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Inorg. Chem. 2000, 39, 3386-3391
Cation Disorder in Ga1212 Kevin B. Greenwood, Donggeun Ko, Douglas A. Vander Griend, Gregory M. Sarjeant, John W. Milgram, Elizabeth S. Garrity, Deborah I. DeLoach, and Kenneth R. Poeppelmeier* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113
Paul A. Salvador and Thomas O. Mason Department of Materials Science and Engineering, Northwestern University, 2137 Sheridan Road, Evanston, Illinois 60208-3108 ReceiVed March 9, 2000 Substitution of calcium for strontium in LnSr2-xCaxCu2GaO7 (Ln ) La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 °C results in complete substitution of calcium for strontium in the lanthanum and praseodymium systems and partial substitution in the other lanthanide systems. The calcium saturation level depends on the size of the Ln cation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation size is reached. Site occupancies from X-ray and neutron diffraction experiments for LnSr2-xCaxCu2GaO7 (x ) 0 and x ) 2) confirm that the A-cations distribute between the two blocking-layer sites and the active-layer site based on size. A quantitative link between cation distribution and relative site-specific cation enthalpy for calcium, strontium, and lanthanum within the gallate structure is derived. The cation distribution in other similar materials can potentially be modeled.
Introduction Almost all layered cuprates based on oxygen-deficient perovskite structures (ABO3-δ) contain multiple cations on the A-sites. Copper-oxygen planes, which can support superconductivity up to relatively high temperatures, are the common structural motif in these layered materials. The coordination preferences of the different A-cations stabilize two-dimensional cuprate planes. The site between the planes is 8-coordinate, while those in the blocking layer are variable, often 9-coordinate or higher.1 The substitution chemistry of YBa2Cu3O7-δ (YBCO) has been intensely investigated. Strontium substitutes for barium in YBa2-xSrxCu3O7-δ only partially up to x ) 1 under ambient pressure, but fully up to x ) 2 under 7 GPa of pressure.2,3 The solubility of calcium on the barium site is limited even more, reaching only x ) 0.25 at ambient pressure. Tc decreases linearly with both Sr2+ and Ca2+ substitution.2-4 A novel family of layered cuprates was developed by Vaughey et al.5 and Roth et al.6 by replacing the blocking-layer copper of YBCO with tetrahedrally coordinated Ga3+. All the strontium analogues, LnSr2Cu2GaO7 (Ln ) La-Nd, Sm-Yb, and Y), can be synthesized at ambient pressure. While the substitution of Sr2+ by Ba2+ causes the framework of LaSr2* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Greenwood, K. B.; Sarjeant, G. M.; Poeppelmeier, K. R.; Salvador, P. A.; Mason, T. O.; Dabrowski, B.; Rogacki, K.; Chen, Z. Chem. Mater. 1995, 7, 1355. (2) Felner, I. Thermochim. Acta 1991, 174, 41. (3) Okai, B. Jpn. J. Appl. Phys. 1990, 29 (12), 2180. (4) Fisher, B.; Genossar, J.; Kuper, C. G.; Patlagan, L.; Reisner, G. M.; Knizhnik, A. Phys. ReV. B 1993, 47, 6054. (5) Vaughey, J. T.; Thiel, J. P.; Hasty, E. F.; Groenke, D. A.; Stern, C. L.; Poeppelmeier, K. R.; Dabrowski, B.; Hinks, D. G.; Mitchell, A. W. Chem. Mater. 1991, 3, 935. (6) Roth, G.; Adelmann, P.; Heger, G.; Knitter, R.; Wolf, Th. J. Phys. I 1991, 1, 721.
