Mat. Res. Bull., Vol. 24, pp. 321-330, 1989. Printed in the USA. 0025-5408/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc.

PROPERTIES AND STRUCTURES OF R2_xAxCuO 4 PHASES: R = La, Pr and Nd; A = Sr, Pb and Cd

J. Gopalakrishnan ", M. A. Subramanian, C. C. Torardi, J. P. Attfield, and A. W. Sleight

Central Research & Development Department # E. I. du Pont de Nemours & Co.

Experimental Station, P. O. Box 80262 Wilmington, Delaware 19880-0262

(Received December 15, 1988; Communicated by W.B. White)

ABSTRACT The R2CuO 4 phases for R = Pr and Nd were prepared and found to be semiconducting as previously reported. Crystal structures of both compounds were examined by profile refinements of neutron powder diffraction data. A series of phases of the type R2_xSrxCuO 4 which are isostructural with R2CuO 4 for x up to 0.I were prepared and found to remain semiconducting. Chemical analysis indicates the presence of Cu III, yet these materials are not superconducting. This supports the view that R2CuO 4 phases should be regarded as Mott insulators. A new series of phases possessing the tetragonal K2NiF 4 structure appear in the Pr2_xSrxCuO 4 system for 1.0<x<1.25. The new phases are semiconducting. We also find semiconductivity and not superconductivity for La2_xAxCUO4 where A is Pb or Cd. Reasons are discussed for why Ba, Sr, Ca and Na substitutions into La2CuO 4 lead to superconductivity whereas the Pb and Cd substitutions do not.

MATERIALS INDEX: copper, lead, cadmium, lanthanum, strontium, praseodymium, oxides

Introduction

Understanding the new high temperature superconductors requires an understanding of the family of R2CuO 4 compounds (R =

~Permanent Address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

#Contribution No.: 4843

321

322 J. GOPALAKRISHNAN, et al. Vol. 24, No. 3

La, Pr, Nd, Sm, Eu or Gd) which all contain infinite Cu02 sheets with copper in square-planar oxygen coordination (i). Of these, only La2CuO 4 becomes superconducting on doping with alkaline earths (2) or sodium (3). There are actually several structures to consider for these R2CuO 4 phases. The tetragonal K2NiF 4 structure (Figure la) exists for La2CuO 4 above about 250°C; below this temperature, there is an orthorhombic distortion (4). For the other RsCu04 phases (R = Pr, Nd, Sm or Gd), a different tetragonal structure exists (Fig ib) (i). Oxygens in La2Cu04 have moved from 00z positions to a 0½z position where all oxygens of this 0(2) layer have the same z value, i.e. ¼. Exactly the same Cu02 sheets are found in this structure, but the placement of oxygens between the sheets is different. In La2Cu04, there are four short Cu-O distances (~1.90~) and two longer Cu-0 distances (~2.45~). For copper in the other R2CuO 4 phases, there are again four short Cu-O distances (~1.95~) whereas there are now eight longer Cu-O distances (~3.55~). This difference in structure does not have a significant effect on band structure calculations in the sense of predicting different properties, e. g. metallic properties for La2CuO 4 and semiconducting properties for others. In fact, band structure calculations indicate that all of these phases should be good metals (5). The properties of La2Cu04 indicate that it is semiconducting at low temperatures but metallic at higher temperatures (6,7). The other R2CuO 4 phases, on the other hand, appear to be well localized systems (8 ) .

Substitutions of the type R2_xAxCuO 4 have been studied previously especially for R = La and A = Ba, St, Ca (8-12) which are not the subject of this paper. Instead we focus on A = Pb and Cd where R = La and on A = Sr where R = Pr and Nd. We believe that this study enables us to better understand the structure-property relationships for these compounds and the new superconductors based on copper oxide.

