Systematic copper deficiency in Y-Ba-Cu-O single crystals

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Systematic copper deficiency in Y-Ba-Cu-O single crystals

G. Collin, A.C. Audier, R. Comes

To cite this version:

G. Collin, A.C. Audier, R. Comes. Systematic copper deficiency in Y-Ba-Cu-O single crystals. Journal

de Physique, 1988, 49 (3), pp.383-387. �10.1051/jphys:01988004903038300�. �jpa-00210708�

Short Communication

Systematic copper deficiency ^{in Y-Ba-Cu-O} single crystals

G. Collin, ^{A.C. Audier} and R. Comes

Laboratoire de Physique ^des Solides, ^{U.A. 02} C.N.R.S., ^Bât. 510, Université Paris-Sud, ⁹¹⁴⁰⁵ Orsay, France

(Re;u ^{le 11 ddcembre 1987,} accepti ^{Ie 20} janvier 1988)

Résumé.2014 La structure de cinq cristaux de YBa2Cu3-xO7-6 (0,09) ^~ 1) ^{a été} déterminée _par diffraction des _rayons X. Quatre ^{cristaux sont} orthorhombiques (03B4 0,35) ^{et maclés avec un taux}

de maclage déduit de l’affinement correspondant à ^{54-70% de} l’individu principal. Le dernier est

quadratique (03B4 =1). ^{Un défaut} systématique ^{est trouvé sur le site} Cu(1), de l’ordre de 4-8% , ^alors que les autres sites cationiques ^sont complètement occupés. ^{L’ordre sur} l’oxygène ^{croît avec} la concentration.

Le site Ba est trouvé fortement anharmonique.

Abstract.2014 The structure of five YBa2Cu3-xO7-03B4(0.09 ~03B4 1) ^{crystals is} determined by X-Ray

diffraction. Four crystals ^are orthorhombic (03B4 0.35) and twinned with a twin ratio deduced from refinement corresponding ^{to 54-70% of the main} sample. ^{The last one is} tetragonal (03B4 ⁼ 1). ^{A new}

feature, ^not systematically reported, ^{is a} deficiency ^{on the} Cu(1) ^{site, in the range of 4-8% , whereas}

the other cationic sites remain fully occupied. ^The oxygen ordering increases with concentration. The Ba site is found strongly anharmonic.

Classification

Physics ^Abstracts

61.10 - 61.70B - 74.70

1. Introduction.

The crystal ^{structure of} YBa2Cu307-6

was determined by ^several groups [1-6], quasi- immediately ^after announcement by ^{Wu et al.}

[7] ^{of a high Te} superconductivity in the Y-Ba- Cu-0 system. ^{It is an} ordered defective triple pseudo-perovskite ^{structure built} up from three units : two BaCu02,5 ^{and one} YCU02- ^With respect ^{to the ideal} perovskite ^structure (i) ^the

oxygen in the yttrium plane ^is missing ^and (ii)

in the _copper plane ^{between two} baryum ^units

at most one of the ^two _oxygen positions ^{is oc-} cupied and there is a non-stoichiometry range included between YBa2Cu306 (the ^{two oxygen} positions ^are empty) ^{and YBa2CU307} (one ^of

the two positions ^is occupied).

We have performed ^{a detailed} single crys- tal analysis, using X-Ray diffraction, ^{in or-}

der to investigate ^the problem ^of oxygen ^or-

dering within the homogeneity range. Most of

previous structural works have been performed by ^neutron diffraction on powders because the available single crystals ^were generally tiny ^and

moreover systematically ^twinned. Indeed, ^the

reference structure is tetragonal (space ^group P4/MMM, Z= 1) ^{and for} high oxygen ^concen- tration an orthorhombic distortion ^occurs (space

group P _mmm, Z ⁼ 1), ^{due to a} preferential ^or- dering of additional _oxygen atoms on one of the two available sites [8], which leads to a system- atic twinning : ^{there are two} types ^{of domains}

(i) ^{a b c} domains and (ii) ^{b a c} domains where a, b and c refer of the orthorhombic lattice _pa- rameters.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01988004903038300

