Mat. Res. Bul l . ,Vol . 26, pp . 229-236, 1991. Pr inted in the USA. 0025-5408/91 $3.00 + .00 Copyr igh t (c) 1991 Pergamon Press plc.

STRUCTURAL AND PHYSICAL PROPERTIES OF Tll.xBixSr2CuO5 (0.20<x_<0.50)

Shu Li and Martha Greenblatt* Department of Chemistry, Rutgers University, Piscataway NJ 08855-0939

Allan J. Jacobson Exxon Research and Engineering Company, Annandale, NJ 08801

(Received J anua ry 24, 1991; Communicated by J . B . Goodenough)

ABSTRACT

The crystal structure of T10.5Bi0.5Sr2CuO5 was determined by powder x-ray diffraction with the Rietveld method. The structure of T10.5Bi0.5Sr2CuO5 is analogous to T1Sr;~CuO5, having a tetra, gonal structure with space group P4/mmm and a=3.7527(1)A and c=9.0278(4)A. Similarities and differences between T10.5Bi0.5Sr2CuO 5 and other Tl-based cuprate materials are discussed. Structural and physical properties of solid solutions of Tll.xBixSr2CuO5 (0.20~x<0.50) were also studied as a function of Bi stoichiometry. Superconductivity was not found in any of the compositions; all of the samples show only metal-like properties.

MATERIALS INDEX: Thallium, bismuth, copper, oxide.

The Tl-based high Tc superconducting cuprates can be classified into two families according to their structures. The members of the first family have a general formula T12Ba2Can-lCunO2n+4 (n=l-5), consisting of one double T1-O layer and n Cu-O layers per unit cell of the structure. The second family includes the phases consisting of a single TI-O layer and n Cu-O layers in their unit cell, which can be described with a general formula T1A2Can-lCunO2n+3 (A=Ba or Sr; n=l-5). Structures of the members of the first family have been characterized in detail by several techniques (1-6). However, structures of the second family members are not well established, except T1Ba2Ca2Cu309 (n=3 phase) the structure of which was determined by single crystal x-ray diffraction (7). The primary difficulty encountered in the structure determination is that one has to deal with impure samples. In general, the single TI-O layered phases are much less stable than the double TI-O layered counterparts, so that the synthesis of a pure powder sample or the growth of single crystals are extremely difficult.

It has been realized that the structural instability of the single T1-O layered phases are due to

To whom correspondence should be addressed.

229

230 S . LI, et a l . Vol. 26, Plo. 4-

the high copper valence (8,9) (copper valence can be estimated by a formula VCu=(2n+l)/n) and the dimensional mismatch of the TI-O and Cu-O layers. It is expected that chemical substitutions may reduce the copper valence and minimize the structural mismatch. In fact, pure phases of (T1,Pb)Sr2Can.lCunO2n+3 (n=l-3) (10-12) and (T1,Bi)Sr2CaCu207 (8,13) have been prepared and their structures were determined by single-crystal or powder x-ray diffraction. In this work, we report on the synthesis and the structure determination of T10.5Bi0.5Sr2CuO 5, which is the first member of the single T1-O layered family, analogous to T10.5Pb0.5Sr2CuO5 (10,14) and T1Sr2CuO5 (15). Structural and physical properties of solid solutions of Tll.xBixSr2CuO5 (0.20~.x<0.50) are also discussed.

Pure Tll-xBixSr2CuO5 (0.0~.x<0.50) samples were synthesized by solid state reaction. First, stoichiometric amounts of T1203, Bi203, Sr(NO3)2 and CuO were thoroughly mixed and heated at 600°C for one hour. The preheated products were then ground and pressed into pellets, sealed in a silver container, and heated at 850°C for 12 hours. Intermediate grindings were found beneficial to improve the homogeneity of samples and to minimize the impurity phase, which was identified to be Sr4T1207. Phase determination was carried out in a SCINTAG PAD-V powder x-ray diffractometer, and the lattice parameters of solid-solution samples were determined with the LATCON least-squares-refinement program. The powder x-ray diffraction data for Rietveld analysis were collected with a SIEMENS D500 powder x-ray diffractometer and Cu Karadiation. The diffraction profile was measured in the ~o - 20mode in the range 2.0 < 20< 122.0 ° in steps of 0.02 ° with count times of 30s at each point. The electrical resistivity of samples was measured using the four-probe technique.

