- Email: [email protected]
Phase identification of the new 126 K Ba–Ca–Cu–O superconductor
Physica C 294 Ž1998. 316–326
Phase identification of the new 126 K Ba–Ca–Cu–O superconductor Y.Y. Xue, Y.Y. Sun, I. Rusakova, D.K. Ross, Z.L. Du, N.L. Wu 1, Y. Cao, L. Gao, B. Hickey, C.W. Chu ) Texas Center for SuperconductiÕity at UniÕersity of Houston, Houston, TX 77204-5932, USA Received 31 March 1997; revised 28 May 1997; accepted 19 October 1997
Abstract New superconducting oxides that have a composition of Ba 2 Ca 2 Cu 3ŽCa x Cu y .O 8q d with 0.4 G x G 0.9 and 0.7 G y G 0, and show an optimum Tc ; 126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common ˚ and c s 28.2 A˚ with an Ir4mmm symmetry. tetragonal unit cell among these oxides, which has lattice constants a s 3.85 A The cell consists of a rock-salt ŽBaO. 2 block sandwiched between two perovskite Ca 2 Cu 3 O6 blocks with interstitial Caq2 x and Cuq2 located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial y Ca xCu y . The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H 2 O and CO 2 in open air. The decayed products ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d have an expanded ˚ and a lower Tc ; 90 K. A structure model for the decayed phases was also proposed. q 1998 lattice constant c ; 33.8 A Elsevier Science B.V. PACS: 74.72Jt; 74.62Bf; 61.66Fn Keywords: Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d ; Structure; Structure phase transition
1. Introduction The structures responsible for the high temperature superconductivity in the ŽBa,Sr,Ca. –Cu–O system remain debating issues up to now. The so-called infinite-layered compounds ŽSr y Ca 1yy .CuO 2 w1x and the first three members of the Sr2 Srny1Cu nO 2 nqny d family w2–4x all have been proposed to be supercon)
Corresponding author. Tel.: q1 713 743 8200; Fax: q1 713 743 8201. 1 Present address: Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan.
ductive. However, difficulties exist for a self-consistent model that unequivocally accounts for both the integrity of the superconducting Ž SC . Ca ny 1Cu nO 2 n block and the Cu valence needed. For instance, the value of d in Sr2 CuO4y d has to be 0.7 or larger in order to keep the corresponding Cu valence VCu below a reasonable limit, above which most cuprates not only become non-superconducting, but also decompose due to the energetically unfavorable configuration of Cu3q. A previous neutron diffraction investigation suggested that these oxygen vacancies were located entirely within the SC block for Sr2 CuO4y d , while the oxygen sites in the so-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 1 2 - 1
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
called charge reservoir ŽCR. block of ŽSrO. 2 were fully occupied w5x. As pointed out by Shaked et al. w3x, such broken SC blocks raise serious doubts about the proposed superconductivity in Sr2 CuO4y d . This structural instability against oxygen vacancy in CuO 2 planes might be more prominent if the Sr 2q is to be replaced by a larger cation, such as Baq2 , which leads to longer and hence weaker in-plane Cu–O bonds and makes the vacancy formation even easier. Indeed, there has been no body-centered Ba 2 ŽSr,Ca. ny1Cu nO 2 nq2y d or even any other superconducting Ba–Ca–Cu–O compound reported thus far, despite the experimental efforts in synthesizing them Žfor example, Ref. w6x.. Significant amount of C, whose higher formal-valence reduces the density of the oxygen-vacancies needed, has to be introduced into the CR blocks of reported superconducting Ba– Ca – Cu compounds, e.g. Ž Cu,C . Ba 2 Ca n y 1 Cu nO 2 nq 3q d and Ž Cu,C . 2 Ž Ba,Ca . Ba 2 Ca ny 1 Cu nO 2 nq5q d , in order to stabilize their layer structure and sustain the superconductivity w7,8x. In this paper, we report, for the first time, the identification of a class of high temperature superconducting ŽHTSC. oxides of Ba, Ca and Cu without detectable C-doping. These oxides have a nonstoichiom etric com position of Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d , but show the same optimum Tc ; 126 K and X-ray powder diffraction ŽXRD. pattern. Depending on synthesis conditions, x varies between 0.4 and 0.9 and y between 0 and 0.7, e.g. Ba 2 Ca 2 Cu 3 Ca 0.9 O 8q d on the Cu-deficient side and Ba 2 Ca 2 Cu 3 ŽCa 0.8 Cu 0.7 .O 8q d on the Cu-rich side. One of the oxides, namely, sample A of Ba 2 Ca 2 Cu 3 ŽCa 0.85 Cu 0.68 .O 8q d Žor Ba 2 Ca 2.85 Cu 3.68 O 8q d . shows a Tc of 126 K in its as-synthesized state as reported earlier w9,10x. The XRD patterns of all these oxides can be indexed by a body-centered tetragonal unit cell with essentially the same lattice constants ˚ and c ; 28.2 A. ˚ Structure analysis india ; 3.85 A cates that the unit cell is similar to that of so-called 0223 phase, e.g. the proposed Sr2 Ca 2 Cu 3 O 8q d . The non-stoichiometric compositions, however, suggest significant interstitial cations and site mixing. These defects would likely occur in the charge reservoir ŽCR. block, considering the composition-independent optimal Tc of 126 K. One of the possible interstitial sites will be the 4d position between the two neighboring BaO planes in the CR block
317
Ba 2 Ca x Cu y . To our best knowledge, many of these structure features, i.e. a IIA elements Ca in the C R block of the B a 2 C a 2 C u 3 Ca 0.9 O 8q d , I-symmetry in the Ba–Ca–Cu system, and the possible interstitial cations at the 4d site of Ž1r2,0,1r4., have not been observed before. This non-stoichiometric cation layer of Ca x Cu y offers a new way, which has not been reported before, to adjust the carrier concentration and stabilize the structure. The new superconducting oxides are susceptible to degradation in air and, particularly, in water-containing atmospheres. Irrespective of the cation stoichiometries, i.e. of the values of x and y, all decayed phases formed in air showed a Tc near 90 K ˚ The crystal and a larger lattice constant c ; 33.7 A. symmetry and cation stoichiometry, however, remain the same as those of the starting 126 K compounds. A structural model for the decayed products and a possible degradation mechanism, which is attributed to the insertion of HOy andror CO 32y between the two BaO layers, are proposed. 2. Experiments The synthesis procedures have been reported in detail earlier w9,10x. In brief, samples were synthesized under 5 GPa at 800–9508C using a multi-anvil Walker module from Rockland Research. AgO, acting as an oxidant, was added into the precursor powders, which have a nominal composition of Ba 2 Ca 2qx Cu 3qyOz with 0 F x F 1 and 0 F y F 1, with a ratio of 0.5–1 molerformula. The oxide mixtures were compacted into cylinders and wrapped in Au foil for the high pressure synthesis. Precautions were taken to avoid contamination of CO 2 , which was known to retard the formation of the high-Tc phases under investigation. The superconducting properties were characterized both by the standard four-point method and by a SQUID magnetometer ŽQuantum Design.. X-ray powder diffraction ŽXRD. measurement was carried out by using a Rigaku D-MAXrBIII diffractometer equipped with a specially designed sample chamber, which allows for continuous purging with dry oxygen during data acquisition. Compositions were determined by the wave dispersive spectrometer ŽWDS. of a JEOL JXA 8600 electron microprobe.
