The La (Ba2-xLax)Cu3- yOz single crystals : structures and conductivity

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The La (Ba2-xLax)Cu3- yOz single crystals : structures and conductivity

G. Collin, A.C. Audier, P.A. Albouy, S. Senoussi, R. Comes, M.

Konczykowski, F. Rullier-Albenque

To cite this version:

G. Collin, A.C. Audier, P.A. Albouy, S. Senoussi, R. Comes, et al.. The La (Ba2-xLax)Cu3- yOz single crystals : structures and conductivity. Journal de Physique, 1989, 50 (1), pp.77-90.

�10.1051/jphys:0198900500107700�. �jpa-00210901�

The La (Ba2-xLax)Cu3-yOz single crystals : structures and

conductivity

G. Collin (1), A. C. Audier (1), ^{P. A.} Albouy (1), ^{S. Senoussi} (1), ^{R. Comes} (1),

M. Konczykowski (2) ^{and F.} Rullier-Albenque (2)

(1) Laboratoire de Physique ^des Solides, ^U.A. 02, ^Bât. 510, Université Paris-Sud, 91405 Orsay,

France

(2) Laboratoire des Solides Irradiés, ^Ecole Polytechnique, ⁹¹¹²⁸ Palaiseau, ^France

(Reçu ^{le 3} juin 1988, accepté ^sous forme définitive le 31 août 1988)

Résumé.

Les structures des phases orthorhombique ^et quadratique ^de

La(Ba2-xLax)Cu3-yO6+x/2- y+z ont ^été determinées sur des cristaux maclés. La structure

orthorhombique, observée pour x petit, ^est proche ^{de la structure Y-Ba-Cu-O} (macle

a * b

c-b

a

c), ^mais présente ^un très fort taux de défauts sur le site Cu(1) (~ ³⁰ %). ^Les

corrélations locales (03BE

20 Å) entre atomes de cuivre et lacunes, déduites de la diffusion des rayons X, correspondent ^{à une} ségrégation à ^courte distance des lacunes en chaînes. Comme

conséquence ^{du taux de défauts élevé, ces cristaux sont} des semi-conducteurs non typiques. ^La

structure quadratique, ^{x ~} 0.50, conduit à des cristaux trimaclés avec des fautes à 90°.

a

a

3 a-a

3 a

a-3 a * a

a (a, ^le paramètre ^{de la} perovskite). ^{Dans ces} matériaux les

potentiels des sites cuivre sont fortement anharmoniques. ^{Ceci est dû au} désordre introduit par la substitution La-Ba. Ces cristaux sont également semi-conducteurs avec une loi d’activation en

T-1/4, ^ce qui indique que la conductivité ^est de type variable range hopping, consequence d’une localisation par le désordre.

Abstract.

The crystal ^{structure of the} orthorhombic and tetragonal phases ^of

La(Ba2-xLax)Cu3-yO6+x/2-y+z ^are determined on twinned crystals. The orthorhombic struc- ture, obtained for low x, is close to the regular ^Y-Ba-Cu-O type (twin a ^{* b * c-b}

^a

c), ^{but is} highly copper deficient ^on the Cu(1) ^site (~ ³⁰ %). The local correlations (03BE ~ 20 Å) ^between

copper ^atoms and vacancies, ^as deduced from X-ray ^diffuse scattering, correspond ^{to a short-}

range segregation of vacancies in chains. As a consequence of the large ^{amount of} defects, ^these crystals ^are non-typical semiconductors. The tetragonal structure, ^{x ~} 0.50, ^{leads to} tri-twinned

crystals ^{with 90°} faulting, a

^a

^{3 a-a}

^{3 a}

^{a-3 a}

^{a * a} (a, ^the perovskite ^lattice constant). ^In

these materials the copper sites ^are found to be strongly anharmonic. This is due ^to the disorder introduced by the La-Ba substitution. These crystals ^are also semiconductors with a T-1/4

activation law for the conductivity which indicates that variable range hopping ^is expected ^{to set} in, ^a consequence of localization by the disorder.