R e s u l t s a n d D i s c u s s i o n s

Rietveld (16) analysis of the powder x-ray diffraction data was used to refine the crystal structure of a pure T10.5Bi0.5Sr2CuO5 sample. The model structure for T10.5Bi0.5Sr2CuO5 is shown in Figure 1. It contains SrO-T10-SrO rocksalt layers intergrowing with CuO2 layers. The structural symmetry was assumed to be tetragonal with space group P4/mmm, as determined for single crystal TISr2CuO5 (15). T1 and Bi ions were assumed to be randomly distributed in the (la) positions since they cannot be distinguished by x-ray diffraction in this experiment. The origin of the cell was chosen at the TI(Bi) atom position. The occupancies of atoms were initially confined to their ideal values. The thal l ium-bismuth composi t ion was represented by T1 and Bi atoms each at the (0,0,0) position but each with half-occupancy and thermal parameters constrained to be equal. The thermal parameters of the three inequivalent oxygen atoms, which are not well determined because of their low relative scattering power, were also constrained to be equal. The data were refined with the Larsen and Von Dreele GSAS system (17). The observed, calculated and difference powder x-ray di f f rac t ion prof i les of TI0.5Bi0.5Sr2CuO5 are shown in Figure 2.

FIG. 1

Schematic Structure of T10.5Bi0.5Sr2CuO5

Vol. 26, I'lo. 4 TI-BASED CUPRATES 231

% x

o

1 ,0

0 . 5

I I I I I I I I I

I I I I I I t I l l I I I I I I I i i n a l l I I I N I l l U l I I I I I B I I I I I U l I I I I I I I I | l U H I I l l l l l l l l l l l

I 1 I I I I I I I I I 2 0 . 0 40+0 6 0 . 0 8 0 . 0 1 0 0 0 1 2 0 . 0

t w o t h e t a

FIG. 2

The Observed, Calculated and Difference Powder X-ray Diffraction Profiles for T10.5Bi0.5Sr2CuO5.

The initial refinement indicated the presence of some preferred orientation in the data with the 00l reflections being relatively too intense. In subsequent refinements, preferred orientation was included as a variable assuming the 001 axis perpendicular to the plane of the specimen. A satisfactory refinement was then obtained for the whole profile. The refined parameters listed in Table 1 are identified as Model 1. The refinement led to a negative thermal parameter for the strontium atom that could not be eliminated by adjusting the background and suggested the possibility of some small substitution of one of the heavier atoms (T1 or Bi) at this site. To model possible substitution, the thermal parameter of the Sr atom was fLxed at a value comparable to that of the Cu site and the Sr atom occupation number was refined. The refinement (Model 2) gave significantly lower R factors and also reduced thermal parameters for the other positions. Because of the large difference in scattering power between TI(Bi) and Sr atoms, the degree of substitution is suggested to be only about 5%. Attempts to refine the TI(Bi) occupancy produced no improvement in the refinement. Other attempts to further improve the fit to the data by including T1 and 0 2 at more general positions were also unsuccessful. The metal-oxygen distances calculated with the parameters of Model 2 are listed in Table 2.

Compared with the reported structural data for TISr2CuO5, TI0.sBi0.sSr2CuO5 has a slightly larger unit cell (a=3.753A, c=9.028A for T10.sBi0.sSr2CuO5; a=3.738A and c=9.01A for TISr2CuOs) (15). In general, all of the structural parameters for T10.sBi0.sSr2CuO5 are very similar to those of T1Sr2CuO5 and other Tl-based cuprates. In other single T1-O layered cuprates, structural imperfections, such as ions located at non-ideal positions, ion deficiencies and mixed occupancy of ions at the same crystallographic site have been reported (1-3). Similarly, in T10.sBi0.sSr2CuOs, some evidence is found for Bi/T1 occupancy at the Sr(2h) site, though no TI(Bi) deficiency at the (la) site can be detected. The results are similar to those found for T10.5Pb0.5Sr2CuOs, where the TI/Pb site is fully occupied by only TI/Pb but the Pb ions partially

232 S . LI, e t a l . Vol. 26 , l'io. 4

occupy the Sr(2h) site (14). Therefore, it appears that the T1-O octahedra in T10.5Bi0.5Sr2CuO5 is not as stretched in the a-b direction as in other Tl-based cuprates. The TI-O bonds in the a-b plane are all equivalent, while in other non-substituted .'rl cuprates, there is a statistical distribution of longer TI-O bonds (~3.0A) and shorter ones (~2.3A) in the a-b plane.

TABLE 1

Structural Parameters of TI0.sBi0.sSr2CuOs.

