Bi-2201 phases Synthesis, structures and superconducting properties

PHYSICA® ELSEVIER

Physica C 246 (1995) 241-252

Bi-2201 phases Synthesis, structures and superconducting properties N.R. Khasanova, E.V. Antipov * Department of Chemistry, Moscow State University, Moscow 119899, Russian Federation Received 17 February 1995

Abstract The Bi2+xSr2_~CuO6+ 8 (Bi-2201) phases and La substituted Bi-2201 compounds with general formulas Bi2Sr2-xLaxCuO6+ 8 and Bi2.a_xSrl.7LaxCuO6+ 8 were prepared and characterized by X-ray powder diffraction, electron diffraction, iodometric titration and resistance measurements. The monophasic samples of Bi2+xSr2_~CuO6+ ~ were obtained in air at 850°C for 0.15 < x < 0 . 4 , while synthesis at 740°C in oxygen flow extended this range to the stoichiometric cation composition (0 < x _<0.4). The superconducting transitions were not detected for these monophasic samples in contrast to the La substituted phases which exhibited superconductivity, and the highest T~,ons,t = 33 K was found for the Bi2Srl.6Lao'.4CuO6.33 compound. Superconductivity in the Bi2Sr2_xLaxCuO6+8 series exists in overdoped and underdoped regions. Incommensurately modulated structures of nonsuperconducting Bi2.3Sr1.7CuO6.23 phase and superconducting Bi2Srl.7Lao.3CuO6.2s phase (T~,ons~t = 26 K) were refined from X-ray powder data by a Rietveld technique using the four-dimensional space group P : A 2 / a : - 11. The maximal Cu displacements from the average position in the first structure was found to be sufficiently larger than in the latter one. The distorted structural arrangement of the (CuO 2) layers in the Bi2.3Srl.TCUO6.23 structure can be a reason for the suppression of superconductivity in this phase, while their less corrugated configuration in the La containing Bi-2201 structure leads to the existence of superconductivity.

1. Introduction The Bi based superconducting Cu mixed oxides form a homologous series with the general formula Bi2Sr2Can_lCunO2,+4+~ and they have layered structures with alternating rock-salt and perovskite blocks. The critical temperatures of these compounds increase drastically from Tc = 6 - 1 0 K for the onelayered phase to T¢ = 110 K for the third member. For the T I 2 B a 2 C a , _ I C u , O 2 , ÷ 4 + 8 series, whose structures are close to those of the Bi based super-

* Corresponding author.

conductors, this increase of Tc is not so drastic; for example the first member, T12Ba2CuO6+~, has a high Tc = 97 K. The reason for this dramatic difference in the superconducting properties is not fully understood. The Bi-2201 phase was first discovered among members of this series [1], but this compound has not been fully investigated for several reasons. The synthesis of monophasic Bi-2201 samples is complicated by the phase relations in this system at nearstoichiometric composition. It was found that in air (temperature range 810-850°C) the Bi-2201 phase forms a solid solution with a range that does not include the stoichiometric one [2-9]. A study of the

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242

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

phase diagram near ideal cation stoichiometry revealed the existence of another insulating phase (phase A here and below) with a monoclinic cell (a = 24.473 ,&, b = 5.4223 ,&, c = 21.959 ,&, fl = 105.40 °) and composition close to Bi2Sr2CuOy [3,4,10]. The structure of this compound contains stepped (CuO 2) layers, while the Bi-2201 phase consists of infinite (CuO 2) sheets. The composition regions of the Bi-2201 phase obtained in several studies differ and depend on the conditions of synthesis. It was shown that increasing of the oxygen pressure suppressed the formation of the A phase and extended the stability region of the Bi-2201 phase [5,10,11]. Another problem concerns the reproducibility of the superconducting properties of this system. Superconducting transitions with Tc = 6-10 K were observed only on multiphasic samples, while monophasic samples with an excess of Bi were not superconducting [6,11]. At that time the partial replacement of Sr by large rare-earth elements (La, Nd) results in an increase in the transition temperature to 24-35 K [12,131. The structural investigation of Bi-2201 phase is complicated by the presence of an incommensurate modulation. The modulation in the Bi based superconductors is caused by the incommensurability between the (BiO) layer and the perovskite block, as was suggested by Zandbergen et al. [14]. This mismatch is reduced by cooperative atom displacements from the average positions and by incorporation of extra oxygen in the (BiO) layer. The existence of this extra oxygen was confirmed by studies of commensurate analogues containing Fe rather than Cu, and by X-ray single-crystal and neutron powder diffraction investigation of the Bi-2212 phase [15-17]. Such a reconstruction enables an appropriate bonding to occur of the bismuth cation. At the same time the additional oxygen in the (BiO) layer creates holes in the conducting bond, thereby providing charge carders for superconductivity. The modulation in the (BiO) layer affects the structural rearrangement and leads to corrugation of the (CuO 2) layers, which are responsible for superconductivity. Hence such structural distortions influence the superconducting properties of these compounds. The Bi2201 phase is perhaps the best example to study the effect of modulation on these properties, because in

their structure the modulation is largest among Bi containing superconducting Cu mixed oxides [18,19]. This study aims first to investigate the factors which control the stability of the Bi-2201 phases, and secondly to analyze the correlation between their structures and superconducting properties.

