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Stabilization of superconducting phases in thallium-barium-copper oxide system by fluorine substitution [TlBa2CuO5−xFx]
Pergamon
Materials Reseaz~ Bulletin, Vol. 29, No. 2, pp. 119-125, 1994 Copyright© 1994 Elsevie~ Scieace Ltd Printed in the USA. All rishts reu~ved 0025-5408/94 $6.00 + .00
S T A B I L I Z A T I O N O F S U P E R C O N D U C T I N G P H A S E S IN T H A L L I U M BARIUM-COPPER
O X I D E S Y S T E M BY F L U O R I N E S U B S T I T U T I O N [TIBa2CuO5.xFx]
M. A. Subramanian Central Research and Development Dupont Company Experimental Station Wilmington, Delaware 19880-0328 (USA)
(Received November 29, 1993; Communicated b y A.W. Sleight) ABSTRACT Partial substitution of oxygen by fluorine in T1Ba2CuOs. has been achieved by heating T1203, BaO2, BaO and CuO under one kbar external pressure using TIF as a source for fluorine. A single phase region exists for 0.1 < x < 0.5 and the X-ray powder diffraction data indicated a tetragonal structure (space group: P4/mmm) for all the compounds. The in-plane Cu-O distance calculated from the a-lattice parameter showed a systematic increase as x was varied from 0. I to 0.5. This is due to an increase in the formal oxidation state of copper as F 1" substitutes for 02. in the 1201-type structure. The c-lattice parameter also showed a slight increase, as the fluorine content in the reaction mixture is increased. Superconductivity is observed for x values within the range 0.1 to 0.6 with transition temperatures in the range 35-75 K. MATERIALS INDEX: thallium, barium, copper, oxide, fluorine
Iatr..ea.u.~m~ Although the mechanism of superconductivity in high T¢ copper oxides is far from understood, it is currently well known that the existence of holes (Cu3+/Cu 2+) or electrons (Cu2+/Cu 1+) in the CuO2 planes is essential for the occurrence of superconductivity in those materials. In general, superconductivity occurs when the carder (hole or eleclxon) concentration is in the optimal region, which lies between an antiferromagentic insulator and a metallic phase [I]. Carrier concentration in these materials can be controlled in a number of ways. One Is to change the oxygen concentration (YBa2Cu307-8) and another is by cation substitution, viz. La2.xSrxCuO4, TIBa2.xLaxCuO5. Carrier concentration in copper oxide superconductors can also be altered by anion substitution, viz. 02- by F-. In this case, an electron rather than a hole is injected into the CuO2 sheets and this has been well established in the case of n-type copper oxide superconductors, Ln2CuO4-xFx (Ln=Pr, Nd, Sm) [2]. 119
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Thallium-containing superconductors of the type TlmCan_lBa2CunO2n+m+2 (m = 1,2) have been studied extensively in the last five years. All of these oxides are superconducting with transition temperatures of 80 to 125 K, except for TIBa2CuO5 (where m = 1, n=l) and this has been attributed to the presence of copper in +3 formal oxidation state (over-doped). However, weak or metastable superconductivity with an onset of Tc around 20 K has been seen in some preparations [3]. It has been well established that the superconducting transition temperature of this phase can be drastically enhanced through partial substitution of Ba2+ ions by trivalent rare earths (La3+, Nd 3+) [3-5]. The stabilization and enhancement of the superconducting transition temperatures in T1Ba2.xLaxCuO 5 can be attributed to the transfer of electrons to the Cu-O sheets, when trivalent rare earths substitute the Ba 2+ ions. This results in the reduction of the formal oxidation state of copper from it's over-doped level (Cu ~3+) to an optimum level (Cu 2.2+ to Cu 2.5+) and the resulting compositions arc superconducting. Similar reduction in the hole concentration in the Cu-O sheets should occur if oxygen in TIBa2CuO5 is replaced by fluorine. Here we report on our efforts on the stabilization of superconducting phases in T1Ba2CuOs-system through fluorine substitution. Exl)0rlmental Methods
