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Insulator-superconductor transition in Tm0.4Pr0.6Ba2Cu3O7 by iso-valent substitution at Ba site
Solid State Communications,
Vol. 104, No. 9, pp. 547-55 1, I997 D 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00
Pergamon
PII:SOO38-1098(97)00342-6
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tm0.4Pr0.6Ba2Cu307 BY ISO-VALENT AT Ba SITE L. Colonescu,*
Laboratoire
SUBSTITUTION
J. Berthon and R. Suryanarayanan
de Chimie des Solides, CNRS, Bat. 414, Universid
Paris-Sud,
91405 Orsay, France
(Received 21 May 1997; accepted 9 July 1997 by P.H. Dederichs) We report on the preparation, X-ray diffraction and a.c. susceptibility of Tm0.4Pr0.&-kSrkCu306+~ An orthorhombic to tetragonal transition was observed for k = 0.8. When the samples (for 0 5 k < 1) were heated in argon prior to oxygen annealing, orthorhombicity was found to increase. An insulator to superconductor transition was observed for k 2 0.6, The effect of Sr substitution and the heat treatment may lead to a reduction in Pr-f hybridization with oxygen orbitals and to an increase in oxygen ordering favouring the restoration of superconductivity in this compound. 0 1997-Elsevier Science Ltd
1. INTRODUCTION As early as 1987, it was shown that Pr destroyed superconductivity in Y 1-xPrxBa2Cu307 (YPBCO) when the concentration of Pr(x) reached the critical value of x, = 0.55 [l-3]. Further, the critical concentration of Pr (x,) was found to depend on the ionic size of the rare earth, Ln 13, 41. Whereas for Ln = Nd, the value of x, was found to be as low as 0.3, superconductivity was found to be destroyed in the case of Ln = Tm, with a smaller radius compared to that of Nd, only when the concentration of Pr was close to 0.6. These interesting observations have caused an intense research activity on these materials using a wide variety of experimental techniques [3]. In parallel, several models without much success have been proposed to understand the behaviour of Pr in these cuprates [5-91. It is generally believed that the suppression of superconductivity is associated with a strong hybridization between Pr-f states with oxygen p orbitals of the Cu-02 plane. In order to understand further the crystal chemistry of the Pr containing cuprates, studies were undertaken recently to investigate the influence of ionic size effect at the Ba site. Quite interestingly, a partial substitution of Ba by Sr was found to influence strongly the structural and superconducting properties of Lnr_~Pr,Ba@r30~ (Ln = Y, Gd, Nd). Thus xc could be increased from 0.55 to 0.7 *Permanent address: Laboratoire Faculte de Chimie University Romania.
[ 10,l l] in Y ,_xPr,SrBaCu307 (YPSBCO) and from 0.45 to 0.6 in Gdl__Pr,SrBaCu307 [12]. Further, it was shown that superconductivity could be restored [13] in the insulating compound of Ndo.7Pro.3Ba2-kSrkCu307 for k ?I 0.2. Moreover, the T, values depended on the heat treatment [l 1, 13, 141. In order to examine such effects for Ln = Tm, we report here on the preparation, X-ray diffraction (XRD) and a.c. susceptibility of Tmo.4Pra6Ba2_kSrkCu306+2. In addition we have examined the effect of heating the samples in argon followed by oxygen annealing on the structural and superconducting properties.
