Inorg. Chem. 1993, 32, 383-385 Sr2-,BaXu02(
383
COj): A Series of Antiferromagnetic Layered Oxide Carbonates
Alexandros Lappas and Kosmas Prassides'
School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNl SQJ, U.K.
A. Robert Armstrong Rutherford Appleton Laboratory, Didcot, Oxon OX1 1 OQX,U.K. Peter P. Edwards'
School of Chemistry, University of Birmingham, Birmingham B15 2TT, U.K. Received September IS, I992 appears to be crucial in determining how this compound6c96d will All superconducting copper oxide systems have in common the presence of (Cu02), layers which form part of strongly elongated be doped, leading to hole s~perconductivity.~In this communication, we report that ordered local magnetic moments appear octahedral' C U O ~ ( L ~ ~ - ~ S ~ ~square-pyramidal2 C U O ~ - ~ ) , Cu05(Nd2-,,,CeSrYCu04-d), or square-planar' C U O ~ ( N ~ ~ - ~ C ~ , C U Oat~ the - ~ )Cu2+sites in Sr2Cu02(C03)and its solid solutions with Ba2+. This points once again to the importance of strong units. The undoped parent materials in which the formal Cu correlations and the need to understand the competiiion between oxidation state is equal to +2 are characterized by the presence magnetism and superconductivity in any model for the high-T, of strong antiferromagnetic (AF) correlation^.^,^ An intimate cuprates. relation exists between the magnetic and (super)conducting properties of the Cu02 layers, as the AF long-range order (LRO) Sr2-,Ba,CuOz(C03) compositions(0 Ix 5 2 ) weresynthesized is olwuys removed upon e l e ~ t r o n -or~ hole-doping4 before the by standard solid state reactions, as reported ear1ier.a For onset of superconductivity. Recently, it has been demonstrated compositions involving greater than 50% Sr and for the pure Ba that hole doping of the oxide carbonate SrzCuOz(C03), which compound, the reactions were performed exclusively under also contains infinite Cu02 layers? leads to superconductivity at enclosed conditions; for compositions with less than 50% Sr,the -40 K.7 Sr2CuO2(CO3) differs structurally from La2Cu04 in samples were fired first in air before the final treatment under that the CuO6 octahedra present are so severely elongated that enclosed conditions. X-ray powder diffraction profiles at room the compound may be regarded as consisting of alternating temperature were recorded on a Philips PW 1050 diffractometer. structural motifs of C U O ~ sheets ~and Sr2(C03)2+slabs.6d Powder neutron diffraction measurements were performed beHowever, the presence of bridging oxygens between Cu and C tween 5 and 300 K with the high-intensity medium-resolution D1B diffractometers at the Institut Laue Langevin, Grenoble, * To whom correspondence should be addressed. France, using neutrons with a mean wavelength of 2.522 A. Data ( I ) (a) Day, P.; Rosseinsky, M. J.; Prassides, K.; David, W. 1. F.; Moze, 0.; were recorded for the x = 0,0.5, 1.35, and 1.5 samples for 213 Soper, A. K. J . Phys. C 1987,20, L429. (b) Paul, D. M.; Balakrishan, 10-90° in steps of 0.2O. G.;Bernhoeft, N. R.; David, W. I. F.; Harrison, W. T. A. Phys. Reu. Leu. 1987, 58, 1976. (c) Jorgensen, J. D.; Schuttler, H . B.; Hinks, D. The gross structural details of Sr2Cu02(C03)were described G.;Capone, D. W.; Zhang, K.; Brodsky, M. B.; Scalapino, D. J. Phys. in earlier work.6 The structure is tetragonal, consisting of Rev. Lerr. 1987,58,1024. (d) Cava,R. J.;Santoro,A.; Johns0n.D. W.; Rhodes, W. W. Phys. Rev. B 1987, 35,6716. alternating CuOz and Sr2CO3layers. However, the exact location (2) Sawa, H.; Suzuki, S.;Watanabe, M.; Akimitsu, J.; Kolcirlo, K.; Asaio, and associated orientational ordering of the carbonate groups H.; Izuki, F.; Takayama-Muromachi, E. Nature 1989, 337, 347. proved more difficult to