Appropriate quantities of the reactants (TI203, TIF, BaO, BaO2 and Cue) were intimately mixed together in an agate mortar. The mixtures in the form of pellets were loaded into gold tubes (3/8" dia x 4" length) and sealed. The entire process of weighing, grinding and loading the tubes was done in a nitrogen filled dry box. The tubes were heated at 780-820°C for 6 hrs under one kbar pressure. TIBa2CuOs.8 phases were also prepared under similar conditions for various values of 8 ranging from 0.1 to 0.7. In this case, the oxygen content (5-8) in each sample was fixed by varying the ratio of BaO2 to BaO in the starting reaction mixture. X-ray diffraction data were obtained with a SCINTAG (PAD IV) diffractometer using CuKa radiation. Cell dimensions were refined by least squares. The test for superconductivity was made by the a.c. mutual inductance technique. Electrical resistivity measurements were made by the standard four-probe technique. Fluorine content in the compounds were determined by ionselective electrode method [6]. Results
Successful synthesis of fluorine substituted TIBa2CuOs.xFx phases was achieved by using TIF as a source for fluorine, Our attempts to synthesize them using BaF2 or CuF2 in a sealed gold tube under ambient pressure always resulted in the formation of 1201-type phases with BaF2 and BaCuO2 as major impurity phases [7]. Due to the low melting and boiling point of TIF, the sealed gold tubes were heated under one kbar external pressure to avoid the rupture of the gold tubes from the development of high internal pressures. X-ray diffraction data of T1Ba2CuOs.xFx showed that single phase samples were formed for x = 0.2 to 0.5 under the synthetic conditions employed. The X-ray diffraction lines for all the samples could be indexed on the basis of a tetragonal cell with a - 3.8 and c ~ 9.5 ]~, space group P4/mmm. The unit cell parameters are listed in Table I. The X-ray pattern of the x = 0.6 sample contained a few extra reflections and some of them could be accounted for a BaF2 impurity. When x was increased further, the intensities of the impurity reflections also increased. Fig. 1 shows the powder X-ray diffraction trace for T1Ba2CuO4.TF0.3 and it's indexed powder pattern is given in Table II. The fact that the X-ray powder diffraction patterns of these phases are very similar to that of T1Ba2_xLaxCuOs [5] suggests that these compounds have 1201-type structure (Fig. 2). This structure consists of single sheets of corner-sharing CuO 4 groups in which each copper atom has two additional oxygen atoms positioned above and below the sheet. Sr ions reside above and below the Cu-O sheets in nine-fold coordination with oxygen, and the single TI-O sheets are alternately stacked with the Ba-Cu-O slabs along the c direction [5]. Table I also lists the observed fluorine contents in TIBa2CuOs.xFx samples determined by chemical analysis and are
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TABLE I Unit cell parameters and analytical data for TIBa2CuOs.xFx
Composition
a (~) (+0.001)
c(J,) (+0.002)
TIBa2CuO4.9F0.] TIBa2CuO4.8Fo.2 TIBa2CuO4.7F0.3 TIBa2CuO4.6F0.4 TIBa2CuO4.5F0.5 TIBa2CuO4.4F0.6 K2NbO3F
3.828 3.832 3.836 3.839 3.841 3.842 . . .
9.586 9.585 9.590 9.598 9.602 9.605
Fluorine content (wt%) (obs.)* (cal.) 0.27(6) 0.63(5) 0.94(4) 1.20(5) 1.55(6) 1.89(5) 8.04(8)
.
0.305 0.610 0.915 1.220 1.525 1.830 7.98
The values given m the parenthesis are standard deviation in the last decimal place.
I
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I
5
10
15
•
I
I
!
