Neutron powder diffraction study of the crystal structures of HgBa2CuO4+δ and HgBaO2

PHYSICAG

Physica C 212 (1993) 259-265 North-Holland

Neutron powder diffraction study of the crystal structures of HgBa2CuO4 + 6 and HgBaO2 O. C h m a i s s e m a b c d

a,

Q. H u a n g a, S.N. P u t i l i n b, M. M a r e z i o c,o a n d A. S a n t o r o a

National Institute o f Standards and Technology, Gaithersburg, MD 20899, USA Chemical Department, Moscow State University, 119899, Moscow, Russian Federation Laboratoire de Cristallographie, CNRS-UJF, BP 166, 38042 Grenoble cedex 09, France AT&TBell Laboratories, Murray Hill, NJ 07974, USA

Received 27 April 1993 Revised manuscript received 21 May 1993

The crystal structures of HgBa2CuO4+aand HgBaO2 have been refined at room temperature using neutron powder diffraction data obtained from a sample containing 44% of the first phase, 42% of the second and 14% of Ba2Cu3Os+a.the compound HgBa2CuO4+6crystallizeswith the symmetryof space group P4/mmm and lattice parameters a = 3.8829 (6), c= 9.5129 ( 14) A. The unit cell contains only one CuO2layer and the material is a superconductorwith a value of Tcof 94 K. The oxygen in excess of the 04 stoichiometry (6=0.063 in our sample) is located in interstitial positions on the HgOa layer, and it is the only extra oxygen present in the structure. No mixing of the cations has been detected in this study. The compound HgBaO2 crystallizeswith the symmetry of space group R3m and lattice parameters (hexagonal axes ) a = 4.0991 (6), c= 19.355(3)/~. In this new polymorph of HgBaOz the Ba atoms are octahedrally coordinated while Hg has two-fold coordination.

1. Introduction In a recent paper [ 1 ], superconductivity at 94 K was reported in HgBazCuO4+6. The structure of the compound, determined by profile analysis based on X-ray powder diffraction data, can be described by the layer sequence [2 ]

how many of them can enter into the structure and where they are located. Since X-rays are not sufficiently sensitive to oxygen in the presence of heavy atoms such as Ba and Hg, a neutron powder diffraction study of the title compounds was carried out. The results of this work are described in the following sections.

... [ (BaO)c (HgO6)o (BaO)c (CuO2)o ] (BaO)c... where the square brackets include the content of one unit cell and the subscripts c and o indicate if the cation is at the center or at the origin of the mesh for each layer. The interesting feature of this structure is that it contains one CuOz layer per unit cell and that the separation between two consecutive such layers is about 9.5 A,. U n d e r these conditions, a critical temperature of 94 K is surprisingly high. The value of 6 was found to be 0.10 (3) in the X-ray analysis, with the oxygen located at the center of the Hg layer. Since these extra oxygen atoms are necessary to increase the average oxidation n u m b e r of copper and the concentration of holes required for superconductivity, it is important to know with precision

2. Experimental Neutron diffraction intensity data were collected at room temperature with a single counter powder diffractometer at the Reactor of the National Institute of Standards and Technology, using the experimental conditions shown in table 1. The observed intensities were corrected for absorption using the absorption correction factors for cylinders given in the International Tables [ 3 ] and the value of gR determined experimentally. Indexing of the powder pattern was attempted by using the lattice parameters of HgBazCuO4+a determined in the X-ray experiment [ 1 ]. Numerous peaks

0921-4534/93/$06.00 © 1993 ElsevierSciencePublishers B.V. All rights reserved.

260

O. Chmais.~em el el. / ('O,s'tal .vtructure~ o/' ttgBa :( "u04 +,, and It~BaO,

Fable I Collection of intensity data Monochromatic beam: Wavelength: Horizontal divergences: Sample container: 20 angular range: Scattering amplitudes ( 1O- ~2cm ): Measurement made at room temperature.