decipher. Preliminary evidence for 0 (3) (a) Tokura, Y.; Takagi, H.; Uchida, S . Nature 1989, 337, 345. (b) sharing between Cu and C atoms came from time-of-flight powder Singh, K. K.; Ganguly, P.; Rao, C. N. R. Mater. Res. Bull. 1982, 17, 493. (c) Rosseinsky, M. J.; Prassides, K.; Day, P. Physicu C 1989,161, neutron diffraction studies?d but the CO, groups were found to 21. (d) Lightfoot, P.; Richards, D. R.; Dabrowski, B.; Hinks, D. G.;Pei, be orientationally disordered. More recent work6chas confirmed S.;Marx, D. T.; Mitchell, A. W.; Zheng, Y.; Jorgensen, J. D. Physica the 0 sharing between the Cu and C atoms; however, it also C 1990, 168,627. (e) Rosseinsky, M. J.; Prassides, K. Europhys. Lerr. 1991, 14, 551. identified the presence of a superstructure (space group Za), (4) (a) Vaknin, D.; Sinha, S.K.; Moncton, D. E.; Johnston, D. C.; Newsam, resulting from ordering of the carbonate groups (Figure 1). With J.;Safinya, C. R.; King, H. Phys. Reu. Lerr. 1987,58,2802. (b) Freltoft, a standard Rietveld-Hewat profile refinement program and the T.; Shirane, G.;Mitsuda, S.;Remeika, J. P.; Cooper, A. S.Phys. Reu. B 1988,37, 137. (c) Birgeneau, R. J.; Kastner, M. A.; Aharony, A. Z . above structural model, our medium resolution data refine Phys. B 1988, 71, 57. (d) Rosseinsky, M. J.; Prassides, K.; Day, P. J . satisfactorily in the I4 tetragonal space group for all the Murer. Chem. 1991, I , 597. Sr2-,Ba,Cu02(C03) compositionsstudied and at all temperatures (5) (a) Rosseinsky, M. J.; Prassides, K.; Day, P. J . Chem. SOC.,Chem. Commun. 1989, 1734. (b) Rosseinsky, M. J.; Prassides, K. Physicu C between 5 K and room temperature. The average R factors9over 1989, 162, 522. (c) Rosseinsky, M. J.; Prassides, K. Physicu B 1990, the full temperature range obtained in the refinements, e.g. of the 165, 1187. (d) Rosseinsky, M. J.; Prassides, K.; Day, P. Inorg. Chem. Srl,5Bao,5Cu02(C03) profiles,I0 are as follows: R,, = 6.196, R , 1991,30,2680. (e) Rosseinsky, M. J.; Prassides, K.;Scott, C. A. Inorg. Chem. 1991, 30, 3367. (f) Rosseinsky, M. J.; Prassides, K.; Scott, C. = 4.5%, Re = 1.2%. The Cu02 layers appear to remain strictly A. J . Mugn. Mugn. Murer. 1992, 104, 599. (g) Rosseinsky, M. J.; tetragonal with no evidence of any structural distortion. The Prassides, K. Physicu B 1992,180,408. (h) Endoh, Y.; Matsuda, M.; smooth temperature evolution of the unit cell volume of the Yamada, K.; Kakurai, K.; Hidaka, Y.; Shirane, G.;Birgeneau, R. J. samples with x = 0, 0.5, and 1.35 is shown in Figure 2. Phys. Reo. B 1989,40, 7023. (i) Matsuda, M.; Yamada, K.; Kakurai, K.; Kadowski, H.; Thurston, T. R.; Endoh, Y.; Hidaka, Y.; Birgeneau, Figure 3 shows the normalized expansion in the intra- and R. J.; Gehring, P. M.; Moudden, A. H.; Shirane, G. Phys. Reu. B 1990, interlayer spacings relative to the Sr2CuO2(CO3) end member 42,10098. (i)Skanthakumar,S.; Zhang, H.; Clinton,T. W.; Li, W.-H.; upon (Sr,Ba) solid solution formation. Both the intralayer a and Lynn, J. W.; Fisk, Z.; Cheong, S.W. Physicu C 1989, 160, 124. (k) Luke, G. M.; et al. Phys. Reu. B 1990, 42, 7981. the interlayer c dimensions are normalized to the difference in
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(6) (a) von Schnering, H. G.; Hartweg, M.; Walz, L.;Popp, T.; Becker, T.; Schwarz, Jahresberichr des MPI f i r Fesrkbrperforschung Srurrgarr, MPI fiir FestkBrperforschung: Stuttgart, Germany, 1988; p 94. (b) Muller-Buschbaum, H. Angew. Chem. 1989,28,1472. (c) Babu, T. G. N.; Fish, D. J.;Greaves,C. J . Marer. Chem. 1991,1,677. (d) Armstrong, A. R.; Edwards, P. P. J.SolidSrureChem. 1992,98,432. (e) Miyazaki, Y.; Yamane, H.;Kajitani, T.; Oku, T.; Hiraga, K.;Morii, Y.; Fuchizaki, K.;Funahashi, S.;Hirai, T. Physicu C 1992, 191, 434.