Classification

Physics ^Abstracts

61.10

61.70B

74.70

1. Introduction.

The substitution of non-magnetic ions, especially lanthanum, ^to yttrium ^{in 1-2-3} type

compounds ^{is of} particular ^{interest. This} system ^{was first examined} by Er-Rakko et al. [1] long

JOURNAL DE

PHYSIQUE. -

50,

1,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500107700

before the discovery ^of superconductivity. ^{It was, of course,} recently reinvestigated by ^several

authors [2-28]. ^A systematic ^{search of} phases ^{in the} system La-Ba-Cu-0 was performed by

Torrance et al. [2, 3]. These authors mentioned the existence of non-superconducting phases, La6Ba4Cu3016 ^and La4BaCu5013, in addition to the known materials LaCu03 (perovskite- type) ^{and to the two} superconductors La2-xBaxCu04 (K2NiF4-type) ^and

LaBa2Cu306 + z

(YBa2Cu306 + Z-type). ^The outstanding features of the latter system ^are.

a) ^{In addition to a} total occupancy of the Y-type ^site by lanthanum, ^a partial substitution of

La 3+ ions on the Ba2 + site is observed.

b) ^{Some authors} [8, ^9, 15] ^{mention, for their} samples, ^a partial copper deficiency ^{on the} Cu(l) ^site, equivalent ^{to that} previously ^observed in Y-Ba-Cu-0 single crystals [29, 30].

Therefore, ^{the most} general ^{formula for these materials can be written}

c) ^Two phases ^{are described in this} system :

i) orthorhombic [10, 12-15, 17, 19, 20, 24, 25, 26], type YBa2CU307 (space ^{groupe P mmm,}

Z = 1), ^which corresponds ^to the lowest La-Ba substitution rates [18, 20], ^and

ii) tetragonal [1, ^4-10, 16-28], type YBa2Cu306 (space group P4/mmm, ^Z = 1), ^{for the} highest occupancy of the barium site by ^lanthanum.

For the first modification, ^an orthorhombic « tetragonal phase transition was observed in reference [12] ^{at about 350 °C.}

d) ^However, only ^{one reference was made to} single crystals ^studies [10] and the different structural determinations were exclusively performed ^with powders, using X-ray [1, ^{8, 9, 13,} 15, 27] ^{or neutron} [18-21, 39] diffraction. For X-rays, ^the différence between Ba and La (56

and 57 electrons) ^{is much too small to obtain a} precise refinement on the La-Ba substitution

rate. However, Nakai et al. [9] used anomalous dispersion ^effects (CrKa ^for Ba). ^{In the}

neutron case the difference is larger ^{with neutron} scattering amplitude ^b

^0.525 (Ba) ^and

0.827 (La). ^{This makes} possible ^a refinement on the average occupancy of the baryum ^site [18, 20, 21]. However, ^the problem ^{of the lower limit for x is not} definitively solved ; Segre

et al. [19] ^{indicate that, for x 0.25, the} occurrence of BaCu02 reflections suggests ^{that the}

homogeneity range is 0.50 , x , 0.25, whereas, ^for Sunshine et al. [20], ^this corresponds only

to a partial decomposition and values of x lower than 0.25 can be reached (cf. ^{also Ref.} [24]).

e) ^{For most} of the authors only ^the orthorhombic phase ^{exhibits a} superconductive

transition [10-12, 14, 15, 19, 25, 28] ^with 7c ^{in the 90 K} range. But references [4, 5, 6, 9, 17, 20] suggest that, ^{even in the} tetragonal samples, ^a superconductive transition exists with a

lowered Tc === 50 K for intermediate La/Ba ^ratio (x === 0.25). But, for reference [19], ^the

occurrence of superconductivity is associated with a homogeneity range of the orthorhombic modification up ^{to x}

