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The relationship between the CuO bond length and Tc in the superconducting series La1.85 − xSr0.15NdxCuO4
Journal of the Less Common Metals, 153 (1989)
207
207 - 210
THE RELATIONSHIP BETWEEN THE Cu-0 BOND LENGTH AND T, IN THE SUPERCONDUCTING SERIES La,.,, y Sr, lsNdx Cu04* L. SODERHOLM, E. E. ALP
S. MINI, B. DABROWSKI,
G. W. CRABTREE,
D. G. HINKS
Chemistry and Materials Sciences Divisions, Argonne National Laboratory, IL 60439 (U.S.A.) (Received
and
Argonne,
June 29,1988)
Summary
We have examined the relationship between variations in the Cu-0 bond length and the superconducting critical temperature in the series Comparing lattice constants determined from (La 1.s5-~Ndx)Sr0.&u04. X-ray diffraction, and resistivity determined by four-probe resistance, we see that the maximum in T; does not correspond to the minimum Cu-0 bond length. We conclude that the Cu-0 bond distance is not the only variable which affects T, in this system.
1. Introduction
Since the initial reports of superconductivity near 40 K in Laz- yMyCu04 (M = Ba, Sr, Ca) [l, 21, considerable work has been done on these, and on related oxide systems. While details of the mechanism responsible for such high critical temperatures (T, s) is not well understood, there is substantial evidence that the Cu-0 planes which form in this K2NiF4 type structure [3] are an essential element for the occurrence of superconductivity. The two-dimensional Cu-0 planes are formed by the corner-sharing of Cu-0 octahedra, which are separated in the third (c) dimension by La-O layers. The resulting Cu-0-Cu network is planar, with the Cu-0-Cu distance equal to the unit cell edge a,. By preparing a variety of compounds for which the average ionic radius of the alkaline earth is varied systematically while the total composition is fixed at Lal.9M0.1Cu04 (M E Ba, Sr, Ca), it has been demonstrated that a, does not vary linearly with the average size of M [4], as would be expected. However, T, is seen to increase with decreasing a,, indicating that a, may be influenced by electronic effects.
*Paper presented at the September 12 - 16, 1988. 0022-5088/89/$3.50
18th
Rare
Earth
Research
@ Elsevier
Conference,
Sequoia/Printed
Lake Geneva,
WI,
in The Netherlands
208
We have set out to expand the work done on the alkaline earth substitutions [4] by substituting Nd3+ for La3’ in the series La,_s,_,Nd,Sro.isCuO+ The parent compound of this system, La1.ssSr,i5Cu04, has the highest T,, 38 K, reported for the Laz_ ,M,Cu04 series [5], and it also has the shortest Cu-0 bond length. Furthermore, it has been shown [6] that T, is raised substantially for LalssBao.is Cu04 when it is placed under external pressure. In an effort to mimic these pressure experiments in the strontiumcont~ning compound using chemical techniques we have partially replaced La3+ by the slightly smaller Nd3+, Pure Nd,Cu04 crystallizes with tetragonal symmetry, but in a structure slightly different from La,.s&0.15Cu0~ [7] (designated T’ and T structures respectively). Nd2Cu04 is not superconducting, and has much longer a, (3.943 A) than does La1.ssSr0.,&u04 (3.7765 A). Previous work [8] has shown that some Nd3+ can be substituted into La&uO+ slightly decreasing the Cu-0 bond lengths, before the resulting material converts to the T’ phase.
2. Experimental
details
(3~= 0.1, 0.2, 0.3, 0.4, 0.5) The compounds Lal.sS._ xNd,Sro.i,Cu04 were prepared by intimate mixing of stoichiometric quantities of high purity Laz03, Ndz03, SrC03 and CuO. These mixtures were prefired at 850 ‘C, reground and refired at about 990 “C overnight, followed by furnace cooling under flowing oxygen. X-ray diffraction patterns were obtained using a Scintag PADV theta-theta diffractometer using Cu Kcu radiation, calibrated against a National Bureau of Standards silicon standard. Standard four-probe resistivity measurements, utilizing 0.1 - 1 A cme2 in the 10 - 300 K range, were made using a computer-controlled closed-cycle refrigerator.