de Chimie Physique, of Iasi, 6600 Iasi,
547
2. EXPERIMENTAL
TECHNIQUES
The polycrystalline samples of Tm0.~r0.6Ba2-kSrkCU~O~+~ (z - 1) have been prepared by solid state reaction of the respective oxides or carbonates. The concentration of Pr was fixed at 0.6 since at this concentration, for k = 0, the compound was found to be an insulator. Tmz03, Pr60 I ,. SrCO3, BaC03 and CuO were thoroughly mixed in required proportions and calcined at 950°C in air for a period of 12-18 h. The resulting product was ground, mixed, pelletized and heated in air at 980°C for a period of 16-24 h. This was repeated twice. Two different heat treatments were then employed. First, the pellets were annealed in oxygen at 450°C for about 72 h and furnace cooled. These were denoted as [O]. The second method consisted of heating these pellets in argon at 850°C for 24 h. After cooling down the samples to near room temperature argon flow
INSULATOR-SUPERCONDUCTOR
548
TRANSITION
was shut off and oxygen was let in. Annealing was then carried out in oxygen flow at 450°C for a period of 72 h and the samples were furnace cooled. These were denoted as [AO]. X-ray diffraction (XRD) data of these samples were recorded using Cu K, (40 kV/20 mA) radiation. About 15-20 reflections were taken into account to calculate the lattice parameters a, b and c by least square fitting. The real (X’) and imaginary (x”) parts of the a.c. susceptibility of the samples were recorded in a field of 0.11 Oe and at a frequency of 1500 Hz. The x’ signal obtained at T = 10 K with a fully oxygenated YBa2Cu30, made in our laboratory was taken as -1. The dimensions of the samples reported in this work were the same as that of the YBa2Cu307 sample. 3. RESULTS
AND DISCUSSION
The XRD pattern of Tm0.4Pr0.6Ba*_kSrkCu306+z (z - 1) for some selected compositions (k = 0, 0.6, 0.8 and 1) for both the treatments [0] and [AO] are shown in Fig. I(a, b). Impurity peaks near 28 = 31” start appearing for k = 0.8 and are more evident for k = 1. The orthorhombic splitting was evident only for k = 0 (a) ‘000I
0
30
40
50
60
28
1
1’
4000 c.=
[A01
$“”
5 -2ow
low 0
20
% PO 6%&C=&+z
30
40
50
4 60
20
Fig. 1. X-ray diffraction pattern of Tm~,J’r0.~a2-Q.t30~z (a) [0] oxygen annealed; (b) [AO] argon heated followed by oxygen annealing.
0.0
IN Tm0.4Pr0.sBazCus07
0.2
0.4
0.6
Vol. 104, No. 9
0.6
100
k(Sr) Fig. 2. Orthorhombicity of Tm0.4PT0.6Ba2-kSrkCu306+z as a function of Sr(k) concentration in the case of [0] and [AO] samples. and 0.6 with the [0] treatment. However, the sample with k = 0.8 after the [AO] treatment showed clearly an orthorhombic splitting. The observed and calculated dh k I values for this sample are given in Table 1. The agreement between the observed and the calculated values is extremely good especially for higher angle reflections. The lattice parameters for all the compositions are collected in Table 2. In the case of [0] samples, orthorhombicity E = (b - a)/@ + a) decreased steadily (Fig. 2) and an orthorhombic to tetragonal transition was observed fork = 0.8. The [AO] treatment was found to increase the orthorhombicity for k > 0 and the transition to tetragonal symmetry was observed only for k > 0.8. The real (x’) and imaginary (X”) parts of the a.c. susceptibility of the samples as a function of temperature is shown in Fig. 3(a, b). Superconducting transition was observed only for k(Sr) 2 0.6. For the sample [0] with k(Sr) = 0.6, the diamagnetic onset T, was observed at 19 K and the transition was broad. There was a small increase in T, from 19 to 20 K after the [AO] treatment but the transition was still broader and smaller. For the sample [0] with k(Sr) = 0.8, T, increased to 22 K but the strength of the signal remained almost the same. However, after the [AO] treatment of this sample, there was a remarkable increase in T, from 22 to 30 K and the strength of the signal, at 10 K, increased by more than a factor of 3. Further the intergranular peak also showed a shift from around 12 K to 22.5 K indicating an improved coupling between the grains after the [AO] treatment. As the concentration of Sr was increased further to k = 1, the T, for both the samples [0] and [AO] decreased to 22 K. However, the shielding signal for the sample [A01 was smaller than that observed for the sample [Ol. The [AO] treatment in this case also decreased the coupling between the grains as can be seen from a broadening of