0020-166919311332-0383$04.00/0
(7) Kinoshita, K.; Yamada, T. Nature 1992, 357, 313. (8) Pannetier, J. Chem. Scr. A 1986, 26, 131. (9) Rietveld, H. M. J. Appl. Crysrullogr. 1969, 2, 65. (IO) The average number of observables used in the Rietveld analyses was 220; 14 Bragg reflections were included and 14 structural and instrumental variables refined.
0 1993 American Chemical Society
Communications
384 Inorganic Chemistry, Vol. 32, No. 4, 1993
.2 0.3 0.1
0.2
3 H
x w
P
,o. 1
z.
0.0 0.0
0.2
0.4
0.6
0.0 1.0
0.8
Increase in Alkaline-Earth Radius
Figure 3. Variation of the normalizedintralayer and interlayer expansion in the Sr2-xBaxCu02(C03)series relative to the SrzCuOz(CO3) parent member as a functionof the normalized change in ionicsizeof the alkaline earth metal spacer at room temperature. The shaded region corresponds to samples prepared under oxidizing conditions. [Key to symbols: ( 0 ) X-ray (present study); (v)neutron diffraction data (present study); (*)
Figure 1. Magnetic and crystal structure of Sr2-xBaxCu02(C03). (+) and (-) symbols at Cu2+sites indicate antiparallel spins.
965
.
950 0
3. >
5 K x=0.5
935
n
-
4 4 4 v
to V
920 905
0
0
0
Y
O
D Y
”
280
v vwv
K
0
x=o 0
/
0
°
1250.0 I
I
I
I
I
50
100
150
200
250
I 1
300
Temperature (K)
Figure 2. Unit cell volume of Sr2-xBaxCu02(C03) (x = 0, 0.5, 1.35) plotted as a function of temperature.
ionic radii between the eight-coordinate Ba2+ and Sr2+ ions. Vegard’s law for ideal solid solution formation is exhibited by the evolution of the interlayer spacing as the Ba2+ doping level increases.11 The effect of the change in radius of the alkalineearth metal ion is more indirect on the basal plane lattice dimensions where the strong Cu-0 bonding is present. In the Sr-rich part of the diagram,the size of the basal plane still increases linearly,12 albeit at a smaller rate than the interlayer spacing. However, in the Ba-rich part, where oxidizing preparative conditions were employed, deviations from linearityare apparent, indicating disruption of the Cu02 layers; a much smaller anomaly may be present in the interlayer expansion. Our medium resolution data cannotdistinguish between thevariouspossibilities which may give rise to this effect-we note though that two possible reasons are likely to give rise to this behavior: (i) the alkaline-earth metal ions may substitute on the copper sites, as (1 1) A least-squares fit of the normalized interlayer spacing to a straight line results in a slopeequal to 0.74( l),with a value of 1 expected if the lattice expansion was wholly due to the larger size of Ba2+. (12) The slope of the line fitted to the basal plane dimension in the Sr-rich part is 0.15(1).
I
26.5
I
27.5
I
28.5
I
29.5
30.5
Scattering Angle (2e/Deg)
Figure 4. Powder neutron diffraction profile of Sr,.5Ba&u02(C03) between 26.5 and 29S0 at 5 and 280 K. The peak is not evident at 280 K, which is above TN.
has been found13for the related system Ba4CaCu2.2406.w(CO3)0.5 and/or (ii) excess oxygen may be accommodated in the structure (cf. the existence of oxygen-rich14 temary alkaline-earth metal cuprates Ba2Cu03+a). No anomaly is apparent for the x = 2 sample, which was prepared in a reducing atmosphere. Close inspection of the room temperature and the 5 K powder diffraction profiles of Sr2-,Ba,CuOz(CO3) ( x = 0,0.5, and 1.35) revealed the presence of a weak peak in the vicinity of 28 2 8 O (Figure 4). The magnetic origin of the peak is confirmed by the evolution of its intensity with temperature (Figure 5). It could be also readily indexed on the basis of the chemical unit cell as (1 1l ) ~implying , that the magnetic unit cell coincides with the chemical one (aM= a, cM= c, k = 0 ) and the magnetic moments of the copper ions, related by the body-centring translation vector T = (l/2,l/~,l/2) are aligned oppositely. The AF ordering is thus of G-type15 and is illustrated in Figure 1. It proved impossible to identify any more magnetic reflections as the presence of the
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(13) Greaves, C.;Slater, R. P.J. Muter. Chem. 1991, 1, 17. (14) de Leeuw, D.M.; Mutsaers, C. A. H.A.; Langereis, C.; Smoorenburg, H. C. A.; Rommers, P.J. Physica C 1988, 152, 39.