0.375, ^with T,, decreasing ^{when x} increases. Some authors [10, 15, ^22, 23, 26, 27] report 7c (onset) ^{of 75-90 K for} tetragonal samples ^{which is} probably ^{due to a}

fraction of the orthorhombic phase ^{in the} sample. ^{However, in most cases the} transitions are

broad for these materials, ^{in the} range 8-10 K for the best defined, ^{and the} temperature difference between onset and zero resistivity ^{can be as} large ^{as 20-30 K} especially ^for

materials in the low Tc range. In addition 7c appears ^to be _very sensitive to the oxygen

concentration, ^{more than in Y-Ba-Cu-0 materials as shown in} La,Ba2CU30y ^{[40] and in} La1.1Ba1.9CU30y ^{[41], both} orthorhombic.

Finally, ^{for most of the authors, the} tetragonal modification with large x ^values

(-- 0.50) ^{in the} general formula is a semiconductor without any indication of superconduc-

tivity, ^{even for} completely oxygenated ^materials.

It is rather difficult to compare the different structural results because, ^{in addition to the}

copper deficiency ^mentioned later, ^the reported oxygen concentration varies notably ^between

the authors, ^{from 6.4 to} 7.25 per unit formula in the tetragonal phase ^{and from 6.7 to 7.25 in}

the orthorhombic modification. Moreover, the lattice constants appear ^{to be} somewhat different from one result to the other. In annex 1 we have reported ^{an the} available results

concerning these lattice parameters. ^{For the} tetragonal phase the lattice constants are

respectively ^between 3.90 À and 3.94 A for a and between 11.69 À and 11.84 À for c. For the orthorhombic modification the average value [(ï ⁺ b)/2] is between 3.89 A ^to 3.94 A with a

b-a splitting ^{from 0.002 A to 0.051 À and with the c} parameter ^from 11.70 À to 11.85 À (the

case of Ref. [37], ^this work, ^{will be discussed} below). The determinant parameter ^{is the}

preparation conditions : the largest ^{c lattice constants} correspond ^{to materials} prepared ^and

annealed at higher temperatures (T::. ⁶⁰⁰ °C ) whereas the lowest values are obtained when the materials are reannealed in oxygen ^at lower temperatures (T _ ⁶⁰⁰ °C ). This result is similar to that obtained with Y-Ba-Cu-O in which a low oxygen concentration (high temperatures) ^{leads to} large ^{c lattice constants} while materials carefully reannealed at lower temperatures exhibit smaller ^c parameters. ^In addition, ^this interpretation is confirmed by ^the comparison of the different values for variable parameters ^as deduced from the structure

determinations. Indeed, ^{in the} tetragonal phase ^for example, ^for poorly oxygenated ^materials (9a, 9b, 14, 18a, 39a), ^{the z} coordinates of Ba and the bridging oxygen ^are close to the

corresponding ^{values obtained in} YBa2Cu3 -y06 ^and, for reannealed materials (8, ^{18b, 20, 21,} 39b), they ^are comparable ^to the values of YBa2CU3-yO7-y ^[29].

In this paper ^we investigate structural and transport properties ^of single crystals corresponding ^{to the two} modifications, tetragonal ^and orthorhombic, ^{and we} established that the copper deficiency ^{is a} fundamental parameter ^{of this} system.

2. Experimental.

Single crystals ^were prepared by ^{a flux method} BaCu02 ^and CuO) ^{between 1 000 and 880 °C.}

Two types ^of crystals ^{were obtained.}

a) ^{Some of} them, exclusively ^{on the surface of the} preparation, ^{are thin} platelets ^with typical dimensions 0.5* 0.5* 0.05 mm3 and correspond ^to the orthorhombic modification.

