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Mechanisms for hole doping in thallium cuprate superconductors having single copper-oxygen sheets
240
Materials Chembtry and Physics, 35 (1993) 240-243
Mechanisms for hole doping in thallium cuprate having single copper-oxygen sheets
superconductors
M.A. Subramanian Central Research and D~e~op~nt,
Daunt
Company, P.U. BOX 80328, Bperimental
Station, ~~rn~gton,
DE 298gO-~328 (USA)
Abstract This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-0 sheets interleaved with single and double Tl-0 layers. These studies strongly suggest that, for T1,BazCuOG, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x2-y’ bands, i.e., the Tl-0 layer creates holes in the CuO, layers. This is in contrast to TlBa,CuO&ype phases, where the metal chemistry forces the oxidation of Cu02 layers. However, studies on T1Sr2_JaXCu05 and TIBa,_&LaCu05 systems indicate that for TI-O single rock-salt layer phases with a short in-plane Cu-0 distance, such as TlSrLaCuO,, the Cu02 layer x2-y’ band is raised high enough in energy for the Tl-0 layer to create holes in the CuO, layer.
Introduction of superconductivity in the The discovery Tl-Ba-Ca-Cu-0 system [l] has not only provided compounds with the highest transition temperatures (T,= 125 K) but also revealed a structural pattern that is common to all the superconducting copper oxides. From structural investigations of this system E2-83, it is now known that there exists a very large family of compounds of the type Tl,,,Ba,Ca, _ ,Cu,02, +m+z which contains copper-oxygen sheets interleaved with Tl-0 sheets. The value of m may be 1 or 2. The number of consecutively stacked CuO, layers is indicated by n. So far, the maximum n value reached is 6, for the Ba analogue in the m= 1 series. Phases of the type &M,Ca,_ lCu,O, +,,,+Z (m = 1 or 2) are readily prepared for M =Ba, but our attempts to prepare such phases with M = Sr have not been successful. However, it is now known that T&&a, _ &u,Oz, + 3 (nz = 1) phases can be readily stabilized by substitution of Bi (mixture of Bi’+Bi”+) or Pb (Pb4’) at the Tl site [9-131 or rare earth at the Sr or Ca site [13-171. Although the detailed structural features of these compounds have been revealed by a ~mbination of X-ray and neutron d~ra~tion methods [6], the mechanisms giving rise to Cu(II1) (or holes) and the role played by Tl-0 layers in the creation of charge carriers are some of the issues that are not fully understood. Figure 1 shows, as examples of the Tl,Ba,Ca,, _ ,Cu,O, +m+2 family, crystal structures of the tetragonal n = 1 phases with ~tz= 1 and 2. The structures
-
TI
-
TI
-
TI
-
Ba
-
Ba
-
cu
-
cu
-
Ba
-
Ba
-
Tl
-
Ti
-
II
(a) Fig. 1. The structures of (a) T1Ba2CuOS and @) T12Ba,CuO~. Metal atoms are shaded and Cu-0 bonds are shown.
can be described as perovskite-like slabs alternating with single (Fig. l(a)) or double (Fig. l(b)) Tl-0 rocksalt-type layers. When II is greater than 1, the n sheets of comer-sharing square-planar CuO, groups are oriented parallel to the (001) plane. Additional oxygen atoms, located above and below the consecutive Cu-0 sheets, are bonded to the copper atoms of the outer two Iayers. There are no oxygen atoms between the Cu-0 sheets. Barium ions are found above and below the Cu-0 sheets in ninefold coordination with oxygen and calcium atoms are located between Cu-0 sheets in eightfold coordination. The structures of other members in the family are quite similar and ditIer from one another only in the number of Cu-0 sheets. Creation of holes in the Cu-0 sheets for the TlBa,Ca, _ ,Cu,O, + 3 phases seems to be straightforward.
