X-ray single crystal analysis of HgBa2Ca2Cu3O8+δ and influence of oxygen stoichiometry on the superconducting properties

PIIYSICA® ELSEVIER

Physica C 250 (1995) 213-221

X-ray single crystal analysis of HgBa2Ca2Cu308+ ~ and influence of oxygen stoichiometry on the superconducting properties A. Bertinotti *, D. Colson, J. Hammann, J-F. Marucco, D. Luzet, A. Pinatel, V. Viallet CEA-Saclay, Service de Physique de l'Etat Condense, DRECAM / SPEC, 91191 Gif sur Yvette Cedex, France Received 17 May 1995

Abstract

X-ray diffraction analyses of good-quality single crystals of HgBa2Ca2Cu3Os+8, synthetized by a closed-vessel technique at low pressure, have lead to more definite results with respect to previous structural studies. Samples with Tc = 135 K showed a Cu occupancy of 16.1% on the Hg site at the origin of the unit cell. Excess oxygen appears to be present only in the basal plane at the interstitial site 1, ½, 0 with an occupancy 6 = 0.190 + 0.015. This compound has the shortest copper oxygen apical distance within the mercury family (2.696(3) A) together with an almost complete planarity of the CuO 2 planes. Variations in the excess oxygen content upon different heat treatments were analyzed by thermogravimetry and magnetic-susceptibility measurements. A correlation between ~ and Tc could be derived, showing in particular that superconductivity exists down to a very low interstitial oxygen content.

1. Introduction

The discovery of the mercury cuprates, HgBa 2Ca,_ 1Cu,O6+ 8, in 1993 [1] launched a vast amount of investigation. This family of compounds displays the highest known values for the superconducting transition temperature, T¢ = 135 K for the n = 3 compound (Hg-1223). The studies of these materials, however, were restricted to powder samples before a specific technique for growing single crystals could be designed. Obtaining single crystals, of course, is an important step for the detailed study of directiondependent physical properties, especially in these

* Corresponding author.

highly two-dimensional systems, but also for the precise characterization of the structural phases. Two basic requirements that a crystal-growth technique must fulfil is the achievement of crystal sizes appropriate to the desired physical measurements and the need for defect-free lattices. This last requirement is particularly critical in the case of the mercury cuprates where intergrowth in the same crystal specimen of various members of the family may be a conspicuous and not rare phenomenon. A first attempt at an X-ray structural study, based on micro-crystals of Hg-1212 and Hg-1223 emphasized the need for good-quality crystals in order to answer all the questions raised by the type of structural defects which may be present in these systems [2]. The first work involving large-enough single crystals not only for the X-ray structural work [3] but

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A. Bertinotti et a l . / Physica C 250 (1995) 213-221

primary importance for the physical properties. Structural models have been introduced displaying, for example, a split atom at the barium site [7], or at the oxygen site [2], or displaying excess oxygen atoms out of the basal plane [2,8], or out of the center of the basal plane [2], or displaying the presence [8] (or the absence [7,9]) of substitutional defects at the mercury site. It may be that this variability reflects an intrinsic disorder within the block layer itself. It may also be that the variability depends on the technique of preparation, or on the number of copper layers, i.e. on the particular structure type. The fact remains that a prerequisite for addressing these rather involved structural problems is the availability of sizeable well-ordered single crystals. The second aspect of this paper concerns a systematic study of weight variations by thermogravimetry and the corresponding changes in the magnetic behavior of a Hg-1223 sample submitted to various annealing treatments under flowing oxygen or nitrogen. Associated with the results provided by the X-ray study, the curve relating the dependence of the critical temperature versus oxygen doping will be considered.

