Properties of a new copper ternary compound La2Sr6Cu8O18−δ

0038-1098/90$3.00+.00 Pergamon Press plc

Solid State Communications,Vol. 73, No. 4, pp. 291-295, 1990. Printed in Great Britain.

PROPERTIES OF A NEW COPPER TERNARY COMPOUND La2Sr6Cu8018_8 W.T. Full2 , q. Xul, A.A. Verheijenl L.J. de Jongh

J.M. van Ruitenbeekl,

H.W.

Zandbergen2'3

and

Kamerlingh Onnes Laboratory1 and Gorlaeus Laboratories2, Leiden University P.O. Box 9506, 2300 RA Leiden, The Netherlands National Center for HREM, Laboratory for Material Sciences3 Rotterdamseweg 137, 2628 AL Delft, The Netherlands (Received

September 7. 1989 bv P. Burlet

)

A new ternary compound La Sr6Cu8018_~ has been isolated and characterized by X-ray diffraction and electron Ziffraction. It crystalizes in a tetragonal unit cell, space group P4212 or P421m, with the a-axis related to that of cubic perovskite (ap) by a=242a but with the c-axis slightly shorter than ap. The oxygen content determine 8' from TGA was found to vary strongly, depending on the post-annealing conditions. Air-annealed samples (6~0.4) show metallic conductivities, however airquenched samples (6~2) are considerabaly less conducting, and show a weak upturn in the resistivity at low temperature. No superconductivity is observed down to 4K. Thermogravimetric analysis, d.c. resistivity and a.c. susceptibility data as a function of temperature are presented, and the structural features are discussed.

were able to isolate a new ternary compound rich in alkaline earth, with the formula La2Sr6Cu8018_s. In this paper we describe the preparation and some physical properties of these new materials and discuss their structural features as obtained from X-ray diffraction and electron diffraction experiments.

1. Introduction Copper-based oxygen deficient perovskites and related compounds have been extensively studied during the last years by Raveau and co-workers [l-5]. Their crystal structures and physical properties are known to depend strongly on the amount of oxygen vacancies and their ordering. Interest in these copper oxide compounds has grown tremendously since the discovery of high-temperature superconductivity in this family (6,7]. In the La203-SrO-CuO system three stable ternary compounds have been reported pre(01x<1.34) viously : (1) La2-.pxCu04_~ which has the K2NiF4-type structure, and with near 40K shows superconductivity x=0.15 [3,81; (2) La2-xSrl+xCu206+6 (O
2. Experimental Samples of La2Sr6Cu8018_~ were prepared by solid state reaction in dense alumina crucibles from the appropriate mixtures of dried La203, SrC03 and CuO. The mixtures were first heated in air at 105OOC for 24 hours, followed by regrinding and firing in air at this same temperature for 24 hours to improve the homogeneity; and finally quenched, or furnace-cooled in air, to room temperature. The new compounds were characterized by X-ray diffraction with a Philips PW 1050 diffractometer using Ka radiation. Electron diffraction was carried out with a Siemens Elimiscop 102 electron microscope with a top entry f40° double tilt-lift cartridge, and operating at 100kV. The oxygen content was determined by thermogravimetric analysis (TGA) by heating the samples in the diluted hydrogen (8% H2 in N2) at 95O'C. The electrical resistivity was measured by standard d.c. four-probe technique in the temperature range between 4 and 300K. The a.c. suscebtibility measurements were performed between 1.1 and 200K at 82.9 Hz using the mutual inductance technique [ll].

Most Ln203-BaO-CuO systems (Ln=lathanide or Y) involve an oxygen deficient perovskite with the Ln/Ba ratio rich in alkaline earth. The family LnBa2Cu307_s is notit; well-known series of superconductors, values exceeding 90K [lo]. We have TC studied the compounds in the La203-SrO-CuO system along the phase line joining the SrCuO2 and "LaCu02.5" compositions, and

