A 2212 bismuth cuprate with a non-modulated structure Bi2-xLaxBa2.5La0.5Cu2O8.25

Physica C 209 ( 1993) 449-455 North-Holland

A 22 12 bismuth cuprate with a non-modulated Bi2-xLaxBa2.5La0.5Cu208.2, M. Hervieu,

A.Q. Pham,

C. Michel

structure

and B. Raveau

Laboratoire CRISMAT, CNRS URA 1318 - ISMRa, 14050 Caen Cedex, France

UniversitP de Caen, Boulevard du Mar&ha1 Juin,

Received 26 October 1992 Revised manuscript received 23 February 1993

has been isolated for 0.21x 0.20. Taking into consideration the fact that bismuth and lanthanum are isovalent and exhibit the same size, and that the oxygen content does not vary owing to the experimental conditions, this study demonstrates that the electronic 6s’ lone pair of Bi(II1) should be at the origin of the modulation of the structure in bismuth cuprates. The absence of superconductivity in this phase is discussed by comparing with the 22 12 Sr based cuprates substituted with lanthanides.

1. Introduction Among the different layered cuprates, superconductive or not, the bismuth cuprates exhibit a particular behavior. These oxides, which consist of an intergrowth of ordered oxygen deficient perovskites with distorted rock salt type layers, differ from the corresponding thallium bilayer cuprates by a very strong modulation of their structure. This incommensurate structure has been extensively studied for the 2212 bismuth cuprate BizSrzCaCuzOs+d [ l-61 Bi,Sr2(Ca,_XRE,)Cu208+6 [7-91 and for the 2201 phase BiZSrZCu,06+6 [ 10-141. One observes very complex structures due to the coexistence of a displacive modulation of the atoms and of a modulated distribution of the cations and vacancies in the rock salt layers. The influence of this phenomenon upon the superconducting properties of bismuth cuprates is still not clear, so that it is of capital importance to study it further. Several hypotheses have been put forward to explain its origin, among which are the existence of an excess oxygen in the [ BiO ] o. layers [ 15- 18 ] and the stereoactivity or the 6s2 lone pair of Bi(II1) [ 191. In order to determine the factors which govern the formation of this modulation, sev0921-4534/93/$06.00

era1 cationic substitutions involving a variation of the oxygen content have been carried out. The disappearance of such a modulation was then observed by the substitution of lead for bismuth in the oxides Bi2_,PbXSrzCal-XYXCu208+6 1201. and Bi2_XPb,Sr2_,La,Cu06,6 1211 Bi2 _,Pb,BaLaCuO, k 6 [ 22 1. Nevertheless, it is difficult to draw conclusions from these substitutions owing to the fact that two factors are involved simultaneously which influence the modulation. Indeed, Pb(I1) has a larger size than Bi(II1) and exhibits a divalent state, whereas bismuth is trivalent so that the oxygen content is not really known. The recent investigation of the system Bi-Ba-LaCu-0 [22-241 has shown that besides the modulated 2212 type cuprate Bi2Ba2.&a,,sCu208.25, there exists a second kind of 22 12 type cuprate with a nonmodulated structure. This suggests that a possible substitution of bismuth by lanthanum may lead to a non-modulated structure. Such a substitution, if it exists, has the advantage to discard size and valency effects, since from this view point Bi (III) and La (III) are similar. We report here on the synthesis and structural characteristics of new cuprates with formulation Bil_xLaxBa2.sLao.sCu20s.2s.

0 1993 Elsevier Science Publishers B.V. All rights reserved.

