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Structure of the high-temperature superconductor Ba2YCu3O7 by X-ray and neutron powder diffraction
0038-1098/87 $3.00 4- .00 © 1987 Pergamon Journals Ltd
Solid State Communications, Vol. 63, No. 12, pp 1149-1153, 1987. Pnnted in Great Bntaln
STRUCTURE OF THE H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R Ba2YCu307 BY X-RAY AND N E U T R O N POWDER D I F F R A C T I O N M Franqols, E Walker*, J -L. Jorda* and K. Yvon Laboratolre de Cnstallographie aux Rayons-X, 24 quai Ernest Ansermet, Unlversit6 tie Gen6ve, CH-1211 Gen6ve, Switzerland and P. Fxscher Labor fiar Neutronenstreuung, ETHZ, 5303 Wtirenhngen, Switzerland (Received 28 Aprd 1987 by P Wachter) The average structure at 8 K of Ba2YCu307 (critical temperature Tc = 90 K) was refined by the Rietveld method (space group Pmmm, a = 3817(2)A, b = 3883(2)A, c = 11.633(7)A, Rwp = 0.11, 28 parameters, T = 8 K). It contains two Cu s~tes of which one has square-p~(ramldal ([Cu--O] = 2 × 1.925(1)t~, 2 x 1 959(1)/~ (base), 2 285(6) A (apex)) and the other square-planar ([Cu-O] = 2 x 1.850(5)fl(, 2 × 1 942(1)A) oxygen coordination. The Cu-O bonds are connected to quasi two-dimensional networks forming 10 A thick structural slabs parallel to the ab plane Large displacement amplitudes of one oxygen atom along [1 0 0] ((u 2) = 0.032(5)A ~) and associated short repulswe oxygen-oxygen contacts [0-(3] = 2 682(4) A) suggest a low-temperature structural phase transmon. The average structure can be described as an mtergrowth of tetragonal La2SrCu206 type structure slabs with Cu-O chains INTRODUCTION RECENTLY AN OXO-CUPRATE of approximate composition Ba2YCu309_ ~ having superconductive transmon temperatures of up to Tc = 93 K was reported [1, 2]. The compound was further characterized with respect to composmon and properties [3, 4] As to its crystal structure X-ray powder diffraction [3, 5] and electron diffraction data [5] suggested orthorhomblc symmetry and an atom arrangement which was described [3] as one which derives from the cubic peroskite type by an ordered substitution of the large caUons and removal of two (out of nine) oxygen atoms The resulting threefold superstructure (aor ~ ac, bor ~- at, Cot "~ 3-ac)wassupposed[6]to be related to the tetragonal La2 SrCu206 structure type [7] The purpose of this work was to determine the structure of Ba2YCu3Og_~ m detail by using both X-ray and neutron powder diffraction analysis, and to clarify its relation to La2 SrCu2 06. Preliminary results were reported elsewhere [4] While this work was m progress we have received preprlnts of two mdepen-
* D6partement de Physique de la Mati6re Condens6e
dent neutron powder diffraction studies Ba2YCu307.
[8, 9] on
EXPERIMENTAL Samples of nominal composmon Ba2YCu307 were prepared by the smtenng method described elsewhere [4]. X-ray powder diffraction patterns (Gumler-Nomus camera, T = 298 K, CuK0t radiation, internal standard Si) could be indexed on orthorhomblc cells similar to those reported [3, 5) previously The lattice memc derived from these patterns was such that the long cell axis (c) was almost always exactly three times that of one of the two short axes (b). Thus many (non-equwalent) reflections coincided m these diffraction patterns. Only one pattern showed significant hne splitting whxch indicated that the axial ratio c/b m the other patterns actually differed from three The absence of systematic extractions suggested one of the following space groups" Pmmm, Pmm2, or P222 Intensity data were obtained from densitometer scans and subsequently analysed by the Rletveld method. A sample of 10 g was used for neutron diffracUon studies Its superconducting transition temperature was T, = 90 K It was placed m a cyhndncal vanadium container (O = 7ram) and
1149
1150
H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R Ba2YCu307 Ba2YCu309-x 8 K Pmmm .,~. Wo.,i,.g,. i r l ~ I
-0
--2
-
e=
4
o
6.