These crystals ^{exhibit the usual} twinning (a b c ) - (b ^a c) ^of orthorhombic Y-Ba-Cu-O single crystals ^{as shown on the} peak profile ^{2 0- 0} scan) ^{of the}

²⁰⁰

reflection, ^as compared ^{to the}

unbroadened 006, ^which presents ^a shouldering ^{due to the} partial overlap ^{with the 020}

reflection of the second twin (Fig. 1).

b) Most of the crystals exhibit cubic shape, with linear dimension 0.15-0.5 _mm, and

correspond ^{to the} tetragonal phase. ^These crystals appear ^{to be} systematically tri-twinned.

Indeed the two layers ^hko (Fig. 2) ^{and hol} (not represented) ^are strictly equivalent ^{and lead to}

an apparent ^cubic symmetry ^with a 3 a * 3 a * 3 a unit cell. But the superstructure reflections

are only present along the fundamental rows of the perovskite ^lattice (with ^{two indexes}

=

3 n). ^This corresponds ^{to the} superposition ^{of the} diffraction due to three tetragonal samples with lattice vectors respectively (a ^{a 3} a), (a ^{3 a} a), (3 ^{a a} a) ^{where a refers to the} perovskite ^lattice constant - 3.90 A. A similar pattern ^{was obtained} by several authors [4, 37, 38] by electron diffraction. There is only ^{a weak} broadening ^{of the} substructure Bragg peaks

which indicates that, ⁱⁿ spite ^{of a} tripling ^{of one} of the three parameters ^{in each one of the} samples, ^the symmetry ^{of the} perovskite sublattice remains quasi-cubic. ^In previous ^studies (annex la), ^{this is also the case for most of the} _published data in which these values should not

introduce, ⁱⁿ single crystals, ^an appreaciable broadening ^{of the} substructure reflections

because of the quasi equivalence ^{between 3 a and c.}

Fig. ^{1. - Peak} profiles ^{for the} La(Ba2-.,La,,)CU3-YO6,.,/2,,, orthorhombic crystal (2 (J-(J ^scan,

Mo Ka) : a) ^{the « 200 »} reflection, with the characteristic shouldering ^{due to the} orthorhombic

twinning, ^and b) the 006 reflection which is not affected by ^the twinning.

Fig. ^2.

Precession photograph (Mo Ka) ^{of the hkO} layer ^{of the} La (Ba2 -.,La,,)CU3 - yO6 + xl2 + z

tetragonal crystal with the 90° faulting leading ^to the characteristic tri-twinning ^{of these} crystals.

The data collection for the two types ^of crystals (after reannealing ^{in an} oxygen flux for 3 days ^{at 450} °C) ^was performed ^{on a half} sphere ^of reciprocal space up ^{to 2 0}

65° and 90°

respectively ^{for the} orthorhombic and tetragonal samples (MoKa, ^scan range w

3° which

corresponds, ^{in the} orthorhombic _{case, to} an integration ^{of the two} types ^of reflections, ^hkl

and khl, resulting ^{from the} twinning, Fig. 2). ^After absorption corrections, ^the independent

reflections were obtained as an average of the equivalent reflections : 4 and 8 for the orthorhombic and tetragonal samples, respectively. ^{In both cases} the whole sets of

independent reflections were used in the refinement, including ^{those with zero} intensity.

3. Crystal ^structure determination.

3.1 ORTHORHOMBIC CRYSTALS.

The procédure ^{used was} previously ^{described in} référence [29] ; calculation of Fhkl [(ci, Fhki )2 +(a2 Fkhl)2] and associated derivatives

including ^the expansion ^{of the} Debye-Waller factor up ^to the 4th rank in tensors [31]. ^The

final values of the parameters ^are given ^{in table I.}

The structure of these crystals ^{is identical to that of} YBa2Cu3 - y07 -

^with especially ^a pronounced anisotropy ^{for the} heavy ^atom Debye-Waller factors, Ul,

Un [29]. ^{In addition,}

Table I.

Structural parameters of ^the orthorhombic modification of La (Ba2 _ xLax )Cu2.69 (1)C)6.84 (8) > (crystal Cl ).