Inorganic Chemistry, Vol. 32, NO. 4, 1993 385
Communications
critical exponent analysis in the 200-260 K temperature range for Sr2Cu02(C03)gave @ = 0.32 f 0.09 and TN = 257 f 3 K. (The N k l temperature of the x = 0.5 sample is estimated to be 240 f 5 K.) This result favors a 3D spin Hamiltonian close to the transition (Ising models, @ = 0.325,0.345;Heisenberg model, @ = 0.365) but excludes a mean-field The strong superexchangeinteractionsbetween the Cu2+ions and the bridging equatorial 02-ions are principally responsible for the AF order exhibited by this series of oxide carbonates. The interlayer exchange interactions are crucial in inducing the 3D LRO by coupling the essentially 2D AF Cu2+sublattices. Even though successive Cu02 layers are separated by -7.5-7.8 A, the interlayer interactions are considerably enhanced by the presence of the bridging C032-groups which lead to a pseudooctahedral coordination for Cu. A crude estimate of the interlayer exchange constant, JI 2 K, can be deduced if we assume that 3D LRO occurs whenz3 ~ B T N J 1 ( 5 2 ~ / a ~ u ~where u ) 2 , 52D is the correlation length and acuxuis the Cu-Cu separation in the layers. Reduced Temperature,T/TN Thevalues of the Cu magnetic moment, p 0.3-0.5p ~obtained , in this series are comparable to those obtained for other parent Figure 5. Temperature dependence of the normalized Cu sublattice magnetization in Sr2Cu02(CO& The solid line represents a fit, close members of layer cuprate ~uperconductors.~~5.~6~~~ They display to the transition temperature, to the expression for critical behavior: IM/ a marked reduction from 1.14 p~ predicted from S = l / 2 and g I(0) = (1 - T / T N ) ~with @ , 4 = 0.32(9) and TN = 257(3) K. The dashed = 2.28 for Cu2+ due to zero-point spin fluctuations and strong curve represents the calculated mean-field curve for a S = l / 2 system. covalency. The in-plane Cu-0 bond lengths in the present series (11.95 A) are comparable with the ones found in Nd2Cu04 (Cu (expected) weaker (1 1 3 ) and ~ (1 15)M was masked by the very coordination number 4; 1.9585 A), but considerably larger than intense ( 0 0 4 ) ~and (222)N nuclear peaks. No evidence for any the ones in LazCuO4 (Cu coordination number 6; 1.9036 A). magnetic scattering was found for the Sro.sBal.sCuO~(CO3) Longer Cu-0 bond lengths imply reduced transfer integrals and composition which lies within the shaded region of Figure 3; both reduced covalency. The similarity in the measured moments may excess oxygen (Cu oxidation states >+2) and/or alkaline-earth thus reflect a balance between covalencyand quantum fluctuation metal substitution of copper sites will destroy the N k l state. effects associated with low dimensionality. The slight reduction The magnetic structure of Sr~..,Ba,Cu02(CO~)contains AF in the Cu moment with increased Ba2+concentration may thus Cu02layers identical to the onesobserved in (Ca0,8sSro.~s)Cu02,~~ arise from an increased importance of spin fluctuations, as a La2Cu04: Nd2Cu04,5and YBa2Cu3O6.I7Furthermore, adjacent result of the increased 2D character upon interlayer expansion, layersare also coupled antiferromagnetically. In uniaxial systems, despite the reduced covalency, associated with a slight in-plane powder diffraction cannot lead to unambiguous determination of expansion. the magnetic moment direction; only the angle between the spin In conclusion, we have shown that ordered local magnetic direction and the unique (c) axis can be determined.l* For moments appear at the Cu2+sites of Sr~,BaXCu02(C03), the illustrative purposes, we considered two models for the spin parent of a new series of hole-doped supercond~ctors.