Structure of Ba2 YCu3O6.26 at 20 and 298K by single-crystal neutron diffration

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Structure of Ba2 YCu3O6.26 at 20 and 298K by single-crystal neutron diffration

A. Renault, G.J. Mcintyre, G. Collin, J.-P. Pouget, R. Comes

To cite this version:

A. Renault, G.J. Mcintyre, G. Collin, J.-P. Pouget, R. Comes. Structure of Ba2 YCu3O6.26 at 20 and 298K by single-crystal neutron diffration. Journal de Physique, 1987, 48 (9), pp.1407-1412.

�10.1051/jphys:019870048090140700�. �jpa-00210569�

Structure of Ba2 YCu3O6.26 at ^{20 and 298K} by single-crystal ^{neutron diffration}

A. Renault*, ^G.J. McIntyre+, ^G. Collin++, ^J.-P. Pouget ^{and R. Comes}

Laboratoire de Physique ^{des Solides} (LA02), ^{Bâtiment 510,} Université de Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France +Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, ^France

++UA200, Université René Descartes, ^{4 Avenue de} l’Observatoire, ⁷⁵⁰⁰⁶ Paris, ^France (Reçu ^{le 17 juin 198?’,} accepti ^{le 8} juillet 1987)

Résumé.- La structure cristalline de Ba2YCu3O8-03B4 (6=1,74) ^{a été} déterminée _par diffraction des neutrons sur

monocristal à 20 K et 298 K. Par opposition à ^la phase orthorhombique supraconductrice pour laquelle 6 ~1,1,

ce composé cristallise dans le _groupe d’espace tétragonal P4/m ^{m m} avec 03B1

3,8573(4)Å ^{et c}

11,7913(10)Å.

L’augmentation du nombre de lacunes d’oxygène, ^par rapport ^à Ba2YCu3O8-03B4 (03B4 ~1,1), ^est entièrement située dans le plan médiateur des pairs ^d’atomes de barium les plus proches. ^Aucune différence notable n’a été observée entre les structures à 20 K et 298 K.

Abstract.- The crystal ^{structure of} Ba2YCu3O8-03B4 (03B4=1.74) ^{at 20K and 298K has been} determined by single-crystal

neutron diffraction. In contrast to the superconducting orthorhombic phase for which 6 ~ 1.1, ^this phase crystallizes

in the tetragonal space group P4/MMM ^{wtih 03B1}

3.8573(4)Å ^{and c}

11.7913(10)Å. ^The increase in _oxygen vacancies

compared ^to Ba2YCu3O8-03B4 (6 ~1.1.) ^{is confined} entirely ^to the sites in the plane midway ^between pairs ^{of closest}

barium atoms. No significant differences were observed between the structures at 20K and 298K.

Classification

Physics ^Abstracts

74.70 - 61.12

1. Introduction

The observation by ^{Wu et al.} [11 ^of superconduc- tivity ^{at 93K in a} mixed-phase compound ^{of the Ba-}

Y-Cu-0 system ^has sparked ^frenetic activity ⁱⁿ many laboratories to identify ^and characterize the specific phase responsible ^{for the} high-temperature supercon-

ductivity. ^From studies of the Ba-Y-Cu-0 phase ^di-

agram using powder diffraction and transport ^mea- surements, ^{Rao et al.} [2] ^{and Hinks et al.} [3] ^{have in-} dependently concluded that the superconducting ^ma-

terial is single phase ^with compositionBa2YCu3 . ^The exact oxygen stoichiometry ^determines

the ratio Cu2+ /Cu3+ ^{which in turn seems to govern}

the occurrence of high-temperature superconductiv- ity [4].

It also _appears that the structure is _very sensitive to the _oxygen stoichiometry. ^{Of the numerous diffrac-}

tion structural determinations published ^so far, ^the majority (e.g. Siegrist ^et al., [5] ; Capponi ^et al., [6] ;

Beno et al., [7] ; ^{Beach et al.,} [8]) ^{have reported an}

orthorhombic structure for 6 -1.1. However Hazen et al. [9] ^and LePage ^{et al.} [10] ^{observed tetragonal}

structures for 8 -1.5 and 6 -1.0 respectively. ^{In the} X-ray ^studies [5,9,10] ^the reliability ^{of the} oxygen po- sitions and occupations ^is relatively poor due to the low scattering power of X-rays by oxygen compared

to barium, yttrium ^and copper. The neutron pow- der diffraction studies [6-8,11] ^{overcome this in part,}

but main give unreliable thermal parameters. ^{In view} of the subtle role believed to be played by ^the oxy- gen atoms in the occurrence of superconductivity ⁱⁿ

this system, single-crystal ^neutron diffraction is the

preferred technique. Unfortunately the dimensions of the single crystals grown have until now been _pro-

hibitively ^{small for neutron} diffraction.