20
25
30
i
i
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35
40
•
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45
i
i
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50
55
20 FIG. 1 Powder X-ray diffraction pattern for TIBa2CuO4.7F0.3. The 20 values are in degrees.
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TABLE H
X-ray Diffraction Data for TIBa2CuO4.TFO.3 hk I
dob s (A)
deal (A)
001 002 10 0 10 1 003 10 2 1 10 111 10 3 004 1 13 10 4 200 1 14 10 5 2 11 203 2 12
9.503 4.774
9.5900 4.7950 3.8360 3.5616 3.1967 2.9954 2.7125 2.6101 2.4557 2.3975 2.0682 2.0331 1.9180 1.7963 1.7155 1.6887 1.6446 1.6152
3.554 3.195 2.996 2.7121 2.6100 2.4558 2.3979 2.0680 2.0336 1.9181 1.7960 1.7155 1.6883 1.6445 1.6154
lobs 4 1 24 10 100 58 4 3 15 17 2 20 17 12 3 4 27
Ical* 5 1 <1 22 10 100 54 3 3 17 16 1 21 17 13 2 3 24
* Relative peak height intensities are calculated on the basis of a 1201-type structure with P4/mmm symmetry. Atomic positions (x, y, z) used are taken from reference [5]: 1 TI (0, 0, 0); 2 Ba (1/2, 1/2, 0.295); 1 Cu (0, 0, 1/2); 2 O(1) (0, 0, 0.225); 2 0(2) (1/2, 0, 1/2); 1 0(3) (1/2, 1/2, 0). Disorder of T1 and O (3) atoms are not considered. in good agreement with the calculated values based on the starting compositions. The analytical data for K2NbO3F sample is also included for comparison. Our attempts to synthesize oxygen deficient TIBa2CuO5.8 phases for various values of 8 (0.1 to 0.6) under similar conditions were not successful and always resulted in a multiphase product with BaCuO2 as a major impurity. In some cases, the X-ray diffraction lines corresponding to T1Ba2CuO5-type structure were seen along with other impurity phases. Magnetic flux exclusion measurements for T1Ba2CuO5_xFx phases showed superconducting onset temperatures in the range 35-75 K with large Meissner fractions. Electrical resistivity measurements showed zero resistivities in the range 29-65 K (Table III). However, for T1Ba2CuO5-8 samples, only weak superconducting transitions were observed in some of the preparations and their transition temperatures arc also listed in Table HI.
Earlier attempts on the synthesis of single phase 1201-type T1Ba2CuO5 phase without the presence of suitable substitutions were unsuccessful [8]. This is because such a synthesis would revolve stabilizing Cu in the trivalent state. This communication shows that the tetragonal (P4/mmm) TIBa2CuOs-type phases can be stabilized by partial substitution of oxygen by fluorine. The substitution of 02- by F- should reduce the formal valence state of copper below 3+, thereby
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FLUORINES
123
•
Cu
Q
Tl
0 Ba 0
0
00/F
FIG. 2 Structure of T1Ba2CuO5.xFx. TABLE III Superconductivity data for TIBa2CuOs.xFx and TIBa2CuOs.8 TIBa2CuO$.xFx
TIBa2CuO$.8*
xor~ To K (on set) O.1 0.2 0.3 0.4 O.5 0.6 * Impure phases.
Tc, K (zero)
35 (very weak) 48 75 65 69 50
29 65 55 51 37"
To, K (onset)
To K (zero)
Semieonducfing 10 (very weak) 21 <5 18 <5 Semieonducting Semieondueting
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creating mobile holes in the Cu-O sheets. In other words, electrons are added to the copperoxygen sheets to create Cu 3+ and Cu 2+. This is analogous to Nd2CuO4-xFx, where the fluorine substitutionreduces the formal oxidation state of copper below 2+, thereby introducing electrons in the Cu-O sheets and results in n-type superconductivity. In tetragonal thallium cuprate superconductors, the in-plane Cu-O distance is roughly one-half of the a-latticeparameter. From Table I, itis clearthatas x increasesthe a-latticeparameter thence the in-plane Cu-O distance)also increases. This is due to an increase in the formal oxidation stateof copper in the sheets. If one assumes that thallium rather than copper is reduced, the formation of TI I+ in the latticeshould show a large increase in the c-latticeparameter as the ionic radius of TI I+ is much largerthan that of TI 3+. However, the observed c-latticeparameter showed only a very slightincrease with x (Table I) and is is probably due to an elongation of the apical Cu-O distance resulting from an increase in the Cu 2+ content in Cu-O sheets. It is interesting to compare the superconducting properties of TIBa2.xLaxCuO5 with TIBa2CuOs.xFx. In the case of TIBa2_xLaxCuO5 system, superconductivity was observed for samples in the range x= 0.5 to 0.8, which corresponds to the formal valence state of copper between 2.5 and 2.2. This is in contrast to TIBa2CuOs.xFx system, where the superconducting compositions exist for x = 0.2 to 0.5, which