- Email: [email protected]
New Tl 2201 (Tl,Cr)2Sr2CuO6±δ phase material
Physica C 309 Ž1998. 208–214
New Tl 2201 žTl,Cr /2 Sr2 CuO6 " d phase material Y. Xin a , B.R. Xu b, K.W. Wong b
b,)
a Midwest SuperconductiÕity, 1315 Wakarusa DriÕe, Lawrence, KS 66049, USA Department of Physics and Astronomy, UniÕersity of Kansas, Lawrence, KS 66045, USA
Received 17 August 1998; revised 18 September 1998; accepted 24 September 1998
Abstract Evidence of the existence of the strontium analog of Tl 2 Ba 2 CuO6 , new HTSC material which bears the formula ŽTl,Cr. 2 Sr2 CuO6 " d is obtained. This phase has a transition temperature of 76 K. q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.10 q v; 74.70 y b; 61.10 y i; 74.70Vy Keywords: Tl-2201 phase; Tl–Sr–Cu–O; Transition temperature; X-ray diffraction
1. Introduction The 1201 structure of the thallium cuprate was first discovered in a Tl–Ba–Cu–O sample with a chemical composition of TlBa 2 CuO5 w1x. Soon after this discovery, it was found that the barium atoms in the 1201 structure can be totally replaced by the strontium atoms, i.e., TlSr2 CuO5 also bears the same crystalline structure w2x. The 1201 phase is the smallest in formula weight and unit cell size among all thallium-based superconducting cuprates. Fig. 1a shows the schematic structure of the 1201 crystal. This structure contains two identical blocks with a mirror symmetry about the central ŽCu–O 2 . plane. The Cu and Tl atoms occupy the vertices of each block and the Ba or Sr sits at the center of the block.
)
Corresponding author. Tel.: q1-785-864-4025; Fax: q1-785864-5262; E-mail: [email protected]
Oxygen atoms are positioned around each Cu atom, forming an octahedral. There is also an oxygen atom sitting at the center of the top and bottom planes of the unit cell. Historically, Tl 2 Ba 2 CuO6 instead of TlBa 2 CuO5 is the first thallium cuprate superconductor discovered w3x. It is the starting member of the thallium double layer family and is known as the 2201 phase since the formula contains two thallium atoms, two barium atoms, zero calcium atom, and one copper atom. The schematic crystal structure of the 2201 phase is given by Fig. 1b. This diagram depicts a complete bottom-half unit cell Žthe complete block. and an incomplete top-half unit cell Žthe unfinished block.. For a half unit cell, thallium atoms occupy the corners, copper atoms take the places of the middle of the side edges, barium atoms sit at the center of each of the two cubes, and oxygen atoms sit at the middle of the cube edges except on the top and bottom faces. The top or bottom face can be considered as a square defined by four thallium
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 5 8 4 - X
Y. Xin et al.r Physica C 309 (1998) 208–214
209
Fig. 1. Crystal structure of TlBa 2 CuO5 Ža. and Tl 2 Ba 2 CuO6 Žb..
atoms. Oxygen also stays at the center of the square. The top half unit repeats the same arrangement as the bottom unit except that the whole block slides by Ž a q b .r2. So the thallium atoms in one Tl–O layer are vertically aligned with the oxygen atoms in the adjacent Tl–O layer. This feature characterizes the crystal structure of the double layer thallium cuprate family. In a Tl-1201 structure, as mentioned above, the Ba and Sr atoms are exchangeable. In a Tl-2201 structure, however, such an exchange has never been verified. In this paper, we report the discovery which indicates the existence of the 2201 structure in samples made of Tl–Cr–Sr–Cu–O. Our work was initially set up in hope of improving the Tc of TlSr2 CuO5 material by chromium substitution. It is well-known that neither stoichiometric TlBa 2 CuO5 nor TlSr2 CuO5 is a superconducting material due to excessive hole population, which has an average Cu valency of 3.0. Substitution of higher valence elements Pb or Bi for Tl can convert the TlSr2 CuO5 material into superconducting w4–7x. Pb 4q and Bi 5q have the same electronic configuration as Tl 3q, and their ionic radii and electronegativity are also close to those of thallium. Therefore, substituting the higher ionic state of Pb 4q or Bi 5q for Tl 3q can reduce the hole density of the 1201 phase material and make the material superconductive. Substitution of a rare earth Ž3 q or 4 q in valence. for BarSr Ž2 q . in the 1201 phase has
been successfully demonstrated for both TlBa 2 CuO5 and TlSr2 CuO5 w8–13x. Besides Bi, Pb, and rare earths, chromium is the element which has been proven to be able to bring superconductivity to the 1201 structure w14x. It was established that the Cr substitution not only can bring superconductivity to the 1201 phase but actually can enhance the superconducting transition temperature to above 70 K. This 70 K superconductivity was speculated as coming from the presence of a Tl–Cr–Sr–Cu–O 2212 phase w15x. However, after careful examination of the X-ray diffraction data for our series of Tl–Cr–Sr– Cu–O samples, we can conclude that the phase responsible for the 76 K superconductivity is actually ŽTl,Cr. 2 Sr2 CuO6 " d , a totally new 2201 phase material.
