The crystal structure of a new 84 K superconductor, Bi4Sr4CaCu3O14+x

PHYSICA

Physiea C 215 (1993) 371-374 North-Holland

The crystal structure o f a n e w 84 K superconductor, Bi4Sr4CaCu3014 + x Yu.F. Shepelev, A.A. Levin a n d Yu.I. Smolin Institute of Silicate Chemistry, Russian Academy of Sciences, 199155 Saint Petersburg, Russian Federation

A.A. Bush a n d B.N. R o m a n o v Institute of Radiotechnics, Electronics and Automatics, 117454 Moscow, Russian Federation

Received 27 November 1992 Revised manuscript received 30 June 1993

The crystal structure of a new superconductorin the Bi-Sr-Ca-Cu oxide systemof nominal compositionBi4Sr4CaCu3Oi4has been studied by single crystal X-ray diffraction. A modulation was observed alongthe b-axis. The structure of the fundamental unit cell has been determined in orthorhombic space group Pbmm, a=5.411 (2), b=5.417(3), c=27.75(1 ) A, Z=2. The final R-factoris 0.067 for 167 unique reflectionscollectedup to 20= 60* with AgKit radiation. The essential feature of the structure is the existence of two kinds of copper--oxygenlayers with square-pyramidal (CuO3-1ayer)and square-dipyramidal (CuO4-1ayer) oxygen coordinations of the Cu atoms. Two perovskite-like blocks [Sr-CuO3-Ca-CuO3-Sr]and [Sr-CuO4-Sr] are alternated by double [BiO]2layers. The bonds in the BiO planes form chains Bi-O-Bi extending alongthe b-axis.

1. Introduction The Bi-Sr-Ca-Cu oxide system has attracted much attention in recent years because some compounds of the system exhibit high-temperature superconductivity. Among the superconducting phases known in the system, the compound Bi2(Sr, Ca)aCu2Os+x showing Tc around 90 K has been studied extensively. It has a c-parameter of the orthorhombic unit cell of about 31 A. The oxygen coordination of the Cu cations is square-pyramidal [1 ]. The Bi2(Sr, Ca)4CuaOlo+x phase ( T¢~ 100 K, c ~ 37 A) has layers with square oxygen coordination of Cu atoms in addition to square-pyramidal conducting copperoxygen layers [2]. The compound Bi2Sr2CuOr+x shows the superconducting transition near 20 K. It contains copper-oxygen layers in which the coordination of the Cu cations is square-dipyramidal [ 3 ], the c-parameter of the orthorhombic cell is about 24 A. Recently, a new superconducting phase Bi4Sr4CaCuaOl4+x having Tc (zero) = 8 4 K and ATe=8 K has been discovered by Bush et al. [4]. The structure of the phase has not been determined

and the first problem of the structural study of this bismuth cuprate is the determination of the layer sequence along the c-axis and the identification of the oxygen coordination of Cu in the conducting layers. In this paper, we report the results of the first structural study of a new high-temperature superconductor of the B i - S r - C a - C u oxide system using X-ray single crystal data.

2. Experiment and results The crystals of Bi4Sr4CaCuaO 14+; (4413-phase ) used in our investigation were prepared by the noncrucible zone-melting method described in refs. [ 57 ]. The synthesis and electrical and magnetic properties of the crystals of the 4413-phase have been described in detail in refs. [4,7 ]. The crystals were of a plate-like form with maximum dimensions of up to 1.5×0.5x0.1 mm 3. However, X-ray diffraction showed that the large specimens contained several misoriented single-crystal blocks and many of them were slightly bent. After several attempts, a fairly

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Yu.F. Shepelev et al. / Crystal structure o f a new superconductor