E --B
I
o= 4
0 -
,,
,
,m 20
, 40
60 2-Thefa
80
1(90
120
~40
(Degrees)
Fig 1. Absorption corrected observed and calculated (bottom) and difference (top) neutron diffraction pattern of orthorhomblc Ba2YCu307 at 8K (2 = I 713A). measured on the multldetector powder dlffractometer D M C of the reactor at Wurenhngen (CH) at three d]fferent temperatures 295, 120, 8 K (wavelength 2 = 1 713 A) The pattern at 8 K is presented in Fig 1 Except for systematic hne shifts and intensity differences due to thermal motion effects the patterns recorded at higher temperatures (not represented here) were practically identical to that at 8 K RESULTS AND DISCUSSION The structure refinements on the X-ray data confirmed a perovsk~te hke metal arrangement The two Ba atoms and the Y atom were distributed in an ordered manner on sites 2t (½, ~,1 + z; .~ ~_ 1/6) and 1 h (½, ½, ½), respectively, m centrosymmetrlc space group Pmrnm, whereas the three Cu atoms were placed on sites la (0, 0, 0) and 2q (0, 0, z, z - 1/3) A prehmmary analysis showed that the Ba and Cu atoms on the twofold positions 2t and 2q were displaced along the long cell axis by about 0 21 A (Ba) and 0 26 A (Cu) with respect to their ideal positions in the cubic prototype The oxygen positions were located from analysis of the neutron data For this purpose various models of composition Ba2YCu307 were constructed by assuming a threefold perovsk~te type superstructure from which two oxygen atoms were removed The model which gave the best fit was that m which the Cu atoms were fourfold and fivefold coordinated by O atoms (see below) That model was also adopted by other workers [8, 9] Results of the refinement on the 8 K data are summarized in Table 1 (number or reflections 170, number of parameters refined 19 (atomic), 4 (profile), 3 (latuce), 2 (zeropolnt, scale), R~e = 0 11, R~ = 0 053, R~,o = 0 055, Z2 = 3 84) A l l s t o f l n t e r a tom~c distances is gwen m Table 2 Except for the
Vol. 63, No 12
latUce parameters and some temperature factors (see Table 1) the results of the refinements on the 120 and 295 K data d]d not differ slgmficantly from those on the 8 K data Refinements of the occupancy factors set an upper hmlt of 5 % for possible oxygen defects No evidence for additional oxygen atoms in the structure was found The structure of Ba2Yfu307 is shown in Fig. 2 It contains no octahedral CuOo bulldmgblocks and thus can no longer be considered as a perovsk]te derivative The two Cu sites have square-pyramidal (Cu(2)) and square-planar (Cu(1)) oxygen coordmatlon All C u - O bonds are shorter than 2 0 A except that which is directed toward the apex of the pyramidal umt ([Cu(2)-O(l)] = 2 285(6)A) Square-pyramidal CuO5 units with short base bonds and long apex bonds also occur in La2SrCu206 ([Cu-O] = 1 937(3)A (base), 2.27(4) A (apex) 7) In Ba2YCu307 the shortest C u - O bond ~s that in the square-planar CuO4 umt ([Cu(1)-O(1)] = 1850(5)A) which connects two square-pyramidal units along the long cell axis (Fig 2) This bond is shorter than those observed [10] in the layer-perovsklte superconductor La185Sr015CuO4 (7, = 35K) which contains a square-planar coordinated Cu site ([Cu-O] = 1 89 A) with two further hgands at octahedral apices ([Cu-O] = 2 43 A) Altogether the C u - O bonds in Ba2YCuaO7 are connected to a quas~ two-dimensional network which forms 10 A thick structural slabs parallel to the ab plane The slabs contain Ba atoms and are separated along c by Y atoms (Fig 2) This topology and in particular the interruption of the C u - O bond network along the c direction conveys to the structure strong anlsotroplc