384

2. Experimental.

Several crystals ^were prepared using ^two

different methods, spontaneous crystallisation

from the powders ^{at 970°-980°} C, ^and growth

from a flux (BaCu02 ^and CuO) ^between

1000 and 880° C. Some of these crystals ^were quenched ^from high temperature (C5), ^some

others cooled in air (C4), and the last ones rean-

nealed in an oxygen flux at 900° C for 12 hours,

then slowly cooled down to room temperature,

once for C2 and C3 and twice for Cl. All ex-

hibit a platelet shape with the small dimension

corresponding ^{to the c} orthorhombic ^or tetrag- onal axis. They ^were found orthorhombic ex-

cept ^{C5 which was} perfectly tetragonal. ^For

the orthorhombic samples , reflections such as

h00 or OAO for example, ^are systematically sep- arated into four distinct reflections due to the

twinning ^mechanism occurring ^{on 110 bound-}

aries [9]. ^This corresponds ^to the existence of two types ^of domains, ^{the first ones oriented ac-} cording ^{to an a b c} type description ^{and the}

second ones to the conjugated ^{b a c} type ^de- scription. ^{Each one of these} types ^{of domains} is further split ^{into two} samples depending ^on

the orientation after the twin boundary. ^But fundamentally ^{there are} only ^{two distinct} types of orientation. Under these conditions, ^{the in-} tegration ^{of each set of} four reflections on a 4- circle diffractometer will lead to intensities cor-

responding ^to the incoherent summation of the contributions of the two different kinds of orien- tations [10].

The data collection was performed ^{on a half} sphere ^of reciprocal space up to 28 ⁼ 65" (Mo

Ka with a scan range ^{w =} 3° which leads to the required integration in the 20 _range used

as verified on peak profiles). ^After absorption correction, ^the independent reflections ^were ob- tained as an average of equivalent reflections,

8 or 4 respectively ^{for the} tetragonal ^{and or-}

thorhombic samples. Since all the orthorhom- bic crystals ^were twinned, the refinement _pro- gram ^was modified in order to incorporate ^a

variable rate of twinning : calculation of Fhkl

[(alFhkl)2 ⁺ (a2Fhkl)2] and related deriva- tives including ^an expansion ^{of the} tempera-

ture factors _up to the 4th rank in tensors (from

O.R.J.F.L.S. [11] using ^a tensorial Hermite poly-

nomial expansion). ^{The whole set of indepen-}

dent reflections were kept in the refinement in-

cluding those with zero intensity. ^{When the} occupation factors, anisotropic ^for Y, ^{Ba and}

Cu positions, isotropic ^for oxygen positions. ^In

the orthorhombic _cases, for the two groups of oxygen positions 0(2)-0(3) ^and 0(4)-0(5) (cf.

Tab. II) ^{each one} deduced from a single position

of the tetragonal cell, only ^one isotropic ^tem- perature ^{factor was} used, ^with simultaneously ^a

refinement of occupancies ^for 0(4)-0(5).

Table I.- Lattice parameters (A) ^{and twinning ratio for the} five crystals.

* crystallisation ^from powders, ^**from flux,

***ratio(V1/(V1 +V2» deduced from refinement of the ratio Ql / Q2. ^{The lattice constants are ob-}

tained from a routine reflection centring (25 ^re- flections) ^which systematically underestimates the orthorhombic distorsion because of partial overlap ^{of AM} and khl reflections. However, ^this

does not affect the c lattice constant.

3. Results and discussion.

3.1 COPPER DEFICIENCY.- The first refinement

was carried out, using ^atomic scattering ^fac- tors, ^{with all} occupation factors constrained to their ideal values, except ^for 0(4)-0(5). ^Start-

ing from this result each one of the occupancies

was refined successively. ^{All the atoms} remained

close to their initial value, ^without any signifi-

cant change ^{of the R factor} value, except ^for

Cu(I), ^{the copper} position in-between two Ba

planes, for which a systematic deficiency ^was

found leading ^{to an} important decrease of the R factor. In table III are reported the results and the Ri /R2 ^ratio (respectively without and

with a variation of the Cu(l) occupancy) ^com-

pared ^{to the} minimum value it should exhibit in order to be statistically significant Rb,n-m, ^0.005 [12]. ^{In all cases the} R1 /Ra ratio if much higher

than R. In addition the standard deviation ^on the _occupancy factor of Cu(l) ^are, for the five

crystals, ^{an order of} magnitude lower than the deviation from the ideal value.

Table II.- Structural parameters for ^Cl sample.

This establishes unambigously that, ^{in sin-}

gle crystals, ^the copper (1) ^{site can be appre-}

ciably ^{vacant. A} crystal prepared by ^another

group ^was tested and also revealed a copper (1)

deficiency ^{in the} range of 8% . A similar result

was also previously ^mentioned by Nakai et al.

