Low-pressure synthesis of tetragonal Sr2CuO3 + χ from a single-source hydroxometallate precursor

PHYSICA PhysieaC227 (1994) 279-284

EI~EVIER

Low-pressure synthesis of tetragonal Sr2CuO3+xfrom a single-source hydroxometallate precursor J o h n F. M i t c h e l l *, D . G . H i n k s , J . L . W a g n e r Materials Science Division and Science and Technology Center for Superconductivity, Argonne National Laboratory, Argonne, IL 60439, USA

Received 9 February 1994

Abstract

Synthesis of the tetragonal "high-pressure" form of Sr2CuO3+x at 370°C in 1 atm oxygen from a copper hydroxometallate precursor, Sr2Cu(OH)6 is reported. Results of thermogravimetric analysis, and in-situ X-ray diffraction on the thermal decomposition of the precursor are described. The as-synthesized tetragonal material is nonsuperconducting and is irreversibly converted to the orthorhombic phase by heating at 450°C in 1 atm 02. l . Introducfion

Interest in the so-called infinite layer high-temperature cuprate superconductors has recently been intensified by the discovery of a class of compounds containing only Sr and Cu that superconduct at 70 K and 100 K [ 1 ]. At ambient pressure, Sr2CuO3 forms with an orthorhombically distorted K2NiF4 structure [2,3 ]. This distortion arises from the ordering of the oxygen vacancies in the CuO2 plans, leading to an orthorhombic structure containing CuO3 chains. Hiroi et al. [ 1 ] have synthesized tetragonal Sr2CuO3. l in macroscopic quantities under oxidizing conditions at 900 ° C and 6 GPa in a cubic anvil apparatus. This material adopts a highly oxygen-deficient K2NiF4 structure and is the first member of the series with general formula Sr, + ~Cu,O2, + 1- They also report the presence of the n = 2 phase as an impurity and conclude from susceptibility measurements that the 70 K and 100 K transitions belong to the n = 1 and n = 2 phases, respectively. Lobo et al. [ 4 ] have shown that * Corresponding author.

the low-pressure, nonsuperconducting orthorhombic form of Sr2CuO3 can be oxidized under 160 bar of O2 at ~ 400 ° C to yield the tetragonal phase, which they assign the stoichiometry Sr2CuO3.9, a highly oxygenated analog of the Hiroi compound. This latter material is not superconducting. The common theme of these two groups has been synthesis under elevated oxygen pressure. In an attempt to find an ambient-pressure route to this class of compounds, we have sought single-source precursors which will decompose at low temperatures, obviating the need for high P(O2). In this report we describe the synthesis and thermal decomposition of one such single-source precursor, Sr2Cu (OH) 6, that yields the tetragonal form of Sr2CuO3 +x on dehydration below 400 °C in 1 arm 02. The as-synthesized material is nonsuperconducting, but appears to be isostrucrural with Hiroi's superconducting material. Thermogravimetric analysis and in-situ X-ray diffraction of the decomposition of Sr2Cu (OH)6 to the tetragonal phase raise the possibility that differing oxygen stoichiometry and/or oxygen vacancy ordering may play a role in preventing superconductivity, but other

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J.F Mitchell et al. /Physica C 227 (1994) 279-284

less straightforward possibilities involving the misidentification of the superconducting phase in Hiroi's sample cannot be ruled out, either.

2. Synthesis, structure, and thermal analysis of Sr2Cu(OH)6 Synthesis of the precursor hydroxometallate follows the method described by Scholder [5], and is carried out in highly concentrated sodium hydroxide solution. We note parenthetically that Nadezhina et al. [6] have synthesized small amounts of this material via hydrothermal techniques. All solution manipulations were carried out in teflon containers to prevent silicate contamination, but no special precautions to avoid atmospheric carbon dioxide were exercised. 80 mmol of Cu(NO3)2" 3H20 dissolved in 25 ml of H20 were added to 200 ml of an ice-cold NaOH solution (50% w / v ) . The resulting blue solution was heated to 80°C for several minutes and, after cooling, filtered to remove unreacted CuO. The deep blue filtrate was heated to l 10 ° C, and a hot solution of 8.3 mmol Sr(OH )2" 8H20 in 70 ml H20 was slowly added with vigorous shaking. A light blue precipitate immediately forms. The hot solution was then suction filtered through a polyethylene fritted filter, the clear filtrate indicating that the copper was quantitatively precipitated from the solution. The sky-blue precipitate was washed several times with cold, anhydrous methanol, and then with acetone and finally with anhydrous diethyl ether. The product was allowed to dry at room temperature under a nitrogen atmosphere. No attempt was made to optimize the product yield. As-synthesized Sr2Cu(OH)6 has been characterized using a combination of microscopy and X-ray diffraction. Under an optical microscope, Sr2Cu(OH)6 powder appears as small, irregularly formed light-blue crystallites. EDS analysis using an Oxford Instruments Link Isis system on a Hitachi Model $2700 scanning electron microscope revealed an extremely uniform material with erystallites averaging 1-2 ~tm on an edge. The crystallites analyzed for Sr and Cu in the ratio 2.0: 1. The X-ray powder pattern (Cu Ka radiation) has been indexed on a monoclinic P2~/c cell by Dubler et al. [7] with a = 5 . 7 8 6 ( 1 ) A, b = 6 . 1 5 4 ( 1 ) ~, c=9.744 /~, and