We have been able to grow good quality crystals

of a number of compounds ^{in the Ba-Y-Cu-0} system with volumes approaching ^{1 mm’. In this} paper ^we

report the first determination by single crystal ^neu-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019870048090140700

1408

tron diffraction of the ^structure of the non supercon-

ducting phase ^with oxygen content 6.26 per formula

weight. ^This oxygen content, determined by ^refine-

ment of the neutron diffraction data, is less than the value (6.5) ^{which gives an average Cu charge state of}

2+. Determination of this ^structure should elucidate the role played by ^{the oxygen and oxygen} sites in the

superconducting.

2. Experimental

A powder sample ^{with Y:Ba:Cu} composition ^ra-

tio 1:2:3 was prepared by reaction in air of stoichio- metric proportions ^of Y203, BaC03 ^{and CuO at}

950° C. It was checked that the treated material corre-

sponds ^{to the} Ba2YCu30$_b phase by X-ray ^diffrac-

tion on a Guinier-Laine camera. Small single crystals

were obtained by ^a long annealing ^{of the} powder ^{in air}

at about 900°C, ^then quenched ^{at room} temperature.

It should be noticed that under these conditions the material does not exhibit superconductivity ^{even at}

low temperature (no ^Meissner effect). ^{In the} batch,

an approximately rectangular single crystal ^{with di-}

mensions 1.0x0.3x0.25 mm3 and ^a number of well- defined faces was selected for the neutron diffraction measurements.

The diffraction data were collected on the four- circle multidetector diffractometer D19 on the Hll thermal neutron beam at the Institut Laue-Langevin

[12]. ^The wavelength ^{used was 1.3188A obtained} by

reflection form the (311) ^{planes of a} vertically-focus- sing ^Ge monochromator. Although ^the detector,

which subtends 4° horizontally ^{and 64°} vertically ^at

the sample, permits observation of reflections out of the equatorial plane, the small ratio of unit cell di- mensions to wavelength ^would only permit ^observa-

tion of two reflections simultaneously and under less favourable resolution conditions than in equatorial- plane geometry. ^{The data were} therefore collected in

equatorial-plane geometry. ^All reflections in a quad-

rant of reciprocal space ^to sinO/A

0.68A ^were

scanned at 20K and 298K. For the 20K data collection the crystal ^{was mounted in an Air Products} two-stage Displex cryorefrigerator. The three-dimen-sional ar-

rays of counts observed around each scanned reflec- tion were corrected for background and reduced to

squared ^structure amplitudes using the three-dimen- sional integration method of Wilkinson & Khamis

[13]. Because of the _very small crystal size corrections for sample absorption and extinction were unneces-

sary. At both temperatures ^the integrated intensities

were averaged according ^to the Laue class 4/mmm.

The merging ^{indices are} given in table I.

Optimization ^{of the} accuracy of the structure

factors was essential in this study ^{since the} very small

crystal ^size by ^{normal neutron} diffraction standards meant that for over half the reflections a(F 2) IF 2 ^>

0.1. In the method of Wilkinson and Khamis the limit to the peak extend for strong reflections is found by

reference to the ellipsoidal surface for which cr(7)/7,

where I is the background-corrected ^{sum of the}

counts within the ellipsoid, ^{is a minimum. The} shape

and orientation of the ellipsoid ^are found from the mo-

ment-of-inertia tensor of the stronger peak ^counts.

A shape function of the intensity fraction contained within the ellipsoid ^{as a} function of the ellipsoid ^vol-

ume is also found. For our sample ^{and resolution}

function an enlargement by ^{a factor 4} of the volume which _gave the minimum u(1)/ I ^{was found to contain}

all the peak ^{counts for the} strong peaks. To increase the _accuracy of the structure factors for weak peaks,

their ellipsoid parameters ^and shape ^{functions are de-}

duced by interpolation amongst the values for strong peaks ^{at the same 20. The} peak ^{counts are then}

summed in the statistically optimum volume and a

correction factor to the inegrated intensity ^{is deduced}

from the interpolated shape ^function.