Atom x ), z B z B

Model 1 Model 2 TI(Bi) (la) 0 0 0 2.51(5) 0 1.96(4) Sr (2h) 1/2 1/2 0.2983(1) 4).10(5) 0.2987(1) 1.0" Cu (lb) 0 0 1/2 1.40(8) 1/2 1.07(7) O1 (2g) 0 0 0.232(1) 2.6(2) 0.2261(8) 1.4(2) 02 (lc) 1/2 1/2 0 2.6(2) 0 1.4(2) 03 (2e) 0 1/2 1/2 2.6(2) 1/2 1.4(2) Space group P4/mmm {No. 123), * Sr(B) fixed, O1 (B)=O2(B)---O3(B). Model 1, a=3.7526(2)A, c--9.0279(6)A, Rp = 0.0511, wRp = 0.723. Model 2, a--3.7527(2)/~, c--9.0278(6),/~, Rp -- 0.0468, wRp = 0.653, Sr occupancy 1.106(3).

The Cu-O octahedra in T10.sBi0.sSr2CuO5 are similar to those in other thallium cuprates, but the Cu-O bond distance along the apical direction of the octahedron is much shorter, indicating that the Cu-O octahedra are less distorted. In the double TI-O layered compounds, this bond distance is about 2.7A, while in T10.sBi0.sSr2CuO5, it is only -2.47A. In addition, the Cu-O distance in T10.sBi0.sSr2CuO5 along the a-b direction is much shorter (1.876/~) than those in most cuprate materials (1.90-1.96A), which could be attributed to the very high copper valence in T10.sBi0.sSr2CuOs. From the Cu-O distances obtained (Table 2), one can calculate the copper valence according to the empirical bond-strength-bond-length relationship (18). For TI0.sBi0.5Sr2CuOs, the calculated copper valence is +2.59.

TABLE 2

Metal-Oxygen Distances in TI0.5Bi0.5Sr2CuO 5.

M-O d(A) TISr2CuO~ (15) Tl(Bi)-O1 2.041(7) x2 2.01 Tl(Bi)-O2 2.6535(1) x4 2.67+2.24 Cu-O1 2.473(7) x2 2.489 Cu-O3 1.8763(1) x4 1.867 Sr-O1 2.733(2) x4 2.718 Sr-O2 2.696(1) xl Sr-O3 2.612(1) x4 2.626

Solid solutions of Tll.xBixSr2CuO 5 form in the range of 0.20.~x<0.50. Figure 3 shows the structural variations of Tll.xBixSr2CuO 5 as a function of Bi content. For x--0.20, where a pure phase starts to form, a and V approach minima and then increase with a further increase of x. We suggest that for 0.0~.x_<0.20, TI3+ cations are replaced by smaller Bi 5+ ions so that the structure is stabilized because the valence of copper is reduced from +3.0 to -+2.6. For x>0.20, since the structure is already stabilized, TI3+ cations will be replaced only by larger Bi 3+ ions (rBi3+=l. 17A,

VOI. 2 6 , NO. 4 T I - B A S E D C U P R A T E S 2 3 3

rBis+=0.90A, and rrl3+=l.03/~ for CN=6) (19). In this region, Bi 5+ and Bi 3+ coexsist and the copper valence does not change with increasing Bi content, remaining at -+2.60, which is consistent with the calculated value +2.59 for TI0.50Bi0.50Sr2CuO 5. The lattice dimensions of Tll.xBixSr2CuO 5 are almost saturated between 0.40~.x_q0.50, because the average size of Bi 5+ and Bi 3+ is almost equal to that of TI 3+. However, the c parameter remains rather constant (9.01(1)A) for all values of x, which is consistent with the invariance of the TI-O1 and Cu-O1 bond distances in TI0.50Bi0.50Sr2CuO 5 as compared with T1Sr2CuO 5 (Table 2).

o.<

Cq

3 . 7 8

3 . 7 6

3 . 7 4

3 . 7 2

3 . 7 0

0 . 0

I I I I I

o a

, V

I , I , I , I h I

0 . 1 0 . 2 0 . 3 0 . 4 0 . 5

1 3 0

1 2 9

1 2 8

1 2 7

1 2 6

, 1 2 5

0 . 6

FIG. 3

Structural Variations of Tll.xBixSr2CuO 5 as a Function of x.