YL °(2)- I R;~- ) °
--
) 1 ,
]
F]g 2 Structural slabs and C u - O bond network m Ba2YCu3 07 Arrows point towards nearest Cu nelghbours
Vol 63, No 12
1151
HIGH-TEMPERATURE SUPERCONDUCTOR Ba2YCu307
Table 1 Structural parameters of Baz YCu307 Space group" Pmmm (No 47) cell parameters(A)
8K
120 K
295 K
a = 3.817(2) b = 3 883(2) c = 11 633(7)
3 818(8) 3.883(2) 11.643(7)
3 824(2) 3 888(2) 11.678(7)
atom parameters* at 8 K x
y 1 1
Ba Y Cu(1) Cu(2) O(1)
2t lh la 2q 2q
~1 ~1 0 0 0
0 0 0
0(2)
2s
½
o
0(3) 0(4)
2r le
0 0
z 0 1851(4) 2
0 0 3554(3) 0 1590(4) 0 3767(4) 0 3782(4) 0
1 1
Amsotroplc thermal parameters of 0(4) 8K B,, (A 2) 2 6(5)
B22(A2)
1.5(5)
B33 (/~ 2 )
1.3(6)
occupacy 1.0(-) 1.0(-) 1 0(-) 1 0(-) 1 06(2) 0 97(1) 0.99(2) 0 96(3)
B,so(~2) 0 18(9) 0 09(8) 0.004(95) 0 20(6) 0 7(1) 0.06(11) 0 07(9) 1 9(2)
120 K
295 K
2 5(6) 1.5(5) 2 0(7)
3 3(6) 1 6(5) 3.0(8)
form of temperature factors lsotroplc exp { - B,sosm20/22 } amsotroplc exp {-h2a*ZB. + k2b*2B22 + 12c'2B33)} * description as m [8], different from that m [9] with respect to O(1) and 0(4) which are interchanged
Table 2 Interatomzc dtstances (t~) m Baz YCu3 07 at 8 K Ba
- 4 2 2 2
O(1) 0(4) 0(3) 0(2)
2.739(1) 2 877(4) 2.948(6) 2.956(5)
O(1)
-
Cu(1) Cu(2) 4 Ba 2 0(4)
1 850(5) 2.285(6) 2 739(1) 2.682(4)
Y
- 4 0(3) 4 0(2)
2 377(3) 2.414(3)
O(2)
- 2 Cu(2) Y 2 Ba
1.925(1) 2 414(3) 2 956(5)
Cu(1)
2 O(1) 2 0(4)
1.850(5) 1 942(1)
4 0(3) 2 0(2)
2 723(1) 2 869(7)
Cu(2)
2 O(2) 2 0(3) O(1)
1 925(1) 1.959(1) 2 285(6)
0(3)
- 2 2 2 4
Cu(2) Y Ba 0(2)
0(3) 0(4)
- 2 Cu(1) 4 Ba 4 O(1)
1.959(1) 2 377(3) 2 948(6) 2 723(1) 2.834(7) 1 942(1) 2 877(4) 2 682(4)
HIGH-TEMPERATURE SUPERCONDUCTOR Ba2YCu307
1152
I" o/1',,.o.,¢.,.o/I,,,o'
oVS,uS,Uo. o o o J o
0
i
o
.'1o
o
o
o
0,7 2 I ,,,A,A,," 0000 Z:O 0
0
0
0
q G
b
Lo2SrCu206 Fig 3 Structural relation between tetragonal La~SrCu206 (a) and orthorhombic Ba2YCu307 (b). Large circles La, Sr, Ba, small circles Y, full lines square-pyramidal CuOs units, broken lines squareplanar CuO4 units. character This feature could be of importance for the understanding of certain superconducting properties such as the upper critical fields The Ba-O and Y-O bond lengths are consistent with those in the corresponding binary and ternary oxides The Ba atom has 10 nearest nelghbours with distances in the range [Ba-O] = 2 74 - 2 96 A The Y atom is surrounded by 8 nearest neighbours with significantly shorter distances of [Y-O] = 2.377(3) A (4 × ) and 2 414(3) A (4 x ), as expected from ItS smaller ionic size The shortest metal-metal contacts in the structure are those between Y and Cu atoms ([Y-Cu(2)] = 3 20A) A structural detail of possible importance are the relatively large displacement amplitudes of the 0(4) atom along the [1 0 0] and [0 0 1] directions (Table 1) Those along [l 0 0] do not much change as a function of temperature Since that atom shows a rather short contact distance with another oxygen atom ([O(4)-O(1)] -- 2 682(4)A), it is likely that 0(4) is statically displaced along [1 0 0] This is the direction of empty space in the structure The displacements could for example originate from the very short Cu(1)-O(l) bonds which tend to increase the dosedshell repulsive interactions between the 0(4) and O(1) atoms (see Fig 2) Consequently the P m m m model used in this work (and in other works [8, 9]) describes only an average structure The true symmetry of BazYCu307 at 8 K ~s probably lower than P m m m (or its cell volume larger), and its structure presumably transforms into the modlficaUon with P m m m sym-
Vol. 63, No. 12
metry only above that temperature. The anomalous behavlour of 0(4) (called O(1) in [9]) also appears from the neutron dxffractlon work reported in [8, 9] and warrants further investigations. Notice that removal of that oxygen atom from the structure leads to a compound of composlt10n Ba2YCuaO6 which is expected to have tetragonal symmetry (atetr "~ aor, C,etr -- Cor) and to contain square-planar and linear coordinated Cu atoms A tetragonal high-temperature modification of composition Ba2YCu307_6 was recently found to exist above 1023 K [11]. Finally the orthorhomblc structure of Ba2YCu307 can be derived from the tetragonal La2 SrCu2 06 structure type as follows The latter is cut into structural segments of composition Ba2YCu206 as shown in Fig 3, and these segments are &splaced along [1 1 0], and linked together along [0 0 1] via intercalated hnear-Cu-O-Cu-O-strIngs which run along [1 0 0] The role of these Cu-O strings for superconductivity is a matter of speculation Acknowledgements - - We thank Professors J Muller
(Geneva), Hk Muller-Buschbaum (Klel) and S Kemmler-Sack (Ttiblngen) for useful discussions, correspondence and exchange of information The help of Mrs B Kunzler with the drawings is gratefully acknowledged This work was supported by the Swiss National Science Foundation REFERENCES 1
2. 3
4
5 6 7 8
Z Zhao, L. Chen, Q. Yang, Y Huang, G Chen, R Tang, G LlU, Y NI, C CuI, L Chen, L Wang, S Guo, S. L1, J B I & C Wang, to be published in Kexue Tongbao, 6 (1987) M K Wu, J R Ashburn, C J Torng, P H Hor, R L Meng, L Gao, Z J Huang, Y.Q Wang & C W Chu, Phys Rev Left. 58, 908 (1987) R J Cava, B Batlogg, R.B. van Dover, D W Murphy, S Sunshine, T Slegrlst, J P Remelka, E A Rletman, S Zahurak & G P Espinosa, preprlnt (1987) A Juno& A Bezlnge, T Graf, J L Jorda, J Muller, L Antognazza, D Cattanl, J Cors, M Decroux, O Fischer, M Banovskl, P Genoud, L Hoffman, A A Manuel, M Peter, E Walker, M FranqoIs, & K Yvon, Europhys Lett (1987), submitted Y Syono, M Klkuchi, K Oh-lshl, K Hlraga, H Aral, Y Matsul, N Kobayashl, T Sasaoka & Y Muto, preprint (1987) H Takagi, S Uchlda, H Ishli, H Sato, K Klshlo, K Kltazawa, K Fuekl & S Tanaka, preprlnt (1987) N Nguyen, L Er-Rakho, C Michel, J Cholsnet & B Raveau, Mat Res Bull 15, 891 (1980) J J Capponl, C Challlout, A W Hewat, P Lejay, M Marezlo, N Nguyen, B Raveau, J L
V01.63, No. 12
HIGH-TEMPERATURE SUPERCONDUCTOR Ba2YCu307
Soubeyroux, J.L. Tolence & R. Tournier, Europhys. Left. (1987) to be published. . M A Beno, L. Soderholm, D.W Capone II, D G. Hmks, J.D. Jorgensen, I K Shuller, C.U Segre, K. Zhang & J.D. Grace, Appl. Phys. Lett, (1987) submitted
10.