Vol. 104, No. 9
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tmo.@o.~Ba$Zu307
549
Table 1. Observed and calculated d h k I values for Tmo.llPro.sBaI.ZSr0.8Cu306+~ [0] = oxygen annealed at 450°C. [AO] = heated in argon at 850°C followed by oxygen annealing at 450°C
KY
IA01
hkl
dabs (A)
d,dc CA)
003 100 101
3.8669
3.8532 3.6556
3.8643 3.8442 3.6488
103 110 005 113 105
2.7265 2.7184 2.3187 2.2250 1.9850
2.7253 2.7183 2.3186 2.2233 1.9854
006 200 115 106
1.9323 1.9216 1.7641 1.7257
1.9322 1.9221 1.7640 1.7264
007
1.6568
1.6562
116
1.5744
1.5749
the X“transition and a shift of the maximum from around 15 K to a temperature less than 10 K. We present below a qualitative discussion of our data. An insulator to superconductor transition should result mainly from an increase in the hole density. Such an increase could be in~uenced by several factors. Some of the crystal chemical factors are: increase in oxygen content, changes in structural parameters such as Cul-apical oxygen distance, increase in chain oxygen
hkl
dabs
d,l, (A)
010 003 100 101 013 110 103 005 113 105 020 006 200 115 120 210 007 123 116 213
3.8711 3.8643 3.8306 3.6431 3.7353 2.7233 2.7180 2.3185 2.2250 1.9834 1.9345 1.9321 1.9158 1.7654 1.7275 1.7154 1.6568 1.5759 1.5748 1.5667
3.8691 3.8642 3.8278 3.6348 2.7341 2.7211 2.7195 2.3185 2.2249 1.9831 1.9345 1.9321 1.9139 1.7649 1.7266 1.7155 1.6561 1.5764 1.5754 1.5679
order etc. Let us examine how these factors would influence our data. Though we have not measured the oxygen content in our samples, we assume that the oxygen content might actually decrease from 6.95 + 0.03 to 6.87 +- 0.03 as was found in the case of Ndo.7Pro.3Ba2-kSrkCu307as k increased from 0 to 1 [15]. This decrease in oxygen content did not result in a decrease of T,, on the contrary superconductivity was restored as k(Sr) increased and this was accompanied by
Table 2. Lattiqe parameters, c/a, volume and Z”,of Tm*.~Pr~.~Ba~_~Sr~Cu~O~+~. Error in a and b = i 0.001 A; in c= ?O.O03A k
a
Samples with [O] treatment 0 3.836 0.3 3.831 0.6 3.832 0.8 3.844 1 3.843 Samples with [AO] treatment 0 3.841 0.3 3.834 0.6 3.825 0.8 3.828
1
3.841
b
c/a
v (A3)
3.904
11.692
3.894
11.666 11.623 11.593 11.565
3.047 3.045 3.033 3.016 3.009
175.107 173.991 172.801 171.319 170.812
11.703 11.670 11.614 11.593 11.559
3.047 3.044 3.036 3.029 3.009
175.667 174.547 172.421 171.688 170.573
3.879 3.844 3.843
3.909 3.902
3.881 3.869 3.841
T, W) 0 0 19
22 22
0 0
20 30 22
550
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tm&‘r0.6Ba2Cu~07 35_,,,,,,‘,,,,,,.,,,,, 30 25 z
0.6[0] O.S[AO] 0.8[0] 0.8fAO1
e”
20 15
Vol. 104, No. 9 ,,I
[
L ...0......... ^ ..I.k
_~~..~~~...f.-..-..-__~...~_.“.~”__ _.__ _I * ( ! : . / i .. . _.............._..t .._............ i_. __..i. .. . . . . _,,,._,._y_... k ..._................. /A ; 4 9 .,.,+__.._,.,..,., ... ....” +___._ _...I ^ _ . ‘.$ _.._I._.__+.._ “..l_._” .t_..___ ^..-..f”.._._ I 2
_.._._
frIn[Oj *_..._._I .._.i ^ “,.._.,..,,. &._....._.“._ 3
1101
1WI 10
2b
3’0 T(K)
Fig. 4. T, of Tm0.4Pr0.6Baz-kSrkC~306tz and Ndo.7Pro.3Ba2-kSrkCu30~(taken from [133 as a function of Sr(k) concentration Only the case of [0] samples is shown. O.S[O] 0.8[0] 0.8[AO] 1 [OI 1 IA01
