Nuclear quadrupole resonance of 6365 Cu in the 2223 phase of Tl-Ba-Ca-Cu-O and (Bi, Pb)-Sr-Ca-Cu-O

PhysicaC 161 (1989) 367-372 North-Holland

NUCLEAR QUADRUPOLE RESONANCE OF 63/65Cu IN THE 2223 PHASE OF T I - B a - C a - C u - O AND (Bi, P b ) - S r - C a - C u - O T. OASHI, K. KUMAGAI and H. NAKAJIMA Department of Physics, Faculty of Science, Hokkaido University, Sapporo, 060, Japan

M. KIKUCHI and Y. SYONO Institutefor Materials Research, Tohoku University, Sendal 980, Japan

Received 18 September 1989

Nuclear quadrupole resonance (NQR) of 63/65Cu is observed in the 2223-phase of T12Ba2Ca2Cu3Olo(T¢=121 K) and (BiL6Pbo4)Sr2Ca2Cu30~o (To= 110 K). The electric quadrupole interactions at two Cu sites (in the Cu-O pyramid and in the Cu-O square plane) are distinguished and are comparedwith those in other high T¢superconductors.

I. Introduction

The nuclear magnetic resonance (NMR) and the nuclear quadrupole resonance (NQR) measurements of the various isotopes such as 139La, 89y, 63/65Cu and ~70 have been successfully applied for studying local electronic and magnetic properties [1,2], which have revealed interesting findings on spin fluctuations of Cu moments in Cu-O planes of high-To superconductors such as La2_xSrxCuO4 and YBa2Cu307. In non-rare-earth high-To superconductors of B i - S r - C a - C u - O [ 3 ] and T1-Ba-Ca-Cu-O [4] which show the highest T~= 110~ 125 K, the similar layered structure of CuO2 planes is considered to be responsible for high-To superconductivity. The NMR studies of 2°3/2°5T1 nuclei in the Tl-compounds have been investigated by several groups so far [ 5-9]. The nuclear relaxation time and the valence of T1 ions in the oxides have been discussed. However, the Cu-NQR of the Tl-system has not been reported in spite of the importance for the understanding of origins of high-T~ superconductivity in the layered oxide compounds. In this paper, we report the observation of Cu-NQR in the 2223 phase of the Tl-compound as well as in the 2223 Bi-phase. The electric quadrupole interactions of the present systems are compared with those of Cu having var0921-4534/89/$03.50 © Elsevier SciencePublishers B.V. ( North-Holland )

ious types of structural symmetry in other oxide compounds.

2. Experimental Samples of the Tl-system were prepared by solid state reaction [ 10 ]. Appropriate amounts of T1203, BaCuO2, BaO, CaO and CuO were mixed and pressed into a pellet, 1 mm in thickness. The pellet was wrapped with gold foil and heated in a silica tube at 890°C for 5 min for 2223 composition under an oxygen flow of 120 ~ 150 ml/min, then cooled at a rate of 10°C/min. The X-ray powder diffraction pattern of the present 2223 sample is shown in fig. 1. The observed lines are indexed by a tetragonal unit cell except for a few weak reflections. The Bi-system was also prepared by the solid state method. The powder of the starting materials of Bi203, PbO, SrCO3, CaCO3 and CuO was mixed with cation ratio of B i : P b : S r : C a : C u = 1.6:0.4:2:3:4. The substitution of Bi by Pb was employed to stabilize the 2223 phase [ 11 ]. The mixed powder was calcined at 795°C for 20 h in air, ground, pressed into a pellet and then re-fired at 853°C for 90 h in air. The fired sample was then cooled to 370°C at a rate of 2.5°C/min. Superconducting transition temperatures, To, were

368

T. Oashi et al. / Nuclear quadrupole resonance of 63/65Cu

,~10 t:3

~,=o

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7 b-7

So

'-

"

I I I I I I I I I I I I I I I I I I I I I I I I I I

3

10

20

30

40

50 28 (degree)

Fig. 1. X-ray diffraction pattern of T1EBa2Ca2Cu30~o. i

i

i

i

I

~

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i

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I

L

r

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l[

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0

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r

100

200

o

650u

Z

%

300 T (K)

o

o o

Fig. 2. Magnetic susceptibility as a function of temperature in T12Ba2Ca2Cu30~o.