Fig. 1. Scanning electron micrographs of millimetric Hg-1223 single crystals.

also for magnetic [3,4], electric-transport [5] and specific-heat [6] measurements was made on Hg-1223 samples, with typical sizes of 0.25 × 0.30 × 0.30 mm 3. The crystal-growth technique that we designed has been improved and is now able to provide in a reproducible way high-quality crystals of millimetric sizes, not only for the Hg-1223 compound as shown in Fig. 1, but also for other members of the mercury family. The purpose of this paper is to present results on Hg-1223 using single-crystal X-ray analysis to determine the substitutional defects occurring on the mercury sites and the excess oxygen responsible for the superconducting properties. As a matter of fact, contrasting data have been published up until now conceruing the atomic structure of the block layer in the mercury family, particularly in the basal plane, of

2. X-ray single-crystal analysis Single crystals from a post-annealed batch showing a superconductivity onset at 135 K were selected for the structural X-ray work. As described previously [3,4], intergrowth phases stacked along the c direction give rise to streaks visible on oscillation photographs, each streak corresponding to a constant value of the Bernal chart. A sample with minimal streaking of diffracted spots was chosen with the following dimensions: 0.20(2) × 0.21(9) X 0.08(8) mm 3. The data of the X-ray analysis at 293 K, collected with an ENRAF-NONIUS CAD-4 diffractometer, are given in Table 1. Counter aperture, scan angle and crystal/counter coupling were optimized in order to avoid the overlap along the [00l] direction between neighboring reflections resulting from the presence of intergrowth phases. Reciprocal space was scanned out to Omax = 40 ° with indices from h = - 6 - - * +6, k = 0 - - * +6, l = 0 ~ 2 8 . Three standard reflections were monitored every 4 h. A

A. Bertinotti et al. / Physica C 250 (1995) 213-221 Table 1 X-ray data at 293 K of Hg-1223 a a = 3.851(2) ,~

Tetragonal Mo Ket radiation A = 0.70930 ,~

c = 15.830(7) V = 234.7(9) ,~3

Space group: P 4 / m m m

a Cell parameters derived from 25 reflections ~9 = 10°.6-24°.3.

total number of 1602 reflections were measured, reducing to 511 independent ones. During the datacollection time the loss of intensity was 0.9%. Numerical absorption corrections making use of the crystal shape to calculate the X-ray path length were applied with an inverse absorption length of /x = 323.39 cm-1. Secondary extinction corrections with a coefficient g = 7.35 × 1 0 - 6 w e r e also applied. The refinement based on structure factors, 506 significant observations and 30 parameters led to a reliability factor R = 0.039 (R w = 0.063). The adopted weighting scheme included an additional term to the error due to statistical fluctuations, in order to down-weight the intense reflections. The expression adopted for the variance of the intensity O r 2 ( / ) is of the type t r 2 ( l ) = ~2(1) + ( p / ) 2 with P = 0.02. In a first step, letting the atomic scattering factor of the mercury atom be variable, it became evident that the site at the origin was not fully occupied by

215

an Hg atom. Owing to the fact that our preparation method excluded any possible contamination by CO 3 ions (no carbonate precursor, synthesis in sealed evacuated quartz tubes), only a mixed occupancy by Hg and Cu atoms was considered. Two types of constraints were introduced, one imposing an 100% occupancy for the sum of both atoms, and one imposing the same thermal parameters for both species occupying the same site. Fourier differences gave indication of the presence of a residual electron density at the position 1, 1, 0. This residual peak could support the presence of an interstitial oxygen, but with a partial occupancy of 0.190 + 0.015 associated with a Debye-Waller temperature factor of B = 1.95 + 0.25. No other stable position for the excess oxygen atoms could be detected, without leading to grossly diverging thermal parameters (Table 2, Fig. 2). Our present X-ray single-crystal results should be compared with the two other structural studies on the same compound, one by X-ray diffraction made on a single crystal [2], and one by neutron diffraction on powders [7]. Before investigating the details, some differences are noticeable between the unit-cell parameters (Table 3). There is agreement between the c value of the present study and the value of Finger et al. [2], but a difference of more than 0.04 A with Chmais-