corresponds to the formal valence stateof coppcr between 2.8 and 2.5. Earlier Band structuralcalculations revealed that for TIBa2CuO5-typc compounds, the Tl 6s-block band lieswell above the Fermi levcl and do not participatein the removal of electrons from the Cu-O layer d(x2-y2)band [9]. Itis likelythatthe formal valence of copper in the TIBa2CuO5.xFx system is furtherreduced by the presence of oxygen vacancies A very c o m m o n feature of fluorine in solid-statechemistry is it'sease of substitutionfor oxygen in oxide systems. This is due to the similar size of the 02" and F- anions. This is normally compensated for eitherby reduction of the cationiccharge (e.g.,in F304-xFx, Nd2CuO4-xFx) or by an appropriate hctrovalcnt cation substitution(e.g.,in Pb2-xNaxNb207-xFx) [10]. The atomic scatteringfactorfor oxygen and fluorinearc so similarthat the diffractiontechniques based on Xray as well as neutron arc normally inadequate for the determination of theirexact ratio and site occupancies in a given crystalstructure. The ion-selectiveelectrodemethod used in thisstudy for the determination of fluorine is highly reliablein terms of the totalfluorine content in the sample, but this technique does not exclude the possibilityfor some of the fluorine to be present as amorphous fluorides at the grain boundaries. Due to the above limitations, it is difficultto conclude whether allthe fluorinein the startingreactionmixture has entered the TIBa2CuO5 lattice or not. However, the fact that single phase TIBa2CuO5-type compounds with good superconducting propertiescould bc easilyprepared when the reactionmixtures contained fluorine, clearly shows that the fluorine substitutionindeed occurred. In addition, the a-latticeparameter showed an expected increasedue to the reduction of the formal oxidation stateof copper when F- is substitutedfor 0 2-. In TIBa2CuO5 (1201-type),each copper atom is at the center of an elongated octahedron. It is likelythat all the fluorine atoms substituteat the apical oxygen positions of the octahedron (below and above the Cu-O planar sheets) as shown in Fig. 2. Such an arrangement is not unusual among oxides with a laycrcd structure,and has been seen in K2NiF4-type oxyfluoride compounds, viz. K 2 N b O 3 F [11], Sr2FcO3F 12] and Ca2MnO4-xFx [13],where the fluorine and oxyg.cn atoms arc statisticallydistributedat the apical oxygen positionsof the octahedra along the c-axls. Further studies, such as fluorine-NMR, are being undertaken to fully understand the structure/propertyrelationshipsin TIBa2CuO5.xFx.
For the first time, stabilization of high temperature superconducting phases (Tc > 70 K) in TIBa2CuO5 (1201-type) system has been achieved through the substitution of fuorine for oxygen. The trend in the a- and c-lattice parameter with x in TIBa2CuOs-xFx, indicated that there is a reduction in the formal valence of copper as F- substitutes for 02-. Our attempts to prepare T1Ba2CuO5.8 samples (without fluorine) under identical conditions resulted in the multiphase products with weak superconducting transitions.
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125
Acknowledgements
The author wish to thank T. G. Calvarese for sample preparation, C. M. Foris for X-ray powder diffraction dam and D. M. Groski for magnetic measurements. Thanks are also due to M. L. Plummer for assisting in high pressure work. References
1. A.W. Sleight, Science, 242, 1519 (1988). 2. A . C . W . P . James, S. M. Zahurac and D. W. Murphy, Nature, 338, 240 (1989). 3. H.C. Ku, M. F. Tai, J. B. Shi, M. J. Shieh, S. W. I-Isu, G. H. Hwang, D. C. Ling, T. J. W. Yang and T. Y. Lin, Ypn. J. Appl. Phys. 28, 923 (1989). 4. Y. Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B, 40, 1140 (1989). 5. M.A. Subramanian, G.H. Kwei, J.B. Parise, J.A. Goldstone, and R.B. Von Dreele, Physica C, 166, 19 (1990). 6. H.H. Willard, L. L. Merritt and J. A. Dean, in "Instrumental Methods of Analysis", Von Nostrand, New York, 1974. pp. 575-576. 7. M.A. Subramanian and A. K. Ganguli, unpublished results. 8. M.A. Subramanian, Mater. Phys. Chem., 35, 240 1993. 9. D. Jung, M.-H. Whangbo, N. Hen'on, and C. C. Torardi, Physica C, 160 (1989) 381. 10. P. I-Iagenmuller in "Inorganic Solid Flurorides°', Ed. P. Hagenmuller, Academic press, 1985, pp. 1-15. 11. F. Galasso and W. Darby, J. Phys. Chem., 66, 1318 (1962); M.A.Subramanian, N. Jones and J. C. Calabrese, to be published. 12. F. Galasso and W. Darby, J. Phys. Chem., 67, 1451 (1963). 13. G. Le Flem, R. Colmet, C. Chanmont, J. Claverie and P. I-Iagenmuller, Mat. Res. Bull., 11, 389 (1976).