3. Results and Discussion X-ray powder diffraction 291

investigations

292

PROPERTIES OF A NEW COPPER TERNARY COMPOUND La2Sr6Cu80,8_6

Vol. 73, No. 4

for various compositions corresponding to due to a partial decomposition of the matethe molar ratio (La+Sr)/Cu=l suggest that rials, as will be discussed below. the new compound has the formula La2Sr6Cu8 From the above results it is apparent that the structure of LaiSr,6Cu8Cl,8-& can Cl?-& It crystallizes in a tetragonal unit ce 1 and can be indexed with cell parameeasily accommodate oxygen eficiencies over ters related to that of the cubic perovsthe composition range 0.44<&1.98. An inkite (a teresting aspect of the observed composicell pa?~rn~~e~~*::~E ~n_~a,"'",po"de':ed~fe:~~,'dtion range in oxygen is that the formal tion for air-annealed and air-quenched oxidation state for Cu varies from 2.14 at samples are given in Table I. The a-axis of 8=0.44 [Cu(II) + Cu(III)] to 1.76 at S=1.98 the air-quenched sample is slightly larger [Cu(II)+Cu(I)J. Such a wide oxygen stoichithan that of the air-annealed sample, ometry and the change in the formal oxidawhereas the c-axis is significantly shorter tion state of Cu are also observed for the than that of the latter compound. superconducting compound YBa2Cu307_8 [121, The compound La 2Sr 6Cu 8018-s toleratesThz but differ from other copper-based oxygen wide range of oxygen stoichiometries. deficient perovskites such as La4BaCu5013+g oxygen contents as determined by TGA are and La8-xSrxCu8 VI 141, =a3Ba3Cu6014+6 presented in Table I. The &values were in which the averaged Cu valence found to depend strongly on the preparation 020-? [51'higher than 2+. is a ways procedure. Air-annealed samples have typiAlthough the basic oxygen deficient perocally a &value of 0.44. However, air-quenvskite structure of La2Sr6Cu8018_S exists ched samples have a S-value as high as for 0.4416<1.98, it is found that these 1.98, thus corresponding to the lowest samples are not stable at temperatures oxygen content. The variation of the oxygen between 900' and 1000°C. For example, samcontent is also clearly seen in the thermoples of La2Sr6Cu8018_s can not be prepared gravimetric measurements. In Fig.1 we show directly by reactron of the constituent the TG data for air-annealed La2Sr6Cu80 metal oxides and carbonates in air at temwhen heating in air, where the observes peratures below 1OOO'C. Furthermore the "as weight changes have been converted to oxyprepared" samples, when annealed at tempe(X). The weight losses gen stoichiometry ratures between 900' and 1000°C for several start at a temperature of about 420°C, and hours, appeared to be decomposed into La*_ increase rapidly in the temperature range SrCu02 and CuO. All these $'J$X cu206+6~ 750-85O'C. The decrease in weight continues rn ings point to a thermodynamic effect. to about 9o0°c, whereas further heating The decomposition of the the material at results in an increase in weight, which is temperatures between 900' and 1OOO'C is probably due to the instability of Cu(1) present in the sample. Another explanation is that the structure of La2Sr6Cu8018_s is looo"c. unstable below thermodynamically lower The decomposition at temperatures than 9OO'C is not occuring because of a kinetic effect. Consequently, in order to obtain higher oxygen content, either rapid cooling from 1050° to <900°C, or annealing of the quenched sample directly at low temperatures is required. As mentioned above, the X-ray diffraction patterns of La2Sr6Cu8018_f ca;itrt;zd;z;; in the tetragonal unit ce 1, Such a superparameters a=*d*a and c
Table I. Lattice parameters and

Oxygen

for La2Sr6Cu8018_&.

content

v

(A?

Composition

S

a

La2Sr6Cu801'7.56* ** La2Sr6Cu8016.02

0.44

10.861(3)

3.764(3)

444.0

1.98

11.055(4)

3.652(4)

446.3

* Air-annealed ** Air-quenched

sample sample

(A)

c

(A)

Vol. 73, No. 4

PROPERTIES 0~ A NEW COPPER TERNARY COMPOUND L~~S~~CUSO,S_~

293

ture for La2Sr6Cu8018_6 is shown in Fig.3. The temperature dependence of the air-annealed samples is metallic-like, with a IOO -'5 temperature resistivity of about 1.6 *lO residual also the This value, and Rem. close to ratio's are quite resistivity the L~8-xSrxC"8020-~ those reported for air-quenche FOX materials [13,141. resistivity is a higher samples however, also show latter samples observed. The slight upturns in the resistivity ate temperatures below 100K. We did not observe superconducting transitions in any of the La2Sr6CuSO18_~ samples down to 4K, although some samples are quenched in air from difthe samples of temperatures. For ferent Fig.3 the resistivity data were even extended down to 0.2K, and it was found that the resistivity values hardly change in this temperature region. The results of the a.c. susceptibility measurements are shown in Fig.4, where the measured molar susceptibilities are plotted vs. temperature. The temperature dependence of the susceptibilities can be empirically analyzed in terms of the following equation

Fig.2

Electron diffraction patterns along [OOll (a) and [loo] (b) of La2Sr6Cu8 The [lo01 zone shows systema%&y absent reflections for hO0, h=2n+l.

zone some forbidden reflections can be generated by double diffractions, the crystals were tilted in such way that either hO0 or Ok0 reflections were excited. For these orientations systematically absent reflections were found to occur for hO0, h=2n+l, as can also be seen from the diffraction patterns along the [lo01 orientation (Fig.2). No further systematic absences were found by recording the other zones, suggesting possible two space namely P4212 and Pz21m. groups, It is interesting to note that the compound a composition La2,"f;6c",s,o,l;-6 has similar of ~a8-xS~,C”8020-~ (1.28&11.92), and exibits the same kind of superstructure [5]. However it differs from the latter compound by a fixed La/Sr ratio that is very rich in alkaline earth, and, consequently, it will posses a considerably higher amount of oxygen vacancies with respect to the stoichiometric perovskite. The observed superstructure suggests that the oxygen vacancy ordering in the basal plane might be the same as observed for but could be different LaS_xSrxC"8C20-&~ and should be distributted over the other sites, in particular because of the rather short c-axis compared to that of the normal cubic perovskite. A neutron diffraction study to determine the oxygen distribution in La2Sr6 Cu8018_8 is in progress. In view of the mixed-valence character of copper, it is interesting to examine the transport properties of this compound. The d.c. resistivity as a function of tempera-