Table 2 Refined structural parameters for Hg-1223 at room temperature

Hg Cu Ba Cul Cu2 Ca O1 02 O3 04

Occupancy

x

y

z

Ull

U22

U33

Beq/iso

0.83(9) 0.16(1) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.190

0.0 0.0 0.5 0.0 0.0 0.5 0.5 0.5 0.0 0.5

0.0 0.0 0.5 0.0 0.0 0.5 0.0 0.0 0.0 0.5

0.0 0.0 0.1752(5) 0.5 0.2998(4) 0.3980(3) 0.5 0.3005(2) 0.1295(2) 0.0

0.01725 0.01725 0.01043 0.00610 0.00637 0.00841 0.00928 0.00647 0.02001 -

0.01725 0.01725 0.01043 0.00610 0.00637 0.00841 0.01436 0.01448 0.02001 -

0.01023 0.01023 0.01117 0.00896 0.01000 0.00789 0.01263 0.01663 0.02548 -

1.18 1.18 0.84 0.56 0.60 0.65 0.95 0.99 1.72 1.95

Selected interatomic distances (,~) and C u - O - C u angle Hg-O3 Hg-O4 Cul-O1 Cu2-O2 Cu2-O3 Ba-O2

2.050(3) 2.723(2) 1.925(6) 1.925(6) 2.696(3) 2.764(2)

Ba-O4 Ca-O 1 Ca-Cul Ca-O2 Ca-Cu2 Cu2-O2-Cu2

2.774(3) 2.512(7) 3.165(7) 2.468(0) 3.135(7) 179.35 °

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A. Bertinotti et a l . / Physica C 250 (1995) 213-221

O'

02

Fig. 2. Crystalstructuralof HgBa2Ca2Cu308+8"

sem et al. [7].. Conversely the a parameter agrees within 0.001 A with the neutron value but differs by almost 0.01 .~, with Finger et al. It is well known that the unit-cell parameters, in particular a, correlate directly with the value of Tc. Concerning the fractional occupancy of the mercury atom site our results are in agreement with Finger et al. [2], with a value for copper of 16.1%, slightly lower than the reported value of 18.1%, whereas no copper substitution was seen in the neutron study.

Table 3 Cell parametersof Hg-1223 obtainedby differenttechniques Present study X-raysingle crystal[2] Neutronpowder[7] a = 3.851(2)A a = 3.842(5).A c = 15.830(7).A c = 15.832(8)/~

a = 3.8501 .A c = 15.7837(9),A

Regarding the excess oxygen our results are not in agreement with the sites reported in the previous X-ray study [2] but agree with the interstitial site described by Chmaissem et al. [7]. The occupancy we found, however, appears to be significantly lower; 0.190 + 0.015 versus 0.41(2) [7]. As already observed by previous X-ray studies [2,3], we found no evidence for the presence of a split Ba atom suggested by Chmaissem et al. [7]. A difference is also observed for the position of the apical oxygen, oWe observe the shortest Cu2-O3 distance 2.696 A compared to 2.751 ,A and 2.782 ,A reported, respectively, in Refs. [7] and [2]. ConverselYoWe observe the longest Hg-O3 distance of 2.050 A compared to 1.950 A [7] and 1.975 ~k [2]. The apical distance of 2.696 ,A appears to be the shortest of all those observed in the pyramids of the mercury family (reported to be 2.780 ,~ in Hgla2CuO4 + 8 [8] and 2.775 ,A in HgBa2CaCu206 +o8 [9]), but still greater than the value of 2.298 A observed in YnaECU307. A last feature concerns the deformation of the CuO 2 planes not located in the mirror plane. Our results reinforce the previously observed planarity of the copper oxygen layer [3]. This is illustrated by the out-of-plane distance 8 z of the 0 2 atom with respect to the t]lane formed by Cu2 atoms, which goes from 0.084 A [7] and 0.024 A [2] to 8z -- 0.011 A in the present study. This is evidently reflected also in the bending of the Cu2-O2-Cu2 angle which goes from 175°.47 [7] and 178°.59 [2] to 179°.35.

3. Thermogravimetry and magnetic-susceptibility measurements In order to obtain information about the doping process, samples of Hg-1223 were submitted to various annealing treatments in an Ugine-Eyraud- B 60 thermobalance. This device has a useful sensitivity of 0.015 mg, which corresponds to A 8 = 0.007 for the quantity of sample probed. Details of the experimental apparatus have been previously described [10]. After each heat treatment, the evolution of the sample properties were checked by magnetic-susceptibility measurements. These were derived from DC magnetization measurements performed with a

A. Bertinotti et al. /Physica C 250 (1995) 213-221 Table 4 Thermogravimetry data and Tc measured after each treatment by magnetization measurements Annealing temperature (°C)