Experimental

Members of the La2_xPbxCU04 and La2_xCdxCuO 4 series for various values of x up to x= 0.5 were prepared by heating appropriate mixtures of Las03, Cu0, Pb(NO3)2/PbO 2 or Cd0 in air at 800 - I000°C. Pr2CuO 4 and Nd2CuO 4 were prepared by heating mixtures of the rare earth oxides with CuO at I000-II00°C. Substitution of Sr into Pr2Cu04 and Nd2CuO 4 was investigated by reacting appropriate mixtures of Pr203 or Nd203 with CuO and SrO 2 at 1000-1200°C in air. The Pr203 was prepared by hydrogen reduction of Pr6011 at 750°C. Formation of single phase products was checked by x-ray powder diffraction using a SCINTAG diffractometer with CuK~ radiation. Unit cell parameters were refined by least squares. D.C. resistivities of the sintered pellets were measured by a four probe technique in the 77-300K range. The test for superconductivity was made by the a.c. mutual inductance technique.

Neutron Powder Diffraction

Powder neutron diffraction scans were obtained at the Brookhaven National Laboratory High-Flux Beam Reactor. For these

Vol. 24, No. 3 R2_xAxCUO 4 PHASES 323

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324 J . G O P A L A K R I S H N A N , e t a l . Vol . 24 , No. 3

experiments, pressed and sintered samples of powder in the shape of cylinders approximately 0.9 cm in diameter and 2.5-3.0 cm in height were used. Data for Pr2CuO 4 and NdsCuO 4 were collected at ambient temperature and ii K, respectively, with 0.1 ° step-scan intervals over a 2e range of 5 to 140 °. The experimental configuration consisted of a pyrolytic graphite monochromator and analyzer in the (002) and (004) settings, respectively. The collimation was 20' in-pile, 40' monochromator-sample, 40' sample-analyzer, and 20' analyzer-detector. The neutron wavelength was 2.371 ~, and higher order components were suppressed with a graphite filter.

The diffraction pattern of Nd2CuO 4 showed this material to be single phase. However, the pattern of Pr2CuO 4 contained several very weak peaks that did not fit the body centered tetragonal cell reported for Nd2CuO 4 (13) and Gd~CuO 4 (14). Also, these peaks could not be definitely identified as known phases. They were not included in the refinement, but the possibility that they may belong to a larger or lower symmetry unit cell is presently under investigation.

Neutron diffraction data for both materials were fitted by refinement of the structures in the space group I4/mmm (No. 139) using the Rietveld profile analysis method ,15). Neutron scattering factors were taken from Koester and Rauch (16). Starting atomic positions were taken from the structure of Nd2CuO 4 (13). Regions of the scan that included the diffraction peaks of the aluminum sample container for Nd2CuO 4 were excluded from the refinement. Peaks were treated as Gaussian in shape. Background values were obtained by linear interpolation between average values in regions between peaks. An overall temperature factor was refined for both compounds. The Debye-Waller factor for the Nd compound was corrected for absorption by adding 1.5 ~. This value was estimated using the formula given by Hewat (17) for neutron powder diffraction. Other variables included in the refinement were cell constants, zero point, half-width parameters, and two background parameters. Table I contains the positional and thermal parameters, cell constants, and residuals for both structures. Figure 2 shows the profile fit and the difference between observed and calculated patterns. Interatomic distances are given in Table If.

Results and Discussion

Substitution of Pb as well as Ca, Sr and Ba in La2CuO 4 was first reported by Shaplygin et al (8). We find that Pb and Cd can be substituted for La in La2CuO 4 giving La2_xPbxCuO4__ (0~x~0.2) and La2_xCdxCUO 4 (0~x~0.25). The solid solutions however retain the orthorhombic La2CuO 4 structure (Table III) at room temperature. This is contrary to the result reported by Shaplygin et al (8) for La2_xPbxCuO 4. The cell parameters (Table Ill) indicate however that the extent of orthorhombic distortion decreases somewhat with the Pb or Cd substitution. The distortion nevertheless does not completely disappear at 25°C. This behavior stands in contrast with that of Ca, St, Ba or Na substitution in La2CuO 4 where the substitution suppresses the tetragonal-orthorhombic transition which occurs at 250°C for pure

Vol. 24, No. 3 R2_xAxCUO 4 PHASES 325

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FIG. 2

Profile fit and difference plots for Pr2CuO 4 and Nd~CuO 4. The short vertical marker represents allowed refect10ns.