2. Experimental Two series of samples with the general formula Bi2+xSr2_xCUO6+ 8 (0 < x < 0.4) were prepared from SrCO3, Bi203, CuO using a ceramic procedure. The homogenized mixtures were pressed into pellets, calcinated at an appropriate temperature in "Nabertherm" furnaces and then slowly cooled. The first series was annealed at 800, 820 and 850°C in air. To investigate the effect of the oxygen pressure a second series was prepared at 700 and 740°C in oxygen flow. The La doped series was prepared at 800, 870 and 900°C in air. In all cases the last annealing was repeated several times with intermediate regrindings. The full synthesis time was about 200 h. Some single-phase samples were treated in nitrogen flow (740°C) during 20 h. The Gunier camera (CuKet I radiation, A = 1.54056 .&, Ge-internal standard) was employed for the phase analysis of the samples obtained and for determination of their sublattice and superlattice parameters. X-ray data for the Rietveld refinement were collected by a STADI P powder diffractometer in transmission mode. The scintillation detector was used. The measurements were carded out over the range 6-100 ° (20) with step interval of 0.02 ° (20) and counting time of 40 s per step. For both structural experiments an absorption correction was made. The copper valence of monophasic samples was determined by iodometric titration with an accuracy + 0.02. The amount of oxygen was calculated using these data and assuming the bismuth valence to be +3. An electron-diffraction (ED) study of some samples was performed using JEOL 2000FX. AC susceptibilities of the specimens were measured with an external field amplitude of 1 0 e at a frequency of 27 Hz over the T range 12-100 K. R(T) measurements were carded out using a stan-

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

dard DC four-probe technique in the temperature range 4-270 K.

3. Indexing of X-ray powder patterns The presence of incommensurate modulation leads to the appearance of satellites in the diffraction patterns. In the case of X-ray powder diffraction the satellites often overlap with the main reflections. The possible presence of impurities also makes the indexing difficult. One way to determine the q vector is to use the samples with enhanced preferred orientation. In our case the preferred orientation of crystallites along the c-axis increases the intensity of reflections 0010 and 00l + 1, and in the low-angle region the satellites can be indexed unambiguously. These reflections were used for the preliminary determination of the q vector. The four-dimensional space group P : A 2 / a : - 11 found from the X-ray single-crystal study was used for analysis of the modulated structure of Bi-2201 phase [18,20,21]. While in Bi-2212 phase the modulation vector may be represented by one component along the a direction, the modulation vector of the Bi-2201 phase has two components along the a and c directions: q =a s a* + CsC*. Consequently the diffraction-pattern analysis revealed the presence of some nonoverlapping corn_position-dependent satellite reflections (1111, 1131, 2001, 2021), which were used for determination of the q vector value. In the first step subcell and supercell parameters were refined separately. The subcell parameters which describe the average structure were determined using the orthorhombic group Amaa. For samples of the Bi2+xSr2_xCuO6+ 8 series

Table 1 Lattice constants, copper valence and 6 of the Bi2 + xSr2 xCUO6+ 8 phases prepared in air at 850°C -

x

a (A)

c (~k)

q

0.15 0.20 0.25 0.30 0.40

5.377(1) 5.384(1) 5.388(1) 5.394(1) 5.405(1)

24.630(7) 24.595(8) 24.573(7) 24.550(8) 24.467(7)

0.196(2)a* 0.204(1)a* 0.209(1)a* 0.217(1)a* -

Cu n+ 6 +0.38(1)c* +0.47(1)c* +0.52(1)c* +0.60(1)c*

2.17 2.17 2.•5 2.15 2.08

0.16 0.19 0.20 0.23 0.24

243

Table 2 Lattice constants, copper valence and 6 of the Bi2+xSr2_xCuO6+ phases prepared in oxygen flow at 740°C x

a (A)

c (,A)

q

Cu n+

0,00 0.05 0.10 0.15 0.20 0.30 0.40

5.3637(6) 5.3668(7) 5.3708(7) 5.3730(7) 5.3809(8) 5.3926(5) 5.405(1)

24.698(8) 24.69(1) 24.656(9) 24.643(9) 24.600(7) 24.565(9) 24.467(7)

0.190(1)a* 0.187(2)a* 0.193(2)a* 0.199(1)a* 0.205(1)a* 0.217(1)a* 0.227(2)a*

+0.32(1)c* +0.34(1)c* +0.35(1)c" +0.41(1)c* +0.46(1)c* +0.58(1)c* +0.75(1)c*

2.28 2.28 2.24 2.18 2.15 2.15 2.10

0.14 0.15 0.17 0.17 0.18 0.23 0.25

the orthorhombic splitting of the reflections was too small and unresolvable under the conditions employed. In these cases the indexing was performed using a pseudotetragonal base. The cell parameters obtained were used for the refinement of the supercell parameters. Satellite reflections were indexed in the four-dimensional space group P : A 2 / a : - 1 1 with fl = 90 ° according the following equation [5]: 1

(h+mas) 2

k2

(l+mcs) 2

d2kl m

a2

d- - ~ q-

C2

Only unambiguously indexed reflections in the range of 6-40 ° 20 (about 10-14) were used for the determination of the a s and c s values. Finally both subcell and supercell parameters were refined simultaneously by a standard least-squares refinement procedure.