oo

15

determined by AC susceptibility measurement as 110 and 121 K for the present 2223 Bi- and Tl-compounds, respectively. Fig. 2 shows the temperature dependence of the magnetic susceptibility of the T1compound of 350 mesh powder. The sharp drop of susceptibility at 121 K follows a small amount of diamagnetic tail below 80 K, which indicates that the sample contains a small amount of the low-To (socalled 2212) phase. The Bi-compound contained also the 2212 phase as ~ 40% in volume fraction. A conventional phase-coherent pulse N M R apparatus was used for measurement of N Q R spectra of Cu nuclei. Cu-NQR spectra were obtained from point by point plots with changing frequency in zero external field.

3. Results

Fig. 3 shows the Cu-NQR spectra of the 2223"T1phase at 1.5 K. The intensities of the spectra are

63Cu

Lt

o

20

25

30 35 FREQUENCY (MHz)

Fig. 3. Spin echo spectra of 63/65Cuin Tl2Ba2Ca2Cu3Olo.The intensity is correctedby the T2-reduction.

plotted after the correction of T2-reduction by extrapolating to z = 0 (z is separation time between two pulses). The spectra consists of two groups of signals: 16~22 MHz (hereafter called C u ( I ) ) and 2 8 ~ 3 4 MHz ( C u ( I I ) ) . In both the C u ( I ) and C u ( I I ) spectra, we can see two peaks corresponding to the signals from 63Cu and 65Cu nuclei. The ratio of the peak frequencies in the C u ( I I ) signals is 29.2/ 31.5 = 0.927, which is equal to the ratio of 65Q/63Q for two Cu isotopes (Q is the nuclear quadrupole moment). The ratio of the intensity at each peak is roughly equal to that of the natural abundance of two Cu isotopes (65Cu/63Cu=0.48). The spin echo decay times, T2, is temperature-independent below 4.2 K. T2 of 180 Its for 63Cu(II) is longer than the value of 54 Its for 63Cu(I), suggesting that the spin-spin

T. Oashi et al. / Nuclear quadrupole resonanceof 6z/65Cu

interaction is stronger at the C u ( I ) site than at the Cu (II) site. The integrated intensity of Cu (I), which is directly proportional to the number of Cu nuclei observed, is almost 4 ~ 5 times larger than that of Cu (II) signals. The zero field signals are attributed obviously to the splitting of the nuclear levels due to the electric field gradient (EFG) through quadrupole interaction. The NQR frequency, Vq, which is a direct measurement of the EFG, is given as

369

system. The spin echo decay times at each peak are also obtained to be 140 and 200 ~ts for the C u ( I ) and Cu (II) site, respectively. The integrated intensity of C u ( I ) (after the correction of T2-reduction) is almost five times larger than that of C u ( I I ) signals. From the same procedure as that in the Tl-compound, we obtained the l,q of the 2223-Bi phase as follows:

63Cu(I) : Pq =22.8 MHz,

65Cu(I) : vq =21.1 MHz,

63Cu(II) ://q =33.3 MHz, 65Cu(II) : lpq =30.8 M H z . 3eZqQ Vq= 2 I ( 2 i _ l ) h [ l + -

~ 2

1/2

]

,

where I is 3/2 for Cu nuclei, ~/is an asymmetry parameter and q is electric field gradient. We deduce simply the averaged Vq from possible superposition of signals with the ratio 65Q/63Q. The values of vq at the each peak are obtained as follows: 63Cu(I):Z,q=18.5 MHz,

65Cu(I):vq=17.1 MHz,

The Vq of both the Cu sites in the Bi-system is slightly larger than those of the Tl-compound. This result for the 2223 Bi-system is not consistent with that reported by Fujiwara et al. [12]. They did not detected any signals around 33 MHz in the nominal composition of Bi t.6Pbo.4Ca2Cu301 O-y, but observed Cu-NQR signals with four peaks around 18~24 MHz, which is smaller than the present values.