326 J . G O P A L A K R I S H N A N , e t a l . Vol . 24, No. 3

TABLE I

Atomic Positional and Thermal Parameters for Pr2CuO 4 and NdsCuO 4

Pr2CuO 4 (298K) N_dd2CuO 4 (ii K)

Ln x 0.0 0.0 y 0.0 0.0 z 0.3515(2) 0.3515(1)

Cu x 0.0 0.0 y 0.0 0.0 z 0.0 0.0

0(i) x 0.0 0.0 y 0.5 0.5 z 0.0 0.0

0(2) x 0 . 0 0 . 0 y 0.5 0.5 z 0.25 0.25

B(~2) (overall)

0.52(5) 0.8(1)

a (~) 3.9625(1) 3.9385(1) c (~) 12.2335(3) 12.1465(3)

Rnuc a" i. 6 2.9 R,p 8.7 8.4 R e 4.4 7.5

aDefined in reference 15.

TABLE II

Interatomic Distances (~) in Pr2CuO 4 and Nd2CuO 4

Pr2CuO 4 (RT) Nd2Cu04 (ii K)

Cu-O(1) 1.981 (x4) 1.969 (x4)

Ln-O(1) 2.687(1) (x4) 2.675(2) (x4) Ln-O(2) 2.339(2) (x4) 2.320(2) (x4)

La2CuO 4. Both the La2 xPbxCUO4 _ and La~ xCdxCu04__ series are 0 -- -- ° ~ -

not superconductlng above 6 K unlike thelr Ca, Sr, ~a and Na substituted analogues. This suggests that there might be a correlation between the disappearance of the orthorhombic distortion at room temperature and the appearance of superconductivity in La~_xAxCu04__ for A = Ca, St, Ba and Na at low temperatures. One of us (19)'argued that the appearance of superconductivity in La~_xAxCuO 4 for A = Ca, St, Ba and Na is

Vol. 24, No. 3 R2_xAxCUO4 PHASES 327

TABLE III

Orthorhombic lattice parameters of La2_xAxCUO4 (A = Pb or Cd)

Compound a(~) b(~) c(X)

Lal.95Cdo.osCuO4 Lal.9oCdo.loCUO4 Lal 75Cdo.25Cu04

5.360(2) 5.390(2) 13.144(4) 5.356(2) 5.385(2) 13.141(4) 5.350(2) 5.381(2) 13.134(5)

Lal.95Pbo.osCu04 Lal.9oPbo.loCU04 Lal.85Pbo.lsCU04

5.354(2) 5.382(2) 13.168(4) 5.350(2) 5.378(2) 13.163(5) 5.346(5) 5.377(3) 13.148(7)

La2CuO 4 5.356 5.399 13.167

related to the strong electropositive character of these alkaline earth metals which results in an increase of Cu-O covalency as well as producing charge carriers. Sharer et al. (20) have shown that the hole concentration (formal Cu III content) in La2_xSrxCu04_. equals Sr 2+ content up to x = 0.15. Thus, y is

J close to zero untll x exceeds 0.15. On the other hand, we find that the formal Cu Ill content in La2_xPbxCUO4__ and La2_xCdxCuO4__ is not equal to the value of x. J Chemical analysis showed that the Cu III content in both is about 5% of the total copper and it essentially the same as that of pure 'La2CuO 4' prepared under similar conditions. Thus, it appears that Pb/Cd substitutions in La2CuO 4 create oxygen vacancies rather than causing a significant increase in the Cu III content. This conclusion is supported by the observation that La2_xAxCuO4_y (A = Pb or Cd) remain semiconducting in the temperature region 300-77K. The structure and semiconducting property of these compounds are not affected by annealing the samples in oxygen at 1000 psi and 450"C.