0 1993 - Eisevier Sequoia. AI1 rights reserved
241
For the above series, the formal oxidation state of Cu varies as (2n + 1)/2. For example, the formal valence state of Cu is 3.0 for n = 1, 2.5 for n = 2 and 2.33 for II = 3. In other words, in the above materials, chemistry forces the formal oxidation state of Cu to be above +2 and hence there are holes in the Cu-0 sheets. This is in contrast to stoichiometric Tl,Ba,Ca,_,CunOzn+m+2 (m = 2 series) phases, where the chemical composition implies that the formal oxidation state of Cu is always 2, irrespective of the value of IZ. Several possible mechanisms have been suggested for the introduction of holes into the Cu-0 sheets for hases. Tight-binding band elecTl,Ba,Ca, - Cu,O, +4 P tronic structural calculations for Tl-0 double rock-salt layer phases have shown that the Tl 6s levels lie well below the Fermi level and remove electrons from the x2-3 band of the CuO, sheets [18]. However, this is not the case for the Tl-0 single rock-salt layer phases, where the Tl 6s levels lie well above the Fermi level. Other possible mechanisms suggested are cation vacancies (Tl,_,Ba,Ca,Cu,O,), cation substitutions (Tl,_,Ca,Ba,CaCu,O,) and oxygen nonstoichiometry
[W* In order to get insight into the hole-doping mechanisms in thallium-based superconductors, we have carried out many systematic studies involving substitution of various cations in the crystal structure of TlJWuO, +m (m = 1,2) and its implications for the superconducting properties. Cuprates
with double thallium-oxygen
sheets
Perhaps the most enigmatic of the Tl,Ba,Ca, _ r&,0, +m+2 phases is Tl,Ba,CuO, (m = 2; it= 1). This compound was first synthesized in our laboratory as a single phase in sealed gold tubes and was shown to have T,‘s close to 90 K [4]. However, this compound can be prepared as nonsuperconducting as well as superconducting, with Tc’s varying from 20 to 90 K [20]. This superconductor is unique in the sense that it has the highest T, for any superconductor with one Cu-0 sheet (Fig. l(b)). The composition as written implies a formal oxidation state for Cu of +2, and this composition, containing no excess holes, would not be expected to be superconducting on the basis of’ comparison with other Cu-0 based superconductors. High-resolution neutron powder diffraction studies [21] of 90 K superconducting single-phase Tl,Ba,Cu06 samples (prepared at DuPont, by heating the reactants in sealed gold tubes) have shown no significant oxygen excess or cation vacancies. As suggested by the tightbinding band structure calculations, the creation of holes in this phase is due to the overlap of Tl 6s levels with Cu 3d levels at the Fermi level. As part of our ongoing substitution studies of the thallium-based su-
perconductors, we have studied the effect of Sr substitution on the superconducting properties of Tl,Ba,CuO,. Substitution of St-‘+ for Ba2+ should not affect the electron count on copper. However, substitution of the less electropositive Sr for Ba in Tl,Ba,CuO,, should bring down the Tl 6s level and increase the overlap of Tl6s with Cu 3dx2 -3, leading to an increase in the hole concentration. Oxides of the type Tl,Ba,_,Sr,CuO, were prepared by heating stoichiometric amounts of T&O,, BaO,, SrO, and CuO in a sealed gold tube at 875 “C for 6 h. The samples were cooled to room temperature in the furnace. Single phases could be obtained until x = 1.2 and could be indexed on a body-centered tetragonal lattice (space group 14lmmm) as the parent Tl,Ba,CuO, oxide. Beyond x= 1.2, lines due to impurities started appearing and increased in intensity with further increases in the Sr concentration. Figure 2 shows the change in the superconducting transition temperature T, as a function of X. Until x = 0.8 there is a gradual change in T,, after which T, drops sharply and goes to zero at x = 1.2. The x= 1.2 composition is metallic down to 4.2 K. This is very similar to the behavior observed in La,_,Sr,CuO,, where the superconductor-metal transition is brought about by an overdoping of holes through the substitution of SrZ’ for La3+ (x>O.3) [22]. Since the Cu-0 distance is roughly one half of the a lattice parameter (this is true for thallium cuprates, as the Cu-0 sheets are flat), the Cu-0 distance should decrease as x is increased. In the case of Tl,Ba, _,SrXCu06, neutron diffraction refinements revealed a rapid decrease in the in-plane Cu-0 bond length (one half of the a lattice parameter in Fig. 3) as x is increased, which is mainly caused by size effects. However, it is important to note that the in-plane Cu-0 bond length decreases by 5% as x is varied from 0.2 to 1.2, which is twice the c-axis contraction in the same composition range (Fig. 3). This is probably due to the overdoping of Cu-0 sheets caused by an increase in the overlap of Tl 6s with Cu 3d x2--y’ bands as one substitutes the less electropositive Sr for Ba. 100
a0
Fig. 2. Variation of the superconducting as a function of x for TlzBa,_,SrxCuO,.
transition
temperature
242
3.80 0.2
22.6 0.4
0.6
0.8
1.0
1.2
1.4
X
Fig. 3. Variation of a (mf and c (0) lattice parameters as a function of x for T12Ba2_&Cu06 (derived from neutron diffraction data).