318
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
3. Results and discussion 3.1. SuperconductiÕity Fig. 1 shows the field-cooled ŽFC. magnetization curve of as-synthesized sample A at 10 Oe. The onset Tc is ; 126 K and the transition width Žfrom 10 to 90%. is ; 4 K Žcurve A, Fig. 1.. It should be noted that although the Tc of our samples at the as-synthesis state varies between 110 and 126 K, their optimum Tc falls within a much narrower range, i.e. 124 K F Tc F 126 K. For example, the Tc-onset reached ; 125 K after an annealing at 2508C under 1 atm. O 2 for sample B Žcurve B in Fig. 1., whose Tc-onset was only 120 K in the as-synthesized condition. As mentioned earlier w9,10x, all these samples are not stable in open air, particularly after they were pulverized. The Tc of the pulverized samples drops to ; 90 K, independent of both the as-synthesis Tc and the annealing history of the samples, after they were exposed to air for a few hours ŽFig. 1, inset.. However, the 126 K superconductivity in dense bulk samples kept in a dry desiccator can survive for months, suggesting a process controlled by the OHy and CO 32y diffusions. The transition width of the transformed products remains reasonably narrow with
Fig. 1. x vs. T. A: sample A; B: sample B after the optimum annealing. Inset: x vs. T of a pulverized sample; A: before exposing to air; B: 1 h after that; C: after 4 days in open air.
a FC magnetization at 5 K being nearly the same as that before the transformation ŽFig. 1, inset.. The annealing behavior of the samples, however, is totally different before and after the transformation. Particularly, the decay is irreversible in terms of the O 2 or Ar annealing. The same surface of a bulk sample was observed both before and after the transformation. The average grain size on the fresh surface was ; 5 mm or larger with sharp edges ŽFig. 2a., but the surface was rough with many bumps and cracks after the transformation ŽFig. 2b.. We believe, therefore, the process is rather a phase-transformation than a oxygen intake-andror-release. Details of the annealing and decaying behaviors will be reported elsewhere w11x. The similarity in both the optimum Tc and the transformation behavior hints that the corresponding superconducting phases might be closely related. 3.2. Composition analysis The cation compositions and carbon content were directly determined by WDS. The sample surface was prepared by dry polishing just before measurement, and no conducting coating was applied. Both the FC magnetization and the surface-morphology of the prepared samples show that the 126 K superconducting phase was largely preserved during the sample preparation. Although a thin surface-layer may be degraded, the observed stoichiometry is not expected to change, considering the facts that the cation diffusion rate is low at room temperature and that the acquired volume of WDS is large, i.e. ; 1 = 1 = 1 mm3. To confirm this presumption, compositions were detected and compared between a fresh surface and the same surface after being exposed in air for a few days. In spite of the distinct surface morphologies Žsimilar to Fig. 2a,b., the cation compositions are within an experimental resolution of "3%. While the samples are mostly multi-phased, various phases were easily distinguished in the back scattering images of polished surfaces ŽFig. 3.. The cation ratios in each of these phases were measured using WDS, which was calibrated against several standard oxides and carbonates before each experimental run. To evaluate the reliability of our WDS data, a ŽCu 1y x C x .Ba 2 Ca 2 Cu 3 O 10y d sample, whose XRD pattern showing no impurity phases within our
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
319
Fig. 2. The SEM images of Ža. a freshly broken sample; Žb. the surface after exposing to air.
experimental resolution, was measured. The measured compositions are Ba:Ca:Cu:C s 2:2.22 Ž"0.04.:3.62Ž"0.05.:0.47Ž"0.1.. The standard deviations shown in the parentheses indicate the range of spreading among data acquired from different grains. The composition thus detected is in good agreement with the theoretical stoichiometry of ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d . The differences are within data spreading range except for Ca. The slightly higher Ca reading may be due either to an experimental deviation or to a genuine Ca-surplus in the phase.
For most samples, there was predominately only one ternary metal oxide, in addition to impurity grains that can be attributed to single- and binarymetal oxides, such as BaOrBaO 2 , BaCuO 2 and Ca 2 CuO 3 ŽFig. 3.. The cation ratios of this ternary metal oxide have rather narrow spreading in a given sample. For example, the cation ratios of sample A are Ba:Ca:Cu s 2:2.85Ž"0.06.:3.68Ž"0.06., and most other samples have a similar stoichiometry ŽFig. 4.. The cation ratios, however, do vary considerably among samples synthesized under different conditions, i.e. as Ba 2 Ca 2 Cu 3 ŽCa x Cu y .Oz with 0.4
320
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
Fig. 3. Back scattering image by microprobe on a polished surface.
- x - 0.9 and 0 - y - 0.7 ŽFig. 4.. The large gainsize of our samples, the much narrower data spreading in a given sample, as well as the result of the ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d sample show that the observed cation non-stoichiometry is genuine. The carbon contents within the ternary metal oxide grains were directly determined by WDS and found to be nil in all cases. For instance, C:Ba ratio is ; 0.04 " 0.04 in sample A, which is in great contrast with C:Ba s 0.235 " 0.05 measured in the ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d sample. The nil result was confirmed by our EELS, FTIR and mass spectrum measurements. In fact, the presence of carboncontaining species, such as carbonates, in the precursor powder was found to significantly retard the formation of the 126 K phases w9,10x. All these results either directly or indirectly lead to the conclusion that these ternary-metal phases under investigation are oxides but not carboxyl cuprates, therefore, not belong to the well known ŽCu 0.5 C 0.5 .Ba 2Ca ny 1Cu nO 2 nq 2 and Ž Cu 0.5 C 0.5 . 2 Ba 3 Ca ny 1 Cu nO 2 nq5y families. After the phase-transformation in air, WDS showed a slight increase in carbon content in the ternary oxide grains. In the case of the sample A, for example, the average carbon content was found to
Fig. 4. The cation compositions of several samples measured by WDS. The letters A and B in the figure represent to the compositions of samples A and B, respectively.
increase to C:Ba s 0.065, which remains far lower than that in ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O 9q d . 3.3. X-ray diffraction Due to the fast decay of the 126 K phase in open air, the XRD measurements were conducted in a sample chamber that is constantly purged with dry O 2 . To verify the effectiveness of such a protection measure, two consecutive measurements, each of which lasted for ; 45 min, were taken on the same sample under the protection. The two XRD patterns were found to be the same ŽA and B in Fig. 5a with the vertical scale of B reduced.. Meanwhile, small parts of the XRD sample were removed and subjected to magnetization measurement both before and after the two XRD measurements. The same single-step superconducting transition at 126 K was observed. In contrast, the XRD showed a completely different pattern ŽFig. 5b., and the Tc dropped to 90 K within a few hours of air being introduced into the
Fig. 5. Ža. Sequential XRD patterns of sample A under protection of dry O 2 . A: the first run; B: the second run about 45 min later. The vertical scale of curve B is reduced by 5 times for a better view. Žb. XRD pattern of sample A after the air was introduced. Žc. A: the difference pattern of sample A. The arrow at ; 238 shows the position of the Ž001. line. B: the XRD pattern of a single phase ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d sample. Žd. The difference pattern of sample B. Že. The simulated XRD patterns.