63Cu(II) : l,q= 31.5 MHz, 65Cu(II) : l/q =29.2 M H z . Similar spectra of 63/65Cu-NQR are observed in ( Bi L6Pbo.4) Sr2Ca2Cu301 o ( T~= 110 K) which is isomorphous to the 2223 Tl-phase. As shown in fig. 4, the signals with two peaks are observed around 31 and 33 MHz. The signals with relatively large intensity are broadened between 18 ~ 28 MHz. We denote again the signals around 22 MHz as C u ( I ) and the one around 29-35 MHz as C u ( I I ) for the 2223 Bii i i i i i 1 ~ [

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25

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p

r

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30 35 FREQUENCY (MHz)

Fig. 4. Spin echo spectra of 63/65Cuin (Bit.~Pbo.4)Sr2Ca2Cu3Oto. The intensity is corrected by the T2-reduction.

4. Discussion

We show simply the schematic stacking of layers in oxide superconductors in fig. 5. The slightly distorted tetragonal La2CuO4 has two dimensional (2D) layers of C u - O octahedron with weak coupling between them [ 13 ]. Recent N Q R measurements of the superconducting L a - S r - C u - O show the 63l/q is ~ 36 MHz, depending on Sr concentration [ 14,15 ]. In YBa2Cu307, the 2D layers of C u - O pyramids which are separated by the oxygen vacant Y-layers couple to C u - O chains through C u - O bonds along the c-axis [ 16]. The two distinct Cu sites provide two sets of Cu-NQR signals: 63pq of Cu-chains is 20.3 MHz and 63/1q of Cu planes is 31.5 MHz in YBa2Cu307 [ 17,18 ]. The Vq depends largely on the different oxygen coordination and is obtained to be 30.1 MHz for Cu chains and 22.9 MHz for the Cu planes in the antiferromagnetic YBa2Cu30 6 [ 18 ]. The stacking of the 2212 Bi- and Tl-systems is analogous to the YBa2Cu306 system with non C u - O chains. The 2D pyramidal C u - O layers are formed by the oxygen vacancies in the Ca layers [ 19 ], and the double layers of Bi-O ( T I - O ) fulfil a structural role as a reservoir of holes similar to the C u ( I ) - O chains in the YBa2Cu306. Therefore, vq of Cu in the

370

T. Oashi et al. / Nuclear quadrupole resonance o f 6s/65Cu

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i

/ f

• f

• f /

•

-U

c"

J

(a)

(b)

(c)

(d)

Fig. 5. Stackinglayers in various oxide superconductors. (a) La2CuO4, (b) YBa2Cu307_y, (c) TI2Ba2CaCu208 (Bi2Sr2CaCu2Os), (d) T12Ba2CazCu3Olo(Bi2Sr2Ca2Cu3Olo).

pyramids is expected to be the same order as the Vq in the CuO planes in the YBa2Cu306.The vq of 2212 Bi-phase is near 24 MHz as reported previously [20]. We have not detected any signals around 31-33 MHz for the 2212 Bi-phase. In the case of the 2212 Biphase where there is no square plane Cu but is only Cu in the pyramids, it is quite reasonable to detect one kind of the Cu signal. The broadened signals are considered to be attributed to the modulation of the atomic position due to oxygen displacement [21 ]. The atomic modulation causes the extremely large distribution of EFG at the Cu sites in the C u - O pyramids. In the 2223 phase, the additional C u - O square planes locate between oxygen-vacant Ca layers in addition to the pyramidal C u - O layers [22 ]. Thus, we have crystallographically two inequivalent Cu sites in the 2223 phase, indicating observation of two distinct N Q R signals. Actually, two sets of the Cu-signals are observed in the 2223 phase as mentioned before. Thus, the site assignment of the signals seems to be straightforward as follows: the C u ( I ) signals ( 18 ~22 MHz) to Cu in the pyramids and the Cu(II) signals (around 31 ~ 3 4 MHz) to Cu in the square planes. The spectra of Cu (II) assigned to the square plane in the 2223 phase are relatively narrower than those of the 2212 phase, showing the relatively small distribution of the electric quadrupole interaction of Cu in the square plane in the 2223 Bi- and also T1systems.