T(K) Fig. 3. (a) Real and (b) imaginary parts of a.c. susceptibility of Tmo.4Pro.sBaz-kSrkCu306+2as a function of temperature. an increase in the hole density as revealed by Seebeck effect measurements [ 1.51.The reason for this behaviour may be related to an increase in oxygen ordering of Cut-0 chain. Consider the increasing results obtained by Luetgemeier et al. 1161. They found a distinct v~iation of T, in the parent com~unds N~a~Cu~O~+~ and TrnBa~Cu~O~~~as a function of oxygen content. For example, it was found that for the same oxygen content of (6 + z) = 6.6, T, = 0 for NdBaJZu306+, but around 50 K for TmBa2C!u306+,. The NQR spectra of these compounds showed that though the total oxygen content was the same for both these compounds, the number of oxygen atoms per chain were more in the case of the TmBazCusO~+, compound resulting in a T, of 50 K. It was thus concluded [16] that the arrangement of the oxygen defects depended on the ionic radii of the rare earth ions at the Y site. It is then tempting to propose the ionic size dependence of oxygen defects as one of the causes for the suppression of superconductivi~ in the Pr
containing 123 compounds, in addition to the band structure effects arising from hybridization of Pr-states with oxygen p orbitals [6, 71. Similar arguments can be put forward to account also for the Sr concentration dependence on ionic size at the Y site. Thus in the case of Yo.~ro.~az_kSrkCusO~ [17] and Ndo.7Pro.3Baz_$rkCusO~ [13] both of which are insulators for k = 0, superconductivity could be restored for k 2 0.75 and 0.25 respectively indicating that bigger the ionic size of the rare earth, smaller the concen~ation of Sr required for inducing su~rconductivi~. In the present case of Tm compounds, ionic size of Tm being smaller (Fig. 4) than that of Nd, we found the value of k (0.6) greater than that found for the Nd compound and closer to that found for the Y compound. The effect of [AO] treatment may also result in a similar increase in the number of oxygen atoms per chain resulting in an increase in the hole density and T, since there was no increase in the total oxygen content following the [AO] treatment [ 181. Another influence on the hole density could arise out of changes in Cul-oxygen apical distance according to the charge transfer model [193. Though we have not carried out neutron diffraction studies, one would like to look at such dam reported [20] recently on the Sr rich compound, Y ,_+Pr,Sr~Cu2,sReo.~0,. In this compound, the decrease in the apical distance as x(Pr) increased from 0 to 0.6 was much smaller compared to that found in the Y r_$rXBa2Cu307 for a similar increase in the concentration of Pr indicating important effects of Sr on structural parameters and also on superconductivity since the Ba rich compound was an insulator when x(Pr) increased to 0.6 whereas, in the Sr rich compound, su~rconductivity was observed at 25 K for n = 0.6. One may also invoke reduced hybri~zation between
Vol. 104, No. 9
INSULATOR-SUPERCONDUCTOR
Pr-f states and the oxygen p orbitals in the Sr rich compounds as was pointed out elsewhere [21]. And finally, we would like to add that we could not succeed in inducing superconductivity in Tm0,4Pr0.6Ba2Cu307 by Ca substitution which is known to increase the hole density in the Cu-O2 planes when substituted at the rare earth site [22]. In conclusion, we have shown that an insulator to superconductor transition could be induced in the insulating compound Tm0.$‘r,,.sBazCu307 by iso-valent substitution at the Ba site by Sr. Some of the factors that would favour such a transition are changes produced in structural parameters, reduced hybridization between Pr-f states and the oxygen p orbitals and increase in oxygen ordering, etc. These results point to the fact that detailed heat treatment studies including those under high oxygen pressure [14] would have to be carried out along with neutron structural measurements to further understand the behaviour of Pr in these cuprates.
TRANSITION 6. 7. 8. 9. 10.
11. 12.
13. 14. 15. 16.
REFERENCES 1.
2. 3. 4.
5.
Soderholm, L., Zhang, K., Hinks, D.G., Beno, M.A., Jorgenson, J.D., Segre, C.U. and Schuller, I.K., Nature, 328, 1987, 604. Liang, J.K., Xu, X.T., Xie, S.S., Rao, G.H., Shao, X.Y. and Duan, Z.G., Z. Phys., B69, 1987, 137. For a review, see Radousky, H.B., J. Mat. Res., 7, 1992, 1917. For a detailed discussion, see, Weiyan Guan, Xu, Y., Sheen, S.R., Chen, Y.C., Wei, J.Y.T.,Lei, H.F., Wu, M.K. and Ho, J.C., Phys. Rev., B49, 1994, 15993. Guo, G.Y. and Temmermann, W.M., Phys. Rev., B41, 1990,6372.
17.
18.
19. 20. 21. 22.