Heating atmosphere

Oxygen content z z = 8+ 8

Tc onset (K)

as-grown 300 260 300 400

vacuum O2 02 N2 N2

8.025 a 8.19 (reference) 8.27 8.09 8.02

115 135 133.5 130 110

217

135 130

125~ (3 I-"

120

115 110 8.(30

The oxygen content of the as-grown sample is estimated from Fig. 3. a

e.o5

8.,o s.~s 8.20 8.25 Oxygen Content z

8.30

Fig. 3. Critical temperature Tc as a function of oxygen content z

(z=8+ 8). SQUID magnetometer (Cryogenic Limited S 600) in an applied field of 10 Oe. Classical zero-field cooled (ZFC) and field-cooled (FC) procedures were used. For each run, the sample was first cooled down, from above the critical temperature To, in the residual field of the magnetometer (smaller than 3 mG). The field was applied at the lowest temperature and the ZFC magnetization was recorded as a function of increasing temperatures up to T > To. The sample was subsequently cooled down in the applied field and the FC measurements were taken again with increasing temperatures. The shape of the sample, hence the demagnetizing factor, remained the same for all runs. This factor was determined by assuming

that the largest observed ZFC response at low temperature corresponds to a complete shielding (4"rrX = - 1). The assumption led to a demagnetizing factor consistent with the approximate value estimated from the shape of the sample. The susceptibility data shown throughout this paper have been normalized taking this factor into account. In a first step of our study aggregates of single crystals were placed in an alumina crucible under oxygen at 300°C during 3 days. To our surprise the weight remained constant within A8 = 0.007. The

300

Hg Ba 2 Ca 2 CU 3 O8,z

,-5 iu o

-el

o

c

=

o~

o~

o o

r-

0 20

3'0

4'0

5'0

6'0

70

20 {deg) Fig. 4. Powder X-ray diffraction pattern using Ca Kc~ radiation for an as-grown Hg-1223 sample (impurities of CuO and an unknown phase are marked by ( + ) ) .

218

A. Bertinotti et al. / Physica C 250 (1995) 213-221

0.0

0.0

FC f

~< "~"

-0.5

~-, ZFC

o~

! J_ /

2

'~

-0.5

/',<'

ZFC -1.0

40

j/

-1.0

60

80

1 00

1 20

140

40

60

TEMPERATURE (K) Fig, 5. DC field-cooled (FC) and zero-field cooled (ZFC) susceptibilities of a clump of Hg-1223 aggregated single crystals before (O) and after oxygen annealing at 300°C ( zx) in a field of 10 Oe, after correction for demagnetizing effects.

same clump of aggregated crystals showed, however, an increase of the critical temperature from 115 K to 135 K [4]. This can be seen in Fig. 5 which displays the normalized susceptibility data before and after the annealing process. The ZFC curve of the as-grown crystals shows a flat bump around 90 K which probably reflects the existence of intergrowth phases of Hg1212. In all cases large differences are observed between ZFC and FC curves, i.e. between the shielding and the Meissner effects. Both responses are, however, larger in the oxygenized sample. Meaningful weight variations during the annealing procedures could only be detected after crushing the single crystals and finely grinding the sample. The ground sample was then submitted to a series of treatments in the thermobalance with varying conditions of atmosphere and temperature. The results are summarized in Table 4 and Fig. 3. Only the powder X-ray diffraction pattern of the non-annealed sample is shown in Fig. 4 since this pattern does not change significantly after the specific treatments. Since only relative weight variations could be determined, we took as a reference the value for the oxygen content previously derived by X-rays from a single crystal annealed at 300°C with a Tc = 135 K. This value corresponds to 8 = 0.19. This reference point was the starting point of all the various treatments on the ground sample. Indeed, between any two treatments, the powder was reannealed in pure oxygen at 300°C, which restored the reference values of 8 = 0.19 and Tc = 135 K. Fig. 6 shows the nor-

80

100

120

140

TEMPERATURE (K) Fig. 6. Reference curves of field-cooled and zero-field cooled ground sample annealed under oxygen at 300°C. Same field as for previous curves.