X,(T)

=C/ (T+e)

+x0

where the first term has the form of a Curie-Weiss law and is attributed to paramagnetic centers and the second (P.C.), term describes the temperature independent paramagnetism (T.I.P.). The P.C. concentration, n, can be estimated by assuming the presence of spin-l/2 moments, with a g-factor of 2, thus n(moles

P.C./mole)=C/0.375

The resulting listed in values are the Table II. The values of x0 are of the same magnitude as in the superconducting copper oxides [13], indicating strong antiferromagnetic interactions between the moments associated with the Cu(I1) atoms present. The attribution of the P.C. to the presence of foreign magnetic impurities in the

E60 -

c! 50 c E t 40

b

0.5 1 0

50

100

150 T

Fig.3

200

250

I 300

(K)

d.c. resistivity of La2Sr6Cu8zt8_ii;"_ a function of temperatur annealed sample c&=17.56) and (b) air-quenched sample t&=16.02).

294

Table

PROPERTIES

OF A NEW COPPER

TERNARY

II. Magnetic

susceptibility

data

COMPOUND for

Vol. 73,

La2Sr6Cu80,8_6

No.

La2Sr6Cug018_6.

C

La2Sr6C"8017.56

3.76*1@-2

2.23

7.36*10-4

10

La2Sr6Cu8016.02

5.12*10-2

3.11

3.88*10-4

14

(emu.K./mol)

0004

;

(loo3

i 0.002 Q, x

0.001

50

Fig.4

n (mol%)

Composition

100 -I (K)

150

I 200

Temperature dependence of a.c. susceptibility of La2S;6Cu~018i~ for airannealed and airsample quenched sample (---o---).

samples seems unlikely, since, the relevant impurity phases such as CuO [15], cup, LaCu03 [161 and other related oxides in the system La20 -SrO-CuO [13] in the concentration range 3 ess than 5% cannot give similar order-of-magnitudes of the Curie constant (Table II). It is probably due to the presence of a small amount of isolated Cu(I1) atoms in the samples, in particular because of the high amount of oxygen vacancies. However in the absence of precise structural data, we cannot conclude definitively that the Curie-Weiss behavior is an intrinsic property of the materials. 4. Conclusions and We have presented the preparation some structural and physical properties of La2Sr6Cu8018_&. This new series of compounds confirm the interest of the La203SrO-CuO system, and show that the existence of still other ternary compounds in this system cannot be excluded yet. The metallic properties of but non-superconducting La2Sr6Cu8018_~ clearly demonstrated the important inf uence of crystal structure on

8

(K)

x0

(emu/m011

these physical properties. Because the range of materials can tolerate a wide they can be very oxygen stoichiometries, helpful in understanding the relationship between the crystal structure, charge carrier density and mechanism of conductivity and superconductivity in the copper based high-T, superconducting oxides. The observed metallic but non-superconducting properties in La2Sr6Cu8018_s may be r;i;zzda;; its three-dimensional structure, oxides known copper-based superconducting so far have a two-dimensional structure, of Cu02 planes. In these e.g. the existence CuO2 planes one has an extended structure in which the overlap of of Cu-0-Cu bonds, the copper 3d and oxygen 2p wave functions leading to very large transfer is optimum, Comparing this with the threeintegrals. e.g. found in dimensional structures and La8_xSrxCu802U_ , which La4BaCu5013+5 k ut nonare likewise metallic compounds one notices that in these superconducting, only finite clusters of wellcompounds coupled copper oxygen polyhedra do occur, mutually connected by weak transfer links arising from the fact that across these links neighboring copper 3d orbitals (dxz_ orthogonal to one another. As a 2) are materials F onsequence, there is in these not an infinitely extended array of Cu-0-Cu transfer with a large bonds similarly copper layer-type integral as in the oxides.

Acknowledgements We would like to thank V.A.M. Brabers of the Technical University of Eindhoven for the use of the TG apparatus for the oxygen continuous The content determinations. interaction with the members of the Leiden gratefully Materials Science Center is the is part Of acknowledged. This work F.O.M. Stiching research program of the (Foundation for Fundamental Research on Matter) which is sponsored by NW0 (Netherlands Organization for the Advancement of pur research).

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Vol. 73, No. 4

PROPERTIES OF A

NEW

COPPER TERNARY COMPOUND La2Sr6Cu80,8_6

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