Veal et al. 1231 have examined the system YBa,_,SrXCusO-I and discussed the possible ways in which Sr substitution might lower T,. In their view the depression of T, is indirectly caused by a change in the structure around the Sr ion and due to additional oxygen vacancies at the O(1) sites (chain) in the Srdoped samples. It is also argued that the electronic structure is not affected significantly by the substitution of Ba by Sr. The effect of Sr substitution on the lattice parameters and the superconducting properties is similar in both YBaZ_-xSr&‘u307 and Tl,Ba,_,Sr,CuO,. However, in the case of YBa,_,Sr,Cu,O,, the decrease in T, may be directly related to the oxygen vacancies, whereas the changes in the Cu-0 bandwidth (resulting in an increase in the overlap of Tl 6s at the Fermi level) due to Sr substitution may play an important role in the observed decrease in T, in Tl,Ba,_,Sr,CuO,. Cuprates with single thallium-oxygen
sheets
The phase TlBa,CuO, (nt =1 and n= 1 in TlBa&% - Cu,O, + 3yFig. l(a)) is reported to be a nonsuperconductor [24], presumably owing to the presence of all the copper atoms in the +3 oxidation state. Our attempts to synthesize single-phase samples of this compound were not successful. However, TlM,CuO, (M = Ba, Sr) can be stabilized by the appropriate substitution: Pb or Bi at the Tl site 1121, rare earth at the M site [15-l?‘, 261 or transition metal (Fe, Co) at the Cu site [26]. In the case of TlBa,_,La$uO,, singlephase samples could be prepared for x= 0.5 to 1.0. Superconductivity is observed for samples with x=0.5 to 0.85. A plot of T, versus x showed a maximum (57 K) for the sample with x10.75 (Fig. 4). Electrical resistivity and @R studies [27] showed that the end member, TlBaLaCuO, (x= 1) with all the copper atoms in the + 2 formal oxidation state, is an antiferromagnetic semiconductor. Neutron diffraction studies [28] of single-phase TIBaLaCuOS and TlBa,,LaJZuO, showed that the compounds are stoichiometric with respect to
Fig. 4. Variation of the superconducting transition temperature as a function of x for TiBaz_,La,CuO,.
cation and oxygen content. This shows clearly that the substitution of La for Ba in TlBa,CuO, lowers the formal valence state of Cu to below 3, thereby stabilizing the phases. Superconductivity is observed when the formal oxidation state lies between 2.15 and 2.5, and a maximum T, is observed for TlBa,,,La,,,CuO, with a formal oxidation state of 2.25. Similar investigations on TISr,_,LaXCuOs showed that the single-phase samples are formed for x=0.6 to 1.0 [17]. Magnetic and electrical measurements showed that the compounds are superconducting for all the values of X. The end member TISrLaCuOs with all the copper atoms in the +2 oxidation state is a superconductor, and this is in contrast to TIBa,_,LaXCuO, phases, where the end member TlBaLaCuO, is a semiconductor. Recent neutron diffraction studies showed that the compound TlSrLaCuO, is stoi~hiometric and is similar to TlBaLaCuO, [30]. On the basis of tightbinding band structure calculations, we examined why TISrLaCuOs is a superconductor while its analogue TlBaLaCuO, is not. Our study [31] strongly suggests that, for a Tl-0 single rock-salt layer phase with a very short m-plane Cu-0 distance, such as TlSrRCuO,, the CuO, layer x2-y2 band is raised high enough in energy for the Tl-0 layer to create holes in the CuO, sheet. In order to test this hypothesis, we synthesized solid solutions of the type TlBa,Sr, _,LaCuO, and found that they underwent a superconductor-semi~nductor transition at x= 0.3 1321. It is interesting to note that (T13+)0.5(Pb4+)0.5Sr2CuO~ is a metal and not a superconductor. Neutron diffraction studies [33] showed that the compound is stoichiometric, and hence the formal oxidation state of Cu is very close to +2.5. Recently [34] we have successfully synthesized nearly single-phase TlSr,CuO, (orthorhombic symmetry) and found it to exhibit metallic behavior. Both T&sPbO.,Sr,CuO, and TlSr,CuO, --r have very short in-plane Cu-0 distances (1.865 and 1.867 A, respectively) and are comparable to superconducting TlSrLaCuOs (1.880 A) with all the copper atoms in the +2 formal oxidation state. This suggests that the over-
243
r
Acknowledgements It is my great pleasure to thank Professor M-H. Whangbo and Dr A.K. Gang&i for many fruitful collaborations on thallium cuprate based high-temperature superconductors. References
(8)
fb)
(cl
Fig. 5. Schematic descriptions of the relative energies of the TI 6s block bands and the Cu drr+ block bands in (a) Tl,Ba,CuQ,, (b) TlBaLaCuOs and (c) T1SrLaCu05. q9 is the energy at the Fermi level.