4. Results 4.1. Phase formation The phase stability of the Bi-2201 compound was investigated under two different conditions. The resuits obtained are presented in Tables 1 and 2. It was shown that the region of existence of Bi2+xSr2_ xCuO6+ 8 in air (850°C) is limited to the range 0.15 < x < 0.40. For 0.0 < x < 0.15 the samples were multiphasic and contained the Bi-2201 phase, the A phase and another phase (or phases) in small amounts. The lattice parameters of the Bi-2201 phases in these polyphasic samples were close to those of Bi2.1sSrl.asCuO6.16. Heterovalent substitution of Sr 2÷ by Bi 3÷ leads to an increase of the a parameter and

244

N.R. Khasanova, E. V. Antipov/ Physica C 246 (1995) 241-252

a reduction of the formal copper valence. A contraction of the structure along the c-axis was observed as the B i / S r ratio increased. The wave vector of modulation changes with cation composition. The first component is associated w|th an incorporation of extra oxygen in the (BiO) layer, while the second one can be presented as a stacking period of buckled layers or as a phase slip of one modulated layer relative to another. The c* component of the q vector increased greatly with increasing x in Bi2+xSr2_xCu06+8, while the variation of the a* component was small. The calculated oxygen stoichiometry values,/~, were consistent with the changes in the a s superlattice parameter. The observed evolution of the parameters was in agreement with data published previously [3,5,7,9]. A correct determination o f the superlattice p a r a m e t e r s for Bi2.4Srl.6CuO6.24 was impossible due to broadening of the satellite reflections with increasing x. Such broadening of the satellites was also observed by Fleming et al. [5]. Two samples, Bi2.2Srl.sCuO6.19 and Bi2.aSrl.TCUO6.23, were studied by electron diffraction. Satellites of first and second orders were well pronounced in the diffraction patterns of these two compounds. The values of the supercell parameters (q = 0.2a* + 0.5c* and q --~ 0.21a* + 0.6c* (Fig. l(a)), respectively) were in good agreement with those determined from X-ray powder data.

We tried to find the synthetic conditions which stabilize the Bi-2201 phase with ideal cation stoichiometry. The phase-formation process of the Bi2201 phase was investigated in oxidizing conditions (740°C, oxygen flow). The choice of temperature was made by a preliminary investigation which revealed the decomposition of the Bi-2201 phase with x = 0 at temperature higher than 750°C in air or oxygen flow. It was found that the stability region of the Bi-2201 phase at 740°C and p(O 2) = 1 bar was extended to 0.00 < x < 0.40. Compositional dependences of the sub- and superlattice parameters are presented in Table 2. The character of changes in the parameters was the same as that described above. Moreover, for the phases with identical cation composition prepared in air (850°C) and in oxygen flow (740°C) the lattice parameters and 8 values were practically the same and differences in their values were insignificant. To understand the influence of oxygen pressure on the phase formation the synthesis of a sample with stoichiometric composition was attempted in nitrogen flow (800°C). The sample thus prepared contained practically pure-phase A, the amount of impurities was about 5-7% and no Bi-2201 phase was detected. The diffraction pattern of the phase A with a stepped structure was indexed in the monoclinic space group C 2 / m with a = 24.42(1) .~, b =

i

Fig. 1. Electron-diffractionpatternstaken alongthe [010]zone axis of Bi2.3Srl.7CuO6.23 (a) and Bi2Srl.7Lao.3CuO6.23 (b).

N.R. Khasanova, E, V. Antipov / Physica C 246 (1995) 241-252

\

6.00

~"~'%'~..,. "*|,l,, ~

,

•

•

•

.

• •

4.013

x=0.30

.

a °°a

tlBz~oaooaa

,l~ ,~II~NmII~IMI

o a a a o a a a a a

o a

I O , 0 •~1 ii , , t O • • I I ~ * " • I •

x=O.l,5

x~O'

|0

2.00

f 0 .~' .......

0.0(I 0.00

, ......... 20.00

, .......... 40.00

• ........ (SO.O0

, .....

80.00

T (K) Fig. 2. R(T) for the Bi2+xSr2_xCUO6+ 8 samples with x = 0.1, 0.15 and 0.3 prepared in air at 850°C.