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
321
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
322
chamber. Such simultaneous changes in the XRD pattern and Tc were observed in all samples measured. Hence, it is considered that the XRD pattern acquired before introducing air corresponds to the 126 K phase Žphases. plus some minor impurities, while the XRD pattern after complete decay corresponds to the 90 K phase. Accordingly, ‘difference patterns’ were constructed by subtracting the pattern after decay from the one before ŽFig. 5c.. In these difference patterns, the reflection lines of the 126 K phase Žphases. are expected to show up as positive lines, i.e. lines with positive intensities, while those of the 90 K phase as negative ones. Most of the lines related to the impurity phases should disappear in the difference patterns, which simplifies the data analysis. The 2 u values of most lines will be the same irrespective of whether they are determined from either the original or the difference patterns. However, there are also a couple of lines that do not show significant change in their intensities but small shift in 2 u values after the decay. Such lines can be easily spotted because they lead to the occurrence of a pair of peaks, one positive and one negative, right next to each other in the difference patterns. For these reflections lines, their 2 u values have to be directly read from the original patterns. Almost all of the positive XRD lines, as shown in Fig. 5c and Table 1, can be indexed according to a Table 1 The observed and calculated d-values of the 126 K phase Miller index 004 006 103 008 105 0010 107 110 114 109 116r0012 0014 200 0016 217r0018
Measured ˚. d ŽA
Calculated ˚. d ŽA
Calculated
intensity
Measured
7.07 4.71 3.57 3.50 3.19 2.83 2.74 2.72 2.54 2.44 2.34 2.02 1.93 1.77 1.57
27 28 17 3 14 86 14 100 30 70 46 10 54 20 17
7.05 4.70 3.56 3.53 3.18 2.82 2.78 2.72 2.53 2.43 2.36 2.01 1.93 1.76 1.58
17 32 87 1 77 3 23 100 40 220 100 20 110 15 13
intensity
Table 2 The observed and calculated d-values of the 90 K phase Miller index 002 004 006 008 103 107 110 109 1011 1110r0016 200 0020 1161
Measured ˚. d ŽA
Calculated ˚. d ŽA
Calculated
intensity
Measured
17.11 8.50 5.65 4.24 3.64 3.02 2.72 2.69 2.41 2.12 1.92 1.70 1.68
71 65 7 6 26 16 100 65 19 31 45 23 10
16.98 8.49 5.66 4.25 3.65 3.02 2.72 2.69 2.41 2.12 1.93 1.70 1.67
41 79 2 21 34 43 100 53 71 16 51 5 5
intensity
˚ and body-center tetragonal cell with a s 3.84 A ˚ The Ž00 l . lines with odd l values or c s 28.2 A. Ž10 l . lines with even l values are systematically missing. The only possible exception is a weak line observed at the Ž100.rŽ010. position Žindicated by an arrow in Fig. 5c.. This peak may be due either to an impurity phase, which also decomposes after exposed to air, or to some structure distortions of the 126 K phase. On the other hand, the majority of the negative lines ŽFig. 5c and Table 2. can be indexed according to a body-center tetragonal cell with a s ˚ and c s 34.2 A. ˚ The corresponding prohibit3.84 A ing rules are again followed very well. The XRD pattern of a single phase ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d sample, which has a slightly lower optimum Tc of ; 120 K w12x, is also shown in the bottom of Fig. 5c as a comparison. It is clear that the XRD pattern of ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d , or any known ŽCu 0.5 C 0.5 .Ba 2 Ca ny 1Cu nO 2 nq 2y d and Ž Cu 0.5 C 0.5 . 2 Ba 3 Ca ny 1Cu nO 2 nq5y d phases, is very different from that of either the 126 K phase or the 90 K phase. Sample B has a different cation stoichiometry of Ba 2 Ca 2 Cu 3 Ca 0.9 O 9y d ŽFig. 4., and a slightly lower Tc of 120 K at the as-synthesized state. However, its XRD difference pattern ŽFig. 5d. is similar to that of sample A ŽFig. 5c. with only slight differences in the relative line-intensities, which can be well ascribed to different prefer-orientations. Since the superconducting volume fraction is 30% or larger in both cases, the difference between their XRD patterns should be noticeable if the corresponding phases are
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
323
structurally different. We believe, therefore, that one common non-stoichiometric phase is responsible to the observed 126 K superconductivity in all these samples. 3.4. Structure models All known p-type HTSC cuprates contain periodic stacks of an SC wŽCa,Sr,R. ny 1ŽCuO. n x block, where R is either Y or a rare earth element, and a CR block. With only a few exceptions, such as ŽLa,Ba. 2 CuO4 , the CR block is typically bounded by two BaO ŽSrO. planes. While the value of n characterizes the different members in one HTSC family, HTSC families differ only by the cation composition and arrangement within the CR blocks. The known CR structures are either rock-salt type, e.g. the BaO– Tl 2 O 2 – BaO block in Tl 2 Ba 2 Ca 2 Cu 3 O 10q d , oxygen-deficient perovskite type, e.g. the BaO– ŽCu,C.O–BaO block in ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O 9q d , or fluorite type, e.g. the ŽNd,Ce. 2 O 2 block in ŽNd,Ce. 2 ŽBa,Nd. 2 Cu 3 O 9q d w13x. Previously, the body-centered symmetry has only been observed in cuprates whose CR blocks are either rock-salt type or fluorite type when the total number of the cation layers is even. A structural model for the 126 K was developed along these lines. For almost all the HTSC cuprates, the distance between two neighboring CuO 2 planes in the SC ˚ and the distance between the block is 3.15 " 0.05 A, neighboring BaO and CuO 2 planes, i.e. the distance ˚ Žfor between the SC and CR blocks, is 2.00 " 0.05 A example, Ref. w14x.. Simple calculation immediately ˚ can be acshows that the observed c ; 28.2 A counted for by two half-unit cells which are displaced by Ž1r2,1r2,cr2., and each of which contains one SC block with n no larger than 3. To estimate the value of n, the projected electron density in c-direction was estimated based on the relative intensities of all 00 l lines observed ŽTable 1. using a simple Fourier transform ŽFig. 6, where a temperature factor of 2 was assumed.. Three main peaks are shown between z i s 0 and z i s 0.2, with an additional small bump at z i s 0.07 ŽFig. 6.. We attribute these peaks and the bump to two CuO 2 layer, BaO layer and Ca layer, respectively. This suggests that the 126 K phase is an n s 3 cuprate. The layer distances are in good agreement with those
Fig. 6. The estimated project electron density in c-direction. Only the first half cell is shown.
in known cuprates Žtop of Fig. 6.. In such a case, the overall thickness of the CR block should be ; 3.8 ˚ and its composition Ba 2 ŽCa x Ca y . with 0.4 F x F A, 0.9 and 0 F y F 0.7, e.g. Ba 2 ŽCa 0.85 Ca 0.68 . in sample A and Ba 2 Ca 0.9 in sample B. An obvious candidate is the so-called 0223 phase of Ba 2 Ca 2 Cu 3 O y with some site-mixing since a BaO– ŽCa,Cu.O–BaO arrangement will violate the I-symmetry observed, and a BaO– ŽCa,Cu.O– ŽCa,Cu.O–BaO CR block will be ˚ cell. The composition and too thick for a c s 28.2 A distance, however, are difficult to be reconciled without significant interstitial cations. The cation ratio of Cu:ŽBa q Ca. ; 0.61 in sample B, for example, is significantly less than the ratio of 0.75 in ŽBa,Ca. 2 Ca 2 Cu 3 O y . Dense defects in the CuO 2 layers would be unavoidable in a 0223 model without more than one ŽBa,Ca. ionrcell located at some interstitial sites. In addition, the estimated layer dis˚ would be considerably larger than tance of 3.8 A ˚ either the average Ba–O bond length, ; 2.6–2.8 A, in other cuprates or the ŽLa,Ba.O layer distance of ˚ in ŽLa,Ba. 2 CuO4 w14x, but relatively close to ; 1.8 A ˚ in the CR block thickness of ; 4.4 A ŽCu,C.Ba 2 Ca 3 Cu 4 O 11y d , which has three cation layers in its CR block and the P-symmetry. The electron-density distribution tentatively shows a small
324
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
peak between two BaO layers ŽFig. 6.. We regard this as a hint for the possible interstitial cations. A careful inspection reveals that there are four large tetrahedral cages existing at the 4d site between the two adjacent BaO planes, if the space between them ˚ ŽFig. 7a.. The cages, each of is larger than 3 A ˚ and 5.4 A˚ along the four which is measured ; 4.7 A interlayer O–O edges and the two intralayer O–O edges, respectively, are large enough for a Cu2q ion ˚ ., a Ca2q ion Ž r ; 1.00 A˚ ., or even an Ž r ; 0.62 A 2y ˚ . to fit in comfortably. If these O ion Ž r ; 1.4 A cages are fully occupied by the same cation or
randomly filled by both Cuq2 and Caq2 , the bodycenter symmetry is preserved. Even if they are partially occupied in an ordered fashion, the average structure can still have a body centered symmetry superimposed on a superstructure. Thus, to account for the symmetry and the cation composition, it is proposed that Ca x Cu y are interstitially inserted into two neighboring BaO planes, as shown in Fig. 7b. A simple valence-count suggests that there should also be a significant amount of oxygen surplus, i.e. d ; 0.7. The most likely location for these oxygen ions would be the same 4d site. It should be noted that
Fig. 7. Ža. The cages between two adjacent BaO layers. The ions are drawn with the actual sizes of Ba2q, O 2y and Ca2q, respectively. Žb. The proposed model for the 126 K phase. Žc. The proposed model for the 90 K phase.
