Oxygen ordering and superconductivity in GdBaSrCu3O7−x

Physica C 242 ( 1995) 55-62

ELSEVIER

Oxygen ordering and superconductivity in GdBaSrCu307_x X.Z. Wang a, B. Hellebrand a,., D. Biiuerle a, M. Strecker b, G. Wortmann b, W. Lang c • Angewandte Physik, Johannes-Kepler-Universitiit Linz, A-4040 Linz, Austria b Fachbereich Physik, Universitiit-GH Paderborn, D-33095 Paderborn, Germany c Ludwig Boltzmann Institut flit Festk6rperphysik, Kopernikusgasse 15, A- 1060 Wien, Austria and I~titut fiir Festki~rperphysik der Universitiit Wien, A-1060 Wien, Austria

Received 4 October 1994 Abstract

A special preparation process has been developed to stabilise GdBaSrCu307_x compounds in both tetragonal and orthorhombic structures with oxygen deficiency x close to zero. Powder X-ray diffraction analysis (XRD) gives lattice parameters of a= 3.839 A and c= I 1.572 A for a tetragonal structure, and a = 3.804 A, b= 3.863 A and c= 11.588 A for a typical orthorhombic structure. 155Gd-M~issbauer spectroscopy reveals via the electric quadrupole interaction an asymmetry parameter ~/=0.14(6) for the tetragonal phase (T-phase) and ~/= 0.31 (5) for the orthorhombic phase (O-phase). The small asymmetry parameter for the T-phase indicates an oxygen disordering in the basal plane. Superconducting transition temperatures Tc above 80 K were verified for both tetragonal and orthorhombic phases by resistivity and susceptibility measurements. Despite of a similar Tovalue the normal-state transport properties studied by resistivity and Hall effect measurements revealed higher resistivity in the tetragonal phase, which may be caused by both the destruction of the CuO chains and an enhanced scattering introduced by disorder of oxygen atoms in the basal plane.

1. Introduction

At present the YBa2Cu307_x (Y-123) system is among the most extensively studied high-To superconducting materials [1]. In the case of x ~ 0 the C u ( 1 ) and O ( 1 ) atoms in the basal plane are ordered into chains along the b-direction to give an orthorhombic structure [2,3]. x can be varied from about 0 to 1. Accordingly the crystal structure changes from orthorhombic to tetragonal and superconducting properties are replaced by semiconducting behaviour. Both superconductivity and orthorhombicity vanish as the oxygen deficiency x approaches about 0.5. It was consequently suggested that not only the oxygen content is important for superconductivity but also the ordering o f oxygen atoms in the basal planes * Corresponding author.

[ 4-6 ]. However, experimentally only little is known about the independent influence on superconductivity arising from oxygen disordering in the CuO basal planes. The reason is that the structural transition is often accompanied by a change in oxygen content [ 7 ]. For Y- 123 oxygen disordering was artificially created in the basal planes by ion irradiation [8,9]. However at the same time other crystal defects, which influence superconductivity as well, may be created too. Therefore searching for a compound, which can be stabilised in both tetragonal and orthorhombic structure with the same oxygen content is crucial for an investigation of the relationship between oxygen ordering and superconductivity. Recently developed REBaSrCu307_~ compounds (RE -= rare earth) belong to the same structural family as Y-123 [10]. In this system both the orthorhombic and the tetragonal structure were identified

0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved. SSDI0921-4534(94)02370-0

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x.z. Wang et aL / Physica C 242 (1995) 55-62

for x close to zero. The orthorhombic structure is found for compounds with rare earths of small ionic radius. Interestingly the maximum transition temperature Tc is observed for RE---Dy and Gd, where the crystal structure changes from tetragonal to orthorhombic. In a further investigation we found that it is possible to stabilise the compounds in both the tetragonal and the orthorhombic structure with about the same oxygen content for RE-- Dy and Gd. Preparation and characterisation of DyBaSrCu306.94 was reported previously [ 11 ]. In this work we report on a detailed investigation of the GdBaSrCu3OT_x system in both oxygen ordered and disordered forms with oxygen deficiencies x close to zero. A special preparation technique has been developed to manipulate the oxygen ordering in the basal planes. Determination of the crystal structure by powder XRD, analysis of the microscopic information from gadolinium (~55Gd) Mrssbauer spectroscopy and resistivity measurements permit us to exclude the existence of so-called orthorhombic microdomains in the tetragonal phase. Susceptibility and resistivity studies show that both tetragonal and orthorhombic phases are superconducting with similar transition temperatures above 80 K. For comparison some studies of GdBa2Cu307_x(Gd-123) are also included.