malized susceptibility in this reference state with an almost complete shielding effect and a Meissner effect of about 40%. Heating the annealed sample at 260°C under oxygen shows a further oxygen uptake leading to 8 = 0.27. A slight decrease of T~ to 134 K is observed in Fig. 7, but the screening effect has become complete and the Meissner effect has increased to 45%. In another procedure, the annealed sample was heated to 300°C in a nitrogen atmosphere. A loss of weight is recorded corresponding to 8 = 0.09, and a simultaneous decrease of Tc down to 130 K is observed (Fig. 7). The shielding effect is still about 90%. In a final procedure, the annealed sample was heated up

0.0

-0.5

40

60

8'0

100

120

140

TEMPERATURE (K) Fig. 7. DC field-cooled (FC) and zero-field cooled (ZFC) susceptibilities of the Hg-1223 ground sample after annealing at 260°C ( v ) under oxygen, after annealing at 300°C ( O ) and 400°C (zx) under nitrogen atmosphere, in a field of 10 Oe, after correction for demagnetizing effects.

219

A. Bertinotti et al. /Physica C 250 (1995) 213-221

to 400°C again under nitrogen. A weight loss has been found down to $ = 0.02 and two distinct critical temperatures at 133 K and 110 K are apparent on the corresponding curve in Fig. 7. The small and constant value of the diamagnetic signal observed between 133 K and 110 K is probably due to a weak proportion of the powder (about 5%) which has not been reduced in the procedure. The main transition occurs at 110 K. A great portion of the ZFC curve (from 40 K to 110 K) is superposable on the corresponding curve of the as-grown sample. This indicates that the oxygen content of the as-grown sample with Tc = 115 K is close to ~ = 0.02. This value is consistent with the interpolated value ~ = 0.025 derived from the curve of Fig. 3.

4. Discussion The interest of the present study relies on our specific method of single-crystal synthesis; two main causes of imperfection in the mercury family, namely contamination by impurity atoms and intergrowth phases, have been better controlled and verified. The latter problem is best exemplified by comparing X-ray spectra of the Hg-1223 phase with singlecrystal spectra of the Hg-1201 phase, which we have also synthesized in our laboratory. Even if sizeable and well shaped single crystals of Hg-1223 displaying good-quality X-ray spectra could be obtained by our low-pressure technique [4], careful inspection always showed some extra peaks in the vicinity of the 001 reflections. Moreover, general reflections are not as sharply defined as in the case of the Hg-1201 single crystals which, due to the absence of Ca atoms, do not develop intergrowth phases. Examples of a variable amount of streaking, i.e. of a variable amount of intergrowth among crystals of the same batch have been shown [3,4] imposing a careful selection of suitable specimen previous to X-ray work. The second cause of imperfection concerns the defects of various kinds which can affect the occupancy of the mercury site. A typical example of such difficulties is the reported study of a mercury/lead compound with n = 4, obtained in a high gas pressure system, where as much as four different types of atoms, i.e. Hg, Pb, Cu, CO 3 are thought to populate

the same site [11]. Likewise, extra peaks appear in the unit cell. They have been explained in terms of ghost images arising from a superposition of unit cells of the n - 1 or n + 1 phase, considered to be present in the measured spectrum as a result of stacking faults. The advantage of the synthesis technique we have developed is to operate in an evacuated dosed vessel, making use of pure oxides as starting material, thus avoiding unwanted contamination. Such contamination by impurity atoms may be a cause not only of local disorder, but also of stacking faults, and as a result, aberrant peaks. The truism that good single crystals are a prerequisite for reliable structural parameters is particularly important in the case of the mercury system, where one has to determine the fractional occupancy of interstitial oxygen in the neighborhood of heavy atoms like Hg and Ba. The higher precision obtained reinforce a characteristic already pointed out by Radaelli et al. in Hg-1212 of the remarkable planarity of the CuO 2 sheets in the mercury family [9]. A further observation by these authors is that long copper-apical oxygen distances appear to be associated with flatness of the sheets and higher Tc. This statement has to be moderated by the fact that although it is true that the copper-apical oxygen distances are generally greater in the mercury family than, for example, in the LaCuO and YBaCuO families there is, within the mercury family itself, a decrease of this distance with increasing Tc. In Hg-1223 the apical distance is the shortest of all those observed in the pyramids of the mercury family (Table 5). The total H g - C u distance displays only a slight decrease with increasing n whereas a decrease of almost 0.1 ,A occurs for the C u - O distance from n = 1 to n = 3, associated with an increase of the H g - O distance (Fig. 8; see Table 5 and references therein). This trend appears to