5.430(1) .~, c = 21.978(7) .~, /3 = 105.32(3) °. Chemical titration for this sample gave the oxidation number of copper as + 1.88(2). 4.2. R(T) measurements For all the samples obtained resistance measurements were carried out. Monophasic samples obtained in air showed a semiconducting behavior. Superconductivity with Tc about 6 - 8 K were detected only for multiphasic samples with the follow-

4.00 3.50

O

245

ing composition: x = 0 . 0 5 , 0.1. The experimental R ( T ) curves for the Bi2+xSr2_xCUO6+ ~ samples with x = 0.1, 0.15, 0.3 are shown in Fig. 2. No superconducting transitions were observed in the case of phases synthesized in oxygen flow. In the range 0.0 < x < 0.15 the samples exhibited a metallic-like type of conductivity, while in the range 0.2 < x < 0.4 they showed semiconducting properties. Fig. 3 shows the temperature dependence of R for the samples with x = 0 and 0.2. Monophasic samples with 0.0 < x < 0.15 were treated in nitrogen flow (600°C, 4 h). No measurable changes in cell parameters were observed by X-ray diffraction, and no superconductivity was detected after this post-annealing treatment. However, a nitrogen treatment of these samples for a longer time ( 1 0 - 1 5 h) led to decomposition of the phases. 4.3. Investigation of Bi2Sr 2 _ xLaxCu06 + The effect of heterovalent substitution of L a 3 + for Sr 2+ was investigated at 900°C. The La containing Bi-2201 phases, Bi2Srz_xLaxCuO6+ 8, are formed in the range 0.2 < x < 0.8. The experimental results are shown in Table 3. The supercell parameters were not refined because the superstructure became less pronounced with increasing La content. In accordance with R ( T ) measurements superconductivity was found in the samples with 0.2 < x < 0.5. The dependence of the superconducting transition temperature versus La content has a parabolic-like behavior with maximum Tc, onset = 33 K observed for the sample with x = 0.4 (Fig. 4). It should be noted that the copper valence decreases with the increase of the La content.

~=~

~O@O@@OOO

@ @ O@@@

@@

O @ x~0"2

"-" 3.00

%

2.50 x=O.O •*••

• •*

* * •*••*U

**•*•***I*m•*•

***

2.00

1.50 . . 0.00

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O . . '.

20.00

40.00

60.00

8

.00

T (K)

Fig. 3. R(T) for the Bi2+xSr2_xCuO6+~ samples with x = 0.0 and 0.2 prepared in oxygen flow at 740°C.

Table 3 Lattice constants, copper valence, 6 and Tc.o.set of the Bi2Sr2_xLaxCuO6+ 8 phases x 0.20 0.30 0.40 0.50 0.60 0.80

a(A) 5.398(1) 5.403(1) 5.417(2) 5.423(1) 5.430(2) 5.448(2)

b(A) 5.375(1) 5.377(1) 5.381(1) 5.385(1) 5.393(1) 5.418(2)

c(A) 24.567(8) 24.496(8) 24.39(1) 24.32(1) 24.286(6) 24.10(1)

Cu~+ 8 ~ ..... t 2.29 0.25 18 2.25 0.28 26 2.26 0.33 33 2.17 0.33 10 2.12 0.36 2.04 0.42 -

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

246

8.00

Bi2Sr2.xLaxCuOe+s i

!

i

!

35

|

Tc (K)

2.30 -

6.00

3o

Vcu

,_,

2.25 25

~:~,~ 4.oo

~

2.20

'.

2.10

•

~

A • 4

0.00

~, D

2.05 -

2.00 0.0

, 0.2

, 0.4

9 0.6

• ----

0.00

IiI

0.0

'

.

. . . . . .

•

• x=O.0

,,.,.,,,,,,*** "*

,=0.3

ota

10 t

~

~e"

2.00

15

L

./f"

gg

20 2.15 -

,

, :." . J . . ,

.........

20.00

, .........

40.00

, .........

60.00

r .....

80.00

T (K)

5

Fig. 5. R(T) for the Bi2.3_xSrl.7LaxCuO6+ a samples with x = 0.0, 0.1, 0.2 and 0.3.

0 1.0

X

Fig. 4. Copper valence (left) and Te..... t (right) vs. La content (x) for the Bi2Sr2_xLaxCuOr÷ 8 phases.

4.4. Investigation o f Bi2. 3 _ xSr].7LaxCu06 + a

To understand the reasons causing the appearance of superconductivity in the La substituted Bi-2201 phase an investigation of solid-solution Bi2.3_ xSrl.7LaxCuO6 + a was attempted. The choice was determined by the following reasons. T w o extreme members of this solid solution, Bi2.3Srl.TfUO6.23 and Bi2Sr].7La0.3CuO6.28 , exhibit different physical properties: the first one is a semiconductor, while the second phase shows a superconducting transition with Tc = 26 K. The results of the investigation are presented in Table 4. The substitution of Bi 3÷ by La 3÷ increases the orthorhombic splitting and the difference between the a and b parameters increases with increasing x. It is coupled with an increase in the copper oxidation number and the ~ value. The changes of the 8 value are in agreement with the

decrease in modulation period along the a direction. The Bi2Sr1.TLao.3CuO6.28 sample was studied by ED; only some of first-order satellites were observed in the ED picture along the (010)-axis and the q vector was determined as q = 0.23a* + 0.7c* (Fig. l(b)). X-ray analysis indicated that the satellites became broader with La substitution. A superconducting transition was detected for all La containing samples (Fig. 5). The highest superconducting transition temperature was achieved for Bi2Sr].7Lao.3CUO6.28.