The spin-spin relaxation rate, 1 / T2, of Cu (I) is enhanced by four times compared to that of C u ( I I ) . This fact implies that the Cu nuclei in the Cu (I) sites couple stronger with each other than those of the Cu (II) site. The coupling strength between nuclear spins is proportional to the nuclear number in the system. As the number of Cu nuclei in the unit cell are two and one in the C u ( I ) and C u ( I I ) sites, respectively, the above site assignment is, quantitatively speaking, supported from the nuclear relaxation result. The one difficulty for this site-assignment is that the ratio of the integrated intensity of Cu ( I ) / C u (II) is not equal to 2, the ratio of the Cu number for each site in the unit cell, but is more than 4 ~ 5. The correction of intensity from TE-reduction may have some ambiguities for the exact comparison of the intensity. A very short relaxation time may make invisible a part of the NQR signals. The NQR intensity must be corrected by taking into account the different ~/ values. However, the most plausible interpretation seems to be owing to the following fact. As demonstrated in the magnetic susceptibility measurement, the present T1- and Bi-sarfiples contain some amount of the 2212 phase in which the Cu-NQR signals are observed only around 24 MHz but not around 31-35 MHz [20]. Therefore, the too-large intensity of Cu (I) is considered to be attributed to the superposition of the signals from the 2212 and

T. Oashi et aL / Nuclear quadrupole resonance of 63/6~Cu

371

Table I Summary of the quadrupole resonance frequency, 63pq, and spin echo decay time, T2, of 63Cunuclei in the various oxide superconductors. Lat.ssSro.15CuO4

YBa2Cu307

YBa2Cu306

octahedron chain Cu(I) vq(MHz) T2(tts) ref.

~36 35 [ 14,15]

pyramid Cu(II)

chain Cu(1)

31.54

30.1

22.87

24

752 **

90 **

200

20.25

360 * 87 * ~440 ~ 190 [ 18]

pyramid Cu(II)

Bi2Sr2CaCu2Os Tl2Ba2Ca2Cu30~o Bi2Sr2Ca2Cu3Olo pyramid pyramid square plane Cu(I) Cu(II)

[ 18]

[20]

18.5(T1) 31.5(T1) 22.8(Bi) 33.3(Bi) 54(T1) 180(TI) 140(Bi) 200(Bi) this work

* values of single crystal from ref. [25]. ** values in the antiferromagnetic state from ref. [26]. 2223 phases, while the signals o f C u ( I I ) are from only the Cu nuclei in the square planes o f 2223 phase. Although the m e a s u r e m e n t for a better quality sample (purely single phase o n e ) would give a definite interpretation for this point, we m a y conclude here that in the 2223 T1- a n d Bi-systems, the C u ( I ) signals are from the Cu in the p y r a m i d s a n d Cu ( I I ) signals are from those in the C u - O square planes. The results o f C u - N Q R are s u m m a r i z e d for the various oxide superconductors in table I. The electric field gradient ( E F G ) d e p e n d s sensitively on the electronic charge distribution in a crystal. The particular s y m m e t r y o f Cu in the octahedron, pyramid, chain a n d square plane in various c o m p o u n d s is expected to give quantitatively the different E F G at the Cu site. The ionic c o n t r i b u t i o n is evaluated as the c o n t r i b u t i o n from charges on the surrounding lattice, a n d a simple p o i n t charge m o d e l calculation indicates that this c o n t r i b u t i o n is small to account for the N Q R frequencies. P r e l i m i n a r y calculation o f the E F G by the simple point-charge m o d e l shows the E F G at Cu site is at least 10% o f the observed values in the Bi- a n d Tl-systems as well as other systems [23 ]. The m a i n source o f E F G is considered to be intraatomic in origin. E F G is d e t e r m i n e d by the aspherical density distribution o f the valence electrons, which d e p e n d s largely on the changes o f the electronic state owing to the charge transfer such as from dx2_y2 to dz2 or from 3dx2_y2 to 2p O states [24]. Therefore, in o r d e r to account for the experimental N Q R results s u m m a r i z e d here, the reliable charge distribution base on the proper b a n d calculation with

full-potential will be required. The i n f o r m a t i o n o f the E F G from the C u - N Q R m e a s u r e m e n t s will be helpful to u n d e r s t a n d the local electronic configuration at Cu and O sites.

Acknowledgement The authors would like to thank Mr. M. A b e for his calculation o f EFG. This work was partly supp o r t e d by G r a n t in A i d for Scientific Research on Priority Area " M e c h a n i s m for S u p e r c o n d u c t i v i t y " from Ministry o f Education, Science and Culture o f Japan.

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T. Oashi et al. / Nuclear quadrupole resonance o f 63/65Cu

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