,,2,
.~
150
Temperature (K)
(d) 3.370
2.729 0~
I00
...+"
'
Fig. 6. Temperature dependence of each bond distance forming the coordination of the CuO 5 square pyramid. (a) 0 2 - 0 2 , (b) C u 2 - O 2 , (c) C u 2 - O 3 , (d) 0 2 - 0 3 .
Tc and then abruptly increases below Tc (Fig. 6c). These results suggest that the superconductivity is correlated with the local structure changes around Cu2. The abrupt increase similar to our data around Tc in the bond distance from copper to apical oxygen
40
M. Hasegawa et a l . / Physica C 267 (1996) 31-44
oO o
the perpendicular axes in the figures are set at 0.08 ~,. Fig. 8a shows the temperature dependence of the distance between Cul and T1 along the c-axis which
a) o~
0.02
T'"l17
. . . . .
0.01
K . . . . . . . . . . . . . . .
:: eel
c-axis/4.5392
i
TT ~.~.::+'+~ •
-0+01 -0.02 -0.03
~'~
-0.04
<~
-0.05
....
-0.06
.
.
.
.
i
Fig. 7. Part of the unit cell around the CuO 5 square pyramid of Tl2Ba2Ca2CU3Olo. Symbol notation of circles is the same as in Fig. 3; small closed circles indicate copper atoms.
.
.
.
]oo
30
i
.
.
.
.
|
.
.
.
.
i
1so 300 Temperature (K)
.
•
i
i
zso
3oo
(b) o.o7 T = 117 K
0.06
0.03
° ~
0.04
m
atom in the CuO 5 square pyramid has also been observed in T12Ba2CaCu208 by V.N. Molchanov et al. [8]. In their paper the abrupt increase was 0.3%, which is much smaller than the present data of 0.7%. They explained the abrupt increase in the CuO 5 square pyramid in TI2Ba2CaCu208 by the JahnTeller effect where the population of local holes in the C u - O bonds changed with temperature. However, it is hard to explain the changes of the C u - O distances in both Tl2Ba2Ca2Cu3Ol0 and Tl2Ba 2 CaCu208 structures by this effect, because the Cu20 2 distance forming a basal plane in the square pyramid shows no significant change around T~, as shown in Fig. 6b as well as in their result of Fig. 4 in Ref. [8]. If the Jahn-Teller effect influences these distance changes, both the Cu2-O2 distance in the TlEBaECa2Cu3Ol0 structure and the Cu-O1 in TlEBaECaCu2Os, forming the basal plane in the CuO 5 square pyramid coordination, have to show a more considerable change than observed in the Molchanov's and our data around Tc. In order to understand the anomalous behavior of the Cu2-O3 bond distance, we, first, pay attention to the distance changes around each Tl and Cul atom along the c-axis, that is, C u l - C u 2 , Cu2-O3 and O3-Tl. Fig. 7 shows a local structure around Cul, Cu2, 0 3 and T1. The temperature dependence of the bond distances is presented in Fig. 8, where the perpendicular axes show differences in the bond distances from those at 290 K. The entire lengths of
A 0
..............
0.03
0.02 0.01 0
(1.99) •
.
.0.0150 • . .
.
i
.
.
.
,
100
.
.
.
.
,
150
.
,
,
,
200
!
,
,
,
300
250
Temperature (K) (C)
o4
o.ol
.....
o (2.70) -0.01
T:
+........... +............. ,++...........
-0.03 -0.04
<3
-0.05
+,L.,,"
-0+02
¢~
.............
'",
,
-0.06 .0,07
.
.
.
.
50
i
.'
.
.
I
100
.
.
.
.
150
i
o
,
,
300
,
I
,
•
,
j
250
300
Temperature (K)
(d)
oo,
o~.
0.02
T c = 117 K
0.03
0.01 0
,,,.q (3,102)
...O ................................ •
-0.0l <~
O l h ,~ ip...+,, -.......... ""
.o.o2 -0.03 -0.04
.
50
.
.
.
'
tO0
•
'"
•
"
+
150
.
.
.
.
'
.
.
200
.
.
a
250
.... 300
Temperature (K) Fig. 8. Temperature dependence of the bond distance along the c-axis (see Fig. 7). The perpendicular axis shows the diffence from the bond distance at 290 K (the value is written in parentheses). All lengths of the perpendicular axes in the figures are 8 ~. (a) total distance of C u l - C u 2 - O 3 - T 1 . The open circles are plots of about one fourth of the c-axis. (b) O3-TI, (c) Cu2-O3, (d) Cu I -Cu2.