2. Experimental Samples for the present experiment were prepared by the solid-state reaction method. According to the stoichiometry of TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, appropriate molar amount of high purity Tl 2 O 3 Ž99.99%., Cr2 O 3 Ž99.7., SrOŽ99.9%., and CuOŽ99.99%., were mixed and ground in an agate mortar. The resultant powder was pressed into pellets of about 25 mm in diameter and about 8 mm in thickness with a hydraulic press. The pellets were placed in a tubular alumina crucible and
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then put in a tube furnace. The power of the furnace was turned on as oxygen flowed slowly through the tube of the furnace. The temperature of the furnace was raised to about 9508C or 9808C at an ascending rate of 208rmin, and kept at the temperature for 10 h then followed by furnace cooling at a descending rate of 18rmin to 6508C. The samples were kept at 6508C for 6 h. Finally, the power of the furnace was turned off and the samples were cooled to room temperature in the furnace. Oxygen was maintained flowing until the room temperature was reached. Resistivity of the samples was measured by the standard four-probe technique with an ac frequency of 400 Hz. Powder X-ray diffraction was performed by Cu–K a radiation using a GEIGERFLEX CN2029 diffractometer. Magnetic property was measured with in the temperature range from 10 to 100 K with a Quantum Design MPMS-5S system.
3. Results and discussion Fig. 2 plots the resistivity vs. temperature curves for a series of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4,
sintered at 9508C. As expected, the parent material TlSr2 CuO5 is not superconductive. Its R–T curve displays a metallic property in the temperature range from 300 to 15 K. The sample with x s 0.1 exhibits an onset superconducting transition at about 45 K and a completed transition at 30 K. This clearly demonstrates the significance of Cr-doping in converting the material to superconductive. The sample with x s 0.2 has a Tc Žzero. of 43 K, while the zero resistance temperatures of samples with x s 0.3 and x s 0.4 are 47 K and 48 K, respectively. These results are similar to those of the Bi, Pb, or rare earth containing Tl 1201 phase materials. A surprising result was found from the Cr-doping work when the sintering temperature was raised from 950 to 9808C as samples with the same nominal compositions demonstrated much higher superconducting transition temperatures. Fig. 3 illustrates the R–T curves for the samples sintered at 9808C. This time the sample with x s 0.1 exhibits an onset superconducting transition at about 75 K and a completed transition at 50 K. While the samples with x s 0.2 and x s 0.3 have almost the same Tc Žzero., of about 76 K, further increase of the ratio of CrrSr suppresses the superconducting transition temperature in this series. Sample with x s 0.4 has a Tc Žzero.
Fig. 2. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4, sintered at 9508C.
Y. Xin et al.r Physica C 309 (1998) 208–214
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Fig. 3. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.1, 0.2, 0.3, 0.4, sintered at 9808C.
of 73 K. The R–T curves for samples with x s 0.5 and 0.6 are plotted separately in Fig. 4. Due to the large magnitude of the resistivities for these two samples these curves are too high to match those plotted in Fig. 3. Nonetheless, the sample with x s 0.5 still has a Tc Žzero. of 72 K. The sample with
x s 0.6 demonstrates a significant deviation in property. The normal state resistivity becomes semiconductive, increasing with the decrease of temperature in the range from room temperature to 120 K. At about 120 K, a semiconductive to metallic transition appears. An onset of a superconducting transition is
Fig. 4. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.5 and 0.6, sintered at 9808C.