~ Although orientation: along the c axis (11) and in the plane (I). However, there are differences in the details of the magnetic structures, (1 1 1 ) ~is the strongest magnetic peak expected whether the similar magnetic behavior is exhibited by the parent members of moment is parallel or perpendicular to the c axis. Thus in the all the families of the high-T, c u p r a t e ~ ; ~ a* universal ~ J ~ J ~ picture present case, the spatial orientation of the magnetic moment is thus emerging of a Mott-Hubbard description of strongly cannot be uniquely determined. In order to estimate the correlated u* electrons in the layers with magnetic LRO magnitude of the ordered Cu magnetic moment, we used19 disappearing upon doping, before the onset of metallic and superconducting behavior. The presence of axial oxygens has I , = ~ ~ , ~ m , , , / ( sei sin n 28) (1) proven crucial in allowing the present system to be doped with normalized with respect to the structure factors of the ( 0 0 2 ) ~ holes, despite the unusually large Cu-0 in-plane distances. We and the ( 0 0 4 ) ~nuclear reflections. ml1 I is the multiplicity of the note, however, that increased Ba substitution leads to enhanced (1 1 1 ) peak ~ with the magnetic structure factor given byI9 FM 2D character of the systems, as both the in-plane Cu-0 bond 8.07dflq) ( q ) , where ( q ) is the magnetic interaction term, 70 distances expand substantially to -2.00 A and the CuO6 units the gyromagnetic ratio of the neutron, and S the Cu2+spin. The become more severely elongated. This may lead to the fascinating antiferromagnetic form factor flq) for Cu was obtained by possibility of successful electron-doping of the Ba-rich members interpolation from the measurements of Freltoft et al.4b The along with hole-doping of the Sr-rich members of the series. average magnetic moment per Cu ion at 5 K is thus estimated Acknowledgment. We thank SERC, EEC (Advanced Mateto be pll= (0.40 f 0.02) p~ and p i = (0.50 f 0.02) p~ for parallel rials Programme), and BP for financial support of this work, the and perpendicular orientations for S ~ ~ C U O ~ ( C Oand ~ ) pi1 ~ O= (0.28 f 0.02) PBand p I = (0.35 f 0.02) p~ for Srl,~Bao,5Cu02- Institut Laue Langevin for provision of neutron time, and J. K. Cockcroft and C. Ritter for help with the experiments. (C03), respectively.21 The temperature evolution of the normalized integrated (20) We also note that the calculated intensities of the (1 1 3 ) and ~ (1 1 5 ) ~ intensity, IM/Z(O), of the (1 1 1 ) peak ~ (Figure 5) is characteristic magnetic reflections relative to the strongest ( 1 1 I),., reflection arc at a minimum for pI(c. In Sr2CuO*(CO,), the ratio 1(113)/1(11 I ) grows of a continuous transition. Near the ordering temperature, the smoothly from 0.27 to 0.73 and I( 1 1 5)/1( 1 1 1) from 0.07 to 0.44, as the intensity was fitted to the equation I M = 1(0)(1 - T / T N ) ~The ~. angle that the spin direction makes with the unique axis c changes from
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~
~~
( I S ) Bertaut, E. F. Acra Crystallogr. A 1968, 24, 217. (16) Vaknin, D.; Caignol, E.; Davies, P. K.; Fischer, J. E.; Johnston, D. C.; Goshorn, D.P. Phys. Reu. E 1989, 39, 9122. (17) Tranquada, J . M.; Cox, D.E.;Kunnmann, W.; Moudden, H.; Shirane, G.; Suenaga, M.;Zolliker, P.; Vaknin, D.; Sinha, S.K.; Alvarez, M. S.; Jacobson, A. J.; Johnston, D. C. Phys. Reu. Leu. 1988, 60, 156. (18) Shirane, G. A d a Crystallogr. 1959, 12, 282. (19) Bacon,G. E.NeufronDif/racrion;ClarendonPress: Oxford, U.K.,1975.
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0 ' to 90'. (21) Two different samples with stoichiometry Sr, iBao sCUO~(COI) were examined, and the given magnetic moments represent the average value observed. For SroasBalISCUO~(CO]), the presenceofan impurity peak near the magnetic reflection precluded any quantitative estimates of the Cu magnetic moment for this sample. (22) de Jongh, L. J.; Miedema, A. P. Ado. Phys. 1974, 23, 1 . (23) Chakravarty, S.; Halperin, B. I.; Nelson, D. R. Phys. Reu. Left. 1988, 60, 1057.