For the weak peaks ^the accuracy of the reduction to structure amplitudes depends very much on an ac-

curate knowledge ^{of the} crystal orientation, ^{since the} integration ellipsoid is centred on the calculated in- strumental reflection coordinates, ^{and on an} accurate ,

knowledge ^{of the} shape functions. With multidetec-

tor data we observe the coordinates of the centre of every strong reflection that is scanned during ^{the data}

collection. Therefore at each temperature ^{the scans}

were analysed ^{in two} passes. The peak ^{centres ob-}

served for the stronger peaks in the first _pass were

used to redetermine the orientation matrix for the second _pass. The first _pas also makes the library ^of shape ^{functions as} complete ^as possible. ^{At both tem-} peratures ^the merging indices after the second _pass

were some 20% smaller than after the first _pass. We also observed a similar improvement in the R factor of the structure refinement against ^the amplitudes ^af-

ter the second _pass compared ^to the refinement after the first _pass.

The _accuracy of the weak-peak reduction also of

depends ^{on the} crystal quality ^{since the} ellipsoid pa- rameters and shape ^functions applied ^{to a} particular

weak peak ^{will in} general ^{be based on} strong peaks

that are not necessarily ^{close in} reciprocal space ^to the weak peak ^nor scanned around the same axis.

Anisotropic ^mosaic broadening ^or spitting ^{of the re-}

flections due to twinning would decrease the _accuracy of the weak-peak ^{reduction more or less} randomly.

The excellent goodness-of-agreement ^{in the} merging

of the reflections at both temperatures implies ^that

neither phenomenon ^{occurs to an} observable extent

in this samples.

Table L- Lattice parameters, ^and refined fractional ^atomic coordinates, occupation factors ^{and thermal} pa- rameters for Ba2YCu307 J61 ^{and the} corresponding parameters ⁱⁿ Ba,2YCu307 [6]. ^{The units of both the}

isotropic ^and anisotropic ^thermal parameters are )L2 . The anisotropic temperature factor, defined for F, ^has The residuals ^are defined as

where Fo ^and F, ^{are the} observed and calculated structure factors, F> is the _average of ^{the set} of equivalents

to Fo, ^{n is the number} of observations and _p is the numbers of parameters, (g.o.a.

goodness of agreement ; g.o.f.

goodness of fit). ^The parameters of this table are for anisotropic refinement. ^{We present the} isotropic

thermal parameters for comparison ^with Ba2yCU307-

1410

3. Structure

The observed cell parameters ^and comparison ^of

the intensities indicated clearly ^a tetragonal ^diffrac-

tion pattern. ^The tetragonal ^cell parameters ^refined from the observed instrumental coordinates of the centres of the N200 _stronger reflections scanned at each temperature ^are given in table I. Refinement of

an orthorhombic cell _gave a and b values that differed

by ^{no more than} 0.002Á, ^{much less than the differ-}

ence of 0.06Å observed for -I. 1 [6,7,8]. ^{The c axis at}

room temperature ^is significantly larger than that ob- served for the superconducting phase (8 -1.I) ^which

is in agreement with the reduced oxygen content of

our sample.

There are five space groups with no systematic

absences in the tetragonal Laue class 4/mmm. ^Of

these, ^three (PA/mmm, ^{P42m and} P422) ^are equiva-

lent for the subsets of special positions that describe the structure ofBa2 YCU3 08-6. Refinements were

therefore performed ^{in the most} symmetric ^{of these} (P4/mmm) and in the remaining ^two groups. P4mm and P4m2, using ^the program UPALS [15]. ^{Since dif-}

ferent ordering ^{of the} oxygen ^on the various sites over

distances comparable ^{to or} larger than the extinction distance could _occur, refinements were also carried out in the orthorhombic space groups Pmm2, ^P222

and Pmmm, allowing ^for twinning ^{of the orthorhom-}

bic cell to give ^a tetragonal diffraction pattern. ^The refinements in all space groups ^were well behaved but

even with larger numbers of adjustable parameters they ^{did not} significantly improve ^the agreement ^com- pared with refinement in the highest-symmetry space group. We therefore report the results of the final refinement in P4/mmm, ^Z

^{1 with Ba on the sites} 2h(1/2, 1/2,z), ^{Y on ld} (1/2, 1/2, 1/2), ^{Cul on la} (0,0,0), ^{Cu2 on 2g} (0,0,z), ^{01 on 2g} (0,0,z), ^{02 on}

4i (1/2, ^0, ;?) ^{and 04 on 2f} (0, 1/2, 0). ^{Here we}

have adopted the atomic nomenclature of reference

[5] (Fig.l). ^In P4/mmm, ^{02 and 03} crystallographi- cally equivalent, ^and similarly ^{04 and 05. No} signifi-

cant differences were observed between the structures at 20K and 298K.