Anisotropic temperature ^factors (Â2 ^x 104) ^for crystal ^Cl

a

3.936(1) Â*, b

3.930(1) À*, ^c

_11.677(3) ^{Â. R}

^{2.09 %} for the 428 independent

reflections (1.94 % ^{for the 362} reflections with 1 :> 3 00). Twinning ^ratio

Vl/(vi + V2)

^0.393(7).

*

The lattice constants are obtained from a routine reflection centering (25 reflections) ^which

underestimates the orthorhombic distorsion because of the partial overlap of hkl and khl reflections.

Fig. ^3.

Monochromatic Laue X-ray ^diffuse scattering pattern (Mo Ka) ^{of the}

La(Ba2-.,La,,)CU3-yO6,,,/2-,,, orthorhombic single crystal. X-ray beam normal ^to the a-b plane. ^The

diffuse spots correspond ^to the intersection of diffuse rods with the Ewald sphere.

in these particular crystals, ^the Cu(l) site appears ^to be highly deficient, up ^to 30 % vacancies

(-- ^{3-8 % in} Y-Ba-Cu-O), whereas the other sites do not reveal any deficiency. ^This suggests, in agreement with the observations relative to the preparation, ^that the orthorhombic crystals

grow in the high temperature part ^{of the} crystallisation process, far from the stability range of the

stoichiometric

material, ^even in the presence of ^{an excess} of CuO (from ^the flux). ^The

copper defects lead i) ^{to a} disorder in the oxygen occupancy of 0(4)-0(5) ^{sites, even for}

oxygen annealed crystals (in ^{the same} conditions Y-Ba-Cu-O crystals ^are quasi-perfectly

ordered in chains Cu(1)-0(4) ^with empty 0(5) ^site (29)), ii) ^{to a} high ^vibration amplitude ^of

the Cu(l) site with Ul, -- U22, ^{whereas one} observed smaller values with U11:::> U22 ^{in the} yttrium materials and iü) ^{to the} occurrence of a 2-D short-range ^order giving ^{rise to the} X-ray

diffuse scattering pattern ^of figure ^3.

The diffuse spots ^{observed are the} intersection of diffuse rods with the Ewald sphere, ^which corresponds ^to two-dimensional local correlations, ^{with a coherence} length %. ²⁰ Â. These diffuse spots, ^which correspond ^to reflections (h ± 1/3 ), k ^and h, (k ± ^1/3 ), ^{define a local 2-D} superstructure cell, involving ^a superposition ^{of domains} with lattice vectors 3 a* a and a* 3 a

(a ^{is the} perovskite ^lattice constant). The best fit of the diffraction pattern (using ^{the Von}

Laue formalism) ^is obtained with local segregations ^{of -.r} 4-5 copper vacancies in chains ^on nearest neighbour ^{sites. All the other} models introduce diffuse scattering ^at positions ⁱⁿ reciprocal space ^where nothing ^{is observed.} However, ^{it is} impossible ^to get ^the exact contrast between reflections because the occurrence of chains of vacant copper sites introduces relaxation on the adjacent ^sites (Cu, ^{0 and} perhaps Ba), relaxation difficult to appreciate given ^{the weak} intensity of the diffraction pattern.

We should mention that we investigated ^another crystal ^{of this} _type, non-reannealed in an

oxygen flux. This leads exactly ^{to the same} result ; final formula LaBa2Cu2.7o(i)06.go(8)’ ^{with a}

R factor 3.0 % and interatomic distances strictly equivalent ^{to those} given in table III.

Table II. - Structural parameters o f ^the tetragonal modification o f ^La (Ba2 - xLax )Cu3 - y07 - z (crystal C2).

Anisotropic temperature ^factors (Â2 ^x 104) ^for crystal ^C2

a

3.920(1) Á, ^c

11.752(3) ^{Â. R}

2.95 % for the 511 independent reflections (2.08 ^%

for the 309 reflections with 1:::. 3 a-). Twinning ^ratio (Vil.!i Vi) V khl

0.338(3), V1hk

0.331(3), Vklh

0.330(3).