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shown at about 70 K. However, the superconducting transition does not finish until below 50 K. Nevertheless, it should be noticed that all the samples in the later series show an onset of superconducting transition at temperatures above 70 K. Fig. 5 shows the X-ray diffraction patterns for the three samples, TlSr1.6 Cr0.4 CuO5 sintered at 9508C, TlSr1.8 Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C. From the X-ray data, we can immediately recognize that the sample TlSr1.6 Cr0.4 CuO5 sintered at 9508C has a pure 1201 structure. Both TlSr1.8-
Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C are multiphasic. The 1201 phase exists in either of the two samples. Careful examination uncovered that almost all the rest of the peaks in either sample can be attributed to a Tl-2201 structure. Several key arguments should be pointed out here: Ža. all the significant peaks not belonging to the 1201 phase in the X-ray patterns can be assigned to the 2201 phase; Žb. the intensity distribution in the presumed 2201 set of peaks is similar to that of Tl 2 Ba 2 CuO6 , among which the Ž 105 . peak at about 29.88 in the 2 u plot is
Fig. 5. X-ray diffraction patterns for the samples TlSr1.6 Cr0.4 CuO5 sintered at 9508C ŽA., TlSr1.8 Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C ŽB and C, respectively.. The indices without parentheses are for the 1201 phase and the indices in parentheses are for the 2201 phase.
Y. Xin et al.r Physica C 309 (1998) 208–214
the most intensive peak; Žc. when the indexed Ž 105 . peak becomes the most intensive peak in the diffraction pattern ŽFig. 5C., the intensity of most of the presumed 2201 peaks also increases; Žd. considering the overlap of some peaks between the 1201 phase and the 2201 phase, for the multiphasic samples, the intensity peaks at the 2 u angle of the overlap taking place should be increased. This can be checked out with some comparisons between a possible overlapped peak and a neighboring non-overlapped peak. Taking three pairs for the comparison, the 110 at about 33.58 and the 102 at about 31.28, the 112 at about 38.68 and the 004 at about 41.28, the 113 at about 45.88 and the 114 at about 54.18. It is irrefutable that for the multiphasic samples in Fig. 5B and C, the intensity ratio of first peak over the second peak significantly increases, which is a clear evidence of the overlapping of the 2201 phase’s Ž 110 . peak and the 1201 phase’s 110 peak, the 2201 phase’s Ž 0010. peak and the 1201 phase’s 112 peak, and the 2201 phase’s Ž 118 . peak and the 1201 phase’s 113 peak; Že. all the prominent peaks of the 2201 phase can be identified in the X-ray pattern in Fig. 5C, and the order in peak intensity is in agreement with the known Tl 2201 structure. It should be mentioned that from our X-ray data, we cannot find any evidence that supports the claim of the possible existence of a Tl 2212 phase which was proposed by Ref. w15x. Based on the indexed peaks of the 2201
213
phase in Fig. 5C, the lattice parameters of a tetragonal unit cell with I4r m m m symmetry were refined, of ˚ and c s 23.4565Ž4. A. ˚ which a s 3.7598Ž2. A The magnetization temperature dependency of the sample with x s 0.3 is shown in Fig. 6. The magnetic measurement confirms the 76 K superconducting transition temperature, and also indicates the multiphasic feature of the sample. From the above analysis, we conclude that the presence of Cr is essential to the formation of the 2201 crystal structure. It is difficult, however, to experimentally determine the position of the Cr atoms in the crystal structure from such a multiphasic sample. It should also be mentioned that the starting composition TlSr2y x Cr x CuO5 may not have an impact on the position of the Cr atoms in the crystal structure. Considering the ionic radius and electronegativity of Cr ions, we believe that very likely the Cr can partially replace Tl in the 2201 crystal structure, which was claimed by the earlier Cr substitution work w14,15x. The formula for the new 2201 phase, therefore, should be written as ŽTl,Cr. 2Sr2 CuO6 " d . Experimental determination of the cations distribution and the oxygen content in this structure is very instructive when a single phase sample is available. However, because the sintering temperatures of the 1201 and the 2201 are very close, it is quite difficult to achieve a 2201 pure phase.