M. Hasegawa et al./Physica C 267 (1996) 31-44
is equal to the total distance of Cul-Cu2-O3-T1, and to about one fourth of the c-axis length. It is found that these two data items from the local structure analysis and from the lattice constant determination (Fig. 2b) behave in almost the same way. Figs. 8b, c and d show the temperature dependence of each two-atom bond distance between Cul and TI. It is found that both the Cu2-O3 and O3-TI distances change drastically below T~ in the opposite directions. On the other hand, the C u l - C u 2 distance shows no drastic change below Tc. From these resuits, it is concluded that the local movement of 03 except Cu2 occurs during cooling and the anomalous behavior of the Cu2-O3 distance as shown in Fig. 6b is attributable to this anomalous movement of the 03 atom. In order to understand the anomalous behavior of the 0 3 atom, we pay attention to the local behavior of the in-plane 0 2 oxygen atom in the basal plane of the CuO 5 square pyramid, and try to visualize the relative changes in the 0 2 atom with temperature
41
towards 03 and Cu2 on the basis of the experimental results on bond distances in Tables 5.2, 5.3. Since from the structure analysis the structure of the T12Ba2Ca2Cu3O10 single crystal in this study was found to keep the I 4 / m m m symmetry down to 90 K, the 03, the Cu2 and 0 2 atoms in the CuO 5 square pyramid form a triangle and are located at the two-dimensional rectangular coordinates as shown in Fig. 9. There the Cu2 atom is fixed at (0, 0) because of the conclusion on the mobile 0 3 atom in the above paragraph, and because the 03 and 0 2 atoms are located at (0, Y3) and ( x z, Y2), respectively. One can obtain the relative position of each atom and angles in the triangle at each temperature by the following calculations based on the geometric relations of the O3-Cu2-O2 triangle. If a = 03-02,
(1)
b = Cu2-02,
(2)
c = O3-Cu2,
(3)
we have the following equations from the geometric relations:
(a)
03
a2 = x2 + (Y3 - Y2) 2,
(4)
b 2 = x~ +y22,
(5)
c=Y3
(6)
(Y3 > 0 ) -
From Eqs. (4)-(6), we can obtain each position and angle as follows: 02 02 02 02'
Cu2 = (0, 0),
(7)
03 = (0, c),
(8)
0 2 = (¢b 2 - ( c 2 - 02 + b2)2/4c 2 ,
(b)
( c 2 - a 2 + b2)/2c), 03(0, Ys)
(9)
/_ O3-Cu2-O2 = cos[(b 2 + c 2 - a 2 ) / 2 / O / c ] .
(m)
Cu2(O, O) ~
02(xv Y~)
Fig. 9. (a) CuO5 square pyramid, (b) O3, Cu2 and 02 atoms in the CuO5 square pyramid forming a triangle in the two-dimensional rectangular coordinates.
The temperature dependence of the relative behavior of each atom, Cu2-O2 bond, and /_ 0 3 Cu2-O2 angle is shown in Fig. 10, where (a) and (b) show the relative atom positions in two-dimensional rectangular coordinates in units of A. It is found from Fig. 10a that the local structure changes at each temperature are far smaller than the bond distances. Fig. 10b indicates that the in-plane oxygen 0 2 atom
M. Hasegawa et al./Physica C 267 (1996) 31-44
42
(a) o<~
atom position in O3-(Cu2-O2) pyramid 3.0
.
, . . .
,
.
.
.
,
.
.
.
~ .
T 2.5
I
r"2
2.0
[o3~ I -I
~
1.O
"~
0.5
,
.
O : A:
~
~
.
.
,
.
290 K 190 K
[3: 13OK O : 120 K • : 11s K
~
Co2 0
•~ ,~
.
= 117 K
~
~"
.
lm
-O.S
am
. . . . .
'
0
. . . . . . .
0.4
'
0.8
.
.
.
.
.
1.2
.
.
'
1.6
-
2.0
a-axis position from Cu2 (~)
(b) 0
~
.o.o2
~
-0.04
~x.
~
"~
• ! 11s K
290 0 2 behavior
190
-0.07 . . . . * . . . . ' . . . . ' . . . . ' . . . . ' . . . . 1.925 1.926 1.927 1.928 1.929 1.93 1.931
a-axis position from Cu2 (A)
(c) 0.01
-O.Ol
90
-0,02
-0.03 ~, ~,
-0.04
/ A: 190 K ]- [] : 130 K / © : 12o~
-0.05
•
.
* .
.
.
0
r,.)
.
.
l
|
~ . .
.... .
,.~ .
..I
|
/
115
b
-0.07
C
|
~
: 115 K
.~ -o.o6
(d)
~"-
~
.
.
.
~
.
.
.
t
.
.
.
,f._.f
.
.
.
,4 O
.
.
J
2.0
. . . .. . . . . . . . . . .
.•,
~, o .............. • ...... . . . . . . .
9t.s
9Lo
t
0.4 o.s 1.2 1.6 a-axis position from Cu2 (A)
C
O "
i,
179
90.0
. . . . 50
* . ,...