2. Sample preparation and experimental GdBaSrCu307_x and GdBa2Cu307_x were prepared by the conventional solid-state reaction methods. Stoichiometric amounts of Gd203, BaCO3, SrCO3, and CuO, with purity better than 99.9%, were ground in an agate ball mill for one hour and pressed into pellets at a pressure of 600 MPa. The first calcination was carried out in air at 920°C for 24 h. After furnace cooling down to room temperature, the calcined pellets were ground for another hour and pelletised again. The second calcination was carried out at 940°C for 24 h under oxygen atmosphere. The cooling rate down to room temperature was less than 100 °C per hour in oxygen atmosphere. Further thermal treatment for GdBaSrCu3OT_x was made in two steps: firstly, for oxygen desorption, a treatment in argon atmosphere at 940°C for two hours followed by cooling down to room temperature, secondly, an

annealing process in oxygen atmosphere at different temperatures for each sample, varying from 400°C to 950 °C. The time of annealing was 12 h at the maximum temperature, the cooling rate down to room temperature was less than 100 °C per hour. Oxygen deficient samples with 6.1 oxygen per chemical formula were obtained by annealing the corresponding compounds in argon atmosphere at 940°C for about 2 h. All the annealing process were done under ambient pressure. The oxygen content of all samples, except for oxygen deficient samples, was determined by iodometric titration analysis using a similar procedure as reported in Ref. [ 12 ]. The oxygen content for oxygen deficient samples was determined by the weight loss during the annealing in argon atmosphere. Structural determination was carried out by the powder XRD technique at room temperature using Cu Ke radiation. The instrument was calibrated by silicon powder. Lattice parameters were calculated by using a least-squares refinement. The ~55Gd-M6ssbauer spectra were taken in a He bath cryostat at temperatures between 1.5 and ~ 40 K. The source, ~SSEu(SmPd3), was moved sinusoidally with respect to the absorber. The 86.5 keV gamma rays were detected by a large-area intrinsic Ge diode with typical count rates of 3 × 104 Hz. The various GdBaSrCu3OT_x and, for comparison, GdBa2Cu307_x absorbers were prepared with a thickness of about 20 m g / c m 2 of natural Gd. The hyperfine spectra at 1.5 K with a combined magnetic dipole/electric quadrupole interaction allowed for a full diagonalisation of the hyperfine Hamiltonian. More details about ~55Gd-M/Sssbauer spectroscopy and data analysis, especially in the high-Tc superconductors, may be found in Refs. [ 13-17 ]. The transport measurements were performed on thin, bar-shaped samples, which were cut from the ceramic pellets. Contacts were established in the usual five-probe geometry with Ag paste and gold wires. The measurements were carried out in a closed-cycle refrigerator. Temperature was controlled by a platinum resistor and precision temperature measurements were made with a calibrated GaA1As diode placed close to the sample. AC current with a frequency of 80 Hz and a current density of 10 A / c m 2 was used. A lock-in technique was performed for the detection of the Hall voltage and the resistive voltage drop along

57

x.z. Wang et al. / Physica C242 (1995) 55-62

the sample. The temperature variation of the Hall voltage was measured during a slow down-sweep in a magnetic field B = 0.7 T generated by an electromagnet, which was reversed for each datapoint to exclude any offset voltage. The AC magnetic susceptibility measurements were performed with an AC susceptometer at a magnetic field of 1 Oe and a frequency of 680 Hz from a temperature of 10 K up to about 120 K.

i

i

i

i

i

i

i

i

i

T

{2oo}

T >F--

o

(006) (020)