IN Tm0.4Pr0.6Ba2Cu307
551
Fehrenbacher, R. and Rice, T.M., Phys. Rev. Lett., 70,1993,3471. Lichtenstein, A.I. and Mazin, I.I., Phys. Rev. Lett., 74,1995, 1000. Khomskii, D., J. Superconductivity, 6, 1993, 69. Muroi, M. and Street, R.A., Physica, C253, 1995, 205. Guanghan Cao, Yitai Qian, Zuyao Chen, Xiaojun Li, Hongkai Wu and Yuheng Zhang, Phys, Z&t., A196,1994, 263. Das, A. and Suryanarayanan, R., J. de Physique Z. France, 5, 1995, 623. Guanghan Cao, Yitai Qian. Xiaojun Li, Hongkai Wu, Zuyao Chen and Yuheng Zhang, Physica, C248, 1995,92. Das, A. and Suryanarayanan, R., Mat. Res. Bull. (USA.), 30, 1995, 1363. Das, A., Suryanarayanan, R. and Dabrowski, B., Solid State Commun., 99, 1996, 413. Zelenay, I., Das, A. and Suryanarayanan, R., Phys. Len., A222, 1996, 451. Luetgemeier, H., Heinma, I., Wagener, D. and Hosseini, S.M., in Phase Separation in Cuprate Superconductors (Edited by E. Sigmund and K.A. Milller), p. 225. Springer-Verlag, Berlin, 1994. Cao, G., Qian, Y., Li, X., Chen, Z., Wang, C., Ruan, K., Qiu, Y., Cao, L., Ge, Y. and Zhang, Y., J. Phys. Condens. Matter., 7, 1995, L287. Samson, L., Berthon, J., Suryanarayanan, R. and Revcolevschi, A., Eur. J. Solid State Znorg. Chem., 33, 1996,751. Cava, R.J., Science, 247, 1990, 656. Taylor, C.R., Das, A., Zelenay, I., Greaves, C. and Suryanarayanan, R., Physica, C261, 1996, 135. Guanghan Gao, Qian. Y., Li, Y., Chen, Z. and Zheng, Y., J. Phys. Chem. Solids, 56, 1995, 981. Manthiram, A., Lee, S.J. and Goodenough, J.B., J. Solid State Chem., 73, 1989, 278.
Vol. 104, No. 9, pp. 547-55 1, I997 D 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00
Pergamon
PII:SOO38-1098(97)00342-6
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tm0.4Pr0.6Ba2Cu307 BY ISO-VALENT AT Ba SITE L. Colonescu,*
Laboratoire
SUBSTITUTION
J. Berthon and R. Suryanarayanan
de Chimie des Solides, CNRS, Bat. 414, Universid
Paris-Sud,
91405 Orsay, France
(Received 21 May 1997; accepted 9 July 1997 by P.H. Dederichs) We report on the preparation, X-ray diffraction and a.c. susceptibility of Tm0.4Pr0.&-kSrkCu306+~ An orthorhombic to tetragonal transition was observed for k = 0.8. When the samples (for 0 5 k < 1) were heated in argon prior to oxygen annealing, orthorhombicity was found to increase. An insulator to superconductor transition was observed for k 2 0.6, The effect of Sr substitution and the heat treatment may lead to a reduction in Pr-f hybridization with oxygen orbitals and to an increase in oxygen ordering favouring the restoration of superconductivity in this compound. 0 1997-Elsevier Science Ltd
1. INTRODUCTION As early as 1987, it was shown that Pr destroyed superconductivity in Y 1-xPrxBa2Cu307 (YPBCO) when the concentration of Pr(x) reached the critical value of x, = 0.55 [l-3]. Further, the critical concentration of Pr (x,) was found to depend on the ionic size of the rare earth, Ln 13, 41. Whereas for Ln = Nd, the value of x, was found to be as low as 0.3, superconductivity was found to be destroyed in the case of Ln = Tm, with a smaller radius compared to that of Nd, only when the concentration of Pr was close to 0.6. These interesting observations have caused an intense research activity on these materials using a wide variety of experimental techniques [3]. In parallel, several models without much success have been proposed to understand the behaviour of Pr in these cuprates [5-91. It is generally believed that the suppression of superconductivity is associated with a strong hybridization between Pr-f states with oxygen p orbitals of the Cu-02 plane. In order to understand further the crystal chemistry of the Pr containing cuprates, studies were undertaken recently to investigate the influence of ionic size effect at the Ba site. Quite interestingly, a partial substitution of Ba by Sr was found to influence strongly the structural and superconducting properties of Lnr_~Pr,Ba@r30~ (Ln = Y, Gd, Nd). Thus xc could be increased from 0.55 to 0.7 *Permanent address: Laboratoire Faculte de Chimie University Romania.
[ 10,l l] in Y ,_xPr,SrBaCu307 (YPSBCO) and from 0.45 to 0.6 in Gdl__Pr,SrBaCu307 [12]. Further, it was shown that superconductivity could be restored [13] in the insulating compound of Ndo.7Pro.3Ba2-kSrkCu307 for k ?I 0.2. Moreover, the T, values depended on the heat treatment [l 1, 13, 141. In order to examine such effects for Ln = Tm, we report here on the preparation, X-ray diffraction (XRD) and a.c. susceptibility of Tmo.4Pra6Ba2_kSrkCu306+2. In addition we have examined the effect of heating the samples in argon followed by oxygen annealing on the structural and superconducting properties.