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
these interstitial cations of Ca2q and Cu2q have the same distance to the adjacent four Ba2q and four O 2y ions in this model, if the BaO layers are not distorted locally. Their net electrostatic interaction, therefore, is weak, and the 126 K phase might be rather unstable against distortions in the CR block. On the other hand, this arrangement allows cations and O 2y ions to occupy the same crystalline site without a large penalty of the electrostatic energy. Therefore, the structure may offer a new way to adjust the Cu valence. However, strong local distortions are expected in such a model, which may affect the XRD pattern severely. By simply adopting the typical plane spacing Ži.e. ˚ between adjacent CuO 2 layers, and 2 A˚ 3.15 A between the BaO layer and the nearby CuO 2 layer., XRD pattern was simulated using a commercial software of Ca.R.INe crystallography 3.0 ŽFig. 5e, and Table 1.. It agrees reasonably well with our experimental data. However, the local environments inside the CR block should be very different from this rough average structure due to the interstitial cations, and modifications and refinements are needed. 3.5. The phase transformation and the 90 K phase The XRD pattern of the 90 K phase can be indexed by a tetragonal unit cell that has the same ˚ but with I-symmetry and lattice constant a ; 3.85 A, ˚ a c ; 6 A longer as compared with the 126 K phase. ˚ expansion suggests that approximate two This 6 A extra layers are inserted into a unit cell. Since the transformation process occurs readily at room temperature, no long-range migration of cations is expected. Indeed, as shown by the WDS analysis and our preliminary TEM data, the Ba:Ca:Cu ratios of the 90 K phase are, in fact, the same as those before decay. To account for these facts, we propose that the formation of the 90 K phase is associated with the short-range re-arrangement of the Ca and Cu ions at the 4d site to form two layers between the BaO planes in a rock-salt type configuration ŽFig. 7c.. The formation of these two planes may be assisted by the incorporation of CO 32y and OHy, as described below. As mentioned earlier, incorporation of C in the form of CO 32y group has been proposed in some superconducting Ba–Ca–Cu–O compounds. Our
325
WDS analysis has indeed showed an increase in carbon content after the phase transformation but the amount increased remains far less than the vacancies created. In order to understand the transformation further, several controlled experiments were conducted. Four samples, ; 10 mg each, were separately sealed in quartz tubes of ø Ž3 mm = 20 mm, which were either filled with 1-atm O 2 , Ar, and CO 2 , respectively, or evacuated to 10 y 3 Torr. Their FC magnetization were measured both at the fresh state and one month after being sealed. It was found that both Tc and the FC magnetization at 5 K, which may be regard as an index of the superconducting volume fraction, did not changed by the one-month storage for all four samples. These results suggest that none of these gases alone can cause the decay process observed. Another sample was placed in flowing moisturized O 2 at room temperature. Its Tc , however, was found to drop to ; 70 K after just one day. The presence of the water molecules clearly has a profound adverse effect on the stability of the 126 K phase. In this case, the final Tc is different from that of the 90 K phase formed in air, presumably because there is a lack of CO 2 to stabilize the 90 K phase. Finally, two pieces from the same batch of sample, one before and the other after the decay, were separately heated up to ; 10008C inside an effusion cell, and the released gases were analyzed
Fig. 8. The gases released during heating a decayed sample.
326
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
by a gas mass-spectrometer w15x. A significant amount of H 2 O was released from the decayed sample around 3008 to 5008, corresponding to a nominal atomic ratio of H:Ba ; 0.1 ŽFig. 8.. All these results are consistent with the assumption that H 2 O, or simply OHy, is incorporated within the 90 K phase. The exact configuration of the incorporated H 2 O is not clear. Accordingly, the 90 K phase is proposed to be have a formula ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d and, as summarized in Fig. 7c, a unit cell consisting of three CuO 2 planes Ž n s 3. within the SC block and a BaO– ŽCa,Cu,H,C,O. – ŽCa,Cu,H,C,O. –BaO CR block. XRD pattern was simulated by assuming the originally interstitial inserted Ca and Cu ions to be evenly distributed within the two new layers and ignoring the possible existence of H, O, and CO 2 , which are expected to have only minor effects due to their low electron densities. Again, the pattern is in good agreement with the data.
4. Summary The structures of the recently discovered 126 K and 90 K phases in Ba–Ca–Cu–O system were investigated. Depending on the synthesis conditions, the 126 K phases show a variable cation stoichiometry of Ba:Ca:Cus 2:2 q x:3 q y with 0.4 F x F 0.9 and 0 F y F 0.7, but the same crystal symmetry, i.e. ˚ I4, and the same lattice constants, i.e. a s 3.85 A ˚ and c s 28.2 A. It is proposed that the unit cells of these oxides consist of a rock-salt ŽBaO. 2 block sandwiched between two perovskite Ca 2 Cu 3 O6 blocks, with additional Caq2 and Cuq2 interstitially x y inserted at the 4d site between the two adjacent BaO planes. They differ only in the occupation factors. These oxides, therefore, may more reasonably be e x p re s s e d b y a c o m m o n fo rm u la Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d , i.e. Ba 2 Ca 2 Cu 3 O 8 with interstitial Ca x Cu y . The 90 K phase, which is formed by exposing the 126 K oxides to moist air at room temperature, has the same cation composition, crystal symmetry and lattice constant a as those of the related 126 K phase, but its lattice constant c ; 33.7 ˚ is ; 6 A˚ longer. The formation process is proA posed to be an intercalation of H 2 O ŽOHy. and CO 2 ŽCO 32y . into two neighboring BaO layers of the 126
K phase. The charge reservoir block of the compound, therefore, will be a rock-salt type block of BaO – ŽCa,Cu,H,CO 2 . – ŽCa,Cu,H,CO 2 . –BaO, and the com pound m ay be w ritten as ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d . Acknowledgements We thank A. Hamed for help in measurement of the released gases. This work is supported in part by NSF Grants No. DMR 95-00625, EPRI RP-8066-04, TCSUH, T.L.L. Temple Foundation and the J & M Mopores Endowment. References w1x M. Azuma, Z. Hiroi, M. Takano, Y. Bando, Y. Takeda, Nature 356 Ž1992. 775. w2x Z. Hiroi, M. Takano, M. Azuma, Y. Takeda, Nature 364 Ž1993. 315. w3x H. Shaked, Y. Shimakawa, B.A. Hunter, R.L. Hitterman, J.D. Jorgensen, P.D. Han, D.A. Payne, Phys. Rev. B 51 Ž1995. 11784. w4x Z.L. Du, L. Gao, Y.Y. Sun, I. Rusakova, C.W. Chu, in: B. Batlogg et al. ŽEds.., Proceedings of 10th Anniversary HTSC Workshop on Physics, Materials and Applications, World Scientific, Singapore, 1996, p. 123. w5x Y. Shimakawa, J.D. Jorgensen, J.F. Metchell, B.A. Hunter, H. Shaked, D.G. Hinks, R.L. Hitterman, Z. Hiroi, M. Takano, Physica C 228 Ž1994. 73. w6x X.J. Wu, S. Adachi, C.Q. Jin, H. Yamauchi, S. Tanaka, Physica C 223 Ž1994. 243. w7x T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 224 Ž1994. 69. w8x T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 227 Ž1994. 95. w9x L. Gao, Z.L. Du, Y. Cao, I. Rusakova, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Mod. Phys. Lett. B 21 Ž1995. 1397. w10x C.W. Chu, Z.L. Du, Y. Cao, Y.Y. Xue, Y.Y. Sun, I. Rusakova, L. Gao, Phil. Mag. Lett. 74 Ž1997. 15. w11x Y. Cao, F.Y. Liu, Z.L. Du, F. Chen, Y.Y. Xue, C.W. Chu, Physica C 282–287 Ž1997. 1243. w12x C. Chaillout, S. Le Floch, E. Gautier, P. Border, C. Acha, Y. Feng, A. Sulpice, J.L. Tholence, M. Marezio, Physica C 266 Ž1996. 215. w13x Y.K. Tao, Y.Y. Sun, P.H. Hor, C.W. Chu, J. Solid State Chem. 105 Ž1993. 171. w14x R.M. Hazen, in: D.M. Ginsberg ŽEd.., Physical Properties of High Temperature Superconductors II, World Scientific, Singapore, 1990, p. 121. w15x A. Hamer, R. Oritiz, H.H. Feng, Z.G. Li, P.H. Hor, Y.Y. Xue, Y.Y. Sun, Q. Xiong, Y. Cao, C.W. Chu, Phys. Rev. B 54 Ž1996. 682.