220 reflection of('u monochromator. 1.540( 1 }:~. 20', 40', [0' of arc for the in-pile, monochromatic beam, and diffracted beam collimators, respecliveb. AI can of about 7 mm diameter. 5 115:, steps: 0.05 . t,(Hg) = 1.266, b(Ba)=0.525, h(Cu)=0.772, h(O)=0.581.

could not be accounted tor by this procedure, and were attributed to impurities. These were later identified as HgBaO2 and Ba2Cu3Os+a. Two p o l y m o r p h s o f the first material are known. One o f these crystallizes with the s y m m e t r y o f space group P6322 and the Ba cations in the structure have a trigonal BaO6 prismatic c o o r d i n a t i o n [4]. The second p o l y m o r p h is isomorphous with HgCaO2 and HgSrO2 [5]. In this phase the alkali-earth cations have octahedral coordination and the s y m m e t r y of the structure has been described in space groups R.3m [ 5,6 ] and P3:21 [7]. The second impurity, Ba2Cu3Os+a has been analyzed by T h o m p s o n et el. [8] who found that the "'average" structure of this phase has symmetry' C m c m and that a m o d u l a t i o n is also present whose wave-vector changes in magnitude and direction with oxygen stoichiometry. The Rietveld refinement o f this mixture o f three phases was carried out with the multiphase program GSAS o f Larson and Von Dreele [9], using as initial parameters for HgBa2CuO4+a, HgBaO2 and Ba2Cu3Os+a, those reported in refs. [1 ], [5] and [8], respectively, and by refining each structure in turn. The final results of these calculations are given in table 2 and the agreement between observed and calculated intensities is shown in fig. 1. In table 3 the relevant b o n d distances are reported for the struclures o f HgBa2CuO4+a and HgBaO2.

3. Results 3. l . H g B a e C u O 4 + a

The structure o f HgBa2CuO4+6 is schematically illustrated in fig. 2. The results shown in table 2 con-

firm the basic structure lound in the X-ray experiment, with the positional parameters agreeing within two standard deviations in the two determinations. The occupancy factor ,l of the oxygen atom O( 3 ) was refined with the temperature factor fixed at B ( O ( 3 ) ) = 0 . 8 ~&2 and was found to be equal to 0.06( 1 ), i.e., not significantly below the X-ray valuc of 0.10(3 ). Since there is high correlation between B and ,7, refinements were also carried out with values o r B ( O ( 3 ) ) ranging from 0.4 to 1.6 .&2. In these calculations the refined values o f n changed fiom 0.06( I ) to 0.07( 1 ), thus confirming that the oxygen content m the sample used in our experiment is between 0.06 and 0.07 atoms per unit cell. In agreement with the X-ray results, the neutron refinements show slightly higher than normal temperature factors for Hg and 0 ( 2 ) , and anisotropic calculations give B.~ =B22> B33 in both cases. These values can be interpreted in terms o f static disorder caused by the need to satist~ the coordination requirements of the Hg atoms. Shifting these into the _vv0 positions gives . \ = 0.028 ( 5 ), reduces the B( Hg } t e m p e r a t u r e factor to 0.8 .&2 and results in a more reasonable H g - O ( 3 ) separation o f about 2.59 ~i~. down from 2.75 .,~ corresponding to the model with Hg at 0, 0, 0. in c o m p o u n d s o f this type, cation disordering is always a possibility and in the case o f HgBa2CuO4 +,5, some Cu may replace Hg. Although such substitution would have been detected in the difference Fourier m a p based on the X-ray data, a model was refined containing about 0.1 a t o m s o f Cu in the Hg positions, the results of this refinement showed an occupancy factor n ( Cu ) slightly negati~ e ( n ( C u ) = - 0 . 0 3 ( 3 ) ) thus giving a strong indication thai no replacement of Hg by Cu takes place in this

O. Chmaissem et al. / Crystal structures of HgBa2CuO4+6and HgBa02

261

Table 2 Refinement results of three phases mixture at room temperature ~)

Space Group a (A) b (A) c (A) v (A~) Phase fraction (wt.%) Hg Site x y z B (A 2) Occupancy Ba Site x y z B (A 2) Occupancy Cu Site x y z B (,~fl) Occupancy O(1) Site x y z B (A 2) Occupancy 0 ( 2 ) Site x y z B (A 2) Occupancy 0(3) Site x y

z

B (A 2) Occupancy

Ba2HgCuO4+6

HgBaO2

Ba2Cu3Os+6

P4/mmm 3.8829(6) 3.8829(6) 9.5129(14) 143.43(6) 43.9(10) la 4/mmm 0 0 0 1.3(2) 1 2h 4mm ½ ½ 0.2988(6) 0.2( 1) 1 lb 4/mmm 0 0 ½ 0.2(2) 1 2emmm 0 ½ ½ 0.4( 1) 1 2g4mm 0 0 0.2073(5) 1.0( 1) 1 lc 4/mmm

