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EXAFS studies of electron-doped Nd1.82−zMzCe0.18CuOy: M = Ba, Sr or Ca, z ⩽ 0.18
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Physica B 208&209 (1995) 531 532
EXAFS studies of electron-doped Ndl.82_zMzCeo.18CuOr: M = Ba, Sr or Ca, z 0.18 B.D. Padalia a, Okram G. Singh ~, Om Prakash b, S.J. Gurman ¢'*, J.C. Amiss ~ aDepartment of Physics, Indian Institute of Technology, Bombay-400 076, India bDepartment of Metallurgical Engineering, lndian Institute of Technology, Bombay-400076, India ¢Department of Physics, University of Leicester, Leicester LEI 7RH, UK aBMS, Sheffield Hallam University, Sheffield $1 1WB, UK
Abstract We describe EXAFS studies of electron-doped superconductors based on neodymium cuprate doped with cerium. The mechanism for observed changes in the transition temperature on co-doping with divalent cations is investigated using measurements of the Nd, Cu, Ce, Ba, Ca and Sr absorption edges. Changes in the close-neighbour cation-oxygen distances and the Debye-Waller factors are reported and correlated with the observed changes in To.
In the Nd2_xCexCuO r compounds having the T'-type structure superconductivity (SC) occurs through electron doping by substitution of a mixed (3 + , 4 + ) valent Ce atom for some of the Nd atoms. The highest T~ ( ~ 24 K) occurs for x = 0.15 with y < 4 [1, 2]. The increase in electron concentration with increasing Ce content makes the samples non-superconducting when x/> 0.18. In these non-superconducting compounds co-substitution of divalent alkaline earth atoms (Ba, Ca, Sr) at the (trivalent) Nd site revives the SC [3]. This is presumably due to compensation of the excess electrons. In order to obtain information on the valence states and local environment of the cations, we have made X-ray absorption measurements on an absorption edge of all the cations present in samples of Ndl.s2_zMzCeo.lsCuO r where M = B a , Ca or Sr and z~<0.18, together with Nd2-xCexCuO r with x = 0.00 and 0.15. The samples were prepared by solid state reaction [1, 2]. The XRD patterns showed a T'-structure with negligible impurities. The lattice parameters derived from *Corresponding author.
the diffraction data and the measured Tc values are given in Table 1. We note that Sr 2÷ (r = 1.12A [4]) doped samples have very similar lattice constants to the SC Ndl.ssCeo.lsCuO3.97 reference compound (ionic radii: Nd 3+ _ 1.00/~; Ce 3+ 1.03A and Ce 4 ÷ - 0.92/~) whereas Ca 2÷ (r = 0.99 A) and Ba 2 ÷ (r = 1.34/~) doped samples show lattice contraction and expansion, respectively. The Tc values show noticeable changes on Ba or Ca doping but are little changed on Sr doping. It therefore appears that variations in the lattice are manifested in the SC behaviour. The K edges of Cu, Ca and Sr and the L3 edges of Nd, Ba and Ce were measured in transmission or fluorescence (for the dopants) mode at the SRS, Daresbury using a Si(1 1 1) crystal with harmonic rejection set at 50%. Beam currents during data taking were about 250mA and the samples were held at 80 K. The EXAFS data were background-subtracted and normalized in the usual way and analysed using the standard Daresbury package EXCURV92. Electron scattering parameters were calculated within the program using a Hedin-Lundqvist potential. Tests on standard samples of Nd203, CeO2
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 4 0 - 3
B.D. Padalia et al./Physica B 208&209 (1995) 531-532
532
Table 1 The lattice parameters derived from the diffraction data and the measured Tc values Composition
a(,A)
c(A)
T,(K) + 1
Nd2CuO4
3.9380(6)
12.174(2)
-24 23 11.973(9) 21 12.2(1) 19
Ndl.ssCeoAsCuO3.97 3.9500(1) 12.092(2) Ndl.TsSro.o6Ceo.lsCuO3.993.9476(7) 12.079(3) Ndl.TsCao.o6Ceo.1 sCuOr 3.925(3) Ndl.~sBao.o6Ceo.lsCuO3.963.98(1)
Table 2 Bond
RI(,~)
R2(fiO
0,2(10 ,),~2
tr22(lO ,*),~2
Nd-O Cu-O Ce-O Ba-O Ca-O Sr-O
2.32 1.96 2.25 2.66 2.26 2.43
2.64 -2.64 2.82 2.42 2.64
60 ___40 30 _+ 20 50 + 30 40 __+100 60 + 80 50 + 30
100 _+ 50 -120 + 50 20 + 100 20 _ 40 80 + 60
Note: Uncertainty on R + 0.02 A. All shells have coordination number n = 4 + 1. All uncertainties are + 2 standard deviations.