Fig. 6. Magnetization vs. temperature curve for the sample with x s 0.3, sintered at 9808C.
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4. Conclusion In summary, we have found the experimental evidence demonstrating the existence of a new 2201 phase Tl-based cuprate superconductor, ŽTl,Cr. 2 Sr2 CuO6 " d . This new material is superconducting at about liquid nitrogen temperature and shows for the first time that strontium can also be used to fabricate such a double layer Tl cuprate superconductor.
References w1x R. Beyers, S.S.P. Parkin, V.Y. Lee, A.I. Nazzal, R. Savoy, G. Gorman, T.C. Huang, S.La. Placa, Appl. Phys. Lett. 53 Ž1988. 432. w2x J.S. Kim, J.S. Swinnea, M. Steinfink, J. Less Common Met. 156 Ž1989. 349. w3x Z.Z. Sheng, A.M. Hermann, Nature 332 Ž1988. 55.
w4x C. Martin, D. Bourgault, C. Michel, J. Provost, M. Hervieu, B. Raveau, Eur. J. Solid State Inorg. Chem. 26 Ž1989. 1. w5x J.C. Barry, Z. Iqbal, b.L. Ramakrishna, R. Sharma, H. Eckhardt, F. Reidinger, J. Appl. Phys. 65 Ž1989. 5207. w6x M.A. Subramanian, P.L. Gai, A.W. Sleight, Mater. Res. Bull. 25 Ž1990. 101. w7x M.H. Pan, M. Greenblatt, Physica C 184 Ž1991. 253. w8x H.C. Ku, M.F. Tai, J.B. Shi, M.J. Shieh, S.W. Hsu, G.H. Hwang, D.C. Ling, T.J. Watson-yang, T.Y. Lin, Jpn. J. Appl. Phys. 28 Ž1989. L923. w9x T. Manako, Y. Shimakawa, Y. Kubo, T. Satoh, H. Igarashi, Physica C 158 Ž1989. 143. w10x T. Mochiku, T. Nagashima, M. Watahiki, Y. Fukai, H. Asano, Jpn. J. Appl. Phys. 28 Ž1989. L1926. w11x M.A. Subramanian, G.H. Kwei, J.B. Parise, J.A. Goldstone, R.B. von Dreele, Physica C 166 Ž1990. 19. w12x M.A. Subramanian, Mater. Res. Bull. 25 Ž1990. 191. w13x M. Huve, C. Michel, C. Martin, M. Hervieu, J. Provost, B. Raveau, Physica C 179 Ž1991. 214. w14x Z.Z. Sheng, Y.F. Li, Y.Q. Tang, Z.Y. Chen, D.O. Pederson, Solid State Commun. 83 Ž1992. 205. w15x Z.Y. Chen, H.M. Luo, Z.Z. Sheng, B. Qu, Y.T. Qian, L.M. Wang, Phys. Stat. Sol. Ža. 158 Ž1996. 179.
New Tl 2201 žTl,Cr /2 Sr2 CuO6 " d phase material Y. Xin a , B.R. Xu b, K.W. Wong b
b,)
a Midwest SuperconductiÕity, 1315 Wakarusa DriÕe, Lawrence, KS 66049, USA Department of Physics and Astronomy, UniÕersity of Kansas, Lawrence, KS 66045, USA
Received 17 August 1998; revised 18 September 1998; accepted 24 September 1998
Abstract Evidence of the existence of the strontium analog of Tl 2 Ba 2 CuO6 , new HTSC material which bears the formula ŽTl,Cr. 2 Sr2 CuO6 " d is obtained. This phase has a transition temperature of 76 K. q 1998 Elsevier Science B.V. All rights reserved. PACS: 74.10 q v; 74.70 y b; 61.10 y i; 74.70Vy Keywords: Tl-2201 phase; Tl–Sr–Cu–O; Transition temperature; X-ray diffraction
1. Introduction The 1201 structure of the thallium cuprate was first discovered in a Tl–Ba–Cu–O sample with a chemical composition of TlBa 2 CuO5 w1x. Soon after this discovery, it was found that the barium atoms in the 1201 structure can be totally replaced by the strontium atoms, i.e., TlSr2 CuO5 also bears the same crystalline structure w2x. The 1201 phase is the smallest in formula weight and unit cell size among all thallium-based superconducting cuprates. Fig. 1a shows the schematic structure of the 1201 crystal. This structure contains two identical blocks with a mirror symmetry about the central ŽCu–O 2 . plane. The Cu and Tl atoms occupy the vertices of each block and the Ba or Sr sits at the center of the block.