R3m 4.0991 (6) 4.0991 (6) 19.355(3) 281.6( l ) 42.4(5) 3a 3m 0 0 0 2.8(2) 1 3b 3m 0 0 ½ 1.0(2) 1

Cmcm 7.298(3) 4.259(2) 11.421 (5) 355.0(3 ) 13.7(3 )

6c3m 0 0 0.1009(3) 1.0( 1 ) 1

4c m2m 0 0.842 4.6 1 16h 1 0 0.068 7.2 0.375 16hi 0.338 0.420 3.0 0.69

½

0 1.2 0.063 (14)

a~ Rv= 6.69, R~=8.57, X=2.63 c o m p o u n d . In a d d i t i o n , no m o r e oxygen was found, a n d in particular, the p o s i t i o n s 0½0 were f o u n d to be empty.

3.2. HgBa02 I n d e x i n g o f the diffraction lines o f HgBaO2 s h o w e d t h a t the phase p r e s e n t in o u r s a m p l e is the trigonal p o l y m o r p h isostructural w i t h HgCaO2. R e f i n e m e n t s o f this phase were therefore carried out in space group R 3 m (in w h i c h only the z - p a r a m e t e r o f the oxygen

(a)

l

i

r

~

q

I[

4

40001

~ lI

i"

2000 +

i f

k.)

oi

_

_~_ ..I 7L_--__

L7

{

1

~2z72

~

~7~

; 2;k17

]

_~

21

r

&77

z

i

El

_ ZTZ

i2

[

Z

7

7

2

_

i

A

[ 1()

2f

.l/)

{() 2(J ( dug i

(b)

4°°° t : f

J!

!!

2000 --

ol

.L

"~

~e i

i

t

L_

30

i

.

40

511

.

.

.

.

60

20 ( d o g )

(c)

F

I

4000

300

2(X)O

l (X)O

I

J

0-

I

I

- 1000 I

i

50

60

J

70

80

20(deg)

Fig. 1. Observed and calculated intensities after refinement ol a mixture oi"44 o'/oof ttgBa,('u()4 +a- 4_~ o,0 of HgBa( )e and 14 0oolBa( ug),~, , The peaks o f the three phases are indicated in the lower part of the diagram b~ vertical marks. (The first, second and third Imc of the

263

O. Chmaissem et al. / Crystal structures of HgBaeCuO4+a and HgBaOe

{d)

+

200O + +

+

,+

+

1000 r~

r,..)

o

i I

i I

iiiill [

iii I

i i1 Illll I I I I II1~ I

I I

I| ~

I

Ill I

II I I

I

1 70

II lL I

t lI I I [

IEJi

I iI

I I

UIJ I ~ I

I 80

I1 I II I

IIlM I 1

ii i I i i I 11111111 I I I I

I 90

i 100

20 (deg)

(d)

++ +

++ ++

+

: 1000

÷,

+

+

++

+ + + ++

~"

+

+ +

+

.

4-

.

5OO C~

,

*"",,

,

¢",;,,,

7'; ";,,'~'

L

l

I

J

i

i

,,,,

-500 [

I

I

90

100

110

20 (deg) Fig. 1. C o n t i n u e d .

atoms can be varied), and P3221 (in which the xparameters of Hg and Ba and the x, y, z-parameters of oxygen can be varied). The results of these calculations showed that the positional parameters in space group P3221 differ from the corresponding ones in R3m by no more than three standard deviations. In addition, the values of a associated with these parameters tend to be abnormally large and no significant improvement between observed and calculated

intensities is obtained by lowering the symmetry from R3m to P3221. From these data we may therefore conclude that the symmetry of the trigonal polymorph of HgBaO2 is R3m. This result is in agreement with the conclusions of Putilin and Marezio [6 ], who were able to explain that the choice of the primitive space group P3221 was due to the presence of twinning in the sample used in ref. [7].