450

M. Hervieu et al. /A 2212 bismuth cuprate with non-modulated structure

2. Experimental The synthesis of barium based bismuth cuprates requires one to work in an inert atmosphere, as previously shown [ 22-241, in order to avoid the oxidization of Bi”’ in Bi”, i.e., the formation of the perovskite BaBi03. The samples were prepared from mixtures of Bi203, BaCO,, La( NO3 ) 36H20 and CuO in adequate molar ratios according to the general formulation Bt2_-xBa2.5La0.5+xCuG3.25. The mixtures were heated in argon flow at temperatures ranging from 780°C to 800°C depending on the x values, for 14 h. The powders were then reground, pelletized in the form of bars and reheated in the same conditions. The samples are highly sensitive to air exposure and are amorphized after some hours; however, a simple annealing under argon flow for 8 h at the synthesis temperature is sufficient to restore the 22 12 crystallized phase. The powder X-ray diffraction data were quickly collected in the range 10” S213180” by step scanning with an increment of 0.05” (28) with a vertical Philips diffractometer using Cu Ku radiation. Intensities and lattice constants were determined by profile refinement program (Rietveld method) DBW 3.2 [ 251. An electron diffraction investigation was carried out with a JEOL 200 CX electron microscope.

3. Results and discussion For the above experimental conditions, a solid solution ( Biz_xLa,) (Baz.sLa0.s)Cu208.z5 is synthesized for 0.2 Ix< 0.7. The synthesis conditions (under argon flow) allow one to consider that there is no excess of oxygen with regard to the nominal composition 08.25 (calculated from the charge balance). The powder X-ray diffractograms (fig. 1) were indexed in a tetragonal cell whose parameters are characteristic of a 22 12 type structure (table 1). It should be noted that if the tetragonal symmetry 3.9 X 3.9 X 3 1 A3 is a common feature for 2212 thallium cuprates, it is the first time that it is observed in bismuth cuprates. The cell parameters remain approximately constant with x, in agreement with the similar sizes of bismuth and lanthanum and the constant oxygen content. The satellite reflections corresponding to the modulated supercell generally observed in the 22 12

Bi1.5Ba2.5LaCu208.25

Fig. 1.Powder X-ray diffraction pattern for Bi1.5Ba2.5LaCu08.25. The marks symbolize Bragg angle positions. Table 1 Lattice parameters for some compositions Bi,-,La,Ba2.sLao.5Cu208.2S

in the series

x

a (A)

c(A)

0.3 0.5 0.7

3.941(l) 3.938( 1) 3.940( 1)

31.233(5) 31.213(5) 31.230(4)

oxides are not detected in these X-ray patterns. This feature is confirmed by the electron diffraction study of three samples corresponding to x=0.2, 0.5 and 0.7, respectively; in every sample, more than fifty crystals were characterized in order to test their homogeneity. The three samples exhibit a system of very intense reflections. The reconstruction of the reciprocal space on numerous crystals showed that the conditions limiting the reflections are h + k+ l= 2n leading to a I-type tetragonal subcell. An example of a [ 0011 electron diffraction pattern is shown in fig. 2 (a). In numerous crystals weak extra spots are observed, with the reflection conditions MO, h + k= 2n+ 1; such a phenomenon indicates that the I-type conditions of reflection are violated (fig. 2 (b) ). Very weak incommensurate satellites are only observed for x=0.2 (fig. 2(c) ); the component of the modulation vector along b leads to a new periodicity of 5.7x b, i.e. corresponds to the q value observed for the oxides Bi2Baz+xLa,_,Cu,0,+B [23]. For x

M. Hervieu et al. /A 2212 bismuth cuprate with non-modulated

structure

451

different from 0.2 this system of satellites disappears but very weak, diffuse and elongated spots are observed in some crystals; they could be related to order-disorder phenomena in the form of platelet shaped domains, resulting especially from an inhomogeneous distribution of lanthanum over the different sites of the cations. From these observations, several remarks can be made. ( 1) The very weak intensity of the satellites in the x= 0.2 sample suggests, as previously observed [ 221, that the modulations take place only in small areas of the crystals. This hypothesis is confirmed by the observation of [ 1 lo] images (fig. 3); such areas may be correlated to small inhomogeneities in the cation distribution and imply that an excess bismuth would involve the local establishment of the modulation. (2) The appearance of the weak reflections MO h + k= 2n + 1, whose intensity varies from one crystal to the other suggests a loss of symmetry due to small atomic displacements. Note that in the lead rich oxides, such phenomena have been previously reported; in that case the conditions of the F type orthorhombic subcell were systematically violated [ 15171 leading to C and/or A type symmetries. The effects of air exposure were studied also by electron microscopy on the sample with x=0.5. After some hours a diffuse ring is observed, superposed with the sharp reflections (fig. 4(a) ); the dark field image recorded by selecting a part of the ring gives evidence of an amorphization which takes place progressively in the bulk (fig. 4(b) ). After some days, the reflections have disappeared (fig. 4(c) ). The EDS analysis was performed on the x=0.5 sample, B1,.,Baz.sLaCu20s.25, taking BaBiO, [ 261, La8Bi1,,02, [27] and La4BaCu50i3 [ 281 as references. Thirty crystals were analysed; only a small variation of composition was observed from one crystal to the other, leading to the mean formula-