de Chimie Physique, of Iasi, 6600 Iasi,
547
2. EXPERIMENTAL
TECHNIQUES
The polycrystalline samples of Tm0.~r0.6Ba2-kSrkCU~O~+~ (z - 1) have been prepared by solid state reaction of the respective oxides or carbonates. The concentration of Pr was fixed at 0.6 since at this concentration, for k = 0, the compound was found to be an insulator. Tmz03, Pr60 I ,. SrCO3, BaC03 and CuO were thoroughly mixed in required proportions and calcined at 950°C in air for a period of 12-18 h. The resulting product was ground, mixed, pelletized and heated in air at 980°C for a period of 16-24 h. This was repeated twice. Two different heat treatments were then employed. First, the pellets were annealed in oxygen at 450°C for about 72 h and furnace cooled. These were denoted as [O]. The second method consisted of heating these pellets in argon at 850°C for 24 h. After cooling down the samples to near room temperature argon flow
INSULATOR-SUPERCONDUCTOR
548
TRANSITION
was shut off and oxygen was let in. Annealing was then carried out in oxygen flow at 450°C for a period of 72 h and the samples were furnace cooled. These were denoted as [AO]. X-ray diffraction (XRD) data of these samples were recorded using Cu K, (40 kV/20 mA) radiation. About 15-20 reflections were taken into account to calculate the lattice parameters a, b and c by least square fitting. The real (X’) and imaginary (x”) parts of the a.c. susceptibility of the samples were recorded in a field of 0.11 Oe and at a frequency of 1500 Hz. The x’ signal obtained at T = 10 K with a fully oxygenated YBa2Cu30, made in our laboratory was taken as -1. The dimensions of the samples reported in this work were the same as that of the YBa2Cu307 sample. 3. RESULTS
AND DISCUSSION
The XRD pattern of Tm0.4Pr0.6Ba*_kSrkCu306+z (z - 1) for some selected compositions (k = 0, 0.6, 0.8 and 1) for both the treatments [0] and [AO] are shown in Fig. I(a, b). Impurity peaks near 28 = 31” start appearing for k = 0.8 and are more evident for k = 1. The orthorhombic splitting was evident only for k = 0 (a) ‘000I
0
30
40
50
60
28
1
1’
4000 c.=
[A01
$“”
5 -2ow
low 0
20
% PO 6%&C=&+z
30
40
50
4 60
20
Fig. 1. X-ray diffraction pattern of Tm~,J’r0.~a2-Q.t30~z (a) [0] oxygen annealed; (b) [AO] argon heated followed by oxygen annealing.
0.0
IN Tm0.4Pr0.sBazCus07
0.2
0.4
0.6
Vol. 104, No. 9
0.6
100
k(Sr) Fig. 2. Orthorhombicity of Tm0.4PT0.6Ba2-kSrkCu306+z as a function of Sr(k) concentration in the case of [0] and [AO] samples. and 0.6 with the [0] treatment. However, the sample with k = 0.8 after the [AO] treatment showed clearly an orthorhombic splitting. The observed and calculated dh k I values for this sample are given in Table 1. The agreement between the observed and the calculated values is extremely good especially for higher angle reflections. The lattice parameters for all the compositions are collected in Table 2. In the case of [0] samples, orthorhombicity E = (b - a)/@ + a) decreased steadily (Fig. 2) and an orthorhombic to tetragonal transition was observed fork = 0.8. The [AO] treatment was found to increase the orthorhombicity for k > 0 and the transition to tetragonal symmetry was observed only for k > 0.8. The real (x’) and imaginary (X”) parts of the a.c. susceptibility of the samples as a function of temperature is shown in Fig. 3(a, b). Superconducting transition was observed only for k(Sr) 2 0.6. For the sample [0] with k(Sr) = 0.6, the diamagnetic onset T, was observed at 19 K and the transition was broad. There was a small increase in T, from 19 to 20 K after the [AO] treatment but the transition was still broader and smaller. For the sample [0] with k(Sr) = 0.8, T, increased to 22 K but the strength of the signal remained almost the same. However, after the [AO] treatment of this sample, there was a remarkable increase in T, from 22 to 30 K and the strength of the signal, at 10 K, increased by more than a factor of 3. Further the intergranular peak also showed a shift from around 12 K to 22.5 K indicating an improved coupling between the grains after the [AO] treatment. As the concentration of Sr was increased further to k = 1, the T, for both the samples [0] and [AO] decreased to 22 K. However, the shielding signal for the sample [A01 was smaller than that observed for the sample [Ol. The [AO] treatment in this case also decreased the coupling between the grains as can be seen from a broadening of
