Crystal structure of the superconducting infinite-chain cuprate Ba2Cu2.89O6−y

Physica C 311 Ž1999. 239–245

Crystal structure of the superconducting infinite-chain cuprate Ba 2 Cu 2.89 O6yy I.V. Rozhdestvenskaya, T.I. Ivanova ) , O.V. Frank-Kamenetskaya Department of Crystallography, St. Petersburg State UniÕersity, UniÕersity emb. 7 r 9, 199034 St. Petersburg, Russian Federation Received 9 September 1998

Abstract

˚. The orthorhombic structure of Ba 2 Cu 2.89 O6yy Žsp. gr. Pccm, a s 13.065Ž15., b s 20.654Ž21. and c s 11.431Ž8. A contains two nonintersecting symmetrically equivalent sets of parallel CuO 2-chains running along w110x and w110x directions. Along the c-axis the Cu–O sheets are separated by identical layers of Ba-atoms. The Cu–O chains are distorted within the xy plane producing zigzag. Three of 14 positions of copper atoms were found to be partly vacant that caused the significant shifts of the neighbouring Ba atoms in the plane. The determined structure of Ba 2 Cu 2.89 O6yy is compared to that of the monoclinic Ba 2 Cu 3 O6 modification, of another infinite-chain compound Sr0.73 CuO 2 , and to one of the substructures of ladder compounds ŽSr,Ca.14Cu 24O41qy . The superconductivity in Ba 2 Cu 2.89 O6yy is associated with the CuO 2-chains consisting of CuO4-squares sharing edges. The attention is paid to the coincidence of the temperatures of the superconducting transitions in Ba 2 Cu 2.89 O6yy and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr0.73 CuO 2 . q 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 61.66.-f; 61.10.-i; 74.70.-b Keywords: Infinite-chain cuprate; Crystal structure; Superconductivity

1. Introduction High-temperature copper oxide superconductors ŽSC. have a number of common structural features. A question to be resolved experimentally is which of these are intrinsically related to the appearance of SC state. The discovery of SC properties in ladder compounds w1x evidently showed that a perovskite-like crystal structure, where copper–oxygen planar )

Corresponding author. Tel.: q7-812-3289647; E-mail: [email protected]

squares are corner-shared to form infinite CuO 2planes, was not a necessary condition for the occurrence of superconductivity in the cuprates of alkaline-earth metals. The incommensurate composite structure of ladder compounds w2–4x consists of two unique fragments both based on copper–oxygen square planar units but with different mode of their linkage–infinite CuO 2-chains or Cu 2 O 3-planes containing two-legged ladders. It is still a matter of question, which of them is responsible for the transition to SC state occurring at Tc s 12–9 K and pressure 3–4.5 GPa w1x.

0921-4534r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 6 2 9 - 7

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

240

The recent observation of SC transition at Tc F 13 K in the orthorhombic phase of Ba 2 Cu 3yxO6yy w5,6x has prompted a possibility to assign the phase together with the ladder compounds to a new class of one-dimensional superconductors. The lattice constants of the orthorhombic phase when compared to those of monoclinic Ba 2 Cu 3 O6 modification w7x and 2q of the ladder compounds M 14 Cu 24 O41qy w2–4x 2q ŽM s Bi, Sr, Ca. suggested w5x the infinite CuO 2chains to be the main modules of the structure of the

superconducting Ba 2 Cu 3yxO6yy phase. This statement is in agreement with the idea of Karpinski et al. w8x proposed for the number of the compounds like Ca 0.83 CuO 2 , Ba 0.67 CuO 2 and Sr0.73 CuO 2 . The structure of the Sr-based infinite-chain compound with a sharp magnetic transition at 10 K has been recently determined w9x. In this paper, we report the results of the first detailed structure determination of the newly discovered w5,6x infinite-chain superconductor Ba 2 Cu 3yxO6yy .