Fig. :2. [ 0011 ED patterns. (a) A typical pattern correspo to a I-type tetragonal cell, (b) some weak reflections h+k =2n+ 1 violate the I-type symmetry, (c) for x=0.2 weak : satellites are observed along the [ 1 lO]* and [ 1 TO]* tions ,. They are attributed to bismuth rich areas in the tryst

nding with

, very direc.als.

tions Bi1.45(,,,Ba,,,,o,La,.,(,,Cu,.o,(10~08+x in agreement with the nominal composition. These analytical results, together with the absence of the cell parameters variation with x and the disappearance of the modulation, support strongly the hypothesis of the substitution of lanthanum for bismuth in the [ BiO] _ layers. In order to check the validity of this model, a structural study was performed for the composition x= 0.5. For the 28 range

452

M. Hervieu et al. /il 2212 bismuth cuprate with non-modulatedstructure

Fig. 3. [ 1 lo] image of a crystal exhibiting structure (right part of the image).

weak and diffuse satellites

(x=0.2).

Only a small zone of the crystal exhibits

a modulated

Fig. 4. (a) (001 ] ED pattern recorded after some hours’ air exposure; a diffuse ring characteristic of amorphous areas is superposed to the pattern; (b) the corresponding dark field image shows that an amorphization took place in the bulk; (c) after a long air exposure, the pattern gives evidence of a complete amorphization of the grains.

M. Hervieu et al. /A 2212 bismuth cuprate with non-modulated structure

given in the experimental section, 5 1 reflections were involved whose intensities were considered for the calculations. We have considered the tetragonal subcell with the 14/mmm space group. Taking into account the lamellar character of these oxides, a preferential orientation parameter (G) was introduced along the caxis. The different ions were located in the usual sites of the space group 14/mmm for a 2212 type structure (table 2 ). Owing to the low scattering factors of oxygen we considered only 16 oxygen atoms per cell, in the positions of the ideal 22 12 structure, without trying to locate the 0.5 extra oxygens. No shifting from the ideal positions in the ab plane was considered. Positional parameters and isotropic thermal factors were successively refined, except for oxygen atoms whose B factors were fixed at 1 AZ, leading to the different agreement factors R,= 10.9%, R,, = 14.1%, RI = 9.5% for the variable values given in table 2. As stated before, samples became amorphous after air exposure; for this reason, X-ray data were quickly collected. This results in the fact that this study cannot give an accurate determination of the structure and explain the relatively high R values. However, considering the number of variable parameters (see table 2) with regard to the number of reflections, we can consider these results as significant. The calculated interatomic distances are given in table 3. These calculations give only an average view of the structure since the positional parameters of oxygens cannot be obtained with accuracy, nor the localization of the extra oxygens (0.25 atoms per cell). However, two important results concerning the cations can be considered as significant. ( 1) The interlayer Cu-Cu distance (3.7 A) is significantly increased with regard to the BiSrCa cu-

Table 2 Refined parameters obtained for Bi,.5Ba2.&aCu208.2,. Calculations have been performed in the space group 14/mmm ‘) Atom

Site

x

Y

z

B (A*)

Bi/La Ba Ba/La CU O(l) O(2) O(3)

4e 4e 2e 4e 8g 4e 4e

f 0 0 f f f f

t 0 0 f 0 f t

0.2057(3) 0.1184(3) 0 0.0599(S) 0.053(3) 0.697(2) 0.136(3)

2.4(3) 0.4(4) 2.0(5) 0.6(6) 1.0 1.0 1.0

‘) Preferred orientation parameters G=0.90( 1).