lapping of T16s (as well as Pb 6s for Tl,,,Pb,,Sr,CuO,) bands with Cu 3d x2-y2 bands may actually increase the hole concentration either to an optimum level for superconductivity (as in TlSrLaCuO, and TlSr,_,La,CuO,, 0~~~0.2) or to an overdoped region (as in TlO,,Pb,,Sr,CuO, and TlSr,CuO,_,), giving rise to metallic behavior.
1 Z.Z. Sheng and A.M. Hermann,
7 8
9 10 11 12
13 14
From the above studies, it can be inferred that in the case of Tl,Ba,CuO,, Tl6s block bands overlap with Cu 3d bands at the Fermi level, i.e., the 11-O layers create holes in the Cu-0 sheet (Fig. 5(a)). Inco~oration of excess oxygen in T&O, layers (Tl,Ba,CuO,+,), cation substitutions (Tl,_,Ca,Ba,CaCu,O,) or cation vacancies (T1,_,Ba,Ca2Cu,01,,) may play a role in terms of altering the number of charge carriers (holes) in the conduction band and hence the superconducting properties. In many cases, such defects seems to increase the number of holes beyond a certain optimum value (to an overdoped region) and this causes a decrease in the transition temperature or even destruction of superconductivity. In the case of TlBa,CuO,-type phases, the metal chemistry forces the oxidation of Cu-0 sheets. Successful synthesis of semiconducting TIBaLaCuO, with all the copper atoms in the +2 formal oxidation state (Tl-0 single rock-salt layer analogue of superconducting Tl,Ba,CuO,) shows that the Tl 6s block band lies well above the Fermi level and does not remove electrons from the Cu-0 layer d,_,, band (Fig. 5(b)). The observation of superconductivity in TlSrLaCuO, and a superconductor-semiconductor transition in TlSr,Ba,_,LaCuO, strongly suggests that, for a single Tl-0 rock-salt layer phase with a very short in-plane Cu-0 distance, the Cu-0 dXz+ band is raised high enough in energy for the Tl-0 layer to create holes in the Cu-0 sheet (Fig. S(c)).
15 16 17 18 19 20 21
22 23 24 25 26 27 28 29 30 31 32 33 34
Nature (London), 332 (1988) 138. M.A. Subramanian et al., Nature (London), 332 (1988) 420. C.C. Torardi et al., Science, 240 (1988) 631. C.C. Torardi et al., Phys. Rev. B, 38 (1988) 22.5. A.W. Sleight, Science, 242 (1988) 1519. A.W. Sleight, M.A. Subram~ian and CC. Torardi, Mater. Rex Sac. Bull., 14 (1989) 45. C.N.R. Rao and B. Raveau, Ace Cizem. Rex, 22 (1989) 106. R. Beyers and T.M. Shaw, in H. Ehrenreich and D. Tumbull (eds.), Solid State Physics, Vol. 42, Academic Press, 1989, p. 13s. P. Halder et al., J. S~pe~a~d~ct~vi~, I (1988) 21. M.A. Subramanian et al,, Science, 242 (1988) 249. S. Li and M. Greenblatt, Pkysica C, I57 (1989) 365. C. Martin, D. Bourgault, C. Michel, J. Provost, M. Hervieu and B. Raveau, Eur. J. Solid State Inorg. Chem., 26 (1989) 1. M.A. Subramanian, P.L. Gai and A.W. Sleight, Mater. Res. Buff., 25 (1990) 101. M.A. Subramanian, P.L. Gai and M.-H. Whangbo, Mater. Res. Bull., 25 (1990) 899. D. Bourgault, C. Martin, C. Michel, M. Hervieu, J. Provost and B. Raveau, J. Solid State Chem., 75 (1988) 212. A.K. Ganguli, V. Manivannan, A.K. Sood and C.N.R. Rao, Appl. Phys. Lett., 55 (1989) 2664. M.A. Subram~ian, Mater. Res. Bull., 25 (1990) 191. D. Jung, M.-H. Whangbo, N. Herron and C.C. Torardi, Physica C, 160 (1989) 381. M.A. Subramanian, Ceram. Trans., 13 (1990) 59. Y. Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B, 40 (1989) 11400. J.B. Parise, C.C. Torardi, M.A. Subramanian, J. Gopalakrishnan, A.W. Sleight and E. Prince, Pfzysica C, 159 (1989) 239. J.B. Torrance, Y. Tokura, A.I. Nazzal, A. Bezinge, T.C. Huang and S.S.P. Parkin, Phys. Rev. Left., 62 (1988) 1127. B.W. Veal et al., Appl. Phys. Let?., 51 (I 987) 279. R. Beyers et al., Appl. Phys., Lett., 53 (1988) 432. T. Manako, Y. Shimakawa, Y. Kubo, T. Satoh and H. Igarashi, Physica C, 158 (1989) 143. A.K. Ganguh, M.A. Subramanian and G.H. Kwei, 1. Solid State Chem., 91 (1991) 397. Y.J. Uemura and M.A. Subramanian, unpublished results. M.A. Subramanian, G.H. Kwei, J.B. Parise, J.A. Goldstone and R.B. Von Dreele, Physica C, 166 (1990) 19. M.A. Subramanian and A.K. Ganguli, Mater. Res. Ball., 26 (1991) 91. G.H. Kwei and M.A. Subramanian, unpublished results. M.-H. Whangbo and M.A. Subramanian, J. Solid State Chem., 91 (1991) 403. M.A. Subramanian and M.-H. Whangbo, .T.Solid State Chem., 91 (1992) 403. G.H. Kwei, J.B. Shi and H.C. Ku, Physica C, I74 (1991) 180. A.K. Gang& and M.A. Subramanian, J. Solid State Chem., 93 (1991) 250.
Materials Chembtry and Physics, 35 (1993) 240-243
Mechanisms for hole doping in thallium cuprate having single copper-oxygen sheets
superconductors
M.A. Subramanian Central Research and D~e~op~nt,
Daunt
Company, P.U. BOX 80328, Bperimental
Station, ~~rn~gton,
DE 298gO-~328 (USA)
Abstract This article describes some recent studies on the effect of chemical substitution on the superconducting properties of thallium cuprates containing single Cu-0 sheets interleaved with single and double Tl-0 layers. These studies strongly suggest that, for T1,BazCuOG, Tl 6s-block bands lie below the Fermi level, so that they remove electrons from the Cu 3d x2-y’ bands, i.e., the Tl-0 layer creates holes in the CuO, layers. This is in contrast to TlBa,CuO&ype phases, where the metal chemistry forces the oxidation of Cu02 layers. However, studies on T1Sr2_JaXCu05 and TIBa,_&LaCu05 systems indicate that for TI-O single rock-salt layer phases with a short in-plane Cu-0 distance, such as TlSrLaCuO,, the Cu02 layer x2-y’ band is raised high enough in energy for the Tl-0 layer to create holes in the CuO, layer.
Introduction of superconductivity in the The discovery Tl-Ba-Ca-Cu-0 system [l] has not only provided compounds with the highest transition temperatures (T,= 125 K) but also revealed a structural pattern that is common to all the superconducting copper oxides. From structural investigations of this system E2-83, it is now known that there exists a very large family of compounds of the type Tl,,,Ba,Ca, _ ,Cu,02, +m+z which contains copper-oxygen sheets interleaved with Tl-0 sheets. The value of m may be 1 or 2. The number of consecutively stacked CuO, layers is indicated by n. So far, the maximum n value reached is 6, for the Ba analogue in the m= 1 series. Phases of the type &M,Ca,_ lCu,O, +,,,+Z (m = 1 or 2) are readily prepared for M =Ba, but our attempts to prepare such phases with M = Sr have not been successful. However, it is now known that T&&a, _ &u,Oz, + 3 (nz = 1) phases can be readily stabilized by substitution of Bi (mixture of Bi’+Bi”+) or Pb (Pb4’) at the Tl site [9-131 or rare earth at the Sr or Ca site [13-171. Although the detailed structural features of these compounds have been revealed by a ~mbination of X-ray and neutron d~ra~tion methods [6], the mechanisms giving rise to Cu(II1) (or holes) and the role played by Tl-0 layers in the creation of charge carriers are some of the issues that are not fully understood. Figure 1 shows, as examples of the Tl,Ba,Ca,, _ ,Cu,O, +m+2 family, crystal structures of the tetragonal n = 1 phases with ~tz= 1 and 2. The structures
-
TI
-
TI
-
TI
-
Ba
-
Ba
-
cu
-
cu
-
Ba
-
Ba
-
Tl
-
Ti
-
II
(a) Fig. 1. The structures of (a) T1Ba2CuOS and @) T12Ba,CuO~. Metal atoms are shaded and Cu-0 bonds are shown.