3. Results and discussion X-ray powder diffraction patterns of all the samples reveal single phase materials with no observable impurity lines. All the diffraction peaks for all samples can be indexed consistent with the tetragonal structure of La,.ssSr,,sCu04 [3]. However, a close examination of the d~fraction patterns reveals that there is a slight line broadening at lower d spacings which appears with increasing neodymium content. While the origins of this line broadening have not been thoroughly investigated, similar line broadening has been observed in perovskite systems [9], where it has been attributed to strains arising from lattice defects. Since it is known that Nd2Cu04 has different oxygen positions in its structure, it would not be surprising if oxygen lattice defects were incorporated into samples with higher neodymium content. The lattice constants obtained from these X-ray data are plotted against neodymium content in Fig. 1. As expected, the replacement of La3+ by the
209
Nd Cont. (x)
Fig. 1. The lattice constants of Lat.ss_x Nd, Sre. t&u04 obtained neodymium concentration. X-ray peaks were indexed according and at least 15 such lines were then least-squares fitted to obtain
for samples with varying to tetragonal symmetry, these results.
small amounts of Nd3+ results in a decreasing a,, indicating that the T structure type of La,,,,Sr,,&u04 is essentially retained. A plot of T, us. Nd3+ concentration is shown in Fig. 2. T, is at a maximum for a neodymium concentration of x = 0 and falls smoothly until about x = 0.4, after which it levels off. Previous work on the Lal.ss-XNd,Sr,,&u04 system [lo, 111 has shown that the decrease observed in T, cannot be attributed to the effect of a magnetic impurity alone. Figures 1 and 2 are combined to form Fig. 3 to show the change in T, with changing a,. T, decreases with decreasing Cu-0 bond distance, contrary to what could be expected from the previous work on alkaline earth substitutions [4]. However, it is important to note that previous work had as its shortest Cu-0 bond the distance of that in our parent (X = 0) compound, which is the longest in this series. Combining our results with the previous work on Lal.9Mo.,Cu0, [4] leads to the conclusion that the lattice constants a, do not scale with the size of the alkaline earth. This evidence, combined with the fact that there is a peak in the plot of a, us. T, at La,.ssSr,l,CuO, (a, = 3.7765 A), leads to the conclusion that the Cu-0 bond distance is not the only variable to consider, and that electronic effects must be playing a major role. Theoretical support for these results comes from the prediction of a van Hove singularity in the density of states for La2 _ ySryCuO,, at y = 0.15 [ 121. This 380r
380 ~
G ; g 0 I i
340
2.
:
0
260
0
01
02
The variation
the midpoints
0.3
Concentration
0
0
0.4
0 5
*
32 0
2 a
Nd Fig.
z
.
300
22 0
0 e
e
260
kU
" 00
20 0 3 771
(x)
of T, with neodymium
3 775
3773 a,
concentration.
3 j77
(A)
The points plotted
represent
of the transitions.
Fig. 3. The change in T, with a,. The trend of decreasing T, with decreasing a0 is opposite that found for longer values of a, [4]. A combination of this plot with Fig. 2 of ref. 4 shows a maximum in Tc vs. a,, at approx. 3.7765 A, the a, value for Lar.ssSro.rsCuO4.
to
210
peak in the density of states then leads to a maximum in T,, according to simple models, and anything that moves the density of states off the singularity, such as a change in a,, will serve to decrease T,. Therefore, the optimum T, observed for La r~85Sro~15Cu04 is explained as a band structure effect, which manifests itself in the changes in a,, which in turn are observed to correlate with the superconducting critical temperature.
Acknowledgements The authors wish to thank W. T. Carnal1 for a critical reading of this manuscript. This work is supported by U.S.-D.O.E. Basic Energy Sciences, Chemical and Materials Sciences, under contract No. W-31-109-ENG-38.