)
Corresponding author. Tel.: q1-785-864-4025; Fax: q1-785864-5262; E-mail: [email protected]
Oxygen atoms are positioned around each Cu atom, forming an octahedral. There is also an oxygen atom sitting at the center of the top and bottom planes of the unit cell. Historically, Tl 2 Ba 2 CuO6 instead of TlBa 2 CuO5 is the first thallium cuprate superconductor discovered w3x. It is the starting member of the thallium double layer family and is known as the 2201 phase since the formula contains two thallium atoms, two barium atoms, zero calcium atom, and one copper atom. The schematic crystal structure of the 2201 phase is given by Fig. 1b. This diagram depicts a complete bottom-half unit cell Žthe complete block. and an incomplete top-half unit cell Žthe unfinished block.. For a half unit cell, thallium atoms occupy the corners, copper atoms take the places of the middle of the side edges, barium atoms sit at the center of each of the two cubes, and oxygen atoms sit at the middle of the cube edges except on the top and bottom faces. The top or bottom face can be considered as a square defined by four thallium
0921-4534r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 8 . 0 0 5 8 4 - X
Y. Xin et al.r Physica C 309 (1998) 208–214
209
Fig. 1. Crystal structure of TlBa 2 CuO5 Ža. and Tl 2 Ba 2 CuO6 Žb..
atoms. Oxygen also stays at the center of the square. The top half unit repeats the same arrangement as the bottom unit except that the whole block slides by Ž a q b .r2. So the thallium atoms in one Tl–O layer are vertically aligned with the oxygen atoms in the adjacent Tl–O layer. This feature characterizes the crystal structure of the double layer thallium cuprate family. In a Tl-1201 structure, as mentioned above, the Ba and Sr atoms are exchangeable. In a Tl-2201 structure, however, such an exchange has never been verified. In this paper, we report the discovery which indicates the existence of the 2201 structure in samples made of Tl–Cr–Sr–Cu–O. Our work was initially set up in hope of improving the Tc of TlSr2 CuO5 material by chromium substitution. It is well-known that neither stoichiometric TlBa 2 CuO5 nor TlSr2 CuO5 is a superconducting material due to excessive hole population, which has an average Cu valency of 3.0. Substitution of higher valence elements Pb or Bi for Tl can convert the TlSr2 CuO5 material into superconducting w4–7x. Pb 4q and Bi 5q have the same electronic configuration as Tl 3q, and their ionic radii and electronegativity are also close to those of thallium. Therefore, substituting the higher ionic state of Pb 4q or Bi 5q for Tl 3q can reduce the hole density of the 1201 phase material and make the material superconductive. Substitution of a rare earth Ž3 q or 4 q in valence. for BarSr Ž2 q . in the 1201 phase has
been successfully demonstrated for both TlBa 2 CuO5 and TlSr2 CuO5 w8–13x. Besides Bi, Pb, and rare earths, chromium is the element which has been proven to be able to bring superconductivity to the 1201 structure w14x. It was established that the Cr substitution not only can bring superconductivity to the 1201 phase but actually can enhance the superconducting transition temperature to above 70 K. This 70 K superconductivity was speculated as coming from the presence of a Tl–Cr–Sr–Cu–O 2212 phase w15x. However, after careful examination of the X-ray diffraction data for our series of Tl–Cr–Sr– Cu–O samples, we can conclude that the phase responsible for the 76 K superconductivity is actually ŽTl,Cr. 2 Sr2 CuO6 " d , a totally new 2201 phase material.