can be described as perovskite-like slabs alternating with single (Fig. l(a)) or double (Fig. l(b)) Tl-0 rocksalt-type layers. When II is greater than 1, the n sheets of comer-sharing square-planar CuO, groups are oriented parallel to the (001) plane. Additional oxygen atoms, located above and below the consecutive Cu-0 sheets, are bonded to the copper atoms of the outer two Iayers. There are no oxygen atoms between the Cu-0 sheets. Barium ions are found above and below the Cu-0 sheets in ninefold coordination with oxygen and calcium atoms are located between Cu-0 sheets in eightfold coordination. The structures of other members in the family are quite similar and ditIer from one another only in the number of Cu-0 sheets. Creation of holes in the Cu-0 sheets for the TlBa,Ca, _ ,Cu,O, + 3 phases seems to be straightforward.
0 1993 - Eisevier Sequoia. AI1 rights reserved
241
For the above series, the formal oxidation state of Cu varies as (2n + 1)/2. For example, the formal valence state of Cu is 3.0 for n = 1, 2.5 for n = 2 and 2.33 for II = 3. In other words, in the above materials, chemistry forces the formal oxidation state of Cu to be above +2 and hence there are holes in the Cu-0 sheets. This is in contrast to stoichiometric Tl,Ba,Ca,_,CunOzn+m+2 (m = 2 series) phases, where the chemical composition implies that the formal oxidation state of Cu is always 2, irrespective of the value of IZ. Several possible mechanisms have been suggested for the introduction of holes into the Cu-0 sheets for hases. Tight-binding band elecTl,Ba,Ca, - Cu,O, +4 P tronic structural calculations for Tl-0 double rock-salt layer phases have shown that the Tl 6s levels lie well below the Fermi level and remove electrons from the x2-3 band of the CuO, sheets [18]. However, this is not the case for the Tl-0 single rock-salt layer phases, where the Tl 6s levels lie well above the Fermi level. Other possible mechanisms suggested are cation vacancies (Tl,_,Ba,Ca,Cu,O,), cation substitutions (Tl,_,Ca,Ba,CaCu,O,) and oxygen nonstoichiometry
[W* In order to get insight into the hole-doping mechanisms in thallium-based superconductors, we have carried out many systematic studies involving substitution of various cations in the crystal structure of TlJWuO, +m (m = 1,2) and its implications for the superconducting properties. Cuprates
with double thallium-oxygen
sheets
Perhaps the most enigmatic of the Tl,Ba,Ca, _ r&,0, +m+2 phases is Tl,Ba,CuO, (m = 2; it= 1). This compound was first synthesized in our laboratory as a single phase in sealed gold tubes and was shown to have T,‘s close to 90 K [4]. However, this compound can be prepared as nonsuperconducting as well as superconducting, with Tc’s varying from 20 to 90 K [20]. This superconductor is unique in the sense that it has the highest T, for any superconductor with one Cu-0 sheet (Fig. l(b)). The composition as written implies a formal oxidation state for Cu of +2, and this composition, containing no excess holes, would not be expected to be superconducting on the basis of’ comparison with other Cu-0 based superconductors. High-resolution neutron powder diffraction studies [21] of 90 K superconducting single-phase Tl,Ba,Cu06 samples (prepared at DuPont, by heating the reactants in sealed gold tubes) have shown no significant oxygen excess or cation vacancies. As suggested by the tightbinding band structure calculations, the creation of holes in this phase is due to the overlap of Tl 6s levels with Cu 3d levels at the Fermi level. As part of our ongoing substitution studies of the thallium-based su-