References 1 J. G. Bednorz and K. A. Muller, 2. Phys. B, 64 (1986) 189. 2 K. Kishio, K. Kitazawa, S. Kanbe, I. Yasuda, N. Sugii, H. Takagi, S. Uchida, K. Fueki and S. Tanaka, Chem. Lett. (1987) 429. 3 J. D. Jorgensen, H.-B. Schuttler, D. G. Hinks, D. W. Capone II, K. Zhang, M. B. Brodsky and D. J. Scalapino, Phys. Rev. Lett., 58 (10) (1987) 1024. 4 K. Kishio, K. Kitazawa, N. Sugii, S. Kanbe, K. Fueki, H. Takagi and S. Tanaka, Chem. Lett., 4 (1987) 635. 5 D. G. Hinks, B. Dabrowski, K. Zhang, C. U. Segre, J. D. Jorgensen, L. Soderholm and M. A. Beno, Mater. Res. Sot. Symp. Proc., 99 (1988) 9. 6 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang and Y. Q. Wang, Phys. Rev. Lett., 58 (4) (1987) 405. 7 Hk. Muller-Buschbaum and W. Wollschlager, 2. onorg. allg. Chem., 414 (1975) 76. 8 K. K. Singh, P. Ganguly and C. N. Rao, Mater. Res. Bull., 17 (1982) 493. 9 J. G. Greedan, L. Soderhoim and J.-M. Friedt, J. Solid State Chem., 59 (1985) 242. 10 W. K. Kwok, G. W. Crabtree, A. Umezawa, E. E. Alp, L. Morss and L. Soderholm, Jpn. J. A@. Phys., 26 (3) (1987) 1087. 11 G. W. Crabtree, W. K. Kwok, A. Umezawa, L. Soderholm, L. Morss and E. E. Alp, Phys. Rev. B, 36 (10) (1987) 5258. 12 J.-H. Xu, T. J. Watson-Yang, J. Ye and A. J. Freeman, Phys. Lett. A, 120 (1987) 489.
207
207 - 210
THE RELATIONSHIP BETWEEN THE Cu-0 BOND LENGTH AND T, IN THE SUPERCONDUCTING SERIES La,.,, y Sr, lsNdx Cu04* L. SODERHOLM, E. E. ALP
S. MINI, B. DABROWSKI,
G. W. CRABTREE,
D. G. HINKS
Chemistry and Materials Sciences Divisions, Argonne National Laboratory, IL 60439 (U.S.A.) (Received
and
Argonne,
June 29,1988)
Summary
We have examined the relationship between variations in the Cu-0 bond length and the superconducting critical temperature in the series Comparing lattice constants determined from (La 1.s5-~Ndx)Sr0.&u04. X-ray diffraction, and resistivity determined by four-probe resistance, we see that the maximum in T; does not correspond to the minimum Cu-0 bond length. We conclude that the Cu-0 bond distance is not the only variable which affects T, in this system.
1. Introduction
Since the initial reports of superconductivity near 40 K in Laz- yMyCu04 (M = Ba, Sr, Ca) [l, 21, considerable work has been done on these, and on related oxide systems. While details of the mechanism responsible for such high critical temperatures (T, s) is not well understood, there is substantial evidence that the Cu-0 planes which form in this K2NiF4 type structure [3] are an essential element for the occurrence of superconductivity. The two-dimensional Cu-0 planes are formed by the corner-sharing of Cu-0 octahedra, which are separated in the third (c) dimension by La-O layers. The resulting Cu-0-Cu network is planar, with the Cu-0-Cu distance equal to the unit cell edge a,. By preparing a variety of compounds for which the average ionic radius of the alkaline earth is varied systematically while the total composition is fixed at Lal.9M0.1Cu04 (M E Ba, Sr, Ca), it has been demonstrated that a, does not vary linearly with the average size of M [4], as would be expected. However, T, is seen to increase with decreasing a,, indicating that a, may be influenced by electronic effects.