264

O. Chmaissem et al. / Crystal structures of HgBa2CuO4+a and HgBaO:

Table 3 Selected bond distances (A) of HgBa2CuO4+a and HgBaO2 BazHgCuO4+a Hg-O(2) Hg-O(3) Cu-O( I ) Cu-O(2) Ba-O( 1) Ba-O(2) Ba-O(3) HgBaO2 Hg-O Ba-O

O-O

.972(5) 2.7457(4) 1.9415(3) 2.784(5) 2.727(4) 2.880(2) 2.842(5)

×2 ×(4X0.063) ×4 X2 X4 X4 X0.063

1.956(5) 2.686(3) 3.472(8)

X2 X6

C

ea Ba @ Hg ()

+ ;:b

O 0(3)

0(2)

•

f

:

iCu O(1)

O

" L

i

• -

Fig. 3. Schematic representation of the unit cell of the trigonal polymorph of HgBaO2. according to the values given in ref. [8].

@ ,,"

JS

©

a

Fig. 2. Schematic representation of the unit cell HgBazCuO4+a. The positions indicated as 0(3 ) in the figure are partially occupied (6.3% full and 93.7% empty). 3.3. B a 2 C u s O s + a

Due to the complexity of the atomic arrangement in this c o m p o u n d and to the fact that only about 10% of our sample is composed of this impurity, no refinement of the average structure was attempted and the atomic parameters, temperature factors etc. of the atoms were kept fixed in the final calculations

4. D i s c u s s i o n

and conclusions

The c o m p o u n d HgBa2CuO4+a may be considered as the first m e m b e r of the homologous series HgBa2R,, ,CunO2,,+2+,~, where R is a rare-earth element. The second member, HgBa2RCuzO6+a has also been synthesized [10] and it has been found to be non-superconducting although its structure contains a block of two CuO2 layers separated by an oxygen deficient layer of R atoms (similar blocks are present in the structures of superconductors such as YBazCu307, T12Ba2CaCu2Os, etc.) The above homologous series is similar to, but not identical with. the series TIBa2Ca,, ~CunO2~+3 a, the difference being that the T10~_a layers contain only a few oxygen vacancies, while the layers HgOa are practically oxygen deficient. The copper atoms in HgBazCuO4+a are in octahedral coordination, and the octahedra are very elongated, with an apical distance

O. Chmaissem et al. / Crystal structures of HgBa2Cu04+6 and HgBaO2

C u - O (2) of 2.784 (5) A, compared with the in-plane C u - O ( 1 ) distance of 1.9415(3) A. Most of the Ba atoms have eight-fold coordination. However, those located below and above the O ( 3 ) atoms are ninecoordinated and the coordination polyhedron may be considered as a m o n o capped square antiprism. The Hg atoms are generally two-coordinated, except those located on the layers containing the O (3) atoms, which have a coordination that depends on how the atoms O (3) are distributed in the planes. In the second c o m p o u n d refined in this study, HgBaO2, the Ba atoms have octahedral coordination while Hg is two-coordinated with the two oxygen atoms arranged in the usual " d u m b b e l l " configuration (fig. 3).

265

References [ 1] S.N. Putilin, E.V. Antipov, O. Chmaissemand M. Marezio, Nature (London) 362 (1993) 226. [2] A. Santoro, F. Beech, M. Marezio and R.J. Cava, Physica C 156 (1988) 693. [3] International Tables for X-ray Crystallography, vol. II (Kynoch, Birmingham, 1959). [4] M. SoUand Hk. Mueller-Buschbaum,J. Less-CommonMet. 162 (1990) 169. [ 5 ] S.N. Putilin, M.G. Rozova, D.A. Kashporov, E.V. Antipov and L.M. Korvba, Russ. J. Inorg. Chem. 36 ( 1991 ) 928. [6 ] S.N. Putilin and M. Marezio, to be published. [7] M. Solland Hk. Mueller-Buschbaum,J. Less-CommonMet. 175 (1991) 295. [8l J.G. Thompson, J.D. FitzGerald, R.L. Withers, P.J. Barlow and J.S. Anderson, Mater. Res. Bull. 24 (1989) 505. [9]A.C. Larson and R.B. Von Dreele, General Structure AnalysisSystem, Univ. of California (1985). [ 10] S.N. Putilin, I. Bryntse and E.V. Antipov, Mater. Res. Bull. 26 (1991) 1299.