Table III.

Interatomic distances (Â).

All the standard deviations are -- 0.01 (À).

^{and **, one or} both sites are partially ^vacant (cf. ^{Tabs. I}

and II). For distances involving 0(2), 0(3), 0(4) ^and 0(5) positions, ^{see remark} (*) ^{table 1.}

3.2 TETRAGONAL CRYSTALS.

The data collection was performed using ^{one of the}

a * a * 3 a cells. Under these conditions there are two types of reflections :

-

the substructure reflections, ^{with 1}

³ n, which are at the intersection of the reciprocal

rows corresponding ^{to the three} samples ^{of the twin} (Fig. 2) ^{and on which the} contributions of the three orientations are summed, ^and

-

the superstructure reflections, ¹ #= 3 n, which correspond ^to only ^one sample.

The refinement program ^was adapted ^{in order to} incorporate ^this particular ^{case of} twinning ^{with the normal} calculation for the superstructure reflections calculated and substructure reflections corresponding ^to

with the associated derivatives including ^the twin ratio ( a i ) ^and expanded Debye-Waller

factors. The validity ^{of this model was tested} using ^two separate ^{scale factors for the} substructure and superstructure reflections. After a few refinement cycles ^a spontaneous

equalization ^{of the two} scale factors was obtained which is the confirmation that the crystals correspond ^{to the} expected tri-twinning ^of tetragonal samples.

The first step ^of refinement led to an R factor 3.4 %. However, ^the U33 components ^of the Cu(l), partially ^{vacant, and} Cu(2), fully occupied, ^{sites were close to zero. This} problem

was previously ^mentioned by ^{Izumi et al.} [13] ^on X-ray powder refinement and these authors

were obliged ^{to constrain the} temperature ^{factors to} arbitrary fixed values. Contrary ^{to the}

first thought ^{which comes to} mind, this does not correspond ^to incomplete absorption

corrections because calculations with increased absorption ^factors (IL

300 cm - 1) ^{led to the}

same result but with increased R factor. Moreover, ^{the same result was obtained with}

different crystals and appears ^to be a fundamental characteristic of this tetragonal system especially ^{since it was found} systematically ^on isomorphic praseodymium ^materials [32].

Fourier syntheses ^were calculated with a set of untwinned reflections obtained using ^the approximation

and unchanged superstructure reflections. The R factor corresponding ^{to this new set of}

reflections was 3.8 %, ^{in the same} range ^as the initial one.

A difference synthesis (F.b, - F.1c) ^{revealed a} strongly anharmonic behaviour of the copper ^sites with non-symmetric ^residual positive ^and negative electronic densities. A full tensor expansion ^was introduced in the refinement for these two sites. The convergence ^was slow but, ^{after several} cycles, ^a positive ^{value for the} U33 component ^{of the} temperature ^factor

was obtained for both Cu(l) ^and Cu(2) positions. These results are to be compared ^{to the} corresponding ^ones in orthorhombic La-Ba-Cu-0 (crystal Cl, ^this work) ^{and Y-Ba-Cu-0} [29]

which do not exhibit _any appreciable deviation from harmonic distribution for the

corresponding ^sites.

Since a similar behaviour was observed in the tetragonal crystals ^of

Pr(Ba2-,,Pr,,)CU3-yO7:,, ^and of the solid solution Y1-vPrv(Ba2-xPrx)Cu3-y07:tz ^with

v -- 0.5 [32], ^{this means} that the effect of (large) ^Ba-L substitution introduces, ^{in addition to a} tetragonal symmetry, ^a deep modification of the electronic distribution of the copper sites with respect ^to weakly ^or non-susbtituted materials. Indeed, this effect does not result only

from the tetragonal symmetry because in the tetragonal ^material YBa2Cu2,9a06 [29] ^{such an}

anharmonicity of the copper sites ^was completely ^{absent as it is in} orthorhombic crystals. ^{It is}

probably ^{due to different Coulombic} repulsions ^for Cu-Ba2 + and Cu-La3 + which leads to different equilibrium positions according ^{to the nature of the ion present on the Ba site.}

Moreover, ^{this is} associated with large in-plane thermal vibration amplitudes, especially ^for Cu(l) and the coordinated oxygen positions, 0(1), 0(4-5), appreciably larger ^{than in Y-Ba-}

Cu-0 single crystals.