4.5. Rietveld refinement of Bi2.3Srl.7Cu06.23 and Bi 2Sr t.TLao.3Cu06.28 The Rietveld refinement of the monophasic Bi2.3Srl.7CuO6.23 sample was carried out using the GJANA program [17]. This program presumes the superspace group approach. The position of atoms were given by the average-lattice position plus a displacement caused by modulation. The structure

Table 4 Lattice constants, copper valence and ~ of the Bi2.3_xSrl.7LaxCuOr+a phases x

a (~,)

b (,~)

c (,~)

q

0.00 0.10 0.20 0.30

5.394(1) 5.402(1) 5.403(1) 5.403(1)

5.394(1) 5.3910(8) 5.383(1) 5.3765(9)

24.550(8) 24.560(6) 24.544(9) 24.496(8)

0.217(2)a* + 0.225(1)a* + 0.229(2)a* + 0.229(1)a * +

0.60(1)c* 0.70(1)c* 0.71(1)c* 0.73(1)c *

Cu n÷

8

2.15 2.15 2.17 2.25

0.23 0.23 0.24 0.28

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

refinements were carried out up to 95 ° ( 2 0 ) because of the low intensity of reflections at higher angles. The pseudo-Voight function was used for the fitting of the experimental intensities. The background was modelled by a polynomial function with four variable parameters. The preferred orientation along the (001) direction was considered via a March correction. The peak asymmetry correction to 30 ° (2 0) was applied and a zero-shift parameter was also refined. The four-dimensional space group P : A 2 / a : - 1 1 and the starting model were taken from a singlecrystal study [20,21]. The modulations of cations and the oxygen atom in the (BiO) layer were considered in the refinement. The displacements of cations were described by first-order harmonics. The higher-order ones were not employed in the refinement due to the relatively small number of experimental intensities. The modulation of the oxygen atom in the (BiO) layer was represented by a sawtooth-shaped function, which allows a description to be made not only of shifts of this atom, but also of the appearance of extra oxygen in the (BiO) layer. The value of additional oxygen was fixed in accordance with chemical titration. The positions of atoms other than oxygen in the (BiO) layer were assumed to be fully occupied and their occupancies were fixed. The partial replacement of Sr 2÷ by Bi 3÷ (15%) was considered for the Sr position. No substitutional modulations, which may take place in these compounds, were

247

involved in the refinement. The thermal parameters of all oxygen atoms were fixed. The refinement method was similar to that used in Ref. [22]. First the average structure (without modulation) was refined to the following values of agreement factors: R m = 0.17, Rp = 0.19, Rwp = 0.21. Such a refinement was not correct due to the presence of relatively strong satellites which often overlap with the main reflections. The modulation waves of copper were restricted due its special position, and only sin waves were used to describe the displacements of this atom. All other atoms were located at general positions and no restrictions should be applied. However, it was found that the modulation amplitudes of all the metal atoms along the b-axis were insignificant (in the range of standard deviations). Modulations of the atoms in this direction were neglected in subsequent stages of the structural analysis in order to reduce the number of refinable parameters. This procedure is consistent with the character of the modulation in these compounds, because - as was found by HREM and previous structural investigations - the main atom displacements appear along the directions of the modulation vector (in our case the a and c directions) [18,19, 23-25]. At the last step the modulation of the 0 3 atom in the (BiO) layer was considered, the initial parameters for this atom being taken from Ref. [22]. Only the modulation along the a-axis was consid-

Table 5 Summary of crystallographic data and experimental conditions for Bi2.3Srl.TCuO6.23 and Bi2Srl.7La0.3CuOr.28 a Bi2.3Sr1.7CuO6.23

Bi2 Srl.7 Lao.3CuO6.28

Modulation vector Space group Z 20 range (degree) Step (degree) Step counting time (second) Temperature Roy Rm

a = 5.3907(2), b = 5.3907(2) c = 24.534(2), /3 = 90.0(0)° q = 0.217(1)a * + 0.608(8)c * P:A2/a: - 11 4 6-95 0.02 40 ambient 0.104 0.071

a = 5.4015(2), b = 5.3781(3) c = 24.501(2), /3 = 89.830(1) q = 0.228(1)a * + 0.72(1)c * P:A2/a:- 11 4 10-95 0.02 40 ambient 0.096 0.06

Rsa t

0.20

0.198

Rp Rwp

0.099 0.127

0.095 0.124

Unit cell parameters (,~)

a Agreement factor determined as: Ro~, including all reflections, Rm, including main reflections, Rsat, including only satellite reflections, Rp, profile factor, Rwp , weighted profile factor.

248

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252 Absolute intensity

12000 t

9000

6001 -~

3o001 ~i

/t ] I I II II Ill IIIli lilll] lill]ldlll IIHHJHIImliUllllllallllilllll|llll|nllJIIIl|ll|lUilllHOliHHHam|lllmnr

10

]

i

i

i

i

i

20

30

40

50

60

70

,

i 80

90

2 Theta, degree Fig. 6. Observed X-ray diffraction pattern of Bi2.3Sr1.7CuO6.23. The difference curve and theoretical reflection positions are shown at the bottom of the figure.

ered for this atom. Finally the structure was refined down to Roy = 0.104, R m - - - 0 . 0 7 1 , Rsa t = 0 . 2 0 , Rp = 0.099, Rwp = 0.127. The structural parameters obtained are listed in Table 5 and 66, and the final profile fitting is shown in Fig. 6. The structural refinement revealed that the structure of Bi2.3Srl.TCUO6.23 is greatly modulated. The displacements of the Bi atoms are considerable, esl~ecially in the a direction (ux = 0.46 ,~, u z = 0.18 A), while for the Cu atom the modulation in the c direction is predominant (uz = 0.49 ,~). The displacements of the Cu atoms within the (CuO 2) layers