and C u O showed good agreement with standard crystallographic parameters. Results for the close oxygen neighbours of the cations are shown in Table 2. The N d O, C e - O and C u - O parameters found were the same, within errors, for all samples. The N d environment is the same as that found in Nd2CuO4 with 4 + 1 neighbours at 2.33 + 0.02 A and a further 4 + 1 at 2.64 + 0.02 fi,. The first Ce O distance is somewhat shorter, agreeing with the value found in CeO2, although the second C e - O distance is very similar to that of N d O. Thus the Ce atom perturbs the layer structure around it, although the overall T'-symmetry is retained. The S r - O bonds are slightly longer than are N d - O . The C a - O bonds are considerably shorter, and the B a - O bonds considerably longer, than are the Nd O bonds, in line with the ionic radii. In fact, all of these first
neighbour distances are consistent with the standard ionic radii 1-4] if we take the radius of 0 2- to be 1.32 .A
[4]. The C u - O distances do not change within the uncertainty. The value found for this distance implies that most of the Cu atoms are in the 2 + valence state (ionic radius 0.72 A,): this is considered in more detail in Ref. [5]. The C u - C u distance along the a-axis (which contains a strong multiple-scattering contribution, included in our calculations) is also constant at 3.93 + 0.02 ~, in line with the crystallographic data. Doping does not increase the disorder around the N d atom, although it slightly increases that around Cu (a 2 for C u - O in N d 2 C u O , is (10 + 20)x 10-4.A2). In general, the disorder around the doping atoms is no greater than that found around N d in Nd2CuO4. We note the comparative rigidity of the C u - O bonds. Our results suggest that the local structure has a significant influence on the superconducting behaviour of electron-doped T'-structures. Co-doping with Sr, which has a similar environment to Ce, restores the Tc almost to its maximum value (Table 1). Co-doping with Ba or Ca, which leads to large local distortions of the layer structure, leads to lower values of To. Thus our EXAFS studies have shown the influence of local structure on the SC behaviour of these doped materials. The oxygen concentration is also significant [5]. We thank Jyoti K. Shridkar for her assistance. We acknowledge with thanks the support of the DST, New Delhi (Project: SBR 24).
References [1] H. Tagaki, S. Uchida and Y. Tokura, Phys. Rev. Lett. 82 (1989) 1197. [2] O.G. Singh, B.D. Padalia, Om Prakash, K. Suba, A.V. Narlikar and L.C. Gupta, Physica C 219 (1994) 156. [3] O.G. Singh, Om Prakash, B.D. Padalia and A.V. Narlikar, Phys. Rev. B 48 (1993) 13 182. [4] R.C. Weast (ed.), Handbook of Chemistry and Physics (CRC, Baton Rouge, 1980). [5] B.D. Padalia, O.G. Singh, S.J. Gurman, K. Suba and Om Prakash, Physica B 208&209 (1995) 533.
Physica B 208&209 (1995) 531 532
EXAFS studies of electron-doped Ndl.82_zMzCeo.18CuOr: M = Ba, Sr or Ca, z 0.18 B.D. Padalia a, Okram G. Singh ~, Om Prakash b, S.J. Gurman ¢'*, J.C. Amiss ~ aDepartment of Physics, Indian Institute of Technology, Bombay-400 076, India bDepartment of Metallurgical Engineering, lndian Institute of Technology, Bombay-400076, India ¢Department of Physics, University of Leicester, Leicester LEI 7RH, UK aBMS, Sheffield Hallam University, Sheffield $1 1WB, UK
Abstract We describe EXAFS studies of electron-doped superconductors based on neodymium cuprate doped with cerium. The mechanism for observed changes in the transition temperature on co-doping with divalent cations is investigated using measurements of the Nd, Cu, Ce, Ba, Ca and Sr absorption edges. Changes in the close-neighbour cation-oxygen distances and the Debye-Waller factors are reported and correlated with the observed changes in To.
In the Nd2_xCexCuO r compounds having the T'-type structure superconductivity (SC) occurs through electron doping by substitution of a mixed (3 + , 4 + ) valent Ce atom for some of the Nd atoms. The highest T~ ( ~ 24 K) occurs for x = 0.15 with y < 4 [1, 2]. The increase in electron concentration with increasing Ce content makes the samples non-superconducting when x/> 0.18. In these non-superconducting compounds co-substitution of divalent alkaline earth atoms (Ba, Ca, Sr) at the (trivalent) Nd site revives the SC [3]. This is presumably due to compensation of the excess electrons. In order to obtain information on the valence states and local environment of the cations, we have made X-ray absorption measurements on an absorption edge of all the cations present in samples of Ndl.s2_zMzCeo.lsCuO r where M = B a , Ca or Sr and z~<0.18, together with Nd2-xCexCuO r with x = 0.00 and 0.15. The samples were prepared by solid state reaction [1, 2]. The XRD patterns showed a T'-structure with negligible impurities. The lattice parameters derived from *Corresponding author.