*Paper presented at the September 12 - 16, 1988. 0022-5088/89/$3.50
18th
Rare
Earth
Research
@ Elsevier
Conference,
Sequoia/Printed
Lake Geneva,
WI,
in The Netherlands
208
We have set out to expand the work done on the alkaline earth substitutions [4] by substituting Nd3+ for La3’ in the series La,_s,_,Nd,Sro.isCuO+ The parent compound of this system, La1.ssSr,i5Cu04, has the highest T,, 38 K, reported for the Laz_ ,M,Cu04 series [5], and it also has the shortest Cu-0 bond length. Furthermore, it has been shown [6] that T, is raised substantially for LalssBao.is Cu04 when it is placed under external pressure. In an effort to mimic these pressure experiments in the strontiumcont~ning compound using chemical techniques we have partially replaced La3+ by the slightly smaller Nd3+, Pure Nd,Cu04 crystallizes with tetragonal symmetry, but in a structure slightly different from La,.s&0.15Cu0~ [7] (designated T’ and T structures respectively). Nd2Cu04 is not superconducting, and has much longer a, (3.943 A) than does La1.ssSr0.,&u04 (3.7765 A). Previous work [8] has shown that some Nd3+ can be substituted into La&uO+ slightly decreasing the Cu-0 bond lengths, before the resulting material converts to the T’ phase.
2. Experimental
details
(3~= 0.1, 0.2, 0.3, 0.4, 0.5) The compounds Lal.sS._ xNd,Sro.i,Cu04 were prepared by intimate mixing of stoichiometric quantities of high purity Laz03, Ndz03, SrC03 and CuO. These mixtures were prefired at 850 ‘C, reground and refired at about 990 “C overnight, followed by furnace cooling under flowing oxygen. X-ray diffraction patterns were obtained using a Scintag PADV theta-theta diffractometer using Cu Kcu radiation, calibrated against a National Bureau of Standards silicon standard. Standard four-probe resistivity measurements, utilizing 0.1 - 1 A cme2 in the 10 - 300 K range, were made using a computer-controlled closed-cycle refrigerator.
3. Results and discussion X-ray powder diffraction patterns of all the samples reveal single phase materials with no observable impurity lines. All the diffraction peaks for all samples can be indexed consistent with the tetragonal structure of La,.ssSr,,sCu04 [3]. However, a close examination of the d~fraction patterns reveals that there is a slight line broadening at lower d spacings which appears with increasing neodymium content. While the origins of this line broadening have not been thoroughly investigated, similar line broadening has been observed in perovskite systems [9], where it has been attributed to strains arising from lattice defects. Since it is known that Nd2Cu04 has different oxygen positions in its structure, it would not be surprising if oxygen lattice defects were incorporated into samples with higher neodymium content. The lattice constants obtained from these X-ray data are plotted against neodymium content in Fig. 1. As expected, the replacement of La3+ by the
209
Nd Cont. (x)
Fig. 1. The lattice constants of Lat.ss_x Nd, Sre. t&u04 obtained neodymium concentration. X-ray peaks were indexed according and at least 15 such lines were then least-squares fitted to obtain
for samples with varying to tetragonal symmetry, these results.
small amounts of Nd3+ results in a decreasing a,, indicating that the T structure type of La,,,,Sr,,&u04 is essentially retained. A plot of T, us. Nd3+ concentration is shown in Fig. 2. T, is at a maximum for a neodymium concentration of x = 0 and falls smoothly until about x = 0.4, after which it levels off. Previous work on the Lal.ss-XNd,Sr,,&u04 system [lo, 111 has shown that the decrease observed in T, cannot be attributed to the effect of a magnetic impurity alone. Figures 1 and 2 are combined to form Fig. 3 to show the change in T, with changing a,. T, decreases with decreasing Cu-0 bond distance, contrary to what could be expected from the previous work on alkaline earth substitutions [4]. However, it is important to note that previous work had as its shortest Cu-0 bond the distance of that in our parent (X = 0) compound, which is the longest in this series. Combining our results with the previous work on Lal.9Mo.,Cu0, [4] leads to the conclusion that the lattice constants a, do not scale with the size of the alkaline earth. This evidence, combined with the fact that there is a peak in the plot of a, us. T, at La,.ssSr,l,CuO, (a, = 3.7765 A), leads to the conclusion that the Cu-0 bond distance is not the only variable to consider, and that electronic effects must be playing a major role. Theoretical support for these results comes from the prediction of a van Hove singularity in the density of states for La2 _ ySryCuO,, at y = 0.15 [ 121. This 380r
380 ~
G ; g 0 I i
340
2.