Table 6(a) Positional and isotropic thermal parameters for Bi2.3Srl.7CuO6.23

are small (u x = 0.09 ,~). The Sr atoms shift mainly in the c direction, but the displacement amplitude !n the a direction was also significant (u x = 0.24 A, u z = 0.42 ,~). The oblique q vector in the Bi-2201 phases causes a relative shift of the displacement waves of translation-related layers with different z coordinates. The behavior of the modulation is similar to that found in previous studies. The amplitudes of the cations modulations are close in magnitude to those observed by Gao et al. [19] and Leligny et al. [21]. The refinement of the modulation of the 0 3 Table 6(b) Amplitudes (,~) of displacive modulation for Bi2.3Srl.TCuO6.23 Atom

Atom

x

y

z

B (,~2)

Bi

Bi Sr/Bi a Cu O1 02 03

- 0.012(2) 0.505(3) 0 0.31(2) -0.02(2) 0.53(4)

0.268(1) 0.250(2) ¼ 0.45(2) 0.27(1) 0.39(1)

0.0650(3) 0.1797(3) ¼ 0.264(3) 0.151(2) 0.072(3)

2.3(2) 1.2(1) 1.3(2) 1.5 b 1.5 b 2.0 b

Sr/Bi

a Occupancy G = 0.85Sr +0.15Bi. b Fixed value.

Cu

sin(2"tt x 4 ) cos(2wx 4) sin(2"tr x 4) cos(2"lr x 4 ) sin(2 ~rx 4 ) COS(2--tr

03

u0 x4°, A a

X4 )

ux

Uy

Uz

0.361(9) -0.291(11) 0.070(13) -0.226(9) 0.09( 1)

0 0 0 0 0

-0.160(12) -0.076(10) - 0.417(14) 0.069(12) - 0.496(16)

--

- 1.2(1) 0.60(3)

_

_

0 1.115

0 -

a x 0 is the coordinate of the 0 3 atom in fourth direction, A the periodicity of the sawtooth-shaped function, fixed value [16].

249

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252 Table 7 Interatomic distance ranges for Bi2.3Srl.7CuO6.23and Bi2Srl.TLao.3CuO6.2s

Bi2.3Srl.7CuO6.23

Distance (~.)

Bi2Srl.7Lao.aCuO6.28

Distance (A)

Bi-O3 × 2 Bi-O3 × 2 Bi-O3 a Bi-O2 Sr/Bi-O3 Sr/Bi-O2 Sr/Bi-O2 Sr/Bi-O2 Sr/Bi-O2 Sr/Bi-O1 × 2 Sr/Bi-O1 × 2 Cu-O1 × 2 Cu-O1 × 2 Cu-O2

2.07-2.46 2.61-3.74 2.90-3.53 2.055-2.395 2.52-2.98 2.298-2.625 2.928-3.382 2.843-3.103 2.627-2.927 2.611-3.293 2.473-3.228 1.892-2.146 1.918-2.120 2.041-3.034

Bi-O3 × 2 Bi-O3 x 2 Bi-O3 a Bi-O2 Sr/La-O3 Sr/La-O2 Sr/La-O2 Sr/La-O2 Sr/La-O2 Sr/La-O1 × 2 Sr/La-O1 × 2 Cu-O1 × 2 Cu-O1 × 2 Cu-O2

2.19-2.50 2.67-3.70 2.75-3.43 2,106-2.353 2.49-2.87 2.364-2.890 2.990-3.301 2.848-3.045 2.637-2.870 2.625-3.210 2.543-3.169 1.992-2.046 1.894-2.035 2.163-2.740

a 03 atom from adjacent layer.

atom gave a reasonable coordination of the Bi atoms. The ranges of calculated distances are listed in Table 7. The modulations of Bi and 0 3 atoms in the (BiO) layer allowed the coordination of the Bi-atom to be described as three-fold: two bonds (2.07-2.46 ,~) with the 0 3 atoms in the (BiO) layer and one with the 0 2 atom from the (SrO) layer. Other distances

are too large to be considered as bonding. A s one can see from Table 7 the range of the B i - O 3 distances is large, because it presents the distances of two extreme positions of the 0 3 atom. The modulations of the other oxygen atoms (O1 and 0 2 ) were not considered in the refinement and this resulted in a large variation of the distances, especially for the

Absolu~intcnsi~ 18000

15000

12000

9000

6000

ooo

o

10

20

30

40

50 2 Theta,

60

70

80

90

degree

Fig. 7. Observed X-ray diffractionpattern of Bi2Srl.7La0.3CtlO6.28. The difference curve and theoretical reflectionpositions are shown at the bottom of the figure.