z I.M F-Z

3. R e s u l t s

t,3 t~ 45 t~6 t~7 4B 49 50 51

3.1. Phase formation and oxygen content

20

The powder XRD spectra show that all GdBaSrC u 3 0 7 _ x and GdBa2Cu307_x samples investigated are single phase and isostructural with Y-123. According to the different thermal treatments different XRD spectra were observed for GdBaSrCu3OT_x. The differences are reflected most pronounced by changes of the intensities and positions of the (006), (200) and (020) Bragg peaks as shown in Fig. 1. According to different annealing temperatures in oxygen atmosphere both the tetragonal and the orthorhombic phase could be obtained. The tetragonal phase, denoted by T in Fig. 1, was formed at an annealing temperature higher than 920 °C followed by slow cooling in oxygen. One observes the orthorhombic structure after heat treatment in argon at 940 ° C and annealing in oxygen at a temperature lower than 900°C. A typical powder XRD spectrum for an orthorhombic sample, denoted by O, which was annealed under oxygen atmosphere at 500°C, is shown in Fig. 1. A clear separation of the (200) and (020) peaks indicates well defined oxygen ordering in the basal planes. Repeating the argon and oxygen annealing process at different temperatures the O- and T-phases could be reversibly obtained. For the O-phase the lattice parameters a and b approach each other when the oxygen annealing temperature increases from 400°C to 920 ° C, as shown in Fig. 2. This indicates that oxygen disordering increases at higher annealing temperatures. After an annealing process at temperatures of 920°C or higher the oxygen ordering in the basal plane disappears. The oxygen deficient compounds GdBaSrCu306j and GdBa2Cu306A both show a tetragonal structure.

[degrees] - ~

Fig. 1. Powder XRD patterns of (200), (020) and (006) Bragg peaksfor tetragonal (T) and orthorhombic (O) GdBaSrCu3OT_x.

GdBaSrCu307. x

11.65 c 11.60 E-

<

0~0"000--0-~0

0 0 0~00

11.55

3.88:

a

A A AAA--A--A

A A A

3.84

~<

3.80

v v v v v ~ v j V v~v ~z~OO b

i 400

i 500

ANNEALING

i 600

,

i 700

i 800

TEMPERATURE

i 900 [ °C ]

Fig. 2. Lattice parameters a, b and c vs. annealing temperature

for GdBaSrCu3OT_x (0 < x ~<0.06 ). The results of oxygen content analysis by iodometric titration are given in Table 1. All the samples annealed in oxygen have x-values close to zero. The lattice parameters and some physical properties for the two phases, indicated by T and O, are also listed in Table 1. Despite of a change in structure, the volumes of the unit cell are nearly the same for both phases. 3.2. Z55Gd-MOssbauer spectroscopy

It was shown that it is possible by the tSSGd-resonance to distinguish between the tetragonal and or-

58

X.Z. Wang et al. / Physica C 242 (1995) 55-62

Table 1 Some physical parameters of GdBaSrCu3OT_x at room temperature Phase

x

a (A)

b (A)

c (A)

v (A 3)

T~,,

p (mf~ cm)

RH (cm3/C)

T O

0.00 0.06

3.839 3.804

3.863

11.572 11.588

170.5 170.3

80.8 82.6

1.97 0.68

0.9X 103 1.4× 103

thorhombic phases in the GdBa2CuaOT_x system via ~/, the asymmetry parameter of the electric-quadrupole interaction tensor [ 15 ]. In a perfect tetragonal phase r/should be zero due to the four-fold site symmetry and, accordingly, non-zero due to the lower (two-fold) symmetry at the RE site in the orthorhombic structure. A previous study of orthorhombic GdBa2Cu3OT_x revealed that r/is related to the orthorhombicity of the system [ 18 ]. Fig. 3 shows ~55Gd-MSssbauer spectra from orthorhombic GdBa2Cu306.94 (a), GdBaSrCu306.94 (b), and tetragonal GdBaSrCu307 (c) measured at 1.5 and 4.2 K respectively. Table 2 gives the parameters determined from M6ssbauer spectroscopy. The spectra at 4.2 K exhibit a splitting due to the electricquadrupole interaction. As in the GdBa2Cu307_~ system, all investigated GdBaSrCu307_~ samples exhibit, independent of the occurrence of superconduc-

T=4.2K

T=I.5K

I--

"

-4

-2

C

0

""

2

4

Velocity [mm/s] Fig. 3. 155Gd-M~ssbauer spectra from orthorhombic GdBa2Cu306.94 (a), orthorhombic GdBaSrCu306.94 (b), and tetragonal GdBaSrCu307 -x (c).