Materials Reseaz~ Bulletin, Vol. 29, No. 2, pp. 119-125, 1994 Copyright© 1994 Elsevie~ Scieace Ltd Printed in the USA. All rishts reu~ved 0025-5408/94 $6.00 + .00
S T A B I L I Z A T I O N O F S U P E R C O N D U C T I N G P H A S E S IN T H A L L I U M BARIUM-COPPER
O X I D E S Y S T E M BY F L U O R I N E S U B S T I T U T I O N [TIBa2CuO5.xFx]
M. A. Subramanian Central Research and Development Dupont Company Experimental Station Wilmington, Delaware 19880-0328 (USA)
(Received November 29, 1993; Communicated b y A.W. Sleight) ABSTRACT Partial substitution of oxygen by fluorine in T1Ba2CuOs. has been achieved by heating T1203, BaO2, BaO and CuO under one kbar external pressure using TIF as a source for fluorine. A single phase region exists for 0.1 < x < 0.5 and the X-ray powder diffraction data indicated a tetragonal structure (space group: P4/mmm) for all the compounds. The in-plane Cu-O distance calculated from the a-lattice parameter showed a systematic increase as x was varied from 0. I to 0.5. This is due to an increase in the formal oxidation state of copper as F 1" substitutes for 02. in the 1201-type structure. The c-lattice parameter also showed a slight increase, as the fluorine content in the reaction mixture is increased. Superconductivity is observed for x values within the range 0.1 to 0.6 with transition temperatures in the range 35-75 K. MATERIALS INDEX: thallium, barium, copper, oxide, fluorine
Iatr..ea.u.~m~ Although the mechanism of superconductivity in high T¢ copper oxides is far from understood, it is currently well known that the existence of holes (Cu3+/Cu 2+) or electrons (Cu2+/Cu 1+) in the CuO2 planes is essential for the occurrence of superconductivity in those materials. In general, superconductivity occurs when the carder (hole or eleclxon) concentration is in the optimal region, which lies between an antiferromagentic insulator and a metallic phase [I]. Carrier concentration in these materials can be controlled in a number of ways. One Is to change the oxygen concentration (YBa2Cu307-8) and another is by cation substitution, viz. La2.xSrxCuO4, TIBa2.xLaxCuO5. Carrier concentration in copper oxide superconductors can also be altered by anion substitution, viz. 02- by F-. In this case, an electron rather than a hole is injected into the CuO2 sheets and this has been well established in the case of n-type copper oxide superconductors, Ln2CuO4-xFx (Ln=Pr, Nd, Sm) [2]. 119
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Thallium-containing superconductors of the type TlmCan_lBa2CunO2n+m+2 (m = 1,2) have been studied extensively in the last five years. All of these oxides are superconducting with transition temperatures of 80 to 125 K, except for TIBa2CuO5 (where m = 1, n=l) and this has been attributed to the presence of copper in +3 formal oxidation state (over-doped). However, weak or metastable superconductivity with an onset of Tc around 20 K has been seen in some preparations [3]. It has been well established that the superconducting transition temperature of this phase can be drastically enhanced through partial substitution of Ba2+ ions by trivalent rare earths (La3+, Nd 3+) [3-5]. The stabilization and enhancement of the superconducting transition temperatures in T1Ba2.xLaxCuO 5 can be attributed to the transfer of electrons to the Cu-O sheets, when trivalent rare earths substitute the Ba 2+ ions. This results in the reduction of the formal oxidation state of copper from it's over-doped level (Cu ~3+) to an optimum level (Cu 2.2+ to Cu 2.5+) and the resulting compositions arc superconducting. Similar reduction in the hole concentration in the Cu-O sheets should occur if oxygen in TIBa2CuO5 is replaced by fluorine. Here we report on our efforts on the stabilization of superconducting phases in T1Ba2CuOs-system through fluorine substitution. Exl)0rlmental Methods