Table 1 Refined atomic co-ordinates, temperature factors and occupancies in Ba 2 Cu 2.89 O6yy structure Atom

Position

xra

yrb

zrc

˚ Ueqa A

Occupation

Ba1 Ba2 Ba3 Ba4 Ba5 Ba6 Ba7 Ba8 Cu1 Cu2 Cu3 Cu4 Cu5 Cu6 Cu7 Cu8 Cu9 Cu10 Cu11 Cu12 Cu13 Cu14 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14

2g 2f 4k 4i 4l 4j 8r 8r 2a 4q 4q 4q 4q 4q 4q 4q 4q 4q 4q 4q 4q 4q 4n 8r 8r 8r 8r 8r 8r 8r 8r 8r 8r 8r 8r 8r

0 1r2 0 0.1623Ž7. 1r2 0.3283Ž6. 0.1654Ž7. 0.3348Ž8. 0 0.297Ž2. 0.209Ž2. 0.085Ž2. 0.588Ž4. 0.475Ž2. 0.382Ž1. 0.258Ž1. 0.096Ž2. 0.223Ž2. 0.677Ž1. 0.563Ž1. 0.958Ž2. 0.853Ž1. 1r2 0.248Ž6. 0.1426 0.0170 0.3479 0.5441 0.430Ž5. 0.3203 0.176Ž3. 0.992Ž2. 0.832Ž8. 0.710Ž4. 0.654Ž9. 0.890Ž4.

1r2 0 0.1720Ž4. 0 0.3343Ž4. 1r2 0.3319Ž5. 0.8344Ž4. 0 0.0229Ž9. 0.146Ž1. 0.354Ž2. 0.078Ž2. 0.1825Ž9. 0.3030Ž7. 0.4053Ž9. 0.891Ž1. 0.788Ž1. 0.335Ž1. 0.447Ž1. 0.368Ž1. 0.4873Ž7. 1r2 0.076Ž3. 0.8013 0.3125 0.9691 0.1400 0.752Ž3. 0.3527 0.452Ž2. 0.930Ž9. 0.157Ž4. 0.268Ž2. 0.393Ž5. 0.425Ž3.

1r4 1r4 1r4 1r4 1r4 1r4 0.2448Ž7. 0.2522Ž8. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.114 0.112Ž5. 0.113 0.113 0.113 0.113 0.110Ž5. 0.113 0.112Ž3. 0.115Ž2. 0.116Ž7. 0.111Ž4. 0.115Ž1. 0.113Ž5.

1.6Ž2. 1.8Ž3. 1.3Ž2. 1.7Ž2. 1.6Ž1. 1.1Ž1. 1.7Ž1. 2.3Ž1. 3.3 3.4Ž7. 3.4Ž7. 1.9Ž4. 4.2Ž9. 2.0Ž5. 1.0Ž3. 0.4Ž3. 2.6Ž6. 2.3Ž5. 1.7Ž4. 1.7Ž4. 1.9Ž4. 1.0Ž3. 2.3Ž2. b 1.0Ž9. b 4.0Ž2. b 6.3Ž3. b 3.9Ž2. b 5.7Ž8. b 1.1Ž1. b 3.9Ž3. b 0.9Ž1. b 3.2Ž2. b 2.6Ž2. b 1.3Ž1. b 1.0Ž1. b 0.6Ž9. b

1 1 1 1 1 1 1 1 0.66Ž13. 1 1 1 0.85Ž9. 1 1 0.84Ž6. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

a b

Ueq s 1r3wU11 a ) 2 a2 q . . . q2U23 b ) c ) bccos a x. Oxygen atoms were refined in the isotropic approximation.

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

2. Experimental details One of those Ba 2 Cu 3yxO6yy single crystals earlier described and studied in Ref. w5x, which were obtained by high pressure flux crystallisation and had the cation composition of Ba 2 Cu 2.62 – 2.90 , was chosen for the structural study. The X-ray diffraction data for the plate-like crystal with the dimensions of 0.4 = 0.4 = 0.025 mm3 and Tc s 13 K w5x have been collected by v-method in the four-circle autodiffractometer ‘Syntex P2 1’ Žgraphite monochromator, MoK a-radiation.. The intensities of 1372 reflections with I ) 2 s 1 and Žsin url. max s 1.075 were measured in 1r4 of reciprocal space. The systematic extinctions of the 0kl and h0l reflections with l s 2 n y 1 Žin spite of 6 weak peaks violating the conditions. pointed out the orthorhombic lattice with two most probable space groups—Pccm or Pcc2. The following unit cell constants were determined by the least-square method from 24 reflections: a s ˚ 13.065Ž15., b s 20.654Ž21. and c s 11.431Ž8. A; Z s 18; m s 211.1 cmy1 , rcalc s 5.44 grcm3. All the calculations including the absorption correction for crystal shape were performed using the program CSD w10x. The structure was solved in the centrosymmetric sp. gr. Pccm by the direct methods. Not only the metal atoms but almost all the oxygen ones were located at once from the E-map. The least-square refinement of the model using 782 unique reflections converged to the final R-factor: R F s 0.063. The thermal parameters of the cations were refined in the anisotropic approximation whereas the isotropic approximation was used for the refinement of the oxygen atoms. The positions of six oxygen atoms were taken directly from the Fourier difference maps as unrefinable in the presence of heavy atoms of barium. The atomic parameters and the interatomic distances in the final structural model are listed in Tables 1 and 2, respectively.