perconductors, we have studied the effect of Sr substitution on the superconducting properties of Tl,Ba,CuO,. Substitution of St-‘+ for Ba2+ should not affect the electron count on copper. However, substitution of the less electropositive Sr for Ba in Tl,Ba,CuO,, should bring down the Tl 6s level and increase the overlap of Tl6s with Cu 3dx2 -3, leading to an increase in the hole concentration. Oxides of the type Tl,Ba,_,Sr,CuO, were prepared by heating stoichiometric amounts of T&O,, BaO,, SrO, and CuO in a sealed gold tube at 875 “C for 6 h. The samples were cooled to room temperature in the furnace. Single phases could be obtained until x = 1.2 and could be indexed on a body-centered tetragonal lattice (space group 14lmmm) as the parent Tl,Ba,CuO, oxide. Beyond x= 1.2, lines due to impurities started appearing and increased in intensity with further increases in the Sr concentration. Figure 2 shows the change in the superconducting transition temperature T, as a function of X. Until x = 0.8 there is a gradual change in T,, after which T, drops sharply and goes to zero at x = 1.2. The x= 1.2 composition is metallic down to 4.2 K. This is very similar to the behavior observed in La,_,Sr,CuO,, where the superconductor-metal transition is brought about by an overdoping of holes through the substitution of SrZ’ for La3+ (x>O.3) [22]. Since the Cu-0 distance is roughly one half of the a lattice parameter (this is true for thallium cuprates, as the Cu-0 sheets are flat), the Cu-0 distance should decrease as x is increased. In the case of Tl,Ba, _,SrXCu06, neutron diffraction refinements revealed a rapid decrease in the in-plane Cu-0 bond length (one half of the a lattice parameter in Fig. 3) as x is increased, which is mainly caused by size effects. However, it is important to note that the in-plane Cu-0 bond length decreases by 5% as x is varied from 0.2 to 1.2, which is twice the c-axis contraction in the same composition range (Fig. 3). This is probably due to the overdoping of Cu-0 sheets caused by an increase in the overlap of Tl 6s with Cu 3d x2--y’ bands as one substitutes the less electropositive Sr for Ba. 100
a0
Fig. 2. Variation of the superconducting as a function of x for TlzBa,_,SrxCuO,.
transition
temperature
242
3.80 0.2
22.6 0.4
0.6
0.8
1.0
1.2
1.4
X
Fig. 3. Variation of a (mf and c (0) lattice parameters as a function of x for T12Ba2_&Cu06 (derived from neutron diffraction data).
Veal et al. 1231 have examined the system YBa,_,SrXCusO-I and discussed the possible ways in which Sr substitution might lower T,. In their view the depression of T, is indirectly caused by a change in the structure around the Sr ion and due to additional oxygen vacancies at the O(1) sites (chain) in the Srdoped samples. It is also argued that the electronic structure is not affected significantly by the substitution of Ba by Sr. The effect of Sr substitution on the lattice parameters and the superconducting properties is similar in both YBaZ_-xSr&‘u307 and Tl,Ba,_,Sr,CuO,. However, in the case of YBa,_,Sr,Cu,O,, the decrease in T, may be directly related to the oxygen vacancies, whereas the changes in the Cu-0 bandwidth (resulting in an increase in the overlap of Tl 6s at the Fermi level) due to Sr substitution may play an important role in the observed decrease in T, in Tl,Ba,_,Sr,CuO,. Cuprates with single thallium-oxygen
sheets
The phase TlBa,CuO, (nt =1 and n= 1 in TlBa&% - Cu,O, + 3yFig. l(a)) is reported to be a nonsuperconductor [24], presumably owing to the presence of all the copper atoms in the +3 oxidation state. Our attempts to synthesize single-phase samples of this compound were not successful. However, TlM,CuO, (M = Ba, Sr) can be stabilized by the appropriate substitution: Pb or Bi at the Tl site 1121, rare earth at the M site [15-l?‘, 261 or transition metal (Fe, Co) at the Cu site [26]. In the case of TlBa,_,La$uO,, singlephase samples could be prepared for x= 0.5 to 1.0. Superconductivity is observed for samples with x=0.5 to 0.85. A plot of T, versus x showed a maximum (57 K) for the sample with x10.75 (Fig. 4). Electrical resistivity and @R studies [27] showed that the end member, TlBaLaCuO, (x= 1) with all the copper atoms in the + 2 formal oxidation state, is an antiferromagnetic semiconductor. Neutron diffraction studies [28] of single-phase TIBaLaCuOS and TlBa,,LaJZuO, showed that the compounds are stoichiometric with respect to
Fig. 4. Variation of the superconducting transition temperature as a function of x for TiBaz_,La,CuO,.