Appropriate quantities of the reactants (TI203, TIF, BaO, BaO2 and Cue) were intimately mixed together in an agate mortar. The mixtures in the form of pellets were loaded into gold tubes (3/8" dia x 4" length) and sealed. The entire process of weighing, grinding and loading the tubes was done in a nitrogen filled dry box. The tubes were heated at 780-820°C for 6 hrs under one kbar pressure. TIBa2CuOs.8 phases were also prepared under similar conditions for various values of 8 ranging from 0.1 to 0.7. In this case, the oxygen content (5-8) in each sample was fixed by varying the ratio of BaO2 to BaO in the starting reaction mixture. X-ray diffraction data were obtained with a SCINTAG (PAD IV) diffractometer using CuKa radiation. Cell dimensions were refined by least squares. The test for superconductivity was made by the a.c. mutual inductance technique. Electrical resistivity measurements were made by the standard four-probe technique. Fluorine content in the compounds were determined by ionselective electrode method [6]. Results
Successful synthesis of fluorine substituted TIBa2CuOs.xFx phases was achieved by using TIF as a source for fluorine, Our attempts to synthesize them using BaF2 or CuF2 in a sealed gold tube under ambient pressure always resulted in the formation of 1201-type phases with BaF2 and BaCuO2 as major impurity phases [7]. Due to the low melting and boiling point of TIF, the sealed gold tubes were heated under one kbar external pressure to avoid the rupture of the gold tubes from the development of high internal pressures. X-ray diffraction data of T1Ba2CuOs.xFx showed that single phase samples were formed for x = 0.2 to 0.5 under the synthetic conditions employed. The X-ray diffraction lines for all the samples could be indexed on the basis of a tetragonal cell with a - 3.8 and c ~ 9.5 ]~, space group P4/mmm. The unit cell parameters are listed in Table I. The X-ray pattern of the x = 0.6 sample contained a few extra reflections and some of them could be accounted for a BaF2 impurity. When x was increased further, the intensities of the impurity reflections also increased. Fig. 1 shows the powder X-ray diffraction trace for T1Ba2CuO4.TF0.3 and it's indexed powder pattern is given in Table II. The fact that the X-ray powder diffraction patterns of these phases are very similar to that of T1Ba2_xLaxCuOs [5] suggests that these compounds have 1201-type structure (Fig. 2). This structure consists of single sheets of corner-sharing CuO 4 groups in which each copper atom has two additional oxygen atoms positioned above and below the sheet. Sr ions reside above and below the Cu-O sheets in nine-fold coordination with oxygen, and the single TI-O sheets are alternately stacked with the Ba-Cu-O slabs along the c direction [5]. Table I also lists the observed fluorine contents in TIBa2CuOs.xFx samples determined by chemical analysis and are
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TABLE I Unit cell parameters and analytical data for TIBa2CuOs.xFx
Composition
a (~) (+0.001)
c(J,) (+0.002)
TIBa2CuO4.9F0.] TIBa2CuO4.8Fo.2 TIBa2CuO4.7F0.3 TIBa2CuO4.6F0.4 TIBa2CuO4.5F0.5 TIBa2CuO4.4F0.6 K2NbO3F
3.828 3.832 3.836 3.839 3.841 3.842 . . .
9.586 9.585 9.590 9.598 9.602 9.605
Fluorine content (wt%) (obs.)* (cal.) 0.27(6) 0.63(5) 0.94(4) 1.20(5) 1.55(6) 1.89(5) 8.04(8)
.
0.305 0.610 0.915 1.220 1.525 1.830 7.98
The values given m the parenthesis are standard deviation in the last decimal place.
I
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5
10
15
•
I
I
!