Since the _oxygen occupations appear ^to play a key role in the superconductivity, ^and may be deter- mined to relatively high precision ^{from neutron} data,

we allowed them all to vary. The refinement con-

verged rapidly ^to complete occupation of the sites oc-

cupied by ^{01 and 02 and to} complete absence of oxy- gen in the 2r=l/2 plane. ^{This agrees with the observa-}

tions of Capponi ^{et al.} [6] ^and Siegrist ^{et al.} [5]. ^Our

results do not however support ^{the recent} proposition

of Reller et al. [14] ^{that in} Ba2 YCu308-6 ^{all the} per- ovskite _oxygen sites in the z--1/2 plane ^are occupied.

The refined _occupancy of the 04 and equivalent ⁰⁵

Fig.l.- ^{Unit cell of} tetragonal Ba2 YCU30G.2Û. ^The striped ^cir-

cles show the partly occupied ^{oxygen sites.}

sites in the z=0 plane ^was g--13(2)% , corresponding

to the composition Ba2YCu30c.26’ ^{Even refinement}

of the twinned orthorhombic structures gave ^essen-

tially ^{the same} occupations g=13(2)% ^{for both 04 and}

05. This equal ^occupancy constrasts with the 100%

and 0% ^{found for the 04 and 05 sites} respectively by

neutron powder diffraction in the oxygen-richer ^com- pound Ba2YCu307 [6,7,8]. ^{Our data show clearly}

that it is the sites of the z=0 plane ^{that are} gradu- ally ^{filled as the} proportion ^of oxygen is increased in this system. ^The oxygen occupation ^{of the 04 sites is} probably accompanied by ^{a local} orthorhombic dis- tortion but the diffraction patterns ^show clearly ^that

the average structure remains tetragonal ^{for low} oxy-

gen concentrations.

The temperature factors for all atoms were al- lowed to be anisotropic ^{in the final} refinement. Par-

ticularly high value of the thermal parameters ^were observed for Ba, Cul, ^{01 and 04.} A further remark

concerning ^the thermal vibration of 04 should also be made. Refinement of an isotropic ^thermal parameter

gave ^an estimated error comparable ^{to the} parame- ter itself, ^{while the} anisotropic refinement _gave a very

large ^{value of} Bl1 ^{and a} slightly negative ^{value of} B33.

We interpret ^{this as} indication of disorder in the _po- sition of 04, ^{however we have not been able to obtain} satisfactory convergence for a structure with disor- dered 04 positions. ^{A similar} problem ^{also occurred in}

refinements of the orthorhombic phase Ba2YCu307

[6], ^{except that} large values of both Bll ^and B33 ^are

observed for 04 (see ^Table I).

The final refined atomic coordinates at both 20K and 298K are given in table I. For comparison ^{we also}

include in table I the cell parameters and atomic coor-

dinates observed at room temperature ^for Ba2YCu3 07. ^{Since all} structures reported ^for the orthorhom- bic form (with ^{the exception of the equal 04 and 05}

occupations noted’by Siegrist ^{et al.} [5]) are essen-

tially ^{the same we list} just the results of Capponi ^et

al. [6]. ^Two (X-ray) studies have been performed

on supposedly tetragonal crystals by ^{Hazen et al.} [9]

and LePage ^{et al.} [10]. ^The differences in refined a

and b _axes, the c axis and the Ba z coordinate ob- served by ^{Hazen et al. and} LePage ^{et al. lie} partway between our tetragonal values and the values of Cap- poni ^{et al..} However Hazen et al. and LePage ^et

al. observed slight orthorhombic distortions of the cell parameters, and both noted particularly ^broad

or irregular diffraction peak profiles. As noted above this was not the case in our study. This would _sug- gest ^{that the} crystals ^{of Hazen et al. and} LePage ^et

al. were domain crystals, possibly partly tetragonal

and partly twinned orthorhombic. In fact Izumi et al. [11] ^{have reported a} similar domain effect in ^a