3. Results and discussion The orthorhombic cell of Ba 2 Cu 2.89 O6yy contains two symmetrically equivalent layers of parallel CuO 2-chains separated along the c-axis by the identical layers of Ba-atoms ŽFig. 1a.. The chains in the adjacent Cu–O layers run along w110x and w110x

241

Table 2 ˚ . in Ba 2 Cu 2.89 O6yy structure Selected bond distances ŽA Ba-polyhedra

Cu-polyhedra

O14

=4

2.67Ž6.

Cu1

O10

=4

1.98Ž2.

O9

=4

2.97Ž4.

Cu2

O2 O5

=2 =2

1.81Ž6. 1.83

Ba2

O5 O6

=4 =4

2.61 3.34

Cu3

O3 O2

=2 =2

1.89 2.00Ž6.

Ba3

O3 O10

=2 =2

2.50 2.60Ž2.

Cu4

O3 O4

=2 =2

1.88 1.98

O11 O4

=2 =2

2.72Ž1. 3.31

Cu5

O5 O6

=2 =2

1.82 1.91

O2 O10

=2 =2

2.49Ž6. 2.90Ž2.

Cu6

O6 O7

=2 =2

1.81 1.93Ž7.

O5 O10

=2 =2

2.96 3.05Ž2.

Cu7

O7 O8

=2 =2

1.81Ž7. 1.83

O7 O13

=2 =2

2.57Ž7. 2.80Ž1.

Cu8

O8 O9

=2 =2

1.87 1.91Ž4.

O8 O12

=2 =2

2.85 3.31Ž2.

Cu9

O11 O10

=2 =2

1.86Ž9. 1.95Ž2.

O13 O1

=2 =2

2.70Ž1. 2.73

Cu10

O11 O12

=2 =2

1.86Ž9. 1.89Ž5.

O9 O8

=2 =2

2.73Ž4. 3.37Ž2.

Cu11

O13 O12

=2 =2

1.81Ž1. 1.90Ž5.

O4 O8

=1 =1

2.49 2.56

Cu12

O1 O13

=2 =2

1.89 2.10Ž1.

O14 O12

=1 =1

2.66Ž6. 2.69Ž5.

Cu13

O4 O14

=2 =2

1.89 1.93Ž6.

O4 O9 O13 O3

=1 =1 =1 =1

2.91 2.93Ž4. 3.11Ž1. 3.15

Cu14

O9 O14

=2 =2

1.81Ž4. 1.84Ž6.

O6 O7 O2 O11 O12 O3 O6 O5

=1 =1 =1 =1 =1 =1 =1 =1

2.31 2.63Ž7. 2.66Ž6. 2.72Ž1. 2.75Ž5. 3.02 3.18 3.21

Ba1

Ba4

Ba5

Ba6

Ba7

Ba8

directions with the angle of 115.48 between them. Each layer contains two unique chains separated by ˚ They are shifted by approxithe distance of 3.83 A.

242

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

Fig. 1. The polyhedral representation of the orthorhombic structures of the infinite-chain compounds: Ba 2 Cu 2.89 O6yy Ža. and Sr0.73 CuO 2 Žb..

mately half of Cu–Cu distance in the chain direction ŽFig. 2a. and alternate to form a pseudo-hexagonal motif. The chains are formed by the distorted copper–oxygen squares CuO4 sharing edges parallel to ˚ ŽTable w001x. The average Cu–O distance is 1.89 A ˚ 2.. The average O–O distance is 2.57 A in the w001x ˚ in the w110x direction. The direction and 2.75 A average O–Cu–O angles in the polyhedra are 86.1 and 93.98. Three of 14 unique positions of copper atoms in the structure of Ba 2 Cu 2.89 O6yy were found to be partly vacant with the occupancies ranging from 0.85 to 0.66 Žsee Cu1, Cu5 and Cu8 in Table 1.. This gives the sum of vacancies of ; 0.3 at. per chain fragment consisting of 9 copper atoms ŽFig. 2a. and the Ba:Cu ratio Ž0.69. close to that determined by the electron probe microanalysis w5,6x. The distances between the partly occupied copper sites and those ˚ . than the fully occupied are shorter Ž2.57–2.66 A ˚ Ž . Ž . others 2.71–2.87 A Table 2 . Each Cu–O chain is distorted within the xy plane producing zigzag ŽFig. 2a.. The Cu–Cu–Cu angles range from 168 to 1808 with the average value of 173.98. It should be mentioned that the zigzag angles at the partly vacant