Phase identification of the new 126 K Ba–Ca–Cu–O superconductor Y.Y. Xue, Y.Y. Sun, I. Rusakova, D.K. Ross, Z.L. Du, N.L. Wu 1, Y. Cao, L. Gao, B. Hickey, C.W. Chu ) Texas Center for SuperconductiÕity at UniÕersity of Houston, Houston, TX 77204-5932, USA Received 31 March 1997; revised 28 May 1997; accepted 19 October 1997
Abstract New superconducting oxides that have a composition of Ba 2 Ca 2 Cu 3ŽCa x Cu y .O 8q d with 0.4 G x G 0.9 and 0.7 G y G 0, and show an optimum Tc ; 126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common ˚ and c s 28.2 A˚ with an Ir4mmm symmetry. tetragonal unit cell among these oxides, which has lattice constants a s 3.85 A The cell consists of a rock-salt ŽBaO. 2 block sandwiched between two perovskite Ca 2 Cu 3 O6 blocks with interstitial Caq2 x and Cuq2 located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial y Ca xCu y . The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H 2 O and CO 2 in open air. The decayed products ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d have an expanded ˚ and a lower Tc ; 90 K. A structure model for the decayed phases was also proposed. q 1998 lattice constant c ; 33.8 A Elsevier Science B.V. PACS: 74.72Jt; 74.62Bf; 61.66Fn Keywords: Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d ; Structure; Structure phase transition
1. Introduction The structures responsible for the high temperature superconductivity in the ŽBa,Sr,Ca. –Cu–O system remain debating issues up to now. The so-called infinite-layered compounds ŽSr y Ca 1yy .CuO 2 w1x and the first three members of the Sr2 Srny1Cu nO 2 nqny d family w2–4x all have been proposed to be supercon)
Corresponding author. Tel.: q1 713 743 8200; Fax: q1 713 743 8201. 1 Present address: Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan.
ductive. However, difficulties exist for a self-consistent model that unequivocally accounts for both the integrity of the superconducting Ž SC . Ca ny 1Cu nO 2 n block and the Cu valence needed. For instance, the value of d in Sr2 CuO4y d has to be 0.7 or larger in order to keep the corresponding Cu valence VCu below a reasonable limit, above which most cuprates not only become non-superconducting, but also decompose due to the energetically unfavorable configuration of Cu3q. A previous neutron diffraction investigation suggested that these oxygen vacancies were located entirely within the SC block for Sr2 CuO4y d , while the oxygen sites in the so-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 1 2 - 1
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
called charge reservoir ŽCR. block of ŽSrO. 2 were fully occupied w5x. As pointed out by Shaked et al. w3x, such broken SC blocks raise serious doubts about the proposed superconductivity in Sr2 CuO4y d . This structural instability against oxygen vacancy in CuO 2 planes might be more prominent if the Sr 2q is to be replaced by a larger cation, such as Baq2 , which leads to longer and hence weaker in-plane Cu–O bonds and makes the vacancy formation even easier. Indeed, there has been no body-centered Ba 2 ŽSr,Ca. ny1Cu nO 2 nq2y d or even any other superconducting Ba–Ca–Cu–O compound reported thus far, despite the experimental efforts in synthesizing them Žfor example, Ref. w6x.. Significant amount of C, whose higher formal-valence reduces the density of the oxygen-vacancies needed, has to be introduced into the CR blocks of reported superconducting Ba– Ca – Cu compounds, e.g. Ž Cu,C . Ba 2 Ca n y 1 Cu nO 2 nq 3q d and Ž Cu,C . 2 Ž Ba,Ca . Ba 2 Ca ny 1 Cu nO 2 nq5q d , in order to stabilize their layer structure and sustain the superconductivity w7,8x. In this paper, we report, for the first time, the identification of a class of high temperature superconducting ŽHTSC. oxides of Ba, Ca and Cu without detectable C-doping. These oxides have a nonstoichiom etric com position of Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d , but show the same optimum Tc ; 126 K and X-ray powder diffraction ŽXRD. pattern. Depending on synthesis conditions, x varies between 0.4 and 0.9 and y between 0 and 0.7, e.g. Ba 2 Ca 2 Cu 3 Ca 0.9 O 8q d on the Cu-deficient side and Ba 2 Ca 2 Cu 3 ŽCa 0.8 Cu 0.7 .O 8q d on the Cu-rich side. One of the oxides, namely, sample A of Ba 2 Ca 2 Cu 3 ŽCa 0.85 Cu 0.68 .O 8q d Žor Ba 2 Ca 2.85 Cu 3.68 O 8q d . shows a Tc of 126 K in its as-synthesized state as reported earlier w9,10x. The XRD patterns of all these oxides can be indexed by a body-centered tetragonal unit cell with essentially the same lattice constants ˚ and c ; 28.2 A. ˚ Structure analysis india ; 3.85 A cates that the unit cell is similar to that of so-called 0223 phase, e.g. the proposed Sr2 Ca 2 Cu 3 O 8q d . The non-stoichiometric compositions, however, suggest significant interstitial cations and site mixing. These defects would likely occur in the charge reservoir ŽCR. block, considering the composition-independent optimal Tc of 126 K. One of the possible interstitial sites will be the 4d position between the two neighboring BaO planes in the CR block
317
Ba 2 Ca x Cu y . To our best knowledge, many of these structure features, i.e. a IIA elements Ca in the C R block of the B a 2 C a 2 C u 3 Ca 0.9 O 8q d , I-symmetry in the Ba–Ca–Cu system, and the possible interstitial cations at the 4d site of Ž1r2,0,1r4., have not been observed before. This non-stoichiometric cation layer of Ca x Cu y offers a new way, which has not been reported before, to adjust the carrier concentration and stabilize the structure. The new superconducting oxides are susceptible to degradation in air and, particularly, in water-containing atmospheres. Irrespective of the cation stoichiometries, i.e. of the values of x and y, all decayed phases formed in air showed a Tc near 90 K ˚ The crystal and a larger lattice constant c ; 33.7 A. symmetry and cation stoichiometry, however, remain the same as those of the starting 126 K compounds. A structural model for the decayed products and a possible degradation mechanism, which is attributed to the insertion of HOy andror CO 32y between the two BaO layers, are proposed. 2. Experiments The synthesis procedures have been reported in detail earlier w9,10x. In brief, samples were synthesized under 5 GPa at 800–9508C using a multi-anvil Walker module from Rockland Research. AgO, acting as an oxidant, was added into the precursor powders, which have a nominal composition of Ba 2 Ca 2qx Cu 3qyOz with 0 F x F 1 and 0 F y F 1, with a ratio of 0.5–1 molerformula. The oxide mixtures were compacted into cylinders and wrapped in Au foil for the high pressure synthesis. Precautions were taken to avoid contamination of CO 2 , which was known to retard the formation of the high-Tc phases under investigation. The superconducting properties were characterized both by the standard four-point method and by a SQUID magnetometer ŽQuantum Design.. X-ray powder diffraction ŽXRD. measurement was carried out by using a Rigaku D-MAXrBIII diffractometer equipped with a specially designed sample chamber, which allows for continuous purging with dry oxygen during data acquisition. Compositions were determined by the wave dispersive spectrometer ŽWDS. of a JEOL JXA 8600 electron microprobe.