the diffraction data and the measured Tc values are given in Table 1. We note that Sr 2÷ (r = 1.12A [4]) doped samples have very similar lattice constants to the SC Ndl.ssCeo.lsCuO3.97 reference compound (ionic radii: Nd 3+ _ 1.00/~; Ce 3+ 1.03A and Ce 4 ÷ - 0.92/~) whereas Ca 2÷ (r = 0.99 A) and Ba 2 ÷ (r = 1.34/~) doped samples show lattice contraction and expansion, respectively. The Tc values show noticeable changes on Ba or Ca doping but are little changed on Sr doping. It therefore appears that variations in the lattice are manifested in the SC behaviour. The K edges of Cu, Ca and Sr and the L3 edges of Nd, Ba and Ce were measured in transmission or fluorescence (for the dopants) mode at the SRS, Daresbury using a Si(1 1 1) crystal with harmonic rejection set at 50%. Beam currents during data taking were about 250mA and the samples were held at 80 K. The EXAFS data were background-subtracted and normalized in the usual way and analysed using the standard Daresbury package EXCURV92. Electron scattering parameters were calculated within the program using a Hedin-Lundqvist potential. Tests on standard samples of Nd203, CeO2
0921-4526/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 4 0 - 3
B.D. Padalia et al./Physica B 208&209 (1995) 531-532
532
Table 1 The lattice parameters derived from the diffraction data and the measured Tc values Composition
a(,A)
c(A)
T,(K) + 1
Nd2CuO4
3.9380(6)
12.174(2)
-24 23 11.973(9) 21 12.2(1) 19
Ndl.ssCeoAsCuO3.97 3.9500(1) 12.092(2) Ndl.TsSro.o6Ceo.lsCuO3.993.9476(7) 12.079(3) Ndl.TsCao.o6Ceo.1 sCuOr 3.925(3) Ndl.~sBao.o6Ceo.lsCuO3.963.98(1)
Table 2 Bond
RI(,~)
R2(fiO
0,2(10 ,),~2
tr22(lO ,*),~2
Nd-O Cu-O Ce-O Ba-O Ca-O Sr-O
2.32 1.96 2.25 2.66 2.26 2.43
2.64 -2.64 2.82 2.42 2.64
60 ___40 30 _+ 20 50 + 30 40 __+100 60 + 80 50 + 30
100 _+ 50 -120 + 50 20 + 100 20 _ 40 80 + 60
Note: Uncertainty on R + 0.02 A. All shells have coordination number n = 4 + 1. All uncertainties are + 2 standard deviations.
and C u O showed good agreement with standard crystallographic parameters. Results for the close oxygen neighbours of the cations are shown in Table 2. The N d O, C e - O and C u - O parameters found were the same, within errors, for all samples. The N d environment is the same as that found in Nd2CuO4 with 4 + 1 neighbours at 2.33 + 0.02 A and a further 4 + 1 at 2.64 + 0.02 fi,. The first Ce O distance is somewhat shorter, agreeing with the value found in CeO2, although the second C e - O distance is very similar to that of N d O. Thus the Ce atom perturbs the layer structure around it, although the overall T'-symmetry is retained. The S r - O bonds are slightly longer than are N d - O . The C a - O bonds are considerably shorter, and the B a - O bonds considerably longer, than are the Nd O bonds, in line with the ionic radii. In fact, all of these first
neighbour distances are consistent with the standard ionic radii 1-4] if we take the radius of 0 2- to be 1.32 .A
[4]. The C u - O distances do not change within the uncertainty. The value found for this distance implies that most of the Cu atoms are in the 2 + valence state (ionic radius 0.72 A,): this is considered in more detail in Ref. [5]. The C u - C u distance along the a-axis (which contains a strong multiple-scattering contribution, included in our calculations) is also constant at 3.93 + 0.02 ~, in line with the crystallographic data. Doping does not increase the disorder around the N d atom, although it slightly increases that around Cu (a 2 for C u - O in N d 2 C u O , is (10 + 20)x 10-4.A2). In general, the disorder around the doping atoms is no greater than that found around N d in Nd2CuO4. We note the comparative rigidity of the C u - O bonds. Our results suggest that the local structure has a significant influence on the superconducting behaviour of electron-doped T'-structures. Co-doping with Sr, which has a similar environment to Ce, restores the Tc almost to its maximum value (Table 1). Co-doping with Ba or Ca, which leads to large local distortions of the layer structure, leads to lower values of To. Thus our EXAFS studies have shown the influence of local structure on the SC behaviour of these doped materials. The oxygen concentration is also significant [5]. We thank Jyoti K. Shridkar for her assistance. We acknowledge with thanks the support of the DST, New Delhi (Project: SBR 24).
References [1] H. Tagaki, S. Uchida and Y. Tokura, Phys. Rev. Lett. 82 (1989) 1197. [2] O.G. Singh, B.D. Padalia, Om Prakash, K. Suba, A.V. Narlikar and L.C. Gupta, Physica C 219 (1994) 156. [3] O.G. Singh, Om Prakash, B.D. Padalia and A.V. Narlikar, Phys. Rev. B 48 (1993) 13 182. [4] R.C. Weast (ed.), Handbook of Chemistry and Physics (CRC, Baton Rouge, 1980). [5] B.D. Padalia, O.G. Singh, S.J. Gurman, K. Suba and Om Prakash, Physica B 208&209 (1995) 533.