20
25
30
i
i
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35
40
•
I
45
i
i
I
50
55
20 FIG. 1 Powder X-ray diffraction pattern for TIBa2CuO4.7F0.3. The 20 values are in degrees.
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TABLE H
X-ray Diffraction Data for TIBa2CuO4.TFO.3 hk I
dob s (A)
deal (A)
001 002 10 0 10 1 003 10 2 1 10 111 10 3 004 1 13 10 4 200 1 14 10 5 2 11 203 2 12
9.503 4.774
9.5900 4.7950 3.8360 3.5616 3.1967 2.9954 2.7125 2.6101 2.4557 2.3975 2.0682 2.0331 1.9180 1.7963 1.7155 1.6887 1.6446 1.6152
3.554 3.195 2.996 2.7121 2.6100 2.4558 2.3979 2.0680 2.0336 1.9181 1.7960 1.7155 1.6883 1.6445 1.6154
lobs 4 1 24 10 100 58 4 3 15 17 2 20 17 12 3 4 27
Ical* 5 1 <1 22 10 100 54 3 3 17 16 1 21 17 13 2 3 24
* Relative peak height intensities are calculated on the basis of a 1201-type structure with P4/mmm symmetry. Atomic positions (x, y, z) used are taken from reference [5]: 1 TI (0, 0, 0); 2 Ba (1/2, 1/2, 0.295); 1 Cu (0, 0, 1/2); 2 O(1) (0, 0, 0.225); 2 0(2) (1/2, 0, 1/2); 1 0(3) (1/2, 1/2, 0). Disorder of T1 and O (3) atoms are not considered. in good agreement with the calculated values based on the starting compositions. The analytical data for K2NbO3F sample is also included for comparison. Our attempts to synthesize oxygen deficient TIBa2CuO5.8 phases for various values of 8 (0.1 to 0.6) under similar conditions were not successful and always resulted in a multiphase product with BaCuO2 as a major impurity. In some cases, the X-ray diffraction lines corresponding to T1Ba2CuO5-type structure were seen along with other impurity phases. Magnetic flux exclusion measurements for T1Ba2CuO5_xFx phases showed superconducting onset temperatures in the range 35-75 K with large Meissner fractions. Electrical resistivity measurements showed zero resistivities in the range 29-65 K (Table III). However, for T1Ba2CuO5-8 samples, only weak superconducting transitions were observed in some of the preparations and their transition temperatures arc also listed in Table HI.
Earlier attempts on the synthesis of single phase 1201-type T1Ba2CuO5 phase without the presence of suitable substitutions were unsuccessful [8]. This is because such a synthesis would revolve stabilizing Cu in the trivalent state. This communication shows that the tetragonal (P4/mmm) TIBa2CuOs-type phases can be stabilized by partial substitution of oxygen by fluorine. The substitution of 02- by F- should reduce the formal valence state of copper below 3+, thereby
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FLUORINES
123
•
Cu
Q
Tl
0 Ba 0
0
00/F
FIG. 2 Structure of T1Ba2CuO5.xFx. TABLE III Superconductivity data for TIBa2CuOs.xFx and TIBa2CuOs.8 TIBa2CuO$.xFx
TIBa2CuO$.8*
xor~ To K (on set) O.1 0.2 0.3 0.4 O.5 0.6 * Impure phases.
Tc, K (zero)
35 (very weak) 48 75 65 69 50
29 65 55 51 37"
To, K (onset)
To K (zero)
Semieonducfing 10 (very weak) 21 <5 18 <5 Semieonducting Semieondueting
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creating mobile holes in the Cu-O sheets. In other words, electrons are added to the copperoxygen sheets to create Cu 3+ and Cu 2+. This is analogous to Nd2CuO4-xFx, where the fluorine substitutionreduces the formal oxidation state of copper below 2+, thereby introducing electrons in the Cu-O sheets and results in n-type superconductivity. In tetragonal thallium cuprate superconductors, the in-plane Cu-O distance is roughly one-half of the a-latticeparameter. From Table I, itis clearthatas x increasesthe a-latticeparameter thence the in-plane Cu-O distance)also increases. This is due to an increase in the formal oxidation stateof copper in the sheets. If one assumes that thallium rather than copper is reduced, the formation of TI I+ in the latticeshould show a large increase in the c-latticeparameter as the ionic radius of TI I+ is much largerthan that of TI 3+. However, the observed c-latticeparameter showed only a very slightincrease with x (Table I) and is is probably due to an elongation of the apical Cu-O distance resulting from an increase in the Cu 2+ content in Cu-O sheets. It is interesting to compare the superconducting properties of TIBa2.xLaxCuO5 with TIBa2CuOs.xFx. In the case of TIBa2_xLaxCuO5 system, superconductivity was observed for samples in the range x= 0.5 to 0.8, which corresponds to the formal valence state of copper between 2.5 and 2.2. This is in contrast to TIBa2CuOs.xFx system, where the superconducting compositions exist for x = 0.2 to 0.5, which corresponds to the formal valence stateof coppcr