2. Experimental Samples for the present experiment were prepared by the solid-state reaction method. According to the stoichiometry of TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, appropriate molar amount of high purity Tl 2 O 3 Ž99.99%., Cr2 O 3 Ž99.7., SrOŽ99.9%., and CuOŽ99.99%., were mixed and ground in an agate mortar. The resultant powder was pressed into pellets of about 25 mm in diameter and about 8 mm in thickness with a hydraulic press. The pellets were placed in a tubular alumina crucible and
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then put in a tube furnace. The power of the furnace was turned on as oxygen flowed slowly through the tube of the furnace. The temperature of the furnace was raised to about 9508C or 9808C at an ascending rate of 208rmin, and kept at the temperature for 10 h then followed by furnace cooling at a descending rate of 18rmin to 6508C. The samples were kept at 6508C for 6 h. Finally, the power of the furnace was turned off and the samples were cooled to room temperature in the furnace. Oxygen was maintained flowing until the room temperature was reached. Resistivity of the samples was measured by the standard four-probe technique with an ac frequency of 400 Hz. Powder X-ray diffraction was performed by Cu–K a radiation using a GEIGERFLEX CN2029 diffractometer. Magnetic property was measured with in the temperature range from 10 to 100 K with a Quantum Design MPMS-5S system.
3. Results and discussion Fig. 2 plots the resistivity vs. temperature curves for a series of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4,
sintered at 9508C. As expected, the parent material TlSr2 CuO5 is not superconductive. Its R–T curve displays a metallic property in the temperature range from 300 to 15 K. The sample with x s 0.1 exhibits an onset superconducting transition at about 45 K and a completed transition at 30 K. This clearly demonstrates the significance of Cr-doping in converting the material to superconductive. The sample with x s 0.2 has a Tc Žzero. of 43 K, while the zero resistance temperatures of samples with x s 0.3 and x s 0.4 are 47 K and 48 K, respectively. These results are similar to those of the Bi, Pb, or rare earth containing Tl 1201 phase materials. A surprising result was found from the Cr-doping work when the sintering temperature was raised from 950 to 9808C as samples with the same nominal compositions demonstrated much higher superconducting transition temperatures. Fig. 3 illustrates the R–T curves for the samples sintered at 9808C. This time the sample with x s 0.1 exhibits an onset superconducting transition at about 75 K and a completed transition at 50 K. While the samples with x s 0.2 and x s 0.3 have almost the same Tc Žzero., of about 76 K, further increase of the ratio of CrrSr suppresses the superconducting transition temperature in this series. Sample with x s 0.4 has a Tc Žzero.
Fig. 2. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.0, 0.1, 0.2, 0.3, 0.4, sintered at 9508C.
Y. Xin et al.r Physica C 309 (1998) 208–214
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Fig. 3. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.1, 0.2, 0.3, 0.4, sintered at 9808C.
of 73 K. The R–T curves for samples with x s 0.5 and 0.6 are plotted separately in Fig. 4. Due to the large magnitude of the resistivities for these two samples these curves are too high to match those plotted in Fig. 3. Nonetheless, the sample with x s 0.5 still has a Tc Žzero. of 72 K. The sample with
x s 0.6 demonstrates a significant deviation in property. The normal state resistivity becomes semiconductive, increasing with the decrease of temperature in the range from room temperature to 120 K. At about 120 K, a semiconductive to metallic transition appears. An onset of a superconducting transition is
Fig. 4. Resistivity temperature dependency of samples having nominal compositions TlSr2y x Cr x CuO5 , where x s 0.5 and 0.6, sintered at 9808C.