copper sites ŽCu1, Cu5 and Cu8. are closer or equal to 1808 on the contrary to those fully occupied. Ba atoms located at the vertices of rhombi of side ˚ are surrounded by eight oxygen atoms each. 4.07 A The polyhedra of Ba atoms are the distorted orthorhombic antiprisms with four shorter Ž2.31–2.75 ˚ . and four longer Ž2.80–3.37 A˚ . Ba–O distances A ŽTable 2, Fig. 3.. Three of eight Ba atoms are significantly shifted in the xy plane from their ideal positions distorting two rhombi adjacent to partly vacant copper sites ŽFig. 2a.. The Ba rhombus around the most vacant Cu1 site is elongated in w010x direc˚ . but is shortened in w100x tion Ž7.08 instead of 6.88 A ˚ .. The Ba6 atoms direction Ž4.23 instead of 4.35 A are sifted toward partly vacant Cu8 sites in w100x ˚ direction. The Ba6–Ba6 distance increases to 4.51 A in this direction. Similar infinite CuO 2-chains were found in the structures of Ba 2 Cu 3 O6 monoclinic modification w7x, of another infinite-chain compound Sr0.73 CuO 2 w9x and of the ladder compounds M 14 Cu 24 O41qy w2–4x ŽTable 3.. On the contrary to the Ba 2 Cu 2.89 O6yy , in the structures of all the compounds just listed the chains propagate along the coordinate axis Žnamely,

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

243

Fig. 2. The arrangement of the cations in the number of the infinite-chain structures projected onto Ž001. plane. The atoms locating at different levels are depicted in different shades of grey. Filled circles represent fully occupied copper sites, half-filled circles represent partly vacant sites. Ža. orthorhombic Ba 2 Cu 2.89 O6yy . Figures are the numbers of copper atoms in Table 1. Arrows show the displacements of Ba atoms in the plane. Žb. Monoclinic Ba 2 Cu 3 O6 . Žc. Sr0.73 CuO 2 .

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

244

than those in the structure of Ba 2 Cu 3 O6 monoclinic ˚ . ŽTable 3.. Besides, CuO4modification Ž2.82 A squares, which are significantly distorted in all the orthorhombic infinite-chain structures, have similar O–Cu–O angles. Similar to the Ba 2 Cu 2.89 O6yy structure described above, the adjacent chains are staggered ŽFig. 2a,c. in the Cu–O layers of Sr0.73 CuO 2 and M 14 Cu 24 O41qy structures. This is distinct from the monoclinic structure, where the adjacent Cu-chains are not shifted in the chain direction arranging the rectangular network of copper atoms ŽFig. 2b.. In both the Ba 2 Cu 2.89 O6yy and Sr0.73 CuO 2 structures the chains are corrugated in the xy plane ŽFig. 2a,c. that originates from the mismatch between the networks of the copper and alkaline-earth metal. The variations of the Cu–Cu and M–M distances ŽTable 3. denote the adaptation of the networks to each other. As a result, the pseudo-commensurate supercell appears w9x putting together 15 copper subcells and 11 strontium ones ˚ per 41 A˚ along the with the difference of 0.78 A a-axis in the structure of Sr0.73 CuO 2 . The mismatch between copper and barium networks in the structure of Ba 2 Cu 2.89 O6yy is by order of magnitude lower

Fig. 3. The layer of barium polyhedra in the structure of Ba 2 Cu 2.89 O6yy .

the a-axis., setting the corresponding unit cell constants as the multiplies of the distance between the neighbouring copper atoms along the chains. The average values of the Cu–Cu distance in the structures of the phases showing SC or magnetic transi˚ in the ladder compounds tion at 10–13 K Ž2.75 A ˚ in the orthorhombic Ba 2 Cu 2.89 O6yy w2–4x and 2.72 A ˚ in Sr0.73 CuO 2 w9x. are considerably shorter and 2.74 A

Table 3 The comparison between the crystal structures of orthorhombic and monoclinic modifications of Ba 2 Cu 3yx O6yy , of the infinite-chain compound Sr0.73 CuO 2 and of the CuO 2 subcell of the ladder compound ŽSr,Ca.14 Cu 24 O41qy Ba 2 Cu 3 O6 w7x

Sr0.73 CuO 2 w9x

CuO 2 w4x

Pccm

P2

13.065Ž15. 20.654Ž21.