Vol. 104, No. 9
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tmo.@o.~Ba$Zu307
549
Table 1. Observed and calculated d h k I values for Tmo.llPro.sBaI.ZSr0.8Cu306+~ [0] = oxygen annealed at 450°C. [AO] = heated in argon at 850°C followed by oxygen annealing at 450°C
KY
IA01
hkl
dabs (A)
d,dc CA)
003 100 101
3.8669
3.8532 3.6556
3.8643 3.8442 3.6488
103 110 005 113 105
2.7265 2.7184 2.3187 2.2250 1.9850
2.7253 2.7183 2.3186 2.2233 1.9854
006 200 115 106
1.9323 1.9216 1.7641 1.7257
1.9322 1.9221 1.7640 1.7264
007
1.6568
1.6562
116
1.5744
1.5749
the X“transition and a shift of the maximum from around 15 K to a temperature less than 10 K. We present below a qualitative discussion of our data. An insulator to superconductor transition should result mainly from an increase in the hole density. Such an increase could be in~uenced by several factors. Some of the crystal chemical factors are: increase in oxygen content, changes in structural parameters such as Cul-apical oxygen distance, increase in chain oxygen
hkl
dabs
d,l, (A)
010 003 100 101 013 110 103 005 113 105 020 006 200 115 120 210 007 123 116 213
3.8711 3.8643 3.8306 3.6431 3.7353 2.7233 2.7180 2.3185 2.2250 1.9834 1.9345 1.9321 1.9158 1.7654 1.7275 1.7154 1.6568 1.5759 1.5748 1.5667
3.8691 3.8642 3.8278 3.6348 2.7341 2.7211 2.7195 2.3185 2.2249 1.9831 1.9345 1.9321 1.9139 1.7649 1.7266 1.7155 1.6561 1.5764 1.5754 1.5679
order etc. Let us examine how these factors would influence our data. Though we have not measured the oxygen content in our samples, we assume that the oxygen content might actually decrease from 6.95 + 0.03 to 6.87 +- 0.03 as was found in the case of Ndo.7Pro.3Ba2-kSrkCu307as k increased from 0 to 1 [15]. This decrease in oxygen content did not result in a decrease of T,, on the contrary superconductivity was restored as k(Sr) increased and this was accompanied by
Table 2. Lattiqe parameters, c/a, volume and Z”,of Tm*.~Pr~.~Ba~_~Sr~Cu~O~+~. Error in a and b = i 0.001 A; in c= ?O.O03A k
a
Samples with [O] treatment 0 3.836 0.3 3.831 0.6 3.832 0.8 3.844 1 3.843 Samples with [AO] treatment 0 3.841 0.3 3.834 0.6 3.825 0.8 3.828
1
3.841
b
c/a
v (A3)
3.904
11.692
3.894
11.666 11.623 11.593 11.565
3.047 3.045 3.033 3.016 3.009
175.107 173.991 172.801 171.319 170.812
11.703 11.670 11.614 11.593 11.559
3.047 3.044 3.036 3.029 3.009
175.667 174.547 172.421 171.688 170.573
3.879 3.844 3.843
3.909 3.902
3.881 3.869 3.841
T, W) 0 0 19
22 22
0 0
20 30 22
550
INSULATOR-SUPERCONDUCTOR
TRANSITION IN Tm&‘r0.6Ba2Cu~07 35_,,,,,,‘,,,,,,.,,,,, 30 25 z
0.6[0] O.S[AO] 0.8[0] 0.8fAO1
e”
20 15
Vol. 104, No. 9 ,,I
[
L ...0......... ^ ..I.k
_~~..~~~...f.-..-..-__~...~_.“.~”__ _.__ _I * ( ! : . / i .. . _.............._..t .._............ i_. __..i. .. . . . . _,,,._,._y_... k ..._................. /A ; 4 9 .,.,+__.._,.,..,., ... ....” +___._ _...I ^ _ . ‘.$ _.._I._.__+.._ “..l_._” .t_..___ ^..-..f”.._._ I 2
_.._._
frIn[Oj *_..._._I .._.i ^ “,.._.,..,,. &._....._.“._ 3
1101
1WI 10
2b
3’0 T(K)
Fig. 4. T, of Tm0.4Pr0.6Baz-kSrkC~306tz and Ndo.7Pro.3Ba2-kSrkCu30~(taken from [133 as a function of Sr(k) concentration Only the case of [0] samples is shown. O.S[O] 0.8[0] 0.8[AO] 1 [OI 1 IA01
T(K) Fig. 3. (a) Real and (b) imaginary parts of a.c. susceptibility of Tmo.4Pro.sBaz-kSrkCu306+2as a function of temperature. an increase in the hole density as revealed by Seebeck effect measurements [ 1.51.The reason for this behaviour may be related to an increase in oxygen ordering of Cut-0 chain. Consider the increasing results obtained by Luetgemeier et al. 1161. They found a distinct v~iation of T, in the parent com~unds N~a~Cu~O~+~ and TrnBa~Cu~O~~~as a function of oxygen content. For example, it was found that for the same oxygen content of (6 + z) = 6.6, T, = 0 for NdBaJZu306+, but around 50 K for TmBa2C!u306+,. The NQR spectra of these compounds showed that though the total oxygen content was the same for both these compounds, the number of oxygen atoms per chain were more in the case of the TmBazCusO~+, compound resulting in a T, of 50 K. It was thus concluded [16] that the arrangement of the oxygen defects depended on the ionic radii of the rare earth ions at the Y site. It is then tempting to propose the ionic size dependence of oxygen defects as one of the causes for the suppression of superconductivi~ in the Pr