11
1153
M Decroux, A. Junod, A. Bezinge, D. Cattam, J. Cors, J.L. Jorda, A Stettler, M Franqols, K Yvon, O Fischer & J Muller, Europhys Lett 3, 1035 (1987) I K. Schuller, D.G. Hmks, M.A Beno, D.W Capone II, L. Soderholm, J.-P Locquet, Y Bruynseraede, C.U. Segre & K Zhang, Sohd State Commun 63, 385 (1987)
Solid State Communications, Vol. 63, No. 12, pp 1149-1153, 1987. Pnnted in Great Bntaln
STRUCTURE OF THE H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R Ba2YCu307 BY X-RAY AND N E U T R O N POWDER D I F F R A C T I O N M Franqols, E Walker*, J -L. Jorda* and K. Yvon Laboratolre de Cnstallographie aux Rayons-X, 24 quai Ernest Ansermet, Unlversit6 tie Gen6ve, CH-1211 Gen6ve, Switzerland and P. Fxscher Labor fiar Neutronenstreuung, ETHZ, 5303 Wtirenhngen, Switzerland (Received 28 Aprd 1987 by P Wachter) The average structure at 8 K of Ba2YCu307 (critical temperature Tc = 90 K) was refined by the Rietveld method (space group Pmmm, a = 3817(2)A, b = 3883(2)A, c = 11.633(7)A, Rwp = 0.11, 28 parameters, T = 8 K). It contains two Cu s~tes of which one has square-p~(ramldal ([Cu--O] = 2 × 1.925(1)t~, 2 x 1 959(1)/~ (base), 2 285(6) A (apex)) and the other square-planar ([Cu-O] = 2 x 1.850(5)fl(, 2 × 1 942(1)A) oxygen coordination. The Cu-O bonds are connected to quasi two-dimensional networks forming 10 A thick structural slabs parallel to the ab plane Large displacement amplitudes of one oxygen atom along [1 0 0] ((u 2) = 0.032(5)A ~) and associated short repulswe oxygen-oxygen contacts [0-(3] = 2 682(4) A) suggest a low-temperature structural phase transmon. The average structure can be described as an mtergrowth of tetragonal La2SrCu206 type structure slabs with Cu-O chains INTRODUCTION RECENTLY AN OXO-CUPRATE of approximate composition Ba2YCu309_ ~ having superconductive transmon temperatures of up to Tc = 93 K was reported [1, 2]. The compound was further characterized with respect to composmon and properties [3, 4] As to its crystal structure X-ray powder diffraction [3, 5] and electron diffraction data [5] suggested orthorhomblc symmetry and an atom arrangement which was described [3] as one which derives from the cubic peroskite type by an ordered substitution of the large caUons and removal of two (out of nine) oxygen atoms The resulting threefold superstructure (aor ~ ac, bor ~- at, Cot "~ 3-ac)wassupposed[6]to be related to the tetragonal La2 SrCu206 structure type [7] The purpose of this work was to determine the structure of Ba2YCu3Og_~ m detail by using both X-ray and neutron powder diffraction analysis, and to clarify its relation to La2 SrCu2 06. Preliminary results were reported elsewhere [4] While this work was m progress we have received preprlnts of two mdepen-
* D6partement de Physique de la Mati6re Condens6e
dent neutron powder diffraction studies Ba2YCu307.