318
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
3. Results and discussion 3.1. SuperconductiÕity Fig. 1 shows the field-cooled ŽFC. magnetization curve of as-synthesized sample A at 10 Oe. The onset Tc is ; 126 K and the transition width Žfrom 10 to 90%. is ; 4 K Žcurve A, Fig. 1.. It should be noted that although the Tc of our samples at the as-synthesis state varies between 110 and 126 K, their optimum Tc falls within a much narrower range, i.e. 124 K F Tc F 126 K. For example, the Tc-onset reached ; 125 K after an annealing at 2508C under 1 atm. O 2 for sample B Žcurve B in Fig. 1., whose Tc-onset was only 120 K in the as-synthesized condition. As mentioned earlier w9,10x, all these samples are not stable in open air, particularly after they were pulverized. The Tc of the pulverized samples drops to ; 90 K, independent of both the as-synthesis Tc and the annealing history of the samples, after they were exposed to air for a few hours ŽFig. 1, inset.. However, the 126 K superconductivity in dense bulk samples kept in a dry desiccator can survive for months, suggesting a process controlled by the OHy and CO 32y diffusions. The transition width of the transformed products remains reasonably narrow with
Fig. 1. x vs. T. A: sample A; B: sample B after the optimum annealing. Inset: x vs. T of a pulverized sample; A: before exposing to air; B: 1 h after that; C: after 4 days in open air.
a FC magnetization at 5 K being nearly the same as that before the transformation ŽFig. 1, inset.. The annealing behavior of the samples, however, is totally different before and after the transformation. Particularly, the decay is irreversible in terms of the O 2 or Ar annealing. The same surface of a bulk sample was observed both before and after the transformation. The average grain size on the fresh surface was ; 5 mm or larger with sharp edges ŽFig. 2a., but the surface was rough with many bumps and cracks after the transformation ŽFig. 2b.. We believe, therefore, the process is rather a phase-transformation than a oxygen intake-andror-release. Details of the annealing and decaying behaviors will be reported elsewhere w11x. The similarity in both the optimum Tc and the transformation behavior hints that the corresponding superconducting phases might be closely related. 3.2. Composition analysis The cation compositions and carbon content were directly determined by WDS. The sample surface was prepared by dry polishing just before measurement, and no conducting coating was applied. Both the FC magnetization and the surface-morphology of the prepared samples show that the 126 K superconducting phase was largely preserved during the sample preparation. Although a thin surface-layer may be degraded, the observed stoichiometry is not expected to change, considering the facts that the cation diffusion rate is low at room temperature and that the acquired volume of WDS is large, i.e. ; 1 = 1 = 1 mm3. To confirm this presumption, compositions were detected and compared between a fresh surface and the same surface after being exposed in air for a few days. In spite of the distinct surface morphologies Žsimilar to Fig. 2a,b., the cation compositions are within an experimental resolution of "3%. While the samples are mostly multi-phased, various phases were easily distinguished in the back scattering images of polished surfaces ŽFig. 3.. The cation ratios in each of these phases were measured using WDS, which was calibrated against several standard oxides and carbonates before each experimental run. To evaluate the reliability of our WDS data, a ŽCu 1y x C x .Ba 2 Ca 2 Cu 3 O 10y d sample, whose XRD pattern showing no impurity phases within our
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
319
Fig. 2. The SEM images of Ža. a freshly broken sample; Žb. the surface after exposing to air.
experimental resolution, was measured. The measured compositions are Ba:Ca:Cu:C s 2:2.22 Ž"0.04.:3.62Ž"0.05.:0.47Ž"0.1.. The standard deviations shown in the parentheses indicate the range of spreading among data acquired from different grains. The composition thus detected is in good agreement with the theoretical stoichiometry of ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d . The differences are within data spreading range except for Ca. The slightly higher Ca reading may be due either to an experimental deviation or to a genuine Ca-surplus in the phase.
For most samples, there was predominately only one ternary metal oxide, in addition to impurity grains that can be attributed to single- and binarymetal oxides, such as BaOrBaO 2 , BaCuO 2 and Ca 2 CuO 3 ŽFig. 3.. The cation ratios of this ternary metal oxide have rather narrow spreading in a given sample. For example, the cation ratios of sample A are Ba:Ca:Cu s 2:2.85Ž"0.06.:3.68Ž"0.06., and most other samples have a similar stoichiometry ŽFig. 4.. The cation ratios, however, do vary considerably among samples synthesized under different conditions, i.e. as Ba 2 Ca 2 Cu 3 ŽCa x Cu y .Oz with 0.4
320
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
Fig. 3. Back scattering image by microprobe on a polished surface.
- x - 0.9 and 0 - y - 0.7 ŽFig. 4.. The large gainsize of our samples, the much narrower data spreading in a given sample, as well as the result of the ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d sample show that the observed cation non-stoichiometry is genuine. The carbon contents within the ternary metal oxide grains were directly determined by WDS and found to be nil in all cases. For instance, C:Ba ratio is ; 0.04 " 0.04 in sample A, which is in great contrast with C:Ba s 0.235 " 0.05 measured in the ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O10y d sample. The nil result was confirmed by our EELS, FTIR and mass spectrum measurements. In fact, the presence of carboncontaining species, such as carbonates, in the precursor powder was found to significantly retard the formation of the 126 K phases w9,10x. All these results either directly or indirectly lead to the conclusion that these ternary-metal phases under investigation are oxides but not carboxyl cuprates, therefore, not belong to the well known ŽCu 0.5 C 0.5 .Ba 2Ca ny 1Cu nO 2 nq 2 and Ž Cu 0.5 C 0.5 . 2 Ba 3 Ca ny 1 Cu nO 2 nq5y families. After the phase-transformation in air, WDS showed a slight increase in carbon content in the ternary oxide grains. In the case of the sample A, for example, the average carbon content was found to
Fig. 4. The cation compositions of several samples measured by WDS. The letters A and B in the figure represent to the compositions of samples A and B, respectively.
increase to C:Ba s 0.065, which remains far lower than that in ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O 9q d . 3.3. X-ray diffraction Due to the fast decay of the 126 K phase in open air, the XRD measurements were conducted in a sample chamber that is constantly purged with dry O 2 . To verify the effectiveness of such a protection measure, two consecutive measurements, each of which lasted for ; 45 min, were taken on the same sample under the protection. The two XRD patterns were found to be the same ŽA and B in Fig. 5a with the vertical scale of B reduced.. Meanwhile, small parts of the XRD sample were removed and subjected to magnetization measurement both before and after the two XRD measurements. The same single-step superconducting transition at 126 K was observed. In contrast, the XRD showed a completely different pattern ŽFig. 5b., and the Tc dropped to 90 K within a few hours of air being introduced into the
Fig. 5. Ža. Sequential XRD patterns of sample A under protection of dry O 2 . A: the first run; B: the second run about 45 min later. The vertical scale of curve B is reduced by 5 times for a better view. Žb. XRD pattern of sample A after the air was introduced. Žc. A: the difference pattern of sample A. The arrow at ; 238 shows the position of the Ž001. line. B: the XRD pattern of a single phase ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d sample. Žd. The difference pattern of sample B. Že. The simulated XRD patterns.