The refinement on the occupation factor of the bridging oxygen, 0(1), ^{led to a value}

0.98 (5) which indicates that the site is completely ^{filled in} agreement ^with Nakai et al. [9] ^and

in contradiction with Izumi et al. [8] ^{who mentioned an} occupancy of 0.82 (9) for this site.

Another crystal, non-reannealed, ^{was also} investigated ^{and led to} the formula

LaBa2CU2.83 (1)06.8o (8) with a R factor 3.0 % and atomic parameters quite ^{identical to those} reported in table III.

The copper deficiency ^{in these} tetragonal crystals ^occurs again exclusively ^{on the} Cu(l) ^site

but is smaller than in the orthorhombic modification which results from a lower growth

temperature. However, ^{the amount} of vacancies appears ^to be systematically higher ^{than in}

the Y-Ba-Cu-0 single crystals.

3.3 INTERATOMIC DISTANCES.

The interatomic distances reported in table III are very similar for the two types ^of crystals. Especially ^{for the} Ba(La) ^{sites, it is not} possible ^{to deduce}

any indication for a difference in the substitution level. A comparison ^with published ^results

shows that the average Ba(La)-Oxygen ^distance depends ^{more on} the oxygen composition

than on the substitution level and is shorter for the highest oxygen concentration in the

general ^formula [18, ^20, 21] ^{than for more deficient materials} [8, ^9, 13]. The results of refinement indicate that the La-Ba substitution is present ^{in both} types ^of crystals, ^{with an}

excess of oxygen with respect ^{to the} LaBa2CU3-y(:)7-y formula, with, ^as expected, ^a larger

excess in the tetragonal modification which corresponds closely ^{to x}

^0.5.

The Cu(2)-0(1) ^distance, along ^{the c} axis, ^is short, = ^2.20 A for high oxygen concentration and appreciably larger - ^2.37 À in the opposed ^case (2.43 ^{A in} YBa2Cu3 - y06). ^{Our materials}

correspond ^to intermediate values = 2.30 A, ^{in the same} range ^as observed in

YBa2Cu3 _ x0, . ^{This is} probably ^{due to the} highly defective character of the Cu(l) ^{site which}

leads to Cu(l)-0(1) ^distance appreciably ^{shorter than in} the other structures (and consequently larger Cu(2)-0(1) distances). For the other distances, ^{all the} results, including

those of this work, give ^{values close to one another.}

4. Resistivity.

The resistivity measurements were carried out on both types ^of samples. The indium contacts

were ultrasonically ^{soldered on the corners of the} samples. Typical dimensions of the crystals

were 0.5

0.5 * 0.05 mm 3 for the orthorhombic samples ^{and 0.25}

^{0.25 * 0.25} mm 3 for the

tetragonal ^{ones. For both} types ^of crystals, ^{below 80} K, ^{the contact} resistances becomes

prohibitive ^to any reliable measurement.

In figures 4a and b the typical temperature dependences of the resistance are shown for orthorhombic and tetragonal samples respectively. ^Both types ^{exhibit a} semiconductor

like behaviour. Some anomalies (breaks) ^{are observed} between 100 and 200 K, depending ^{on the} sample. ^{No clear} explanation ^{of these} anomalies could be found which could correspond ^to

structural phase transitions, magnetic ordering, ^etc. However, ^the polaron ^effect explanation

is also possible [25].