:
0
260
0
01
02
The variation
the midpoints
0.3
Concentration
0
0
0.4
0 5
*
32 0
2 a
Nd Fig.
z
.
300
22 0
0 e
e
260
kU
" 00
20 0 3 771
(x)
of T, with neodymium
3 775
3773 a,
concentration.
3 j77
(A)
The points plotted
represent
of the transitions.
Fig. 3. The change in T, with a,. The trend of decreasing T, with decreasing a0 is opposite that found for longer values of a, [4]. A combination of this plot with Fig. 2 of ref. 4 shows a maximum in Tc vs. a,, at approx. 3.7765 A, the a, value for Lar.ssSro.rsCuO4.
to
210
peak in the density of states then leads to a maximum in T,, according to simple models, and anything that moves the density of states off the singularity, such as a change in a,, will serve to decrease T,. Therefore, the optimum T, observed for La r~85Sro~15Cu04 is explained as a band structure effect, which manifests itself in the changes in a,, which in turn are observed to correlate with the superconducting critical temperature.
Acknowledgements The authors wish to thank W. T. Carnal1 for a critical reading of this manuscript. This work is supported by U.S.-D.O.E. Basic Energy Sciences, Chemical and Materials Sciences, under contract No. W-31-109-ENG-38.
References 1 J. G. Bednorz and K. A. Muller, 2. Phys. B, 64 (1986) 189. 2 K. Kishio, K. Kitazawa, S. Kanbe, I. Yasuda, N. Sugii, H. Takagi, S. Uchida, K. Fueki and S. Tanaka, Chem. Lett. (1987) 429. 3 J. D. Jorgensen, H.-B. Schuttler, D. G. Hinks, D. W. Capone II, K. Zhang, M. B. Brodsky and D. J. Scalapino, Phys. Rev. Lett., 58 (10) (1987) 1024. 4 K. Kishio, K. Kitazawa, N. Sugii, S. Kanbe, K. Fueki, H. Takagi and S. Tanaka, Chem. Lett., 4 (1987) 635. 5 D. G. Hinks, B. Dabrowski, K. Zhang, C. U. Segre, J. D. Jorgensen, L. Soderholm and M. A. Beno, Mater. Res. Sot. Symp. Proc., 99 (1988) 9. 6 C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang and Y. Q. Wang, Phys. Rev. Lett., 58 (4) (1987) 405. 7 Hk. Muller-Buschbaum and W. Wollschlager, 2. onorg. allg. Chem., 414 (1975) 76. 8 K. K. Singh, P. Ganguly and C. N. Rao, Mater. Res. Bull., 17 (1982) 493. 9 J. G. Greedan, L. Soderhoim and J.-M. Friedt, J. Solid State Chem., 59 (1985) 242. 10 W. K. Kwok, G. W. Crabtree, A. Umezawa, E. E. Alp, L. Morss and L. Soderholm, Jpn. J. A@. Phys., 26 (3) (1987) 1087. 11 G. W. Crabtree, W. K. Kwok, A. Umezawa, L. Soderholm, L. Morss and E. E. Alp, Phys. Rev. B, 36 (10) (1987) 5258. 12 J.-H. Xu, T. J. Watson-Yang, J. Ye and A. J. Freeman, Phys. Lett. A, 120 (1987) 489.