Cu2GaO7 to expand,7 the effects of Ca2+ substitution for Sr2+ have not been investigated. Compared to Sr2+, Ca2+ is smaller and prefers A-sites in layered cuprates such as the 8-coordinate site between cuprate planes, as in HgBa2CaCu2O6+δ8 and Tl2Ba2CaCu2O8.9 However, the situation is not so simple. Mixing of the A-cations can thwart superconductivity when it occurs in sufficient degree. Attfield et al. have demonstrated that Tc varies inversely with the variance in A-cation size for constant mean A-cation radius in A2CuO4 materials.10 In order to understand cation disorder in the Ga1212 phases, the A-cation distributions in LaSr2-xCaxCu2GaO7 for x ) 0 and x ) 2 have been analyzed on the basis of neutron diffraction. The results can be used to generate a predictive model for cation distribution in the Ga1212 structure type. Experimental Section Sample Preparation. Many compositions of LnSr2-xCaxCu2GaO7, Ln ) La, Pr, Nd, Gd, Ho, Er, Tm, Yb, and Lu, 0 e x e 2, were prepared by solid-state reaction of the appropriate ratio of lanthanide oxide, yttrium oxide, strontium carbonate, calcium carbonate, copper oxide, and gallium oxide. More specifically, for Ln ) La, samples with x ) 0.20, 0.40, 0.75, 1.00, 1.25, 1.50, and 2.00 were prepared. For each of the Ln cations smaller than lanthanum, usually three or more samples were prepared using a consecutive step size of 0.1 in x near the expected saturation point. The reagents were ground in acetone, pressed into pellets, and fired at 975 °C in an alumina boat for 10 days with three intermediate regrindings. (7) Mary, T. A.; Kumar, N. R. S.; Varadaraju, U. V. J. Solid State Chem. 1993, 107, 524. (8) Hunter, B. A.; Jorgensen, J. D.; Wagner, J. L.; Radaelli, P. G.; Hinks, D. G.; Shaked, H.; Hitterman, R. L.; Von Dreele, R. B. Physica C 1994, 221, 1. (9) Subramanian, M. A.; Calabrese, J. C.; Torardi, C. C.; Gopalakrishnan, J.; Askew, T. R., Flippen, R. B.; Morrissey, K. J.; Chowdhry, U.; Sleight, A. W., Nature 1988, 332, 420. (10) Attfield, J. P.; Kharloanov, A. L., McAllister, J. A., Nature 1998, 394, 157.
10.1021/ic0002667 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000
Cation Disorder in Ga1212 X-ray Diffraction. A Rigaku diffractometer with nickel-filtered Cu KR radiation was used to collect X-ray diffraction data on the polycrystalline samples. These data were used to determine the lattice parameters and ascertain the extent of calcium substitution for strontium in LnSr2-xCaxCu2GaO7 (Ln ) La, Pr, Nd, Gd, Ho, Er, Tm, and Yb). Lattice parameters were calculated by a least-squares technique using the programs XRAYFIT11 and POLSQ12 or refined Rietveld analysis.13 X-ray data for LaCa2Cu2GaO7, which were refined by the Rietveld method, were obtained by counting 10 s every 0.02° from 15° to 90° 2θ. Silicon was used as an internal standard. Forty-one parameters were refined including scale factor, zero-point shift, eight background parameters, six peak shape parameters, unit cell, atomic coordinates, and site occupancies. The mean atomic scattering factors at sin θ/λ ) 0 used for La, Ca, Cu, Gd, and O were 57.00, 20.0, 29.00, 31.00, and 8.00, respectively. Neutron Diffraction. The intense pulsed neutron source (IPNS) at Argonne National Laboratory was used to collect time-of-flight data on 10 g of a polycrystalline sample of LaCa2Cu2GaO7. Data were obtained for 6 h at room temperature and ambient pressure. The data set from d ) 0.5 to 4.0 Å was analyzed by Rietveld methods.14 Fifty parameters were refined, including scale factor, diffractometer constant, zero-point error, five background parameters, six peak shape parameters, unit cell, atomic positions, site occupancies, isotropic thermal factors, and absorption and extinction parameters. The coherent scattering lengths used for La, Ca, Cu, Ga, and O were 8.27, 4.90, 7.72, 7.29, and 5.81 fm, respectively. Thermogravimetric Analysis. A DuPont Instruments 891 thermogravimetric analyzer was used to measure the oxygen content of LaCa2Cu2GaO7. Powdered samples were heated at 650 °C in a flowing mixture of 8.5% hydrogen and 91.5% helium gas until all of the copper species present in the sample had been reduced to copper metal.
