Structure and superconductivity in YBa2Cu3Oy with additives of NaNO3 and NaCl

Physica C 339 (2000) 129±136

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Structure and superconductivity in YBa2Cu3Oy with additives of NaNO3 and NaCl X.S. Wu *, F.Z. Wang, S. Nie, J.S. Liu, L. Yang, S.S. Jiang Department of Physics, National Laboratory of Solid State Microstructures, Institute of Solid State Physics and Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, People's Republic of China Received 23 March 2000; accepted 24 April 2000

Abstract YBa2 Cu3 Oy (YBCO) cuprates with the additives of NaNO3 and NaCl were synthesized by the standard solid-state reaction technique. For the samples with addition of NaNO3 , the superconducting transition temperature Tc0 remains almost unchanged although the lattice constants of the unit cell decrease monotonically with an increase in the additive of NaNO3 . The superconducting transition temperature Tc0 for NaCl doping in YBCO, on the other hand, decreases with an increase in the content of NaCl. The lattice constants of a, b, and c increase slightly for NaCl doping in YBCO. We compared these results with that of the KNO3 added in YBCO and believe that the e€ects of lattice strain and chlorine replacing for Ba on superconductivity and structure should be considered in these system. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: NaNO3 ; NaCl-added YBa2 Cu3 Oy ; Transport property; Structure

1. Introduction The unit cell of the YBa2 Cu3 O6‡d (YBCO) contains three perovskite-like blocks, i.e., BaCuO2‡d=2 / YCuO2 /BaCuO2‡d=2 . The perovskite-like block of YCuO2 is sandwiched by the two similar BaCuO2‡d=2 perovskite-like blocks. The CuO2 sheets, which connect the three blocks, are believed to be the superconducting layers, and the Cu±O chains, the charge reservoirs, which supply the su-

*

Corresponding author. Fax: +86-25-3300-535. E-mail address: [email protected] (X.S. Wu).

perconducting carriers (holes) to the CuO2 sheets. The oxygen doping generates the mobile carrier [1,2], and the mobile carrier is realized to be supplied from the Cu±O chain and other charge reservoirs. The chemical doping e€ect of the compounds of many di€erent elements in YBCO has been studied intensively so as to explore the possibility to obtain superconductivity at even higher temperatures or ease processing conditions. Among the di€erent elements used as dopants, the alkali metals have been investigated [3±14]. The alkali elements can replace Y, Ba, or Cu in YBCO. However, the data published are contradictory. For example, for potassium replacing barium in YBa2ÿx Kx Cu3 Oy (YBKCO), the reported optimum superconducting transition temperature is 135 K

0921-4534/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 0 3 3 8 - 5

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with an additive of K2 CO3 [6], or 124 K with the additive of KOH [7], or 86 K with the additive of KNO3 (x ˆ 0.50) [8]. On the other hand, some authors showed that the presence of K2 CO3 additive during the synthesis allows one to obtain YBCO with Tc up to 94 K [9]. The e€ect of K2 CO3 is on the grain size and improves the homogeneity of the end product. Few reports mentioned the e€ects of sodium doping in YBCO. Some authors believed that the sodium can replace yttrium and barium in YBCO. In the case of sodium substitute for yttrium in YBCO, the superconducting transition temperature, Tc , remains unchanged in the whole orthorhombic symmetry range [10,11]. Tc decreases, but higher than 77 K, with an increase in the content of Na in YBa2ÿx Nax Cu3 Oy (YBNCO) (with addition of K2 CO3 ) with x 6 1:0 [12±14] for the sodium replacing barium in YBCO. Recently, there is some important progress on the enhancement of superconductivity for cuprates with applied stress on the unit cell. Locquet et al. [15] found that the superconducting temperature can be improved doubly for La1:9 Sr0:1 CuO4 using epitaxial strain. Att®eld et al. [16] have proved that the superconducting transition temperature increases with the increase in the average radius of La site for (La1ÿx Mx )2 CuO4 (M represents one or several dopants) compound, although the disorder due to M replacing La in La2 CuO4 suppresses Tc . That the lattice strain induces the increase of Tc has also been proved in RBa2 Cu3 Oy system, where with increasing radii of R ion in RBa2 Cu3 Oy (R ˆ Yb, Tm, Ho, Dy, Gd, Sm, and Nd), Tc increases from 88±94 K [17]. To prove the above viewpoint, we synthesized the YBNCO cuprates with di€erent additives. Our results show that with the increase the contents of additives, Tc decreases monotonically for NaCl doping in YBCO but is almost unchanged for NaNO3 doping in YBCO. Structural studies show that the structure of the volume of the unit cell increases slightly for the NaCl doped samples, but decreases for the sodium doped samples. We compared these results with those of the KNO3 doped samples. We believe that the Tc depression is due to the competition of the hole doping and the lattice stress.