powder study ^of Ba2YCu308_x. They ^{observed two} phases ^{in their} sample ; ^a tetragonal phase ^{of com-} position Ba2YCu30e.32 ^whose structure agrees with ours, and on orthorhombic phase ^with composition Ba2YCu30e.64’ Bearing in mind the uncertainty ⁱⁿ determining 6, particularly by refinement of the _oxy- gen occupations ^from X-ray data, ^we suggest ^{that for} 6>1.5 ^a single-phase crystal ^will adopt ^the tetrago- nal structure reported here, ^{while for ê 1.5 the or-}

thorhombic structure is obtained.

The distances between the heavy ^{atoms and the}

oxygen ^atoms for both Ba2yCU30r,.26 (our work) ^and

Ba2YCu307 [6] ^are given in table II.

The structure of Ba2YCu30e 20 may be descri- bed as an oxygen-deficient perovskite, ^{where the} oxy- gen plane ^at z--1/2 ^{is missing,} and with the cations

Ba2+ and Y3+ ordered along ^{the c} axis in the se-

quence -Ba-Y-Ba-Ba-Y-Ba-. The atom Cul can be

envisaged ^{to lie at the centre of an} octahedron formed

by ^{the 01 sites and the} partially occupied ⁰⁴ sites,

while the atom Cu2 lies at the centre of a nearly ^flat

square pyramid whose base corners are at the sites 02. In these aspects ^{the structure is the same as} Ba2YCu307. ^{The distance} of the Ba from the z=0

plane (2.29A) ⁱⁿ Ba2YCu30e.24 ^{however is} signifi- cantly longer ^{than the distance} observed in Ba2YCu3 07 (2.15Å). The result is that the plane containing

the Ba and 01 atoms is less puckered ^{in the} tetragonal

structure than in the orthorhombic. The increase in the Ba -Ba separation arises from an increase in the

Ba2+ -Ba2+ repulsion by emptying ^{the 04 sites. In}

fact the partial occupancy of the 04 (05) sites in the

tetragonal ^{structure leads to a} considerable disorder

as shown by ^the high Debye-Waller factors for the Ba,

01 and Cul atoms. The difference in the interatomic

separations ^{Cul - 01 and Cu2 - 01 in} Ba2YCu30e.26 implies ^a stronger interaction between 01 and Cul than between 01 and Cu2. This difference is larger

than that observed in Ba2YCu307 (see ^table I). ^As

Table IL- Selected interatomic ’distance (Å) ⁱⁿ tetragonal Ba YCu306.26 ^{and the} corresponding ^{distances in}

orthorhombic Ba2YCu307 [6].

1412

well as an increase in the Ba - Ba distance, ^{we also}

observe an increase in the Cul - Cu2 distance from

4.14A in Ba2YCu307 ^to 4.24Á in Ba2 YCu306.26.

The Cu2 - Cu2 distance decreases slightly ^from 3.39A

to 3.3lA ; ^{in both} structures it is clearly ^{too short to}

accomodate an oxygen atom ^at 2r=l/2 ^{as proposed by}

Reller et al. [14].

4. Conclusion

The structure of the tetragonal phase Ba2YCu3 06.26 has been determined for the first time by single- crystal neutron diffraction. Comparison ^{with the}

structure of the orthorhombic superconducting ^phase Ba2YCuo3r ^{allows us to identify the oxygen sites}

whose occupations adjust ^{to the} proportion ^{of oxy-}

gen and which play a key ^role in the existence of su-

perconductivity ^{in this} system. ^This study ^indicates

unambiguously ^{that these are the sites on the} plane midway ^between pairs ^{of closest Ba atoms.} Although

there _may be local orthorhombic distortions due to the partial ^{but still} significant (13% ) occupation ^of

these sites, ^the average structure is clearly tetragonal.

Analysis ^{of a similar} experiment ^on single crystals ⁱⁿ

which the _oxygen content has been increased is in progress.

We gratefully acknowledge ^the particular sup-

port ^{of the directors of the} I.L.L. for this work. We

especially thank Prof. J. Lajzerowicz-Bonneteau ^for

sage counsel on all aspects of the refinements. We also wish to thank Dr Sax Mason for his considerable

advice, ^Mr Michel Berneron for technical assistance

during ^the experiment, ^{and Dr R. Moret for construc-}

tive discussions.

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