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
321
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
322
chamber. Such simultaneous changes in the XRD pattern and Tc were observed in all samples measured. Hence, it is considered that the XRD pattern acquired before introducing air corresponds to the 126 K phase Žphases. plus some minor impurities, while the XRD pattern after complete decay corresponds to the 90 K phase. Accordingly, ‘difference patterns’ were constructed by subtracting the pattern after decay from the one before ŽFig. 5c.. In these difference patterns, the reflection lines of the 126 K phase Žphases. are expected to show up as positive lines, i.e. lines with positive intensities, while those of the 90 K phase as negative ones. Most of the lines related to the impurity phases should disappear in the difference patterns, which simplifies the data analysis. The 2 u values of most lines will be the same irrespective of whether they are determined from either the original or the difference patterns. However, there are also a couple of lines that do not show significant change in their intensities but small shift in 2 u values after the decay. Such lines can be easily spotted because they lead to the occurrence of a pair of peaks, one positive and one negative, right next to each other in the difference patterns. For these reflections lines, their 2 u values have to be directly read from the original patterns. Almost all of the positive XRD lines, as shown in Fig. 5c and Table 1, can be indexed according to a Table 1 The observed and calculated d-values of the 126 K phase Miller index 004 006 103 008 105 0010 107 110 114 109 116r0012 0014 200 0016 217r0018
Measured ˚. d ŽA
Calculated ˚. d ŽA
Calculated
intensity
Measured
7.07 4.71 3.57 3.50 3.19 2.83 2.74 2.72 2.54 2.44 2.34 2.02 1.93 1.77 1.57
27 28 17 3 14 86 14 100 30 70 46 10 54 20 17
7.05 4.70 3.56 3.53 3.18 2.82 2.78 2.72 2.53 2.43 2.36 2.01 1.93 1.76 1.58
17 32 87 1 77 3 23 100 40 220 100 20 110 15 13
intensity
Table 2 The observed and calculated d-values of the 90 K phase Miller index 002 004 006 008 103 107 110 109 1011 1110r0016 200 0020 1161
Measured ˚. d ŽA
Calculated ˚. d ŽA
Calculated
intensity
Measured
17.11 8.50 5.65 4.24 3.64 3.02 2.72 2.69 2.41 2.12 1.92 1.70 1.68
71 65 7 6 26 16 100 65 19 31 45 23 10
16.98 8.49 5.66 4.25 3.65 3.02 2.72 2.69 2.41 2.12 1.93 1.70 1.67
41 79 2 21 34 43 100 53 71 16 51 5 5
intensity
˚ and body-center tetragonal cell with a s 3.84 A ˚ The Ž00 l . lines with odd l values or c s 28.2 A. Ž10 l . lines with even l values are systematically missing. The only possible exception is a weak line observed at the Ž100.rŽ010. position Žindicated by an arrow in Fig. 5c.. This peak may be due either to an impurity phase, which also decomposes after exposed to air, or to some structure distortions of the 126 K phase. On the other hand, the majority of the negative lines ŽFig. 5c and Table 2. can be indexed according to a body-center tetragonal cell with a s ˚ and c s 34.2 A. ˚ The corresponding prohibit3.84 A ing rules are again followed very well. The XRD pattern of a single phase ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d sample, which has a slightly lower optimum Tc of ; 120 K w12x, is also shown in the bottom of Fig. 5c as a comparison. It is clear that the XRD pattern of ŽCu,C.Ba 2 Ca 2 Cu 3 O 9y d , or any known ŽCu 0.5 C 0.5 .Ba 2 Ca ny 1Cu nO 2 nq 2y d and Ž Cu 0.5 C 0.5 . 2 Ba 3 Ca ny 1Cu nO 2 nq5y d phases, is very different from that of either the 126 K phase or the 90 K phase. Sample B has a different cation stoichiometry of Ba 2 Ca 2 Cu 3 Ca 0.9 O 9y d ŽFig. 4., and a slightly lower Tc of 120 K at the as-synthesized state. However, its XRD difference pattern ŽFig. 5d. is similar to that of sample A ŽFig. 5c. with only slight differences in the relative line-intensities, which can be well ascribed to different prefer-orientations. Since the superconducting volume fraction is 30% or larger in both cases, the difference between their XRD patterns should be noticeable if the corresponding phases are
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
323
structurally different. We believe, therefore, that one common non-stoichiometric phase is responsible to the observed 126 K superconductivity in all these samples. 3.4. Structure models All known p-type HTSC cuprates contain periodic stacks of an SC wŽCa,Sr,R. ny 1ŽCuO. n x block, where R is either Y or a rare earth element, and a CR block. With only a few exceptions, such as ŽLa,Ba. 2 CuO4 , the CR block is typically bounded by two BaO ŽSrO. planes. While the value of n characterizes the different members in one HTSC family, HTSC families differ only by the cation composition and arrangement within the CR blocks. The known CR structures are either rock-salt type, e.g. the BaO– Tl 2 O 2 – BaO block in Tl 2 Ba 2 Ca 2 Cu 3 O 10q d , oxygen-deficient perovskite type, e.g. the BaO– ŽCu,C.O–BaO block in ŽCu 0.5 C 0.5 .Ba 2 Ca 2 Cu 3 O 9q d , or fluorite type, e.g. the ŽNd,Ce. 2 O 2 block in ŽNd,Ce. 2 ŽBa,Nd. 2 Cu 3 O 9q d w13x. Previously, the body-centered symmetry has only been observed in cuprates whose CR blocks are either rock-salt type or fluorite type when the total number of the cation layers is even. A structural model for the 126 K was developed along these lines. For almost all the HTSC cuprates, the distance between two neighboring CuO 2 planes in the SC ˚ and the distance between the block is 3.15 " 0.05 A, neighboring BaO and CuO 2 planes, i.e. the distance ˚ Žfor between the SC and CR blocks, is 2.00 " 0.05 A example, Ref. w14x.. Simple calculation immediately ˚ can be acshows that the observed c ; 28.2 A counted for by two half-unit cells which are displaced by Ž1r2,1r2,cr2., and each of which contains one SC block with n no larger than 3. To estimate the value of n, the projected electron density in c-direction was estimated based on the relative intensities of all 00 l lines observed ŽTable 1. using a simple Fourier transform ŽFig. 6, where a temperature factor of 2 was assumed.. Three main peaks are shown between z i s 0 and z i s 0.2, with an additional small bump at z i s 0.07 ŽFig. 6.. We attribute these peaks and the bump to two CuO 2 layer, BaO layer and Ca layer, respectively. This suggests that the 126 K phase is an n s 3 cuprate. The layer distances are in good agreement with those
Fig. 6. The estimated project electron density in c-direction. Only the first half cell is shown.
in known cuprates Žtop of Fig. 6.. In such a case, the overall thickness of the CR block should be ; 3.8 ˚ and its composition Ba 2 ŽCa x Ca y . with 0.4 F x F A, 0.9 and 0 F y F 0.7, e.g. Ba 2 ŽCa 0.85 Ca 0.68 . in sample A and Ba 2 Ca 0.9 in sample B. An obvious candidate is the so-called 0223 phase of Ba 2 Ca 2 Cu 3 O y with some site-mixing since a BaO– ŽCa,Cu.O–BaO arrangement will violate the I-symmetry observed, and a BaO– ŽCa,Cu.O– ŽCa,Cu.O–BaO CR block will be ˚ cell. The composition and too thick for a c s 28.2 A distance, however, are difficult to be reconciled without significant interstitial cations. The cation ratio of Cu:ŽBa q Ca. ; 0.61 in sample B, for example, is significantly less than the ratio of 0.75 in ŽBa,Ca. 2 Ca 2 Cu 3 O y . Dense defects in the CuO 2 layers would be unavoidable in a 0223 model without more than one ŽBa,Ca. ionrcell located at some interstitial sites. In addition, the estimated layer dis˚ would be considerably larger than tance of 3.8 A ˚ either the average Ba–O bond length, ; 2.6–2.8 A, in other cuprates or the ŽLa,Ba.O layer distance of ˚ in ŽLa,Ba. 2 CuO4 w14x, but relatively close to ; 1.8 A ˚ in the CR block thickness of ; 4.4 A ŽCu,C.Ba 2 Ca 3 Cu 4 O 11y d , which has three cation layers in its CR block and the P-symmetry. The electron-density distribution tentatively shows a small
324
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
peak between two BaO layers ŽFig. 6.. We regard this as a hint for the possible interstitial cations. A careful inspection reveals that there are four large tetrahedral cages existing at the 4d site between the two adjacent BaO planes, if the space between them ˚ ŽFig. 7a.. The cages, each of is larger than 3 A ˚ and 5.4 A˚ along the four which is measured ; 4.7 A interlayer O–O edges and the two intralayer O–O edges, respectively, are large enough for a Cu2q ion ˚ ., a Ca2q ion Ž r ; 1.00 A˚ ., or even an Ž r ; 0.62 A 2y ˚ . to fit in comfortably. If these O ion Ž r ; 1.4 A cages are fully occupied by the same cation or
randomly filled by both Cuq2 and Caq2 , the bodycenter symmetry is preserved. Even if they are partially occupied in an ordered fashion, the average structure can still have a body centered symmetry superimposed on a superstructure. Thus, to account for the symmetry and the cation composition, it is proposed that Ca x Cu y are interstitially inserted into two neighboring BaO planes, as shown in Fig. 7b. A simple valence-count suggests that there should also be a significant amount of oxygen surplus, i.e. d ; 0.7. The most likely location for these oxygen ions would be the same 4d site. It should be noted that
Fig. 7. Ža. The cages between two adjacent BaO layers. The ions are drawn with the actual sizes of Ba2q, O 2y and Ca2q, respectively. Žb. The proposed model for the 126 K phase. Žc. The proposed model for the 90 K phase.