The resistivity ^{of the} orthorhombic samples ^was only measured in the a-b plane. They

exhibit specific resistivity ^{of 150 0* cm at room} temperature. ^Their resistivity increase is

typically ^{of one order of} magnitude ^{between 300 and} 80 K. Moreover no usual law can be

Fig. ^4.

Resistivity ^versus temperature ^for single crystals ^of La (Ba2 - xLax )CU3 - ,06 + x /2 + z : ^{a) in the}

a-b plane for orthorhombic crystals ^and b) ^for tetragonal crystals. ^{The inset} corresponds ^{to a} fit with the

T- "4 law.

fitted to these variations and the best approximation ^{was obtained with} log (R) proportional

to T. A similar behaviour was observed by Tastardi et al. [26] ^{in the} tetragonal ^non- superconducting phase ^of YBa2Cu306 + x.

The tetragonal-type samples present ^a specific resistivity of 1-10 0* cm at room tempera-

ture and their resistance increases more than two orders of magnitude ^{between 300 and 80 K.}

For the temperature range above the kink the resistivity ^{can be fitted} by ^{the relation}

as can be seen in the inset of figure ^{5b. The value of} Q/k ^{is 7 * x 107 K.} This behaviour is similar to that observed in La2Cu04 [35] ^{and in Y} (Ba2 -.,La,,)CU307 +, [36] samples. ^{This can}

be interpreted ^{in terms of variable} range hopping conduction due to the distortion of the copper planes ^induced by ^the random distribution of La 3+ on the Ba2 + sites. This suggests that, ^{in these} materials, ^{with the same} type ^of hybridization ^between copper and oxygen ions

as in YBa2CU307 (the ^{same Cu formal} valency), the carriers are localized by ^{the disorder and}

the materials exhibit a semiconducting behaviour instead of the metallic character of the

yttrium compound. ^{This of course} prevents ^the occurrence of a superconductive transition.

5. Conclusion.

Because of the preparation method which requires ^{much too} high ^a temperature, ^the single crystals ^{of the} La-Ba-Cu-O system ^exhibit large copper deficiency. ^{This is} especially ^{true for}

the orthorhombic modification which is stable only with low La-Ba substitution level.

Experiments ^with powders ^indicate that this substitution level decreases with increasing temperature. Preparations ^at high temperatures ^lead consequently preferentially ^{to this} phase. ^But, under these conditions, ^the

stoichiometric » material is not stable and large

amounts of copper vacancies ^are introduced in the crystals ^{which do not exhibit the} expected superconductive transition. Lowering ^the preparation temperature ^{increases the La-Ba} substitution (which ^{effect is to} interchange ^the packing along ^the tetragonal ^{a and c axes and is} responsible ^{for the} tri-twinning) ^{and also} destroys ^the superconductivity. ^{This does not occur}

in Y-Ba-Cu-O single crystals because Y and Ba are very different and cannot be easily replaced by ^one another, ^{at least} in usual conditions. This is why alternative methods for

crystal growing ^of La-Ba-Cu-O are to be investigated ^{in order to} eliminate this double

inconvenience.

Annex la : Lattice ^constants (À), composition ^and Tc ^{for the} tetragonal samples (space

group P4/mmm).

Neut. for neutron, elect. for electron, P for powder, ^{C for} single crystal. ^For Tc only ^{the onset} temperature ^is given. Values with

are deduced from refinement or from chemical analysis. Empty

columns correspond ^{to no} indication in the given reference.

this work.

Annex lb : Lattice constants (Â), composition ^and Tc ^for the orthorhombic samples (space

group Pmmm).

Neut. for neutron, P for powder, ^{C for} single crystal. ^For T,, only ^{the onset} temperature ^is given.

Values with

are deduced from refinement or from chemical analysis. Empty ^columns correspond ^{to no}

indication in the given reference.

this work. In the latter case, also see remark (*) of table 1.

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