Superconductivity was not found in any of the Tll-xBixSr2CuO5 samples down to 2K, as determined by both resistivity and magnetic susceptibility measurements. Possibly, the copper valence in these compounds (at the minimum +2.59 as calculated from the structural data) is too high for superconductivity. The temperature dependence of resistivity of solid solution samples is illustrated in Figure 4. For x<0.40, the samples show essentially metal-like temperature dependence; but the temperature dependence changes from metallic to slightly semiconducting for x--0.50. However, the highest room-temperature resistivity is observed at x=0.20, even though the sample shows a metal-like temperature dependence.

Although the amount of insulating S r 4 T I 2 0 7 decreases as x increases from 0.0 to 0.20, the resistivity of samples increases by almost an order of magnitude. From x=0.20 to 0.50, the room temperature resistivity of samples decreases first, and then it saturates in the region 0.305.x.q0.50. The temperature dependence of resistivity also varies with increasing Bi content. For the nominal T1Sr2CuO5 sample, it shows metallic behavior; the resistivity decreases almost linearly as temperature decreases. As the Bi content increases, however, a non-linear dependence develops. This non-linear p-T dependence possibly implies a complex conduction mechanism, which cannot be direcdy explained by the change in the formal valences of Bi or copper in the compound.

2 3 4 S . L I , e t a l . Vol: 2 6 , r l o . 4

200

150

100

50

• . ° . ° ° • ° ° ° ° ° ° ' ° ° °

° • o ° •

° ° o o • °

. ° ° . • ° °

x--0.2 -" • . : : ' " x=0.5

. , ° , , ° , ° o , . . ° ° ° ° ° . . . . . . ° ° • ° • , • • ° ° o • ° ° ° ° o ° °

x--0.4 . . . . . . . . . . ". ~ S w * ~ • . ' ' ' z ~ m ~ ' °

x - - o ~ ' . ' " : : : . : " " ' " ' : " ": . . . . . ~ .'. ". • " . . . . . .

x--0.l , . ' " " • ' . " . . . . ° . .

x=0.0 • . . . . . . . . . . . . ° . . . . o ° . . . . o . • • • o ° . ° . . o ° ° . ' ° ° ° ° "

I I I I I

50 100 150 200 250 300

Temperature (K)

FIG. 4

Temperature Dependence of Resistivity of Tll.xBixSr2CuO5 with various values of x.

We have determined the Bi valence state by measuring the Bi-L3 x-ray absorption near-edge structure (XANES) (20). Bi ions in Tll.xBixSr2CuO 5 are mixed-valent (3+/5+), as evidenced by the 2p-6s transition in the XANES spectra. The magnetic susceptibility measurements on Tll_xBixSr2CuO5 show only weak Pauli paramagnetism in the temperature range 4-300K, which suggests that the Bi 6s electrons are either in the states of Bi3+/Bi 5÷ admixture or they are delocalized in the TI(Bi)-O layer. In the former case, the TI(Bi)-O layer might not be a conducting layer while the latter case would imply that the TI(Bi)-O layers in Tll_xBixSr2CuO 5 should be metallic. Now the question is whether there is an electronic correlation between the TI(Bi)-O and Cu-O layers. In the single T1-O layered copper oxides, the distance between the T.1-O and Cu-O layers is shorter (~4.5A) than that in the double TI-O layered counterparts (N4.8A); SO it is not unreasonable to assume an electronic interaction between the T1-O and Cu-O layers via the apical oxygens. In that case, the TI(Bi)-O layers would serve as a charge reservoir, which consists of the partially filled Bi-6s orbitals. The nature of this charge reservoir will depend on the valence state of Bi ions, which could alter the properties of the Cu-O layers.

In summary, the crystal structure of T10.5Bi0.5Sr2CuO5 was determined from the powder x-ray diffraction data using the Rietveld method. The structure of TI0.5Bi0.5Sr2CuO5 is analogous to the 1201-type thallium cuprate. The TI-O octahedra in this compound are less stretched in the a-b direction, and the Cu-O distances are shorter than the corresponding distances in other Tl-based cuprates. Although the formal copper valence in TI0.5Bi0.sSr2CuO5 is very high (+2.59), the structure is stabilized by Bi substitution for TI. A solid solution of Tll.xBixSr2CuO 5 was found in the range of 0.20~.x_<0.50. The physical properties of Tll.xBixSr2CuO 5 are likely related to the Bi valence state in a complex way.

VOI. 26, NO. 4 TI-BASED CUPRATES 235

Acknowledgments

We would like to thank Dr. K. V. Ramanujachary for helpful discussions. This work was supported in part by the Office of Naval Research and by National Science Foundation-Solid State Chemistry Program Grant DMR-87-14072.

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