2. Experimental procedures The samples were prepared using the solid-state reaction technique. The detailed procedures for sample preparation were as follows [18]: According to the chemical formula of YBa2ÿx Mx Cu3 Oy (M ˆ K, Na) with x 6 0.20, stoichiometric quantities of Y2 O3 (99.99%), KNO3 (99.99%), NaNO3 (99.99%), NaCl (99.99%), BaCO3 (99.5%), and CuO (99.9%) were well ground and mixed using petroleum ether. The mixtures were progressively heated to a temperature of 950 C in air and thereby maintained for 12 h. The calcined powders were reground and sintered at 950°C in air for another 12 h. X-ray di€raction (XRD) studies show that each of the sintered powders is almost single phased. The powders were reground and pelletized into several disc-shaped pellets, which were heated up to 950 C in a furnace with ¯owing oxygen (2.0 l/min) and this temperature was maintained for about one day. The temperature of the furnace was then decreased down to 450 and maintained at this level for 12 h in ¯owing oxygen. Finally, the products were cooled down to room temperature. A four-probe method was used to measure the temperature dependence of the resistivity for each sample. XRD studies were performed on Rigaku Dmax ÿ rB di€ractometer. The di€raction data were collected over the di€raction angle range 20± 100 at room temperature (298 K). The measured parameters were: di€racted-beam graphite …0 0 0 2† monochromator, CuKa radiation, a tube voltage of 45 kV and a tube current of 160 mA, and a step scan size of 0.02 , with a counting time of 1 s per point, divergence slit DS ˆ 1 , antiscattering AS ˆ 1 , and receiving slit RS ˆ 0:15 mm. Rietveld re®nements were performed using the standard analyzing program DBWS9411 [19]. The background was re®ned using a ®fth-order polynomial. The re¯ection was described by a pseudoVoigt function. No absorption correction was applied to the raw data. The wavelengths of CuKa1 , CuKa2 and the intensity ratio were 1.5406,  respectively. The typical re1.5444, and 0.497 A, ®ned ``R factors'', which were used as the numerical criteria of ®tting, are Rwp  7:5, Rp  5.5, and

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Rexp  4.5, respectively. The procedure of the re®nements and the de®nition of the ``R factors'' were described in Ref. [19].

3. Results and discussion The dependence of the zero resistance temperature, Tc0 , for YBa2ÿx Mx Cu3 Oy (M ˆ K, Na) with the additives of KNO3 , NaNO3 and NaCl, on x is shown in Fig. 1. Tc0 is almost unchanged for the samples with the additive of NaNO3 . The minimum value for Tc0 in NaNO3 doping system is about 88.6 K for x  0:15. For the sample with additive of KNO3 , Tc0 decreases sharply with x as x 6 0:10 but increases slightly as x P 0:10. Tc0 , on the other hand, decreases monotonically with the increase in the additive of NaCl in YBCO. The resistivities, which are normalized to the value of the room temperature (298 K) for each sample and dependence on temperature in the range of 20±298 K for the additive of NaCl doping system are shown in Fig. 2. For the sodium-free sample, the metallic behavior in the temperature range of 300± 95 K, and a metal-to-superconductor transition at 92.2 K (midpoint) are observed. With M doping, the metallic behavior is destroyed. The overdoped behavior of the dependence of resistivity on temperature for YBa2ÿx Mx Cu3 Oy samples is observed. Fig. 3 shows the measured and re®ned XRD pat-

Fig. 2. The dependence of the normalized resistance (R(T)/ R(298)) on temperature for the NaCl doping system.

Fig. 3. Rietveld re®nement pro®les for samples of YBa2ÿx Mx Cu3 Oy with x ˆ 0:10 for (a) M ˆ Na(NaNO3 ), (b) M ˆ Na-(NaCl) and (c) M ˆ K(KNO3 ), (+) is the measured XRD data. The solid line is the calculated pro®le.