[8, 9] on
EXPERIMENTAL Samples of nominal composmon Ba2YCu307 were prepared by the smtenng method described elsewhere [4]. X-ray powder diffraction patterns (Gumler-Nomus camera, T = 298 K, CuK0t radiation, internal standard Si) could be indexed on orthorhomblc cells similar to those reported [3, 5) previously The lattice memc derived from these patterns was such that the long cell axis (c) was almost always exactly three times that of one of the two short axes (b). Thus many (non-equwalent) reflections coincided m these diffraction patterns. Only one pattern showed significant hne splitting whxch indicated that the axial ratio c/b m the other patterns actually differed from three The absence of systematic extractions suggested one of the following space groups" Pmmm, Pmm2, or P222 Intensity data were obtained from densitometer scans and subsequently analysed by the Rletveld method. A sample of 10 g was used for neutron diffracUon studies Its superconducting transition temperature was T, = 90 K It was placed m a cyhndncal vanadium container (O = 7ram) and
1149
1150
H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R Ba2YCu307 Ba2YCu309-x 8 K Pmmm .,~. Wo.,i,.g,. i r l ~ I
-0
--2
-
e=
4
o
6.
E --B
I
o= 4
0 -
,,
,
,m 20
, 40
60 2-Thefa
80
1(90
120
~40
(Degrees)
Fig 1. Absorption corrected observed and calculated (bottom) and difference (top) neutron diffraction pattern of orthorhomblc Ba2YCu307 at 8K (2 = I 713A). measured on the multldetector powder dlffractometer D M C of the reactor at Wurenhngen (CH) at three d]fferent temperatures 295, 120, 8 K (wavelength 2 = 1 713 A) The pattern at 8 K is presented in Fig 1 Except for systematic hne shifts and intensity differences due to thermal motion effects the patterns recorded at higher temperatures (not represented here) were practically identical to that at 8 K RESULTS AND DISCUSSION The structure refinements on the X-ray data confirmed a perovsk~te hke metal arrangement The two Ba atoms and the Y atom were distributed in an ordered manner on sites 2t (½, ~,1 + z; .~ ~_ 1/6) and 1 h (½, ½, ½), respectively, m centrosymmetrlc space group Pmrnm, whereas the three Cu atoms were placed on sites la (0, 0, 0) and 2q (0, 0, z, z - 1/3) A prehmmary analysis showed that the Ba and Cu atoms on the twofold positions 2t and 2q were displaced along the long cell axis by about 0 21 A (Ba) and 0 26 A (Cu) with respect to their ideal positions in the cubic prototype The oxygen positions were located from analysis of the neutron data For this purpose various models of composition Ba2YCu307 were constructed by assuming a threefold perovsk~te type superstructure from which two oxygen atoms were removed The model which gave the best fit was that m which the Cu atoms were fourfold and fivefold coordinated by O atoms (see below) That model was also adopted by other workers [8, 9] Results of the refinement on the 8 K data are summarized in Table 1 (number or reflections 170, number of parameters refined 19 (atomic), 4 (profile), 3 (latuce), 2 (zeropolnt, scale), R~e = 0 11, R~ = 0 053, R~,o = 0 055, Z2 = 3 84) A l l s t o f l n t e r a tom~c distances is gwen m Table 2 Except for the
Vol. 63, No 12