tivity, magnetic order of the Gd sublattice around TN(Gd)=2.4 K [15,19]. This leads to a magnetic hyperfine field, oriented parallel to the crystallographic c-axis, at temperatures below TN. A detailed study of the magnetic ordering by specific heat and M6ssbauer spectroscopy measurements has been published elsewhere [ 19 ]. The spectra at 1.5 K with combined magnetic dipole/electric quadrupole interaction allow for a full determination of the electric field gradient (EFG) at the Gd site, expressed by V=, the main component of the EFG tensor, and by r/, the asymmetry parameter, defined by r/= ( V ~ - Vyy)/ V= with Vii as the axes of the EFG tensor. As shown in Table 2, ~/= 0.31 (5) for orthorhombic GdBaSrCu306.94 and ~/=0.14(6) for tetragonal GdBaSrCu307. For a tetragonal oxygen-deficient GdBaSrCu306.~ sample ~/is equal to 0.21 (7). For comparison we present also results from GdBa2Cu306.94 and GdBa2Cu306.1 in Table 2. An enhancement of quadrupole splitting by 30% for GdBaSrCuaO7_x compared with Gd-123 was observed, which holds for superconducting samples (x,~ 0) as well as for the non-superconducting samples ( x ~ 0.9). This enhancement can be explained by the decrease of the c/a ratio upon Sr substitution similar to observations on Gd ions substituted into PrBa2Cu3OT_6 [ 16 ]. The transition from tetragonal to orthorhombic structure found in the XRD spectra of GdBaSrCu3OT_x is reflected by an increase of the asymmetry parameter r/. Small values of 1/in both Tphase GdBaSrCu307 and oxygen-deficient GdBaSrCu306.1 (t/= 0.21 (7) ) point to a local distortion of the lattice by Sr substitution. The GdBaSrCu307_x spectra exhibit a considerably larger experimental line width than the Gd-123 spectra. This can be attributed again to the lattice distortion causing a distribution of the EFG values. The present ~55Gd-M6ssbauer studies as well as EXAFS investigations at the Gd site [ 19 ] give no evidence for a site-mixture between Gd and Sr, which

59

X.Z. Wang et al. / Physica C 242 (1995) 55-62

Table 2 Compilation of hyperfineparameters from the Mtissbauer measurementsof GdBaSrCu307_xand GdBa2Cu3OT_x Sample

Phase

Isomershift (ram/s)

Vzz ( 1017V/cm2)

q

Linewidth (HWHM) (ram/s)

GdBaSrCu306.1

T

0.514(3)

- 5.60(2)

0.21 (7)

GdBaSrCu3OT.o

T

0.507(2)

-6.77(2)

0.14(6)

GdBaSrCu306.94

O

0.512(2)

--6.70(2)

0.31 (5)

GdBa2Cu306.~ GdBazCu306.94

T O

0.530(2) 0.508(2)

-4.20(3) -4.90(2)

0.03(5) 0.57(2)

0.42(8) " 0.36(2) b 0.42(8) " 0.39(2) b 0.46(8) " 0.40(2) b 0.35(2) b 0.33(2) b

The isomer shift is given relative to the SmPd3source. The values for the electric-field-gradient tensor, V= and q, were derived with Q(3/2) = 1.59barn for the tSSGdquadrupole moment. "At 1.5 K. b At 4.2 K. was speculatively assumed on the basis of the XRD analysis [20]. The EXAFS investigations in Ref. [19], however, prove a local disorder originating from the substitution of Ba by Sr.

. . . .

2.0

i

. . . .

'

. . . .

i

. . . .

~

. . . .

i

. . . .

'

. . . .

i

. . . .

'

. . . .

G

3.3. Electrical and magnetic properties t9

Measurements of the resistivity p show metallic behaviour for the GdBaSrCu307_x compounds for both the O-phase and the T-phase (Fig. 4), as it is widely noticed in fully oxygenated cuprate superconductors. A linear decrease o f p from room temperature down to 110 K, where superconducting fluctuations become evident, is observed. The orthorhombic Ophase has a Tco (zero-resistance temperature) of 82.6 K, while it is 80.8 K for the T-phase. The resistivity of the T-phase is larger by a factor of about three throughout the whole temperature range. Magnetic susceptibility measurements confirm the bulk superconductivity for both phases as shown in Fig. 5. The onset temperature of the diamagnetic signals for Ophase samples is above 85 K, while for the T-phase it is above 82 K. A smaller transition width for the Ophases than the T-phase indicates that the former samples are more homogeneous. This behaviour is very similar to that observed for DyBaSrCu307 samples [ 1 1 ]. The results of the Hall effect measurements are presented in Fig. 6. The Hall coefficient RH is positive (hole-like) and around 1.4× 10 -3 cm3/C in the O-phase and 0.9× 10 -3 c m 3 / C in the T-phase, respectively, at room temperature. Like in other ce-