Table 5 Characteristic distances in the mercury compoundfamily Hg-1201 a Hg-1212b Hg-1223c HgPb-1234d Hg-Cu (~,) 4.757 Cu-O (.~) 2.780 Hg-O (.~) 1.977

4.761 2.775 1.986

a From Ref. [8]. b From Ref. [9]. c This work. d From Ref. [11].

4.746 2.696 2.050

4.712 2.704 2.008

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A. Bertinotti et al. / Physica C 250 (1995) 213-221 135

4.8 t tr~

o

0

1 30

4.7~

v

A

p-

2.8

!.it 1 n o A o

125 1 20 115 3.851

"

2.6

3.853

3.fl55

Fig. 9. a-axis lattice parameteras a functionof To, measuredon Hg-1223 single crystals.

2 3 4 Number of CuO2 sheets Hg-Cu distance Cu-O apical bond length Hg-O bond length

Fig. 8. Dependenceof the interatomicdistances on the number n of Cu-O planes. be obeyed also by the HgPb-1234 compound, in spite of the fact that two different apical oxygens are thought to be bound to Hg and Pb, respectively, and where CO 3 together with Cu ions are possibly also present on the Hg site [11]. The availability of single crystals has also allowed us to establish the relation between the decrease of the a parameter of the unit cell measured by X-rays, and the increase of T~ measured directly on the single crystal (Fig. 9). A general decrease of all the lattice parameters is also observed between the two isomorphous compounds T1-1223 (T¢ -- 123 K) [12] and Hg-1223 (T~ --135 K), together with a reduction of the plane deformation and the copper apex distance (Table 6). The average copper-oxidation states of both isomorphs are somewhat contrasting. In the ideal T11223 compound the mean value is + 2.33, and should be even higher if account is taken of the formula

derived from the X-ray refinement [12]. In Hg-1223 the interstitial oxygen occupancy ~ = 0.190 + 0.015 leads to a mean value of 2.125: 0.02, distinctly lower than for its T1 isomorph. Removal of interstitial oxygen by nitrogen annealing results in a decrease of Tc which is relatively moderate. A reduction of the oxygen content from 8.19 to 8.02 leaves the sample still superconducting (Fig. 3). It is well known that despite very low doping levels, optimal Tc values could be achieved in excess oxygen doped lanthanum cuprate La2CuO4+ 8 [13] and, similarly in Hg-1223, interstitial oxygen could be the only doping mechanism. At the other end of the doping curve (Fig. 3) a slight decrease of T¢ may be the indication that beyond a doping level of 8 = 0.27, we enter the overdoped regime associated with the descending part of the T~ curve. We were not able, even after prolonged annealing, to go beyond this experimental limit. Similar values for the interstitial oxygen content have been obtained for HgBa2CuO4+ 8 using neutrons on powders, and a very low occupancy at the interstitial site is still associated with bulk superconductivity [8]. Owing to the presence on the mercury site of copper substitutional defects ( = 8%) along

Table 6 Lattice parametersof the two isomorphouscompoundsT1-1223[12] and Hg-1223(this work)

T1-1223 Hg-1223

3.857

a (A)

a(/~)

c (A)

Inte~beetD.

Cu(2)-O(3)

O(2)-Cu(2)-O(2)

3.853 3.851

15.913 15.831

3.198 3.169

2.732 2.696

177.4 179.3

A. Bertinotti et al. / Physica C 250 (1995) 213-221

with additional oxygen, which Wagner et al. were able to see in the neighborhood of the position 1 ~, 0, 0 but not located in the basal plane, they suggest that in Hg-1201 this defect may be also a source of doping. In Hg-1223, we were able to see about 16% copper on the mercury site but no oxygen bonded to it. Consequently another crude way to interpret the role of copper which represents a sizeable impurity in the basal plane, is to admit that like Hg, it is two-fold coordinated and as such could enter the bond in a Cu +1 oxidation state.