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

250

Table 8(a) Positional and isotropic thermal parameters for Bi2Srl.7ta0.3CuO6.28

Atom

x

y

z

Bi Sr//La a Cu O1 02 03

0.000(2) 0.500(5) 0 0.25(1) -0.061(1) 0.63(1)

0.271(1) 0.250(2) ¼ 0.50(1) 0.24(1) 0.36(1)

0.0656(3) 2.1(2) 0.1780(4) 1.3(2) ¼ 1.3(3) 0.261(3) 1.5 b 0.149(3) 1.5 ~ 0.0764(4) 2.0 b

B (~2)

a Occupancy G = 0.85Sr +0.15La. b Fixed value.

This is understandable because the shifts of the Bi atom allowed the incommensuratability to be reduced and the lattice parameters of the two phases are close. The replacement of Bi 3÷ by La 3+ in the case of Bi2Srl.TLa0.3CuO6.28 results in some decreasing of the cation displacements in this layer (u x = 0.18 ,~, u z = 0.36 ,~). The main difference between the two structures is a considerable reduction of the copper modulation amplitude along the c direction ( u x = 0.04 A, u z = 0.38 ,~) in Bi2Sr].7La0.3CuO6.28 .

5. Discussion bonds with the 0 2 atom. Taking into account the significant shifts of the Sr and Cu atoms one may propose large displacements of this atom as well. The same procedure was applied for the structural investigation of Bi2Sr~ 7La0.3CuOr.2s. No impurities were detected by X-ray analysis. The value of the oxygen excess was fixed in accordance with chemical analysis. A partial occupancy of the Sr position by La 3+ (in accordance with the starting composition) was assumed. The refinement of the average structure without modulation resulted in the following values for the agreement factors: Rrn = 0.17, Rp = 0.22 Rwp = 0.24. The agreement factors were improved after inclusion of the modulation in the refinement (Roy = 0.096, R m = 0.06, Rsat = 0.198, Rp = 0.095, Rwp --- 0.124). Structural parameters are presented in Tables 5, 7 and 8, and the final profile fitting is shown in Fig. 7. The results obtained indicate that the modulation behavior is similar in both compounds. The modulation of the Bi atom is practically the same (u x = 0.48 ,~, u z = 0.17 ,~).

Table 8(b) Amplitudes (A) of displacive modulationfor Bi2Srl. 7 Lao.3CuO6.28 Atom Bi Sr

Cu 03

sin(2"tr x 4) c o s ( 2 ~ x 4) sin(2 Ir x 4) eos(2"Ir x 4 )

sin(2 wx 4) cos(2~x4) u0 x4°, A a

ux

uy

uz

0.400(9) -0.268(9) 0.102(12)

0 0 0

- 0.122(8) -0.120(9) -0.348(11)

-0.151(13) 0.04(2) - 1.0(1) 0.49(4)

0 0 _ 0 1.14 a

-0.115(13) - 0.38(3) _ 0 _

a x o is the coordinate of the 0 3 atom in the fourth direction, A the periodicity of the sawtooth-shaped function, fixed value [16].

The structures of Bi based Cu mixed oxides contain stacks of perovskite and rock-salt blocks along the c-axis. Stabilization of such intergrowth structure requires matching between the layers. Perfect matching is attained when the ratio ( A - O ) / ( C u - O ) is equal to v/-2-. However, the ideal (AO) bond length determined by this ratio is considerably longer than the sum of the ionic radii of Bi 3÷ and 0 2-. This mismatch is reduced through strong atom shifts in the (BiO) layer and an incorporation of additional oxygen in this layer (approximately in every fourthfifth unit cell), that increases the effective size of the (BiO) layer. On the other hand size of perovskite slab in the (a, b) plane is largely determined by the in-plane C u - O bond, which is sensitive to the hole concentration of the o" * antibonding band. Hence a matching between the slabs can be achieved by a variation of the hole concentration of this band. Redundant oxygen in the (BiO) layer introduces holes into this band that lead to compression of the C u - O bond and structure stabilization. The presence of additional oxygen in the (BiO) sheet is necessary for structural stability of Bi based Cu mixed oxides and these compounds exist only if a sufficient amount of oxygen is inserted into the (BiO) layer. It is necessary to point out that the Bi-2201 phases with identical cation composition obtained at different temperatures in air or oxygen flow possess practically the same values of lattice parameters, q vector and oxygen stoichiometry. Therefore we can conclude that the amount of extra oxygen depends slightly on the synthesis conditions and the 8 value is determined by the cation composition and can vary over a narrow range.