~

1.5

l.O

U-

0.5

.:.. 0.0

"" 50

l"

100

150

200

T E M P E R A T U R E

250

300

[K]

Fig. 4. Temperaturevariation of the resistivityin the orthorhombic (O) and the tetragonal (T) phase of GdBaSrCu307_x. ramie high-temperature superconductors, RH is temperature dependent for both phases and increases with decreasing temperature down to 100 K. At still lower temperatures, RIa changes its slope and goes rapidly to zero, exhibiting a sign change in the vicinity of Tco. Values for the room temperature resistivity p, Tco and RH at 300 K are summarised in Table 1.

X.Z. Wang et al. / Physica C 242 (1995) 55-62

60

•

>["

T

F-

i

= g

GdBaSrCu307_ x

I

.~

/

g 10

20

30

40

50

60

TEMPERATURE

70

80

90

100

[K]

Fig. 5. A C m a g n e t i c susceptibility vs. t e m p e r a t u r e f o r t e t r a g n n a l (T) and orthorhombic (O) GdBaSrCu3OT_x.

4.0

....

, ....

, ....

~'u-

.t

3.0

, ....

, ....

GdBaSrCuaOT-x

-..

2.0

"'"'--."

o

7

1

1.0

o

T

,.~

0.0

-1.0

! . . . .

50

i

100

. . . .

i

150

. . . .

i

.

.

.

200

TEMPERATURE

,

i

250

. . . .

300

[K]

Fig. 6. H a l l coefficient RH as a f u n c t i o n o f t e m p e r a t u r e for the orthorhombic (O) and the tetragnnal (T) phase of GdBaSrCu3Ov_x.

4. Discussion

The XRD technique usually reflects an average information over a certain dimension of the sample under investigation. Orthorhombic domains with ordered oxygen chains of a size less than some hundred may not be detectable by XRD. But such an orthorhombicity is detectable by M0ssbauer spectroscopy.

For GdBaSrCu307_x compounds the non-zero asymmetry parameter may have two origins: (i) a distorted lattice by Sr substitution for Ba or (ii) orthorhombicity which originates from the oxygen ordering in the basal plane. It is unlikely that the oxygen ordering in the basal plane exists for GdBaSrCu306.1, so we attribute the non-zero asymmetry parameter q to a lattice distortion caused by Sr substitution. Similarly we believe that the non-zero for T-phase GdBaSrCu3OT_x is also caused by Sr substitution but not by ordered oxygen chains in the basal planes. This fact is also supported by the larger room temperature resistivity of the T-phase compared with the O-phase which will be discussed below. For the Y-123 compound it is well known that the crystal structure is dependent on the oxygen content [6 ]. Typically the phase transition from O-phase to T-phase takes place at an oxygen content of about 6.5. With a special treatment there might exist another Ophase with an oxygen content of about 6.5 [4,5 ]. On the other hand the O-phase transforms to the T-phase at around 685°C depending on the partial oxygen pressure [21 ]. Nevertheless an orthorhombic structure is stable for Y-123 compounds when the oxygen content exceeds 6.5 per chemical formula. For GdBaSrCu3OT_x compound, with x close to zero, both orthorhombic and tetragonal structures are obtained when different preparation processes are employed. A slow cooling rate from 940°C to 600°C under ambient oxygen pressure is important for obtaining the T-phase. The O-phase with well-defined orthorhombicity can only be obtained by oxygen absorption at relative low temperature. From our experiment it is suggested that oxygen absorbed in the lattice at high temperature tends to be disordered. However, for obtaining ordered CuO chains it is necessary to remove the oxygen from the lattice at high temperature before reintroducing it at relative low temperature. The two phases, T- and O-phase, can be reversibly obtained from each other by an adequate thermal treatment. As has been discussed before we believe that the oxygen disordering in the basal plane is caused by a structural deformation introduced mainly by the Sr substitution into the Ba site [ 22 ]. This structural deformation was confirmed by the present M6ssbauer and previous EXAFS studies [ 19 ]. The GdBaSrCu307_x compounds may supply an