5. Conclusion Well shaped and suitably large single crystals of Hg-1223 with Tc = 135 K have allowed a better determination to be made of structural features conceruing the block layer and the CuO 2 sheets. Substitutional defects are present in the block layer at the mercury site, displaying a mixed occupancy of 16.1% copper atoms. Only one interstitial site could be detected in the basal plane at the position 1, 1, 0 with an occupancoy of 0.190 + 0.015. An out-of-plane distance of 0.01 A for the oxygen atoms belonging to the CuO 2 sheets gives evidence of the great planarity of the conducting layers. In Hg-1223 it is associated with the shortest copper-apical oxygen distance existing among the mercury family. The function relating the unit-cell parameter a to the transition temperature has been determined experimentally. Likewise we have determined the dependence of the critical transition temperature on the excess oxygen content. It appears that superconductivity persists down to very low doping levels. The highest oxygen doping we could reach, ~ = 0.27, shows an incipient decrease of Tc suggesting that we are crossing the limit of optimal doping. Evidently, it would be interesting to extent the experimental oxygen doping curve at lower and higher doping regions in order, in particular, to be able to suppress the superconducting state without the degradation of the material. It must be recalled that the determination by X-ray diffraction of oxygen fractional occupancy in the neighborhood of heavy atoms like Hg and Ba is

221

not an easy task. Other techniques providing structural information about oxygen, located in the block layer, should not be neglected, in particular neutron diffraction. The prospect opened by the possibility of growing large enough well-ordered single crystals gives hope that this is no longer an unreachable goal.

Acknowledgement The authors would like to thank F. Holtzberg for useful comments and for correcting the manuscript and L. Schmirgeld-Mignot for the micrographs.

References [1] S.N. Putilin, E.V. Antipov, O. Chmaissem and M. Marezio, Nature (London) 362 (1993) 226. [2] L.W. Finger, R.M. Hazen, R.T. Downs, R.L. Meng and C.W. Chu, Physica C 226 (1994) 216. [3] A. Bertinotti, D. Colson, J. Hammann, J.F. Marucco and A. Pinatel, Proc. Int. Conf. HTSC, July 1994, Physica C 235240 (1994) 891. [4] D. Colson, A. Bertinotti, J. Hammann, J.F. Marucco and A. Pinatel, Physica C 233 (1994) 231. [5] A. Carrington, D. Colson, Y. Dumont, C. Ayache, A. Bertinotti and J.F. Marucco, Physica C 234 (1994) 1. [6] A. Carrington, C. Marcenat and D. Colson, unpublished. [7] O. Chmaissem, Q. Huang, E.V. Antipov, S.N. Putilin, M. Marezio, S.M. Loureiro, J.J. Capponi, J.L. Tholence and A. Santoro, Physica C 217 (1994) 265. [8] J.L. Wagner P.G. Radaelli, D.G. Hinks, J.D. Jorgensen, J.F. Mitchell, B. Dabrowski, G.S. Knapp and M.A. Beno, Physica C 210 (1993) 447. [9] P.G. Radaelli, J.L. Wagner, B.A. Hunter, M.A. Beno, G.S. Knapp, J.D. Jorgensen and D.G. Hinks, Physica C 216 (1993) 29. [10] J.F. Marucco, P. Gerdanian and M. Dode, J. Chim. Phys. (Paris) 66 (1969) 674. [11] H. Schwer, J. Karpinski, K. Conder, L. Lesne, C. Rossel, A. Morawski, T. Lada and A. Paszewin, Physica C 243 (1995) 10. [12] M.A. Subramanian, J.B. Parise, J.C. Calabrese, C.C. Torardi, J. Gopalakrishnan and A.W. Sleight, J. Solid State Chem. 77 (1988) 192. [13] J. Beille, R. Cabanel, C. Chaillout, B. Chevalier G. Demazeau, F. Deslandes, J. Etourneau, P. Lejay, C. Michel, J. Provost, B. Raveau, A. Sulpice, J.L. Tholence and R. Tournier, C.R. Acad. Sci. Paris 304 (1987) 1097.