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shown at about 70 K. However, the superconducting transition does not finish until below 50 K. Nevertheless, it should be noticed that all the samples in the later series show an onset of superconducting transition at temperatures above 70 K. Fig. 5 shows the X-ray diffraction patterns for the three samples, TlSr1.6 Cr0.4 CuO5 sintered at 9508C, TlSr1.8 Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C. From the X-ray data, we can immediately recognize that the sample TlSr1.6 Cr0.4 CuO5 sintered at 9508C has a pure 1201 structure. Both TlSr1.8-
Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C are multiphasic. The 1201 phase exists in either of the two samples. Careful examination uncovered that almost all the rest of the peaks in either sample can be attributed to a Tl-2201 structure. Several key arguments should be pointed out here: Ža. all the significant peaks not belonging to the 1201 phase in the X-ray patterns can be assigned to the 2201 phase; Žb. the intensity distribution in the presumed 2201 set of peaks is similar to that of Tl 2 Ba 2 CuO6 , among which the Ž 105 . peak at about 29.88 in the 2 u plot is
Fig. 5. X-ray diffraction patterns for the samples TlSr1.6 Cr0.4 CuO5 sintered at 9508C ŽA., TlSr1.8 Cr0.2 CuO5 and TlSr1.7 Cr0.3 CuO5 sintered at 9808C ŽB and C, respectively.. The indices without parentheses are for the 1201 phase and the indices in parentheses are for the 2201 phase.
Y. Xin et al.r Physica C 309 (1998) 208–214
the most intensive peak; Žc. when the indexed Ž 105 . peak becomes the most intensive peak in the diffraction pattern ŽFig. 5C., the intensity of most of the presumed 2201 peaks also increases; Žd. considering the overlap of some peaks between the 1201 phase and the 2201 phase, for the multiphasic samples, the intensity peaks at the 2 u angle of the overlap taking place should be increased. This can be checked out with some comparisons between a possible overlapped peak and a neighboring non-overlapped peak. Taking three pairs for the comparison, the 110 at about 33.58 and the 102 at about 31.28, the 112 at about 38.68 and the 004 at about 41.28, the 113 at about 45.88 and the 114 at about 54.18. It is irrefutable that for the multiphasic samples in Fig. 5B and C, the intensity ratio of first peak over the second peak significantly increases, which is a clear evidence of the overlapping of the 2201 phase’s Ž 110 . peak and the 1201 phase’s 110 peak, the 2201 phase’s Ž 0010. peak and the 1201 phase’s 112 peak, and the 2201 phase’s Ž 118 . peak and the 1201 phase’s 113 peak; Že. all the prominent peaks of the 2201 phase can be identified in the X-ray pattern in Fig. 5C, and the order in peak intensity is in agreement with the known Tl 2201 structure. It should be mentioned that from our X-ray data, we cannot find any evidence that supports the claim of the possible existence of a Tl 2212 phase which was proposed by Ref. w15x. Based on the indexed peaks of the 2201
213
phase in Fig. 5C, the lattice parameters of a tetragonal unit cell with I4r m m m symmetry were refined, of ˚ and c s 23.4565Ž4. A. ˚ which a s 3.7598Ž2. A The magnetization temperature dependency of the sample with x s 0.3 is shown in Fig. 6. The magnetic measurement confirms the 76 K superconducting transition temperature, and also indicates the multiphasic feature of the sample. From the above analysis, we conclude that the presence of Cr is essential to the formation of the 2201 crystal structure. It is difficult, however, to experimentally determine the position of the Cr atoms in the crystal structure from such a multiphasic sample. It should also be mentioned that the starting composition TlSr2y x Cr x CuO5 may not have an impact on the position of the Cr atoms in the crystal structure. Considering the ionic radius and electronegativity of Cr ions, we believe that very likely the Cr can partially replace Tl in the 2201 crystal structure, which was claimed by the earlier Cr substitution work w14,15x. The formula for the new 2201 phase, therefore, should be written as ŽTl,Cr. 2Sr2 CuO6 " d . Experimental determination of the cations distribution and the oxygen content in this structure is very instructive when a single phase sample is available. However, because the sintering temperatures of the 1201 and the 2201 are very close, it is quite difficult to achieve a 2201 pure phase.
Fig. 6. Magnetization vs. temperature curve for the sample with x s 0.3, sintered at 9808C.
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4. Conclusion In summary, we have found the experimental evidence demonstrating the existence of a new 2201 phase Tl-based cuprate superconductor, ŽTl,Cr. 2 Sr2 CuO6 " d . This new material is superconducting at about liquid nitrogen temperature and shows for the first time that strontium can also be used to fabricate such a double layer Tl cuprate superconductor.
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