Inorganic Chemistry, Vol. 39, No. 15, 2000 3387
Figure 1. Structure of LaCa2Cu2GaO7, which is isomorphic with LaSr2Cu2GaO7.
Results The X-ray diffraction data sets for LnSr2-xCaxCu2GaO7 (Ln ) lanthanide, Y) could all be indexed on a body-centered x2ap x x2ap x 6ap (ap is the lattice parameter of a simple cubic perovskite, i.e., ∼4 Å) orthorhombic unit cell, similar to the parent compound LaSr2Cu2GaO7 (see Figure 1).6 Each diffraction pattern showed the presence of impurity phases on the order of a few percent. The calcium saturation point was therefore defined as the highest calcium concentration at which the fraction of impurity phases did not exceed a few percent. Based on this criterion, calcium substituted for strontium in LaSr2-xCaxCu2GaO7 and PrSr2-xCaxCu2GaO7 over the entire range and resulted in a linear contraction of each lattice parameter as a function of increasing x (see Figure 2). For the rest of the lanthanides, calcium substitutes progressively less with decreasing lanthanide size through YbSr2-xCaxCu2GaO7, in which it saturates at x ) 0.60 (see Figure 3). LuSr2-xCaxCu2GaO7 did not form under the experimental conditions employed for any value of x, consistent with prior attempts to synthesize LuSr2Cu2GaO7 at ambient pressure.5 Lattice parameters at the limit of calcium solubility for LnSr2-xCaxCu2GaO7, Ln ) La, Pr, Nd, Gd, Ho, Er, Tm, and Yb, are given in Table 1. The structure of LaCa2Cu2GaO7 was refined from both X-ray and neutron diffraction data, and the observed and calculated patterns for each are given in the Supporting Information. The structural parameters of LaSr2Cu2GaO7 (Ima2; a ) 22.8014(5) Å, b ) 5.4819(1) Å, c ) 5.3936(1) Å)6 were used as a starting (11) Thiel, J. P.; Peoppelmeier, K. R. XRAY-FIT; Department of Chemistry, Northwestern University: Evanston, IL, 1991. (12) Kezler, D.; Ibers, J. Modified POLSQ; Department of Chemistry, Northwestern University: Evanston, IL, 1983. (13) Wiles, D. B.; Sakthivel, A.; Young, R. A. RietVeld Analysis Program, Version DBWS-9006; School of Physics, Georgia Institute of Technology: Atlanta, GA, 1990. (14) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System; Los Alamos National Laboratory: Los Alamos, 1994.
Figure 2. Orthorhombic lattice parameters for LaSr2-xCaxCu2GaO7 as a function of x.
Figure 3. Calcium saturation points in LnSr2-xCaxCu2GaO7 materials versus the 9-coordinate ionic radii of the Ln3+ cations.
model for the analysis of the X-ray data. The structure refined from the X-ray data was then used as a starting model for the analysis of the neutron data. The reflections for both sets of data exhibit conditions consistent with the non-centrosymmetric space group Ima2 (No. 46) and the centrosymmetric space group
3388 Inorganic Chemistry, Vol. 39, No. 15, 2000
Greenwood et al.