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
these interstitial cations of Ca2q and Cu2q have the same distance to the adjacent four Ba2q and four O 2y ions in this model, if the BaO layers are not distorted locally. Their net electrostatic interaction, therefore, is weak, and the 126 K phase might be rather unstable against distortions in the CR block. On the other hand, this arrangement allows cations and O 2y ions to occupy the same crystalline site without a large penalty of the electrostatic energy. Therefore, the structure may offer a new way to adjust the Cu valence. However, strong local distortions are expected in such a model, which may affect the XRD pattern severely. By simply adopting the typical plane spacing Ži.e. ˚ between adjacent CuO 2 layers, and 2 A˚ 3.15 A between the BaO layer and the nearby CuO 2 layer., XRD pattern was simulated using a commercial software of Ca.R.INe crystallography 3.0 ŽFig. 5e, and Table 1.. It agrees reasonably well with our experimental data. However, the local environments inside the CR block should be very different from this rough average structure due to the interstitial cations, and modifications and refinements are needed. 3.5. The phase transformation and the 90 K phase The XRD pattern of the 90 K phase can be indexed by a tetragonal unit cell that has the same ˚ but with I-symmetry and lattice constant a ; 3.85 A, ˚ a c ; 6 A longer as compared with the 126 K phase. ˚ expansion suggests that approximate two This 6 A extra layers are inserted into a unit cell. Since the transformation process occurs readily at room temperature, no long-range migration of cations is expected. Indeed, as shown by the WDS analysis and our preliminary TEM data, the Ba:Ca:Cu ratios of the 90 K phase are, in fact, the same as those before decay. To account for these facts, we propose that the formation of the 90 K phase is associated with the short-range re-arrangement of the Ca and Cu ions at the 4d site to form two layers between the BaO planes in a rock-salt type configuration ŽFig. 7c.. The formation of these two planes may be assisted by the incorporation of CO 32y and OHy, as described below. As mentioned earlier, incorporation of C in the form of CO 32y group has been proposed in some superconducting Ba–Ca–Cu–O compounds. Our
325
WDS analysis has indeed showed an increase in carbon content after the phase transformation but the amount increased remains far less than the vacancies created. In order to understand the transformation further, several controlled experiments were conducted. Four samples, ; 10 mg each, were separately sealed in quartz tubes of ø Ž3 mm = 20 mm, which were either filled with 1-atm O 2 , Ar, and CO 2 , respectively, or evacuated to 10 y 3 Torr. Their FC magnetization were measured both at the fresh state and one month after being sealed. It was found that both Tc and the FC magnetization at 5 K, which may be regard as an index of the superconducting volume fraction, did not changed by the one-month storage for all four samples. These results suggest that none of these gases alone can cause the decay process observed. Another sample was placed in flowing moisturized O 2 at room temperature. Its Tc , however, was found to drop to ; 70 K after just one day. The presence of the water molecules clearly has a profound adverse effect on the stability of the 126 K phase. In this case, the final Tc is different from that of the 90 K phase formed in air, presumably because there is a lack of CO 2 to stabilize the 90 K phase. Finally, two pieces from the same batch of sample, one before and the other after the decay, were separately heated up to ; 10008C inside an effusion cell, and the released gases were analyzed
Fig. 8. The gases released during heating a decayed sample.
326
Y.Y. Xue et al.r Physica C 294 (1998) 316–326
by a gas mass-spectrometer w15x. A significant amount of H 2 O was released from the decayed sample around 3008 to 5008, corresponding to a nominal atomic ratio of H:Ba ; 0.1 ŽFig. 8.. All these results are consistent with the assumption that H 2 O, or simply OHy, is incorporated within the 90 K phase. The exact configuration of the incorporated H 2 O is not clear. Accordingly, the 90 K phase is proposed to be have a formula ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d and, as summarized in Fig. 7c, a unit cell consisting of three CuO 2 planes Ž n s 3. within the SC block and a BaO– ŽCa,Cu,H,C,O. – ŽCa,Cu,H,C,O. –BaO CR block. XRD pattern was simulated by assuming the originally interstitial inserted Ca and Cu ions to be evenly distributed within the two new layers and ignoring the possible existence of H, O, and CO 2 , which are expected to have only minor effects due to their low electron densities. Again, the pattern is in good agreement with the data.
4. Summary The structures of the recently discovered 126 K and 90 K phases in Ba–Ca–Cu–O system were investigated. Depending on the synthesis conditions, the 126 K phases show a variable cation stoichiometry of Ba:Ca:Cus 2:2 q x:3 q y with 0.4 F x F 0.9 and 0 F y F 0.7, but the same crystal symmetry, i.e. ˚ I4, and the same lattice constants, i.e. a s 3.85 A ˚ and c s 28.2 A. It is proposed that the unit cells of these oxides consist of a rock-salt ŽBaO. 2 block sandwiched between two perovskite Ca 2 Cu 3 O6 blocks, with additional Caq2 and Cuq2 interstitially x y inserted at the 4d site between the two adjacent BaO planes. They differ only in the occupation factors. These oxides, therefore, may more reasonably be e x p re s s e d b y a c o m m o n fo rm u la Ba 2 Ca 2 Cu 3 ŽCa x Cu y .O 8q d , i.e. Ba 2 Ca 2 Cu 3 O 8 with interstitial Ca x Cu y . The 90 K phase, which is formed by exposing the 126 K oxides to moist air at room temperature, has the same cation composition, crystal symmetry and lattice constant a as those of the related 126 K phase, but its lattice constant c ; 33.7 ˚ is ; 6 A˚ longer. The formation process is proA posed to be an intercalation of H 2 O ŽOHy. and CO 2 ŽCO 32y . into two neighboring BaO layers of the 126
K phase. The charge reservoir block of the compound, therefore, will be a rock-salt type block of BaO – ŽCa,Cu,H,CO 2 . – ŽCa,Cu,H,CO 2 . –BaO, and the com pound m ay be w ritten as ŽCa,Cu,H,CO 2 . 2 Ba 2 Ca 2 Cu 3 O 8q d . Acknowledgements We thank A. Hamed for help in measurement of the released gases. This work is supported in part by NSF Grants No. DMR 95-00625, EPRI RP-8066-04, TCSUH, T.L.L. Temple Foundation and the J & M Mopores Endowment. References w1x M. Azuma, Z. Hiroi, M. Takano, Y. Bando, Y. Takeda, Nature 356 Ž1992. 775. w2x Z. Hiroi, M. Takano, M. Azuma, Y. Takeda, Nature 364 Ž1993. 315. w3x H. Shaked, Y. Shimakawa, B.A. Hunter, R.L. Hitterman, J.D. Jorgensen, P.D. Han, D.A. Payne, Phys. Rev. B 51 Ž1995. 11784. w4x Z.L. Du, L. Gao, Y.Y. Sun, I. Rusakova, C.W. Chu, in: B. Batlogg et al. ŽEds.., Proceedings of 10th Anniversary HTSC Workshop on Physics, Materials and Applications, World Scientific, Singapore, 1996, p. 123. w5x Y. Shimakawa, J.D. Jorgensen, J.F. Metchell, B.A. Hunter, H. Shaked, D.G. Hinks, R.L. Hitterman, Z. Hiroi, M. Takano, Physica C 228 Ž1994. 73. w6x X.J. Wu, S. Adachi, C.Q. Jin, H. Yamauchi, S. Tanaka, Physica C 223 Ž1994. 243. w7x T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 224 Ž1994. 69. w8x T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 227 Ž1994. 95. w9x L. Gao, Z.L. Du, Y. Cao, I. Rusakova, Y.Y. Sun, Y.Y. Xue, C.W. Chu, Mod. Phys. Lett. B 21 Ž1995. 1397. w10x C.W. Chu, Z.L. Du, Y. Cao, Y.Y. Xue, Y.Y. Sun, I. Rusakova, L. Gao, Phil. Mag. Lett. 74 Ž1997. 15. w11x Y. Cao, F.Y. Liu, Z.L. Du, F. Chen, Y.Y. Xue, C.W. Chu, Physica C 282–287 Ž1997. 1243. w12x C. Chaillout, S. Le Floch, E. Gautier, P. Border, C. Acha, Y. Feng, A. Sulpice, J.L. Tholence, M. Marezio, Physica C 266 Ž1996. 215. w13x Y.K. Tao, Y.Y. Sun, P.H. Hor, C.W. Chu, J. Solid State Chem. 105 Ž1993. 171. w14x R.M. Hazen, in: D.M. Ginsberg ŽEd.., Physical Properties of High Temperature Superconductors II, World Scientific, Singapore, 1990, p. 121. w15x A. Hamer, R. Oritiz, H.H. Feng, Z.G. Li, P.H. Hor, Y.Y. Xue, Y.Y. Sun, Q. Xiong, Y. Cao, C.W. Chu, Phys. Rev. B 54 Ž1996. 682.