1O0
L~
,4 O
i
i
lS0
. . . .
i
. . . .
200
i . . . . 250
180 300
Temperature (K) Fig. 10. T e m p e r a t u r e dependence o f the atom positions in the O 3 - C u 2 - O 2 triangle a n d angle / O 3 - C u 2 - O 2 in the C u O 5 square pyramid. (a), (b) and (c) are in two-dimensional rectangular coordinates in ,~ units. (a) O 3 - C u 2 - O 2 triangle, (b) in-plane o x y g e n 0 2 , (d) C u 2 - O 2 b o n d , (d) angle / O 3 - C u 2 - O 2 in the C u O 5 square pyramid.
moves in two steps. At the first step, the 0 2 atom approaches toward the Cu2 atom above Tc up to within about 1.927 ~,, as is well shown in Fig. 6b,
which is almost equal to the sum of the ionic radii of Cu 2+ and 0 2 - . Then, it is noteworthy that at the second step the 0 2 atom moves up largely and abruptly around Tc. Accordingly, it is concluded that this abrupt movement of the in-plane oxygen 0 2 atom results in the anomalous upward movement of the apical oxygen 03 atom around Tc. As mentioned above, the mobile in-plane oxygen 0 2 atom is found to hold the key to the local structure change around the CuO 5 square pyramid. From the viewpoint of the correlation of the local structure change with the superconductivity, it should be noted that the most attractive point is shown in Figs. 10c, d. That is, the abrupt upward movement of the 0 2 atom results subsequently in making the "exactly planar" CuO 4 basal plane in the CuO s square pyramid just below Tc, as shown in Fig. 10c. This scheme is also shown in Fig. 10d. The angle/_ O 3 - C u 2 - O 2 suddenly tends to become 90 ° just below Tc which also means that the angle Z_ 0 2 Cu2-O2' (opposite side) becomes 180°. This movement of the 0 2 atom results in changing the electronic structure of the basal CuO 4 plane in the CuO 5 square pyramid and is thought to be advantageous for the electric conduction. Another notable point on the correlation with the superconductivity is that the mobile 0 2 atom induces the continuous decrease of the 0 3 - 0 2 distance down to 90 K and the further decrease below Tc, as shown in Fig. 6c, even though the Cu2-O3 distance increases below T~. As mentioned in the introduction, the local proximity between the apical oxygen atom and in-plane oxygen atom in the CuO 5 square pyramid was reported to play an important role on the superconductivity, experimentally by Egami et al. [2,6,10]; and theoretically by Andersen et ai. [ 13,14]. They pointed out that the shortening of the apical and in-plane oxygen atoms in the CuO 5 square pyramid could make the oxygen band in higher energylevel over the Fermi level. As a result, this scheme could produce holes as carriers and then stabilize such a short distance of the apical oxygen atom and in-plane oxygen one in the superconducting state. The energy gap due to the superconducting state makes its electronic energy and also its total energy lower, and subsequently keeps such a local structure more stable. Consequently, the mobile in-plane oxygen 0 2 atom, which induces the exactly planar CuO 4
M. Hasegawa et aL /Physica C 267 (1996) 31-44
basal plane and the continuous decrease of the 0 3 0 2 distance, can stabilize the changed local structure around the CuO 5 square pyramid. From this consideration, it is concluded that the mobile in-plane oxygen atoms in the CuO 5 square pyramid lead to the local structure changes in and around the pyramid and induce the change of the electronic structure. Besides, they would be exactly correlated with the superconductivity in all of the cuprate superconductors.
4. Conclusions The local structure changes between 290 K and 90 K of a Tl2Ba2Ca2Cu30~0 single crystal have been investigated using the X-ray diffraction technique. The linear thermal expansion coefficients were obtained to be ot~= 1.0(1)x 10 -5 K -~ and a , . = 1.9(2) × 10 -5 K-~ in the a- and c-axes, respectively. The final crystallochemical requirement, charge neutrality, leads to the chemical formula of this crystal to be TI 3+ 2+ (Cao.848T10.152)2Cu: 2+ 3+ 2+ 1.8o6Ba2 2+ 3+ 2(Cuo.722Cu0.278)O~0 . The oxygen atoms in the thallium-oxygen layers were found to exist at the disordered two split sites above 120 K and the ordered one site below 1 15 K. It was also found that the mobile in-plane oxygen atoms in the CuO s square pyramid induced the local structure changes in and around the pyramid. Besides, it should be noted that they were extremely correlated with the superconductivity of the cuprate superconductors.
Acknowledgments The authors are indebted to M. Tamura, Institute for Solid State Physics, the University of Tokyo, for his supports on employing SQUID magnetometer systems. They also thank to F. Sakai and M. Koike for their assistance throughout this work.
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43
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