Sr substitutes readily for Pr and Nd in Pr2CuO 4 and Nd2CuO 4 to give Ri.gSro.lCUO 4 (R=Pr and Nd). When x>0.1, the compositions were biphasic. With praseodymium, a single phase solid with the tetragonal K2NiF 4 structure is formed in the composition range PrSrCuO 4 - Pro.75Srl.25CuO 4. Indexed powder X-ray diffraction data for PrSrCuO 4 are given in Table IV. With Nd, a new phase Nd2SrCu206 which is isostructural with La2SrCu206 (21) is formed. The unit cell parameters of the new phases are given in Table V.

D.C. electrical resitivity measurements reveal that R1.gSro.iCuO 4 (R=Pr and Nd) are semiconducting having room temperature resitivities around 102 ohm cm. Resistivities of the Sr-substituted phases are in fact higher than those of the parent R2CuO 4 by at least an order of magnitude at room temerature. This result indicates that the lack of metallic properties in R~CuO 4 is due to the greater ionicity of Cu-O bonds rather than slmply a filled band situation. The latter should have produced highly conducting R1.gSro. ICUO 4. This supports the view that

328 J . G O P A L A K R I S H N A N , e t a l . Vol . 24, No. 3

TABLE IV

X-ray powder diffraction pattern of PrSrCuO 4

dob.(~) dcal(~) I/I o h k 1

6.440 6.450 4 0 0 2 3.587 3.588 ii 1 0 1 3.220 3.225 20 0 0 4 2.818 2.820 100 1 0 3 2.641 2.642 46 1 1 0 2.149 2.149 23 0 0 6 2.122 2.122 21 1 0 5 2.044 2.043 23 1 1 4 1.868 1.868 24 2 0 0 1.668 1.667 18 1 1 6 1.653 1.652 21 1 0 7 1.615 1.616 9 2 0 4

TABLE V

Tetragonal lattice parameters for some of the new phases formed in the Pr2_xSrxCuO 4 and Nd2_xSrxCuO 4 systems

Compound a(~) ~(~)

PrSrCuO 4 Pro.goSrl.loCU04 Pro.8oSrl.2oCu04 Pro.7sSrl.25CuO4

3.736(1) 12.899(3) 3.730(1) 12.879(5) 3.734(1) 12.856(3) 3.724(1) 12.853(3)

Nd2SrCusO s 3.836(2) 19.640(5)

R2CuO 4 should be regarded as Mott insulators (19,22) despite band structure calculations which indicate otherwise (5).

It is significant that tetragonal Pr2_xSrxCuO 4 phases (i~x~1.25) are not superconducting although the structure and unit cell parameters of these phases are very similar to those of the superconducting La2_xSrxCuO 4. Members of Pr~_xSrxCuO 4 (i~x~1.25) series are semiconducting and, interestingly, the resistivity increases with increasing Sr content. A possible expla~iO~vOf t~@Tre~it is as follows: PrSrCuO 4 probably exists as Prl_SPr 8 SrCul~Cus-O 4 with 8 << 1.0. Increasing substitution of Sr forces all the copper to the formal trivalent state making the sample increasingly insulating. The decrease of a and c parameters with increasing Sr content (Table V) supports thTs view. It is also possible that the compositions become oxygen-deficient in the Sr-rich region, Pr2_~SrvCuOA_~ for x > 1.0. Presence of praseodymium in the mixed ~alSnt ~t~te most probably destroys superconductivity just as it does in PrBa2Cu307 (23).

Vol. 24, No. 3 R2_xAxCUO 4 PHASES 329

Acknowledqements

The authors thank D. E. Cox for valuable discussions on the neutron diffraction refinements and R. B. Flippen for the a.c. mutal inductance measurements.

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330 J . G O P A L A K R I S H N A N , e t a l . Vol . 24, No. 3

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