good crystal with dimensions 0.5 X 0.16 × 0.028 m m 3 was selected for the diffraction experiment. The diffraction picture showed the main reflections defining the fundamental orthorhombic cell with parameters a = 5.411 (2), b = 5.417(3), c = 27.75 ( l ) A and satellites of the first and second order given by the incommensurate modulation vector 0.213 (4) b*, the modulation period being equal to 4.7 ( 1 ) b. It should be noted that the satellite reflections were significantly wider than the fundamental ones. The integrated intensities of the main reflections were measured using a three-circle automated diffractometer with graphite-monochromatized Ag Ka radiation (2=0.56083 A). A perpendicular-inclination technique was used. The a-axis of the crystal was oriented along the w-axis of the diffractometer. The w-step-scan mode was used with a scanning speed of 1.20 min -1 and a counter aperture of 20= 1.5 °. Only the unique reflections of one octant of reciprocal space were measured. The background was subtracted using the Oatley and French algorithm [ 8 ]. The observed intensities were corrected for Lorentz and polarization factors. Absorption corrections based on the crystal morphology were applied (/t= 325 c m - ~). 218 reflections with I > 4 a ( I ) were detected in the investigated part of reciprocal space. The profile analysis of the reflections showed that some of them were overlapping and such reflections were omitted. In total, we obtained 167 fundamental reflections. The structure was determined by direct methods using the crystallographic program AREN-90 [ 9 ] and the modified least-squares program ORFLS [10]. The atomic scattering factors for neutral atoms with dispersion correction [ 11 ] and Cruickshank's weighting scheme [ 12 ] were used. The positions of the Bi, Sr and Cu atoms were found from the E-map. The Ca atom was localized from subsequent Fourier synthesis. The oxygen atom sites were found using a combination of difference electron density syntheses with blocked full matrix least-squares refinements. The refinement of the anisotropic thermal parameters of the Bi atoms showed that their displacement ellipsoids were very elongated along the b-axis. It is obvious that the large displacements of Bi cations are due to the modulation of the structure. Therefore, we have split the Bi sites by displacing them from the mirror plane. This resulted in a significant drop of

the discrepancy factor R from 0.107 to 0.091. The subsequent difference synthesis showed that the oxygen atoms in the BiO layers were shifted from the special positions on the twofold axes and although the disordering of these atoms did not lead to a noticeably lower R value, we placed them in the general positions. The refinement of the structure with anisotropic thermal parameters for Bi and Cu atoms and isotropic ones for the others gave a final R value of 0.067 (Rw-- 0.072). The structure model without disordering of the Bi atoms gave a final R-value of 0.082 and Rw=0.088. Thus, in the fundamental unit cell, the splitting of the Bi sites can be used as a suitable approximation of modulation displacements of Bi atoms. Fitting of the site populations was also made. In order to avoid a great correlation between the thermal and population parameters, the site populations were refined using the low order reflections with (sin 0)/2 < 0.35 A-1. All populations showed only slight shifts during refinement, not more than twice the standard deviations. Therefore, we fixed the full populations for all atomic sites with the exception of the bismuth and oxygen sites of the BiO layers displaced from their special positions; half of the population was assigned to them. The results of the X-ray diffraction study of the Bi4Sr4CaCu30~4+x superconductor are presented in table 1. The scheme of the structure is shown in fig. 1. Table 2 lists the interatomic distances in the structure.

3. Discussion

The structure is built of two perovskite-like blocks alternating with a double [BiO ]2 layer. The first block includes one copper-oxygen layer. Strontium cations are situated just above and below the layer. The Cu 1 atom in the layer has four oxygen atoms in the plane z - - 0 with distances of 1.87 and 1.94 A and two apical oxygen atoms placed above and below the plane with longer distances of 2.42 kit. Therefore, the coordination of Cul cations in the first block is a square-dipyramid or an elongated octahedron. The Cu 1 octahedra are connected by vertices in the plane z--0. This layer is named CuO4 in fig. 1. The second perovskite-like block contains two conducting copper-oxygen layers connected by a

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Yu.F. Shepelev et al. / Crystal structure o f a new superconductor

Table 1 Positional (x, y, z ), temperature (B) parameters of atoms and populations of positions (P), sp.gr. Pbmm Atom

P

x

y

z

B (A2)

Bil Bi2 Srl Sr2 Cul Cu2 Ca O1 02 03 04 05 06 07 08

0.5 0.5 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 1. 0.5 0.5

0.247(2) 0.222 ( 1) 0.236(4) 0.231(3) 0.259(8) 0.255(5) 0.268(9) 0.5 0. 0.240(22) 0. 0.5 0.806(13) 0.599(26) 0.949(34)

0.703(2) 0.196 ( 1) 0.25 0.75 0.25 0.75 0.25 0. 0. 0.25 0. 0. 0.25 0.000(40) 0.055(32)