8.480

Cmcm 40.968Ž9. 6.823Ž1.

Fmmm 2.753Ž1. a 12.928Ž3.

11.431Ž8.

12.154 110.4

11.026Ž2.

11.376Ž2.

˚ 3. V ŽA Z rcalc Žgrcm3 . Sample R-factor

3084.6 18 5.44 Single crystal 0.063

708.1 4 5.30 Powder 0.175

3082.0 60 5.19 Single crystal 0.33

404.9Ž3. 4 – Single crystal 0.097

˚. Distances ŽA Cu–Cu along the chain Between the chains Cu–O M 2q–O M 2q–M 2q O–Cu–O angle Ždeg.

2.57–2.87 3.83 1.81–2.10 2.31–3.37 4.02–4.12 86.1 and 93.9

2.82 3.66 2.00 2.36–3.34 4.23 90

2.68–2.77 3.41 1.863–1.935 2.38–3.50 3.65–3.76 86.5 and 93.5

2.75 6.46 1.89 –

Ba 2 Cu 2.89 O6yy Žour data. Space group ˚. a ŽA

˚. b ŽA ˚ Ž c A. b Ždeg.

a

For ease of comparison the a and c axes have been exchanged.

7.330

86.3 and 93.7

I.V. RozhdestÕenskaya et al.r Physica C 311 (1999) 239–245

˚ per the w110x diagonal of 24.5 A˚ in length. Ž0.064 A that allows them to co-exist within the commensurate cell with 6 atoms of barium fitting to 9 copper sites along a chain. The primary difference between the structures of different infinite-chain compounds appears to be in the mode of stacking of the Cu–O layers along c-axis. They are shifted in the w100x direction by 1r4 of Cu–Cu distance Žin the monoclinic cell of Ba 2 Cu 3 O6 . or by 1r2 of it Žin Sr0.73 CuO 2 and in the CuO 2-substructure of ladder compounds. but are rotated by 115.48 in the orthorhombic cell of Ba 2 Cu 2.89 O6yy . This defines the differences in the symmetry of their crystal structures but suggests the ˚ . normal to the similarity of the translations Ž; 11 A layers ŽTable 3.. We now consider Ba 2 Cu 2.89 O6yy and Sr0.73 CuO 2 as the compounds with general form ula M 2 Cu 3yxO6yy , whose orthorhombic structures have different mode of stacking of Cu–O layers separated by the identical layers of M 2q atoms ŽBa or Sr, respectively. ŽFig. 1.. The neighbouring Cu–O layers Žthose with Cu–O chains going along w110x direction and those with the chains going along w110x. in the structure of Ba-based infinite-chain compound can be mapped onto one another by the rotation around twofold axis going along either x- or y-axis in the plane of Ba atoms. Disappearance of any of these symmetry elements results in the advent of the new translation T s cr2 in the structure Žsince the neighbouring Cu–O layers become identical. but leaves unchanged the positions of Ba atoms, their co-ordination number and Ba–O distances. The same is true for the structure of the Sr-based compound, if the shift of the adjacent Cu–O layers in the w100x direction does not occur. This gives a principal possibility to expect a certain degree of disorder due to the random disappearance of twofold axes or shifts while stacking the Cu–O sheets along the c-axis and, therefore, allows the orthorhombic infinite-chain compounds to fall into the class of ODstructures w11x. Thus, it has been shown that CuO 2-chains formed by copper–oxygen squares sharing edges are responsible for the superconductivity occurring in Ba 2 Cu 2.89 O6yy single crystals. Now it is a matter of doubt whether superconductivity in Sr0.4 Ca 13.6 -

245

Cu 24 O41qy w1x is associated with spin ladders. The coincidence of the temperatures of the superconducting transitions in Ba 2 Cu 2.89 O6yy and in ladder compound with the temperature of the sharp magnetic transition in another infinite-chain compound Sr0.73 CuO 2 is intriguing. The role of the vacancies at the number of copper sites deforming the Ba-rhombi in the structure of Ba 2 Cu 2.89 O6yy is not clarified yet.

Acknowledgements The work was supported by INTAS Žproject No. 94-2007. and the fund of Russian Council of Ministers Žproject ‘Poisk’.. The authors are grateful to S.V. Moshkin who kindly placed the sample at our disposal, to S.I. Goloshchapov for the study of its SC-properties and to Yu.L. Kretzer who has performed the electron probe microanalysis of the sample.

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