[13] .

Table III.- Refinement results and occupation factors of Cu(1 ).

For crystal Cl, ^for example, ^the occupancy factors were 1.004(4) ^for Y, 1.002(4) ^{for Ba and} 0.996(6) ^for Cu(2). ^{R1 and R2 are calculated}

with respectively ^{25 and 26 variable} parameters

(occupancy ^{factor of} Cu(l) ⁱⁿ addition in R2).

For crystal C5, tetragonal, the number of vari- able parameters corresponds ^{to 16 and 17.}

This _copper deficiency results from a prepa- ration temperature ^of crystals higher ^{than the}

maximum stability temperature ^{of the stoichio-} metrsc material - 940-950°C. Indeed, ^{above this} temperature the materials exhibit an apprecia-

ble degree ^of decomposition ^and especially ^this

is confirmed by ^the large ^{amount of} green phase

386

in the powders ^{from which the} crystals ^{are ex-}

tracted.

3.2 OXYGEN CONTENT.- The refinement of both 0(4) ^and 0(5) ^{leads to rather} imprecise

results because of the high correlation term be- tween these two parameters, = 0.85, ^with large

standard deviations on each one of the _occupa- tion factors. But the sum of the occupancies

remains constant. It is why ^we constrained the total _occupancy to this value obtained sponta- neously ^{in order to lower the} amplitude ^{of fluc-}

tuation in the first cycles. However, in the final

cycles, the constraint was removed and each site

was refined separately.

Table IV.- Oxygen occupancy.

,

For the crystals ^{untreated or treated} only

once in an oxygen flux the refinement indicates

an appreciable degree of disorder between 0 (4)

and 0(5) ^{sites. However, the crystal Cl, treated}

twice in _oxygen, reveals a quasi-perfect ^{order as-}

sociated with the highest oxygen ^content. This is the regular behaviour of ionic conductors in which the order is mainly ^driven by the correla- tions between mobile ions ; stronger ^{for the} high-

est ionic concentration, Cl, than for the other

crystals. ^{For the} tetragonal crystal, C5, ^{the re-}

finement indicated a completely empty ^site.

3.3 ANISOTROPY AND ANHARMONICITY.- In all orthorhombic crystals the Ba and Cu(l) ^sites

exhibit strongly anisotropic temperature ^factors which are large within the basal plane ^{and lower}

in the c direction. In addition, ^the in-plane ^vi-

bration amplitudes ^are larger in the direction normal to the chains (U11 ) than in the other di- rection (U22) - In the less oxygenated crystals, C3, C4, this is also the case for the yttrium ^site.

This can be explained ^{for the} Cu(l) ^{site be-}

cause, along ^the orthorhombic a axis (normal ^to

the 0(4) chains), ^{the Cu-Cu} repulsions ^{are un-}

screened by ^the vacant oxygen 0(5) site, ^which

enhances the potentials ^and consequently ^{the vi-}

bration amplitudes. However, ^{this is not the}

case for the Ba site. This anisotropy ^is probably

the indication of a correlated motion of blocks

Ba-0(1)-Cu(l). ^{in addition, the potential well}

of the Ba site reveals itself to be strongly ^anhar-

monic : a highly significant decrease of the R factor is obtained by introduction of _{tensors up} to the 4th rank (such ^as the contracted pseudo-

vector and 2nd pseudo-tensor ^{in order to in-}

troduce only ⁴ additional variable parameters).

In all crystals Ri /R2 ratio is in the _range of

1.10, ^much higher than the minimum statistical value Jazz ^{0.005= 1.009. On the} contrary ^the

Y and Cu sites do not exhibit _any appreciable anharmonicity.

Table V.- Anisotropic temperature factors

(A2X104 )Ior ^{crystal Cl.}

4. Conclusion.

In this work we have shown that the ho-

mogeneity range of the Y-Ba-Cu-0 system ^is still more complicated ^than initially presumed

and that the general ^formulas YBa2Cu3-z07-6

with a double non-stoichiometry ^{mechanism on}

Cu(l) ^and 0(4)-0(5) sites. This result obtained

on single crystals ^{is in} agreement with the lower

critical temperature ⁱⁿ crystals ^{than in} powders [14, ^15, 16]. ^{Works are in progress in order to}

determine the exact influence of the _copper de-

ficiency ^{on the} superconductive properties.

References

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