[29] and to the oxide (3.5 A) [23], in agreeBi2Ba2.3La&u208+6 ment with the larger barium content located between the pyramids. (2 ) The refinement of the occupancy factor of the Bi/La site, close to Bi1.59(~0.15~La0.4L(f0.15), confirms that a large amount of lanthanum is located on the bismuth site; it should be noted that calculations carried out without lanthanum on the bismuth site, i.e. Bi,Ba,+, La, _xCuZOB, and for the variable values of table 2 leads to a significant increase of the RI value, up to 11.6%. Ba2+ and La3+ are isoelectronic ions, so they cannot be distinguished in the calculations; however, considering the Bi-0 ( 3 ) distance (2.21 A) and the ionic radius of Ba’+, it is unrealistic to consider the possibility for barium to be partly located on the bismuth site. It results that only La3+ can occupy the bismuth site. These results confirm the previous results which showed that [ BaO], layers can adapt to [ BiO], layers [ 22,24,30], but they definitely show that bismuth can be partially replaced by lanthanum in the [ BiO] m layers. This substitution leads to a rapid disprates

(3.35

A)

Table 3 Selected interatomic distances (in A)

‘) Distances along c.

(Bi,La)-O(2) O(2) O(3)

2.79(1)x4 3.02(7)x1 2.21(9)x1

Ba,-O(2) O(2) O(3)

2.84(6)x4 2.47(7)x 1 2.83(2)x4

453

(Ba,La)-0( cu-O( 1) O(3) cu-cu a) 0, -0 ;’

1)

2.56(5)x8 1.98(1)x4 2.34(9)x1 3.73(3) 3.29( 12)

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M. Hervieu et al. /A 2212 bismuth cuprate with non-modulated structure

appearance of the modulations ( 1 La for 6 Bi). It must be emphasized that this substitution of La( III) for Bi(II1) corresponds to constant ionic radii and comparison of constant charges. The Bi2Ba2.4La0.6Cu2% and B11.7Lao.3Ba2.sLao.508.25 shows that, for very close oxygen contents, the first structure is modulated, whereas the second is not. This definitely demonstrates the important role of the electronic structure of bismuth on the origin of the modulation. This is also consistent with previous results which showed that intercalation of excess oxygens is not the origin of the modulation [ 3 11. These oxides are semiconducting with resistivity at room temperature close to 20 Q cm for x=0.5. The absence of superconductivity in this phase must be compared with the properties of the 22 12 strontium based cuprates Bi2Sr2Cal _,Y,CuZOs [ 32-351 and especially with the x=0.50 oxides. Similarly, in the latter, the superconducting properties decrease dramatically as x increases and vanish close to x= 0.50. Thus, most likely, the disappearance of superconductivity in this phase is not due to barium but to the introduction of lanthanum between the pyramidal layers. The recent XANES study of the cuprate [ 361 strongly supports this Bi2Sr2Cal--xYxCu208+6 view point. It is shown that beyond x=0.50, the introduction of a lanthanide induces a distortion of the BiO, polyhedra without bringing additional holes (8=x/2). As a result, a narrow band model is no more possible for the Bi-0 layers, so that they do not play the role of hole reservoir for the [ CuOz] m layers when x20.50. This is not the case for the nonsubstituted bismuth cuprates Bi2SrzCaCuz08+a, for which the distortion of the BiO, polyhedra may be different due to the local oxidation of Bi(II1) into Bi( V), and for which holes are brought by the additional oxygen to the [ Cu02] co layers. An issue remains unanswered which will be studied by neutron diffraction: the localization of the excess oxygen (0.25). It can be localized either in the bismuth layers or between the two pyramidal layers, the Cu-Cu distance allowing for such an intercalation. Acknowledgement The authors are grateful to Rhone Poulenc financial support.

for its

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