Fig. 1. Comparison of the superconducting transition temperature between NaNO3 , NaCl and KNO3 doping system.

terns for samples with x ˆ 0:10 and additives of NaNO3 , NaCl, and KNO3 . From the measured XRD patterns, only ``123'' phase re¯ections were

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observed for the samples with x 6 0:20. The short vertical lines show the re¯ection position of the ®tted ``123'' structure. They all have the orthorhombic structure with Pmmm symmetry. The re®ned curves ®t well with the measured ones as x 6 0:20. The re®ned structural parameters are listed in Tables 1 and 2. The atomic parameters for KNO3 doping in YBCO are also summarized in Table 3 for comparison. The atomic positions [20] are Y(12, 12, 12), Ba/M(12, 12, z), Cu(1)(0, 0, 0), Cu(2)(0, 0, z), O(1A)(0, 12, 0), O(1B)(12, 0, 0), O(2)(0, 0, z), O(3A)(12, 0, z), O(3B)(0, 12, z). We believe that K and Na enter the Ba site in YBa2ÿx Mx Cu3 Oy for KNO3 and NaNO3 doping system according to the variation of lattice con-

stant of c. In YBCO unit cell, the radii of Y3‡ , Ba2‡ , Cu2‡ , and O2ÿ are 0.893, 1.34, 0.72, and 1.32  respectively. K‡ has a radius similar to that of A, Ba2‡ , the replacement of K‡ for Ba2‡ will not vary the local environments around Ba site except for the charge distribution. The lattice constant of c for K-doped samples remains almost unchanged.  On the other hand, Na‡ has the radius of 0.97 A, 3‡ which is greater than that of Y but smaller than that of Ba2‡ . The lattice constant of c will increase if Na‡ is substituted for Y3‡ in the same system [21]. However, the experimental results indicate that the lattice constant of c decreases with the addition of Na in YBNCO. Therefore, Na‡ must replace Ba2‡ in YBNCO although there is a dif-

Table 1 Re®ned structural parameters for the orthorhombic phase from powder XRD at room temperature for NaCl doping in YBCO samplesa  a (A)  b (A)  c (A) zBa zCu…2† zO…2† zO…3A† zO…3B† NNa NBa Cl NO1A Na

x ˆ 0:00

x ˆ 0:05

x ˆ 0:10

x ˆ 0:15

x ˆ 0:20

3.8856(1) 3.8211(1) 11.6869(2) 0.1844(2) 0.3573(5) 0.1685(21) 0.3824(24) 0.3640(26) 0 0 0

3.8858(1) 3.8239(1) 11.6866(2) 0.1833(3) 0.3585(6) 0.1721(27) 0.3804(23) 0.3647(32) 0.094(7) 0.018(21) 0.039(18)

3.8867(1) 3.8268(1) 11.6915(3) 0.1843(3) 0.3572(6) 0.1623(30) 0.3793(35) 0.3674(43) 0.181(7) 0.034(20) 0.115(17)

3.8858(1) 3.8248(1) 11.6910(3) 0.1840(3) 0.3570(5) 0.1679(23) 0.3923(19) 0.3694(28) 0.211(7) 0.206(21) 0.130(18)

3.8866(1) 3.8295(1) 11.6916(3) 0.1840(2) 0.3611(5) 0.1658(22) 0.3716(28) 0.3639(30) 0.169(8) 0.116(21) 0.105(19)

a

O1A The crystal structure has the Pmmm symmetry. The parameters of NNa , NBa are the occupied numbers of sodium at Ba Cl , and NCl site, chlorine at Ba site and the chlorine at O(1A) site, respectively. The numbers in the parentheses are standard deviations of the last one or two signi®cant digit(s).

Table 2 Re®ned structural parameters for the orthorhombic phase from powder XRD at room temperature for the Na-doped YBNCO samplesa  a (A)  b (A)  c (A) zBa zCu…2† zO…2† zO…3A† zO…3B† NNa

x ˆ 0:00

x ˆ 0:05

x ˆ 0:10

x ˆ 0:15

x ˆ 0:20

3.8856(1) 3.8211(1) 11.6869(2) 0.1844(2) 0.3573(5) 0.1685(21) 0.3878(24) 0.3640(26) 0

3.8824(1) 3.8182(1) 11.6774(2) 0.1843(2) 0.3580(3) 0.1677(15) 0.3851(18) 0.3772(18) 0.05(2)

3.8763(1) 3.8155(1) 11.6609(2) 0.1842(2) 0.3578(5) 0.1672(18) 0.3825(20) 0.3714(20) 0.09(2)

3.8719(1) 3.8101(1) 11.6538(3) 0.1839(3) 0.3588(5) 0.1702(21) 0.3836(26) 0.3721(27) 0.13(3)

3.8684(1) 3.8073(1) 11.6483(2) 0.1835(4) 0.3598(6) 0.1718(24) 0.3817(27) 0.3639(29) 0.17(2)

a The crystal structure has the Pmmm symmetry. The parameter of nNa is the occupied number of potassium at Ba site. The numbers in the parentheses are standard deviations of the last one or two signi®cant digit(s).