cation and oxygen content. This shows clearly that the substitution of La for Ba in TlBa,CuO, lowers the formal valence state of Cu to below 3, thereby stabilizing the phases. Superconductivity is observed when the formal oxidation state lies between 2.15 and 2.5, and a maximum T, is observed for TlBa,,,La,,,CuO, with a formal oxidation state of 2.25. Similar investigations on TISr,_,LaXCuOs showed that the single-phase samples are formed for x=0.6 to 1.0 [17]. Magnetic and electrical measurements showed that the compounds are superconducting for all the values of X. The end member TISrLaCuOs with all the copper atoms in the +2 oxidation state is a superconductor, and this is in contrast to TIBa,_,LaXCuO, phases, where the end member TlBaLaCuO, is a semiconductor. Recent neutron diffraction studies showed that the compound TlSrLaCuO, is stoi~hiometric and is similar to TlBaLaCuO, [30]. On the basis of tightbinding band structure calculations, we examined why TISrLaCuOs is a superconductor while its analogue TlBaLaCuO, is not. Our study [31] strongly suggests that, for a Tl-0 single rock-salt layer phase with a very short m-plane Cu-0 distance, such as TlSrRCuO,, the CuO, layer x2-y2 band is raised high enough in energy for the Tl-0 layer to create holes in the CuO, sheet. In order to test this hypothesis, we synthesized solid solutions of the type TlBa,Sr, _,LaCuO, and found that they underwent a superconductor-semi~nductor transition at x= 0.3 1321. It is interesting to note that (T13+)0.5(Pb4+)0.5Sr2CuO~ is a metal and not a superconductor. Neutron diffraction studies [33] showed that the compound is stoichiometric, and hence the formal oxidation state of Cu is very close to +2.5. Recently [34] we have successfully synthesized nearly single-phase TlSr,CuO, (orthorhombic symmetry) and found it to exhibit metallic behavior. Both T&sPbO.,Sr,CuO, and TlSr,CuO, --r have very short in-plane Cu-0 distances (1.865 and 1.867 A, respectively) and are comparable to superconducting TlSrLaCuOs (1.880 A) with all the copper atoms in the +2 formal oxidation state. This suggests that the over-
243
r
Acknowledgements It is my great pleasure to thank Professor M-H. Whangbo and Dr A.K. Gang&i for many fruitful collaborations on thallium cuprate based high-temperature superconductors. References
(8)
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Fig. 5. Schematic descriptions of the relative energies of the TI 6s block bands and the Cu drr+ block bands in (a) Tl,Ba,CuQ,, (b) TlBaLaCuOs and (c) T1SrLaCu05. q9 is the energy at the Fermi level.
lapping of T16s (as well as Pb 6s for Tl,,,Pb,,Sr,CuO,) bands with Cu 3d x2-y2 bands may actually increase the hole concentration either to an optimum level for superconductivity (as in TlSrLaCuO, and TlSr,_,La,CuO,, 0~~~0.2) or to an overdoped region (as in TlO,,Pb,,Sr,CuO, and TlSr,CuO,_,), giving rise to metallic behavior.
1 Z.Z. Sheng and A.M. Hermann,
7 8
9 10 11 12
13 14
From the above studies, it can be inferred that in the case of Tl,Ba,CuO,, Tl6s block bands overlap with Cu 3d bands at the Fermi level, i.e., the 11-O layers create holes in the Cu-0 sheet (Fig. 5(a)). Inco~oration of excess oxygen in T&O, layers (Tl,Ba,CuO,+,), cation substitutions (Tl,_,Ca,Ba,CaCu,O,) or cation vacancies (T1,_,Ba,Ca2Cu,01,,) may play a role in terms of altering the number of charge carriers (holes) in the conduction band and hence the superconducting properties. In many cases, such defects seems to increase the number of holes beyond a certain optimum value (to an overdoped region) and this causes a decrease in the transition temperature or even destruction of superconductivity. In the case of TlBa,CuO,-type phases, the metal chemistry forces the oxidation of Cu-0 sheets. Successful synthesis of semiconducting TIBaLaCuO, with all the copper atoms in the +2 formal oxidation state (Tl-0 single rock-salt layer analogue of superconducting Tl,Ba,CuO,) shows that the Tl 6s block band lies well above the Fermi level and does not remove electrons from the Cu-0 layer d,_,, band (Fig. 5(b)). The observation of superconductivity in TlSrLaCuO, and a superconductor-semiconductor transition in TlSr,Ba,_,LaCuO, strongly suggests that, for a single Tl-0 rock-salt layer phase with a very short in-plane Cu-0 distance, the Cu-0 dXz+ band is raised high enough in energy for the Tl-0 layer to create holes in the Cu-0 sheet (Fig. S(c)).
15 16 17 18 19 20 21
22 23 24 25 26 27 28 29 30 31 32 33 34
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