latUce parameters and some temperature factors (see Table 1) the results of the refinements on the 120 and 295 K data d]d not differ slgmficantly from those on the 8 K data Refinements of the occupancy factors set an upper hmlt of 5 % for possible oxygen defects No evidence for additional oxygen atoms in the structure was found The structure of Ba2Yfu307 is shown in Fig. 2 It contains no octahedral CuOo bulldmgblocks and thus can no longer be considered as a perovsk]te derivative The two Cu sites have square-pyramidal (Cu(2)) and square-planar (Cu(1)) oxygen coordmatlon All C u - O bonds are shorter than 2 0 A except that which is directed toward the apex of the pyramidal umt ([Cu(2)-O(l)] = 2 285(6)A) Square-pyramidal CuO5 units with short base bonds and long apex bonds also occur in La2SrCu206 ([Cu-O] = 1 937(3)A (base), 2.27(4) A (apex) 7) In Ba2YCu307 the shortest C u - O bond ~s that in the square-planar CuO4 umt ([Cu(1)-O(1)] = 1850(5)A) which connects two square-pyramidal units along the long cell axis (Fig 2) This bond is shorter than those observed [10] in the layer-perovsklte superconductor La185Sr015CuO4 (7, = 35K) which contains a square-planar coordinated Cu site ([Cu-O] = 1 89 A) with two further hgands at octahedral apices ([Cu-O] = 2 43 A) Altogether the C u - O bonds in Ba2YCuaO7 are connected to a quas~ two-dimensional network which forms 10 A thick structural slabs parallel to the ab plane The slabs contain Ba atoms and are separated along c by Y atoms (Fig 2) This topology and in particular the interruption of the C u - O bond network along the c direction conveys to the structure strong anlsotroplc
YL °(2)- I R;~- ) °
--
) 1 ,
]
F]g 2 Structural slabs and C u - O bond network m Ba2YCu3 07 Arrows point towards nearest Cu nelghbours
Vol 63, No 12
1151
HIGH-TEMPERATURE SUPERCONDUCTOR Ba2YCu307
Table 1 Structural parameters of Baz YCu307 Space group" Pmmm (No 47) cell parameters(A)
8K
120 K
295 K
a = 3.817(2) b = 3 883(2) c = 11 633(7)
3 818(8) 3.883(2) 11.643(7)
3 824(2) 3 888(2) 11.678(7)
atom parameters* at 8 K x
y 1 1
Ba Y Cu(1) Cu(2) O(1)
2t lh la 2q 2q
~1 ~1 0 0 0
0 0 0
0(2)
2s
½
o
0(3) 0(4)
2r le
0 0
z 0 1851(4) 2
0 0 3554(3) 0 1590(4) 0 3767(4) 0 3782(4) 0
1 1
Amsotroplc thermal parameters of 0(4) 8K B,, (A 2) 2 6(5)
B22(A2)
1.5(5)
B33 (/~ 2 )
1.3(6)
occupacy 1.0(-) 1.0(-) 1 0(-) 1 0(-) 1 06(2) 0 97(1) 0.99(2) 0 96(3)
B,so(~2) 0 18(9) 0 09(8) 0.004(95) 0 20(6) 0 7(1) 0.06(11) 0 07(9) 1 9(2)
120 K
295 K
2 5(6) 1.5(5) 2 0(7)
3 3(6) 1 6(5) 3.0(8)
form of temperature factors lsotroplc exp { - B,sosm20/22 } amsotroplc exp {-h2a*ZB. + k2b*2B22 + 12c'2B33)} * description as m [8], different from that m [9] with respect to O(1) and 0(4) which are interchanged
Table 2 Interatomzc dtstances (t~) m Baz YCu3 07 at 8 K Ba
- 4 2 2 2
O(1) 0(4) 0(3) 0(2)
2.739(1) 2 877(4) 2.948(6) 2.956(5)
O(1)
-
Cu(1) Cu(2) 4 Ba 2 0(4)
1 850(5) 2.285(6) 2 739(1) 2.682(4)
Y
- 4 0(3) 4 0(2)
2 377(3) 2.414(3)
O(2)
- 2 Cu(2) Y 2 Ba
1.925(1) 2 414(3) 2 956(5)
Cu(1)
2 O(1) 2 0(4)
1.850(5) 1 942(1)
4 0(3) 2 0(2)
2 723(1) 2 869(7)
Cu(2)
2 O(2) 2 0(3) O(1)
1 925(1) 1.959(1) 2 285(6)
0(3)
- 2 2 2 4
Cu(2) Y Ba 0(2)
0(3) 0(4)
- 2 Cu(1) 4 Ba 4 O(1)
1.959(1) 2 377(3) 2 948(6) 2 723(1) 2.834(7) 1 942(1) 2 877(4) 2 682(4)
HIGH-TEMPERATURE SUPERCONDUCTOR Ba2YCu307
1152