N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

The incorporation of extra oxygen (6 is about 0.2) increases the copper valence. The latter should be sufficiently high for the Bi-2201 phase with ideal cation composition and this phase can be obtained only in oxidizing conditions (at lower temperature and higher partial oxygen pressure). For this composition synthesis in air at 850°C did not allow the necessary copper valence to be stabilized: the sample was polyphasic and contained the Bi-2201 phase with a higher content of Bi. Such a substitution of Bi 3÷ for Sr 2÷ reduces the copper valence and resuits in formation of the Bi-2201 phase. Hence, to stabilize this structure not only a sufficient 6 value, but also the stability of the copper with a corresponding high valence are necessary, and the Bi-2201 phase with a composition close to the stoichiometric one can be prepared only under elevated oxygen pressure, as in the case of YBa2Cu40 8. This increasing of the copper valence due to extra oxygen influences largely the stability of Bi-2201 phase, because in this case all holes correspond to the one (CuO 2) layer. The heterovalent substitution of Bi 3+ for Sr 2+ in Bi2+xSr2_xCUO6+ 8 results in an increasing of the oxygen stoichiometry. The increase in the ~ value is consistent with the decrease of the modulation period ( I / a s) along the a-axis. However, the values of 6 and a s are not quantitatively the same, especially for the Bi2 + xSr2- xCuO6 + ~ samples with small x. This may be explained by the appearance of oxygen vacancies in the (SrO) layer. It should be noted that the a s and 6 values depend not only on the ratio of heterovalent substitution but also on the crystallochemical behavior of the cation, located in the (SrO) layer. Partial replacement of Sr 2÷ by La 3÷, a cation which has a larger ionic radius than Bi 3+, increases the amount of inserted oxygen and consequently reduces the modulation period. The superconducting properties of these compounds are influenced by two factors: the hole concentration and structural arrangements of the (CuO 2) layers. As was mentioned above, the necessary charge concentration is created by additional oxygen in the (BiO) layer. In the case of Bi based superconductors, where the Cu valence cannot be changed over a wide range by varying the oxygen content, the heterovalent substitution is an important means for altering the carrier concentration. The study of Bi2Sr2_ x-

251

LaxCuO6+ 8 series revealed a parabolic-like dependence between T¢ and formal copper valence, that is in a good agreement with previous studies [12,26]. It indicates that the superconductivity is suppressed in overdoped and underdoped regions. The copper valence in Bi2SrzCuO6A 4 prepared in oxygen flow is relatively high and this phase exhibits the metalliclike conductivity behavior. The heterovalent substitution of Bi 3+ or La 3+ for Sr 2+ reduces the high oxidation number of copper, but the Bi and La substituted Bi-2201 phases exhibit different properties. In the case of the monophasic Bi2+xSr2_ xCuO6+ 8 samples prepared in oxygen flow, only a metal-insulator transition in the properties was observed. However, partial replacement of Sr 2+ by La 3+ results in the appearance of superconducting properties. We propose that this may be explained by a difference in the structures. The structure of Bi2.3Srl.TCuO6.23 is strongly distorted and the maximal displacement of the Cu atom from the average position is about 0.5 ,~. The value obtained is in agreement with results of the X-ray single crystal study of the Bi-2201 phase and it is significantly larger than the displacements of the copper atom in the superconducting Bi-2212 phase (u z = 0.33 ,~) [16,19]. The large puckering of the CuO 2 layers in the ni2.3Sr1.TCuO6.23 structure can be a reason of the suppression of superconductivity despite the appropriate formal copper valence. The replacement of Bi 3+ in the Sr site by La 3+ considerably reduces the Cu displacements in the Bi-2201 structure, and the (CuO 2) layers have a less distorted structural arrangement, which is appropriate for the existence of superconductivity. Finally we conclude that the superconductivity in the Bi based superconducting Cu mixed oxides is governed by the hole concentration and the structure arrangement of (CuO z) layers. In our study we could only determine the modulation of the cations, and to prove our conclusion and find the correct oxygen modulation neutron diffraction is necessary.

Acknowledgements The authors are grateful to P. Kazin and O. Rozanova for magnetic and resistivity measurements, Yu. Barabanenkov for ED study, G. Mazo for the

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N.R. Khasanova, E. V. Antipov / Physica C 246 (1995) 241-252

chemical analysis, R. Shpanchenko and O. D'jachenko for the technical assistance, L. Akselrud and A. Mironov for helpful discussion. This work was partly supported by Russian Scientific Council on Superconductivity (Poisk) and International Science Foundation (MIG000). References [1] C. Michel, M. Hervieu, M.M. Borel, A. Grandin, F. Deslandes, J. Provost and B. Ravean, Z. Phys. B 68 (1987) 421. [2] Y. Ikeda, H. Ito, S. Shimomura, Y. Oue, K. Inaba, Z. Hiroi and M. Takano, Physica C 159 (1989) 93. [3] R.S. Roth, C.J. Rawn and L.A. Bendersky, J. Mater. Res. 5 (1990) 46. [4] R.S. Roth, C.J. Rawn, B.P. Burton and F. Beech, J. Res. NIST 95 (1990) 291. [5] R.M. Fleming, S.A. Sunshine, L.F. Schneemeyer, R.B. van Dover, R.J. Cava, P.M. Marsh, J.V. Waszczak, S.H. Glarum and S.M. Zahurak, Physica C 173 (1990) 37. [6] B.C. Chakoumakos, P.S. Ebey, B.C. Sales and E. Sonder, J. Mater. Res. 4 (1990) 767. [7] D.C. Sinclair, J.T.S. Irvine and A.R. West, Jpn. J. Appl. Phys. 29 (1990) L2002. [8] B.C. Sales and B.C. Chakoumakos, Phys. Rev. B 43 (1991) 12994. [9] D. Sedmidubsky, and E. Pollert, Physica C 217 (1993) 203. [10] J. Darriet, F. Weill, B. Darriet, X.F. Zhang and J. Etoumeau, Solid State Commun. 86 (1993) 227.

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