Table 1. Orthorhombic Unit Cell Parameters and Average 9-Coordinate A-Cation Size for LnSr2-xCaxCu2GaO7 (Ln ) La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) at the Calcium Saturation Levelsa Ln
x at the Ca satn level
a (Å)
b (Å)
c (Å)
cell vol (Å3)
av A-cation size (Å)
La Pr Nd Gd Ho Er Tm Yb
2.0 2.0 1.9 1.5 1.0 0.9 0.8 0.7
22.359(1) 22.396(1) 22.161(1) 22.118(2) 22.323(1) 22.158(2) 22.377(2) 22.401(2)
5.4795(2) 5.4709(3) 5.4855(1) 5.4906(5) 5.4751(3) 5.4798(5) 5.4649(3) 5.4599(4)
5.3902(2) 5.3691(3) 5.3752(1) 5.3794(4) 5.3691(3) 5.3741(5) 5.3661(3) 5.3655(4)
660.39(5) 657.86(6) 653.44(2) 653.29(9) 656.21(6) 652.53(10) 656.21(8) 656.24(9)
1.19 1.18 1.18 1.18 1.19 1.19 1.19 1.19
a The lattice parameters of all the samples decrease with calcium substitution of the pure strontium analogues similar to the lanthanum case (Figure 2). While subtle trends in the individual parameters with changing lanthanide hint at structure-distribution interactions, all the unit cell volumes converge to within 1% of a constant value at the solubility limit.
Table 2. Refined Structural Parameters of LaCa2Cu2GaO7 in Ima2 (No. 46)a atom
site
x
y
z
A1
4a
0 0 0.148(1) 0.1490(1) 0.074(1) 0.0736(1) 0.25 0.25 0.076(1) 0.0760(1) 0.069(1) 0.0673(1) 0.181(1) 0.1770(2) 0.25 0.25
0 0 0.984(1) 0.9835(7) 0.498(1) 0.4993(5) 0.434(1) 0.4333(8) 0.263(8) 0.2508(14) 0.753(9) 0.7479(15) 0.558(3) 0.5600(6) 0.379(5) 0.3860(11)
0 0 0.999(5) 0.9828(21) 0.997(4) 0.9960(22) 0.029(5) 0.0242(25) 0.255(18) 0.2452(23) 0.748(21) 0.7485(25) 0.962(7) 0.9585(23) 0.403(7) 0.3891(23)
A2
8c
Cu
8c
Ga
4b
O1
8c
O2
8c
O3
8c
O4
4b
100Uisob (Å2) 0.54(16) 0.12(10) 0.13(7) 1.05(16) 0.16(9) 0.12(11) 0.96(13) 1.40(15)
occupancy 0.125(4) La/0.875(4) Ca 0.208(14) La/0.792(14) Ca 0.438(2) La/0.562(2) Ca 0.396(7) La/0.604(7) Ca 1 1 1 1 1 1 1 1 1 1 1 1
The first line of data for each atom is from the X-ray refinement (a ) 22.359(1) Å, b ) 5.480(1) Å, and c ) 5.390(1) Å), and the second is from the neutron (a ) 22.366(1) Å, b ) 5.482(1) Å, and c ) 5.391(1) Å). For refinement of X-ray data: Rp ) 3.47, and Rwp ) 4.57. For refinement of neutron data: Rp ) 5.26, Rwp ) 7.39, R(F2) ) 5.89, and χ2 ) 9.415 where Rp ) 100(Σ|Yobs - Ycalc |/ΣYobs ); and Rwp ) 100x(ΣW(Yobs Ycalc)2/ΣWYobs2) with W ) 1/Yobs; R(F2) ) 100(Σ|IKobs - IKcalc|/ΣIKobs). b For refinement of X-ray data: βoverall ) 0.24(5) Å2. a
Imam (No. 74). In the latter, two oxygen sites are disordered and half-occupied because the two possible orientations of the gallate tetrahedra are averaged together. Krekels et al. discuss how the subtle distinctions have been experimentally differentiated.15 The A-cation sites are the same in either case. Since the non-centrosymmetric space group was used for the refinement of LaSr2Cu2GaO7, it was also used for LaCa2Cu2GaO7. Refinement of the X-ray data accounted for two phases: LaCa2Cu2GaO7 and the internal standard silicon. A small amount of LaCaGa3O7 (