I" o/1',,.o.,¢.,.o/I,,,o'
oVS,uS,Uo. o o o J o
0
i
o
.'1o
o
o
o
0,7 2 I ,,,A,A,," 0000 Z:O 0
0
0
0
q G
b
Lo2SrCu206 Fig 3 Structural relation between tetragonal La~SrCu206 (a) and orthorhombic Ba2YCu307 (b). Large circles La, Sr, Ba, small circles Y, full lines square-pyramidal CuOs units, broken lines squareplanar CuO4 units. character This feature could be of importance for the understanding of certain superconducting properties such as the upper critical fields The Ba-O and Y-O bond lengths are consistent with those in the corresponding binary and ternary oxides The Ba atom has 10 nearest nelghbours with distances in the range [Ba-O] = 2 74 - 2 96 A The Y atom is surrounded by 8 nearest neighbours with significantly shorter distances of [Y-O] = 2.377(3) A (4 × ) and 2 414(3) A (4 x ), as expected from ItS smaller ionic size The shortest metal-metal contacts in the structure are those between Y and Cu atoms ([Y-Cu(2)] = 3 20A) A structural detail of possible importance are the relatively large displacement amplitudes of the 0(4) atom along the [1 0 0] and [0 0 1] directions (Table 1) Those along [l 0 0] do not much change as a function of temperature Since that atom shows a rather short contact distance with another oxygen atom ([O(4)-O(1)] -- 2 682(4)A), it is likely that 0(4) is statically displaced along [1 0 0] This is the direction of empty space in the structure The displacements could for example originate from the very short Cu(1)-O(l) bonds which tend to increase the dosedshell repulsive interactions between the 0(4) and O(1) atoms (see Fig 2) Consequently the P m m m model used in this work (and in other works [8, 9]) describes only an average structure The true symmetry of BazYCu307 at 8 K ~s probably lower than P m m m (or its cell volume larger), and its structure presumably transforms into the modlficaUon with P m m m sym-
Vol. 63, No. 12
metry only above that temperature. The anomalous behavlour of 0(4) (called O(1) in [9]) also appears from the neutron dxffractlon work reported in [8, 9] and warrants further investigations. Notice that removal of that oxygen atom from the structure leads to a compound of composlt10n Ba2YCuaO6 which is expected to have tetragonal symmetry (atetr "~ aor, C,etr -- Cor) and to contain square-planar and linear coordinated Cu atoms A tetragonal high-temperature modification of composition Ba2YCu307_6 was recently found to exist above 1023 K [11]. Finally the orthorhomblc structure of Ba2YCu307 can be derived from the tetragonal La2 SrCu2 06 structure type as follows The latter is cut into structural segments of composition Ba2YCu206 as shown in Fig 3, and these segments are &splaced along [1 1 0], and linked together along [0 0 1] via intercalated hnear-Cu-O-Cu-O-strIngs which run along [1 0 0] The role of these Cu-O strings for superconductivity is a matter of speculation Acknowledgements - - We thank Professors J Muller
(Geneva), Hk Muller-Buschbaum (Klel) and S Kemmler-Sack (Ttiblngen) for useful discussions, correspondence and exchange of information The help of Mrs B Kunzler with the drawings is gratefully acknowledged This work was supported by the Swiss National Science Foundation REFERENCES 1
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