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133

Table 3 Re®ned structural parameters for the orthorhombic phase from powder XRD at room temperature for KNO3 doping in YBCO samples [20]a  a (A)  b (A)  c (A) zBa zCu…2† zO…2† zO…3A† zO…3B† NK

x ˆ 0:00

x ˆ 0:05

x ˆ 0:10

x ˆ 0:15

x ˆ 0:20

3.8795(1) 3.8149(1) 11.6608(2) 0.1841(2) 0.3556(5) 0.1651(22) 0.3734(27) 0.3772(26) 0

3.8802(1) 3.8159(1) 11.6570(1) 0.1836(2) 0.3552(5) 0.1661(23) 0.3771(24) 0.3727(24) 0.04(3)

3.8744(1) 3.8127(1) 11.6534(1) 0.1836(2) 0.3575(5) 0.1549(23) 0.3825(20) 0.3810(19) 0.07(3)

3.8753(1) 3.8129(1) 11.6558(3) 0.1835(2) 0.3575(5) 0.1562(25) 0.3784(22) 0.3797(21) 0.12(4)

3.8761(1) 3.8133(1) 11.6598(2) 0.1841(3) 0.3587(5) 0.1524(32) 0.3758(29) 0.3855(29) 0.18(3)

a The crystal structure has the Pmmm symmetry. The parameters of NK is the occupied number of potassium at Ba site. The numbers in parentheses are standard deviations of the last one or two signi®cant digit(s).

ference in radius between Na‡ and Ba2‡ . The reduction of lattice constant of c has also been proven in other system if Ba2‡ ions are replaced by a smaller ion in YBCO [18,22]. Fig. 4 shows the variations of lattice constants of a, b, and c for the three doping systems. For KNO3 doping system, the lattice constants of a, b, and c remain almost unchanged due to the similarity of the ionic radius  and Ba2‡ . For the NaCl doping of K‡ (1.33 A) system, the lattice constants of a, b, and c increase slightly with increasing NaCl content. We believe that the abnormal variation of the lattice constants is due to the e€ect of chlorine which will be discussed below. For the NaCl doping system, the Clÿ ion should be considered during the structural determination due to the higher decomposition temperature. We assumed that the chlorine can replace Y3‡ , Ba2‡ , Cu2‡ , or O2ÿ during the structural re®nements. The ®nal re®ned results show that the chlorine can only replace Ba2‡ at Ba site (most) and O2ÿ at O(1A) site (small). The re®ned occupancies of chlorine replacing Ba at Ba site or O at O(1A) site are listed in Table 1. From the re®ned occupancy of chlorine replacing Ba at Ba site, we can compare the variation of the lattice constants of a, b, and c for NaNO3 doping system with that of the NaCl doping system. The slight increase in the lattice constants for NaCl doping system is due to the occupancy of chlorine for Ba at Ba site. The ionic radius of chlorine (Clÿ ) is  which is much larger than that of about 1.81 A  Ba2‡ ion (1.34 A).

The lattice contraction and expansion of the YBCO unit cell with the additives of NaNO3 and NaCl can also be demonstrated by the variation of the unit cell volume versus x. Fig. 5 shows the dependence of the volume of the unit cell for YBNCO on the nominal sodium content. The unit cell volume decreases with the increase in sodium content for the NaNO3 doping system, which indicates that the dopant of sodium must replace a smaller ion in volume, but increases slightly with the increase in the nominal sodium content for the NaCl doping system, which indicates that the dopants of both sodium and chlorine replace Ba at Ba site. This analysis ®ts the above discussions well. The orthorhombic strain, which is de®ned as gˆ

…a ÿ b† …a ‡ b†=2

seems to relate to the superconducting transition temperature. The orthorhombic strain g decreases with increasing x for NaNO3 doping system with x 6 0:10. The g increases with Na content for x P 0:10. The varied tendency of the orthorhombic strain is the same as that of the superconducting transition temperature. On the other hand, the orthorhombic strain decreases monotonically with the increase of dopant of NaCl in the NaCl doping system. The dependence of the orthorhombic strain on the content of Na in YBNCO is shown in Fig 6. For potassium doped YBKCO, the orthorhombic strain g is almost unchanged although Tc0 decreases sharply with increasing potassium contents. We guess that the orthorhombic strain may

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3 ), verses x for sodiumFig. 5. Unit cell volume, V (unit: A doped YBa2ÿx Nax Cu3 Oy with the additives of NaNO3 and NaCl.