between 2.8 and 2.5. Earlier Band structuralcalculations revealed that for TIBa2CuO5-typc compounds, the Tl 6s-block band lieswell above the Fermi levcl and do not participatein the removal of electrons from the Cu-O layer d(x2-y2)band [9]. Itis likelythatthe formal valence of copper in the TIBa2CuO5.xFx system is furtherreduced by the presence of oxygen vacancies A very c o m m o n feature of fluorine in solid-statechemistry is it'sease of substitutionfor oxygen in oxide systems. This is due to the similar size of the 02" and F- anions. This is normally compensated for eitherby reduction of the cationiccharge (e.g.,in F304-xFx, Nd2CuO4-xFx) or by an appropriate hctrovalcnt cation substitution(e.g.,in Pb2-xNaxNb207-xFx) [10]. The atomic scatteringfactorfor oxygen and fluorinearc so similarthat the diffractiontechniques based on Xray as well as neutron arc normally inadequate for the determination of theirexact ratio and site occupancies in a given crystalstructure. The ion-selectiveelectrodemethod used in thisstudy for the determination of fluorine is highly reliablein terms of the totalfluorine content in the sample, but this technique does not exclude the possibilityfor some of the fluorine to be present as amorphous fluorides at the grain boundaries. Due to the above limitations, it is difficultto conclude whether allthe fluorinein the startingreactionmixture has entered the TIBa2CuO5 lattice or not. However, the fact that single phase TIBa2CuO5-type compounds with good superconducting propertiescould bc easilyprepared when the reactionmixtures contained fluorine, clearly shows that the fluorine substitutionindeed occurred. In addition, the a-latticeparameter showed an expected increasedue to the reduction of the formal oxidation stateof copper when F- is substitutedfor 0 2-. In TIBa2CuO5 (1201-type),each copper atom is at the center of an elongated octahedron. It is likelythat all the fluorine atoms substituteat the apical oxygen positions of the octahedron (below and above the Cu-O planar sheets) as shown in Fig. 2. Such an arrangement is not unusual among oxides with a laycrcd structure,and has been seen in K2NiF4-type oxyfluoride compounds, viz. K 2 N b O 3 F [11], Sr2FcO3F 12] and Ca2MnO4-xFx [13],where the fluorine and oxyg.cn atoms arc statisticallydistributedat the apical oxygen positionsof the octahedra along the c-axls. Further studies, such as fluorine-NMR, are being undertaken to fully understand the structure/propertyrelationshipsin TIBa2CuO5.xFx.
For the first time, stabilization of high temperature superconducting phases (Tc > 70 K) in TIBa2CuO5 (1201-type) system has been achieved through the substitution of fuorine for oxygen. The trend in the a- and c-lattice parameter with x in TIBa2CuOs-xFx, indicated that there is a reduction in the formal valence of copper as F- substitutes for 02-. Our attempts to prepare T1Ba2CuO5.8 samples (without fluorine) under identical conditions resulted in the multiphase products with weak superconducting transitions.
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Acknowledgements
The author wish to thank T. G. Calvarese for sample preparation, C. M. Foris for X-ray powder diffraction dam and D. M. Groski for magnetic measurements. Thanks are also due to M. L. Plummer for assisting in high pressure work. References
1. A.W. Sleight, Science, 242, 1519 (1988). 2. A . C . W . P . James, S. M. Zahurac and D. W. Murphy, Nature, 338, 240 (1989). 3. H.C. Ku, M. F. Tai, J. B. Shi, M. J. Shieh, S. W. I-Isu, G. H. Hwang, D. C. Ling, T. J. W. Yang and T. Y. Lin, Ypn. J. Appl. Phys. 28, 923 (1989). 4. Y. Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B, 40, 1140 (1989). 5. M.A. Subramanian, G.H. Kwei, J.B. Parise, J.A. Goldstone, and R.B. Von Dreele, Physica C, 166, 19 (1990). 6. H.H. Willard, L. L. Merritt and J. A. Dean, in "Instrumental Methods of Analysis", Von Nostrand, New York, 1974. pp. 575-576. 7. M.A. Subramanian and A. K. Ganguli, unpublished results. 8. M.A. Subramanian, Mater. Phys. Chem., 35, 240 1993. 9. D. Jung, M.-H. Whangbo, N. Hen'on, and C. C. Torardi, Physica C, 160 (1989) 381. 10. P. I-Iagenmuller in "Inorganic Solid Flurorides°', Ed. P. Hagenmuller, Academic press, 1985, pp. 1-15. 11. F. Galasso and W. Darby, J. Phys. Chem., 66, 1318 (1962); M.A.Subramanian, N. Jones and J. C. Calabrese, to be published. 12. F. Galasso and W. Darby, J. Phys. Chem., 67, 1451 (1963). 13. G. Le Flem, R. Colmet, C. Chanmont, J. Claverie and P. I-Iagenmuller, Mat. Res. Bull., 11, 389 (1976).