containing 123 compounds, in addition to the band structure effects arising from hybridization of Pr-states with oxygen p orbitals [6, 71. Similar arguments can be put forward to account also for the Sr concentration dependence on ionic size at the Y site. Thus in the case of Yo.~ro.~az_kSrkCusO~ [17] and Ndo.7Pro.3Baz_$rkCusO~ [13] both of which are insulators for k = 0, superconductivity could be restored for k 2 0.75 and 0.25 respectively indicating that bigger the ionic size of the rare earth, smaller the concen~ation of Sr required for inducing su~rconductivi~. In the present case of Tm compounds, ionic size of Tm being smaller (Fig. 4) than that of Nd, we found the value of k (0.6) greater than that found for the Nd compound and closer to that found for the Y compound. The effect of [AO] treatment may also result in a similar increase in the number of oxygen atoms per chain resulting in an increase in the hole density and T, since there was no increase in the total oxygen content following the [AO] treatment [ 181. Another influence on the hole density could arise out of changes in Cul-oxygen apical distance according to the charge transfer model [193. Though we have not carried out neutron diffraction studies, one would like to look at such dam reported [20] recently on the Sr rich compound, Y ,_+Pr,Sr~Cu2,sReo.~0,. In this compound, the decrease in the apical distance as x(Pr) increased from 0 to 0.6 was much smaller compared to that found in the Y r_$rXBa2Cu307 for a similar increase in the concentration of Pr indicating important effects of Sr on structural parameters and also on superconductivity since the Ba rich compound was an insulator when x(Pr) increased to 0.6 whereas, in the Sr rich compound, su~rconductivity was observed at 25 K for n = 0.6. One may also invoke reduced hybri~zation between
Vol. 104, No. 9
INSULATOR-SUPERCONDUCTOR
Pr-f states and the oxygen p orbitals in the Sr rich compounds as was pointed out elsewhere [21]. And finally, we would like to add that we could not succeed in inducing superconductivity in Tm0,4Pr0.6Ba2Cu307 by Ca substitution which is known to increase the hole density in the Cu-O2 planes when substituted at the rare earth site [22]. In conclusion, we have shown that an insulator to superconductor transition could be induced in the insulating compound Tm0.$‘r,,.sBazCu307 by iso-valent substitution at the Ba site by Sr. Some of the factors that would favour such a transition are changes produced in structural parameters, reduced hybridization between Pr-f states and the oxygen p orbitals and increase in oxygen ordering, etc. These results point to the fact that detailed heat treatment studies including those under high oxygen pressure [14] would have to be carried out along with neutron structural measurements to further understand the behaviour of Pr in these cuprates.
TRANSITION 6. 7. 8. 9. 10.
11. 12.
13. 14. 15. 16.
REFERENCES 1.
2. 3. 4.
5.
Soderholm, L., Zhang, K., Hinks, D.G., Beno, M.A., Jorgenson, J.D., Segre, C.U. and Schuller, I.K., Nature, 328, 1987, 604. Liang, J.K., Xu, X.T., Xie, S.S., Rao, G.H., Shao, X.Y. and Duan, Z.G., Z. Phys., B69, 1987, 137. For a review, see Radousky, H.B., J. Mat. Res., 7, 1992, 1917. For a detailed discussion, see, Weiyan Guan, Xu, Y., Sheen, S.R., Chen, Y.C., Wei, J.Y.T.,Lei, H.F., Wu, M.K. and Ho, J.C., Phys. Rev., B49, 1994, 15993. Guo, G.Y. and Temmermann, W.M., Phys. Rev., B41, 1990,6372.
17.
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19. 20. 21. 22.
IN Tm0.4Pr0.6Ba2Cu307
551
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