Fig. 6. The dependence of the orthorhombic strain, g, on the content of sodium in the NaNO3 and NaCl doping YBa2ÿx Nax Cu3 Oy samples.

 of (a) a, (b) b, and (c) c for Fig. 4. Lattice constants (unit: A) the NaNO3 …† and NaCl …† doping system.

relate to both the valence state and the radius of the dopants. The di€erence in superconductivity depression between K doped YBa2ÿx Kx Cu3 Oy and Na doped YBa2ÿz Naz Cu3 Oy is obvious. Fig. 1 shows the Tc0 dependence on the content of dopants for YBa2ÿx Kx Cu3 Oy and YBa2ÿz Naz Cu3 Oy . With x, z 6 0:20, Tc0 varies slightly for NaNO3 doped

YBa2ÿz Naz Cu3 Oy , while Tc0 decreases sharply for potassium doped YBa2ÿx Kx Cu3 Oy . The structure of YBa2ÿx Kx Cu3 Oy is almost unchanged in this doping range. We attribute the strong Tc depression of K substituting Ba in YBKCO to the charge re-distribution. As well known, the Cu±O chains are believed to be the charge reservoir in YBCO. The Cu±O2 sheets are the conducting layers. The oxygen sited at O(2)(0, 0, z) play an important role in transferring hole carriers from Cu±O chains to Cu±O2 sheets. The position of O(2) may be important in superconductivity of YBCO. The role of

X.S. Wu et al. / Physica C 339 (2000) 129±136

charge re-distribution may be played by the shift of O(2). The O(2) (along the z-direction, zO…2† ) shifts to Cu(1)±O layer in K doped YBKCO (Table 3). This shift will suppress the transfer of hole carriers from Cu(1)±O chains to Cu(2)±O2 sheets, and Tc will be depressed. For NaNO3 doping system, YBNCO, O(2) shift from Cu(1)±O chains to Cu(2)±O2 sheets (Table 2). Tc changes very little although more holes are produced due to Na replacing for Ba, which is similar to the case of K substituting for Ba in YBKCO. Unfortunately, the accuracies of the determined O(3) positions are poor due to the weak X-ray scattering power of the oxygen atoms. As described above, K‡ has the same valence state as that of Na‡ but less than that of Ba2‡ . The substitution of potassium and sodium for barium in YBMCO will supply more hole carriers in the unit cell, which may localize at the Ba site. The samples become overdoping state (Fig. 2). The superconducting transition temperature decreases. On the other hand, Na‡ has a radius smaller than that of Ba2‡ ; there must be some contract stress around Ba site, which may increase Tc in cuprates [15,17]. The competition between holon localization and stress e€ect determines the variation of Tc in both Na- and K-doped YBMCO. For NaCl doping system, we have proved that both the elements of sodium and chlorine (most) enter the Ba site. The average radius at Ba site increases slightly on adding NaCl. The lattice shows a slight expansion, and the superconducting transition temperature decreases. The charge distribution must be complex due to the e€ect of chlorine occupying at Ba and O(1A) sites. 4. Conclusion In summary, we investigated the structure and the transport properties on the NaNO3 , NaCl, and KNO3 doping YBa2ÿx Mx Cu3 Oy (M ˆ K, Na) systems with a dopping range of 0:00 6 x 6 0:20). The structural parameters remain unchanged for KNO3 doping system. The lattice constants a, b, and c decrease on adding NaNO3 in NaNO3 doping system. The lattice constants increase slightly on increasing the amount of NaCl in NaCl doping system. We compared the struc-

135

tural changes of the YBa2 Cu3 Oy with additives of NaNO3 , NaCl, and KNO3 , and con®rmed that the elements of potassium, sodium, and most chlorine replace Ba at Ba site. The small chlorine enters O(1A)-site of Cu±O chains. The transport properties for the three doping systems were compared. The superconducting transition temperature decreases sharply with x 6 0:10 for YBa2ÿx Kx Cu3 Oy , but is almost unchanged in NaNO3 doping system. The strong depression in superconductivity for the K-doped samples may be due to the shift of oxygen atoms sited at O(2) sites or hole localization. The decrease in Tc0 in NaCl doping system may be due to the chlorine entering Ba site and the complex charge re-distribution.

Acknowledgements This work has been supported by the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Jiangsu. The authors appreciate the ®nancial support from Ke-li Fellowship of Sanzhu in Shandong.

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