X.Z. Wang et al. / Physica C 242 (1995) 55-62

example to answer the question of the relationship between oxygen ordering in the basal plane and superconductivity. As evidenced by electrical and magnetic measurements the oxygen ordered O-phase shows a Tco only several degrees higher than that of the T-phase. This result is in agreement with the previous experimental results from DyBaSrCu3Or.94 [ 11 ], but it is in contradiction with some theoretical predictions [6]. Our results are also different from experimental results of ion irradiated Y-123 which provides also a tetragonal structure [ 8,9,23 ]. There are two possibilities that may cause this discrepancy. One is that ion irradiation produces not only oxygen disordering but also cation disordering which destroys superconductivity. This is similar to the case of low-temperature prepared Y-123, which is an insulator with a tetragonal structure [ 24]. The second possibility is that in T-phase GdBaSrCu307 the CuO chains still exist in a small scale. However, our experimental results from XRD, MSssbauer spectroscopy and electrical transport measurement do not support such a possibility. Despite the very similar Tco of both phases, the electrical transport properties are remarkably different. The room-temperature resistivity of the O-phase is 0.7 m ~ cm, which is close to the values observed in Y-123 [25]. In comparison, the resistivity of the T-phase is enhanced by a factor of about three. This cannot be explained easily by the absence of a transport mechanism due to missing CuO chains, since the anisotropy between a- and b-axis in untwinned single crystals of Y- 123 is only 2.2 [ 26 ]. On the other hand the room-temperature values for RH are rather similar and again are close to results in ceramic Y-123 [27]. It is noteworthy that the slightly lower oxygen content in the O samples may be responsible for the difference in the room-temperature Hall effect. In Fig. 7 we compare the tangent of the Hall angle versus temperature for both T- and O-phases of GdBaSrCu307_x. In a simple approach involving only one parabolic band the Hall angle is proportional to the free carrier mobility. Although such an oversimplified interpretation is certainly no adequate in the cuprate superconductors and a quantitative interpretation of the Hall effect is controversial, the Hall angle gives a measure for the influence of scattering on mobile charged carriers. It can be noticed that tan OH

0.8

61

. . . .

,

. . . .

,

.

.

.

,

.

.

.

.

,

.

.

.

.

GdBaSrCua07_ x

r,

0.7

.

0.6

%,

%

~-~ 0.5 I

•

o

vv

0 0.4

v~

x,

¢D

\

0.3

",.%.

0.2 T

0.1 0,0

.... 50

'

I00

•

•

,

,

I

150

,

,

,

,

I

i

,

,

200

TEMPERATURE

,

I

250

,

,

,

,

300

[K]

Fig. 7. Tangent of the Hall angle tan OH vs. temperature for orthorhombic (O) and tetragonal ( T ) GdBaSrCu307_x at a magnetic field B = 0 . 7 T.

exhibits a similar temperature dependence in both modifications of GdBaSrCuaO7_x, but is smaller by a factor of approximately six in the T-phase. We take this for an indication of enhanced scattering processes in the T-phase. Presumably the disordered oxygen atoms in the basal plane of the T-phase contribute an additional scattering potential to the free carrier transport properties in the CuO2 planes. This behaviour is also reflected in the higher normal-state resistivity of the T-phase.

5. Conclusion Polymorphic compounds of GdBaSrCu3OT_x have been prepared by a special thermal treatment. Both tetragonal and orthorhombic structures have an oxygen content per chemical formula close to 7. Introducing the oxygen into the lattice at high temperature tends to give a tetragonal structure, while at low temperature it results into an orthorhombic structure. The structural analysis was accomplished by powder Xray diffraction and MSssbauer spectroscopy. For tetragonal GdBaSrCu3OT_x the asymmetry parameter from Mrssbauer spectroscopy is 0.14 (6) in compar-

62

X.Z. Wang et al. / Physica C 242 (1995) 55-62

ison with 0.31 (5) for the o r t h o r h o m b i c phase and 0.57 ( 2 ) for GdBa2Cu306.94. S u p e r c o n d u c t i v i t y was c o n f i r m e d by resistivity a n d susceptibility m e a s u r e m e n t s for b o t h o r t h o r h o m b i c a n d tetragonal phases o f G d B a S r C u 3 0 7 _ x . It was p o i n t e d out that oxygen d i s o r d e r i n g in the basal p l a n e does not destroy highTc s u p e r c o n d u c t i v i t y a l t h o u g h the n o r m a l - state transport p r o p e r t i e s are significantly altered.

Acknowledgements T h e a u t h o r s wish to t h a n k the F o n d zur F 6 r d e r u n g der wissenschaftlichen F o r s c h u n g in tSsterreich for the financial support u n d e r project No. P8848.

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