0.2796(2) 0.1626 (2) 0.3745(4) 0.0612(3) 0. 0.4375(5) 0.5 0. 0. 0.087(2) 0.438(4) 0.445(3) 0.347(2) 0.243(3) 0.200(4)

7.40 3.47 6.02 2.70 7.28 3.05 5.69 1.59 2.38 4.19 3.69 3.64 2.16 4.80 7.19

81 81 4k 4k 2e 4k 2f 2e 2a 4k 4g 4h 4k 81 81

Table 2 Interatomic metal--oxygendistances (A) in Bi4Sr4CaCuaOt4+x

CuO4 BiO o-

BiO

CuOa C~

::. t :o

CuOa S

r

BiO

Bil-O7 -06 -07 -07 -07 -08 --08

1.83 1.90 2.17 2.26 2.55 2.58 2.78

Bi2-O8 -08 -03 -08 -08 -07 --07

1.94 1.96 2.11 2.25 2.39 2.74 3.01

Srl-O6 -04 -05 -06 -06

2.45 2.57)<2 2.78)<2 2.83)<2 3.18

Sr2-O2 -O1 -03 -03 -03

2.51)<2 2.62)<2 2.65 2.80)<2 2.95

Cul-O1 1.88)<2 - 0 2 1.95)<2 --03 2.42)<2 Cu2-O5 1.91)<2 - 0 4 1.93)<2 -06 2.54 Ca-O5 2.39X4 - 0 4 2.62)<4

BiO /

tz

Sr Cu04

Fig. 1. Crystal structure of Bi4Sr4CaCu30~4+~. m i r r o r plane symmetry. The Cu2 cation in the layer is s u r r o u n d e d b y four oxygen a t o m s with distances o f 1.92 a n d 1.94 A and, in a d d i t i o n , there is a fifth contact C u 2 - O with a longer distance o f 2.54 A. Thus, the c o o r d i n a t i o n p o l y h e d r o n a r o u n d Cu2 is a square-pyramid. Each o f the oxygen a t o m s in the CuO plane is shared between two copper cations. The c o m p o s i t i o n o f such a c o p p e r - o x y g e n layer is CuO3. The Ca cations are placed between the CuO3 layers

in the m i r r o r plane. The slab [ C u O 3 - C a - C u O 3 ] is sandwiched by Sr cations. Thus, the 4413-phase structure contains two types o f perovskite-like blocks [ S r - C u O 4 - S r ] a n d [ S r C u O 3 - C a - C u O a - S r ] which exist separately in Bi2Sr2CuO6 a n d Bi2(Sr, Ca)3Cu2Os, respectively. The double [BiO]2 layer is inserted between the perovskite-like blocks. The adjacent [ BiO ] layers in the double layer are not related b y symmetry. W i t h i n the [BiO] layers, the Bi a t o m is b o n d e d to two oxygen a t o m s at distances o f 2.2 A. The oxygen a t o m s 0 7 a n d 0 8 in the BiO layers connect the Bi a t o m s in the chains extending along the b-axis. In addition, each o f the Bi a t o m s forms a b o n d with the apical oxygen a t o m o f the Cu p o l y h e d r o n with a distance

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Yu.F. Shepelev et al. / Crystal structure o f a new superconductor

o f about 2.0 A. The positions o f the 0 7 a n d 0 8 ato m s are displaced f r o m the Bi planes towards the adj a c e n t Bi plane a n d the 0 7 a n d 0 8 a t o m s form contacts with the Bi a t o m s o f the adjoining BiO layer with somewhat longer distances ( f r o m 2.57 to 2.78 A ) . It should be n o t e d that as the 0 7 a n d 0 8 a t o m s are disordered, the n u m b e r o f B i - O contacts presented in table 2 is twice that o f the actual value. Thus, a simplified structural m o d e l o f the new sup e r c o n d u c t o r Bi4Sr4CaCu3Ot4+x can be built from two kinds o f perovskite-like blocks [ S r - C u O a - S r ] a n d [ S r - C u O 3 - C a - C u O 3 - S r ] alternating with the double [BiO ] 2 layers. In the structure, there are two types o f CuO layers with the s q u a r e - p y r a m i d a l a n d s q u a r e - d i p y r a m i d a l c o o r d i n a t i o n s o f Cu cations. F u r t h e r structural investigation on the m o d u l a t e d lattice is in progress a n d will be r e p o r t e d separately.

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