fl= 124. I 5 . The structure, shown in Fig. 1, is a highl3 distorted KxPtCI6 type [8], with Jahn-Teller distorted C u ( O H ) 4 - octahedra. Indeed, the two axial ligands are so far removed from the central Cu ( > 2.8 A) that the coordination polyhedron is better described as square planar with the four C u - O bonds averaging 1.96 ,~, a typical C u - O distance lbund in the cuprate superconductors. We investigated the thermal decomposition of Sr2Cu(OH)6 via T G A using a Perkin-Elmer Model TGA7 Thermogravimetric Analyzer in both oxidizing and reducing environments. In each case, the sample was heated at a rate of l ° C / m i n to 7 0 0 ' ( ' and held there ( 12 h for At, 9 h for O2). All calculated weight percents are based on the assumption of a pure Sr2Cu (OH)6 sample. Previously, lvanov-Emin et al. [9] studied the thermal decomposition of Sr2Cu(OH)6 in air and concluded that the decomposition path followed a three-step sequence: Sr2Cu(OH)6 - + H 2 0 + C u O + 2Sr(OH)2 , Sr(OH)2 + CuO-, H20 + SrCuO2 Sr (OH) 2 ''1-S r C u O 2

~

H 20 + Sr2 CuO3.

They report that XRD confirms the phases proposed in this scheme. Our results under both reducing (Ar, 1 atm) and oxidizing (O2, 1 atm) atmospheres, shown in Fig. 2, are in marked contrast to these conclusions. Under reducing conditions (top panel), two dehydration events are easily observed. The first event occurs at approximately 270°C, with a weight loss ( - 5.3%) corresponding to a single H20 moiety per formula unit of Sr2Cu (OH)6. Immediately after this

Fig. 1. ATOMSdrawingof the monoclinicunit cellof SrzCu(OH)6 emphasizing the Jahn-Teller distorted Cu(OH)~- octahedra. Spheres are Sr2+ ions.

J.F. Mitchell et al. /Physica C 227 (1994) 2 79-284 100.0

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first process, two additional water molecules are evolved at 400 ° C. The final product (weight percent = 83.6%), identified by powder XRD, is the normal "low pressure" orthorhombic form of Sr2CuO3. No phases other than this brown material can he detected in the diffraction pattern. The TGA in oxygen, shown in the bottom panel, is quite different. The first dehydration event, again occurring at 270 ° C, corresponds to a weight loss intermediate between one and two water moieties per formula unit of Sr2Cu (OH) 6 (two H 2 0 = - 10.6%), suggesting an oxygen uptake from the sample environment. At 400 °C the second event begins, yielding a final weight of 85.5%, approximately 1.4% above that of Sr2CuO3 (weight percent = 84.1%), again indicating excess oxygen in the material. Over the next 70°C essentially no further weight loss is observed. Finally, at the highest temperature (700 °C), after a third weight loss event at 510 ° C, a weight of 84.1%, consistent with formation of stoichiometric Sr2CuO3, is achieved; XRD indicates this final material to be the orthorhombic phase. Intrigued by the plateau beginning around 440 ° C, we heated a second sample to 440°C and held it at

this temperature for 12 h. The product was a uniform black powder with small dendritic crystallites with size up to 10 ~tm in length. EDS analysis resulting in Sr: Cu ~ 1.7:1 on all crystallites examined, indicated a Sr deficiency in the as-synthesized material. The powder XRD of this sample showed peaks consistent with those of the tetragonal, high-pressure form of Sr2CuO3+ x reported by Hiroi et al. [ 1 ]. We have succeeded in synthesizing large (5-6 g) samples of this material by heating Sr2Cu(OH)6 at 370°C for 12 h in flowing oxygen. As shown by the diffraction profile in Fig. 3, the sample is essentially phase pure, with the exception of a small amount of SrO that we have been unable to completely eliminate from our bulk samples, consistent with the EDS measurements that showed the main phase ta be Sr poor. Particularly noteworthy in the pattern is the presence of the weak satellite reflections (indicated by the *) of the 4ax//-2 × 4av/-} X c supercell noted by Hiroi et al. [ 1 ], indicating that our material is structurally the same as their superconducting sample. Though hindered by the presence of this modulation, we have refined the X-ray data using the tetragonal subcell proposed by Hiroi in space group I 4 / m m m (Rp=5.20, R,v=7.01), yielding a = 3 . 7 7 3 ( 3 ) A and c = 12.553 ( 1 ) ~,, very close to the lattice constants reported by Hiroi (a=3.764 A, c= 12.548 A). An extensive neutron diffraction study, including a model of the superstructure of this material, will be reported shortly [ I I ]. . . . .

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Sr2Cu(OH) 6. Superlatticereflections (°) and SrO impurity ( ) are marked. •

282

J.tq Mitchell et al. /Physica C 227 (1994) 279-284

3. ln-situ X-ray diffraction To better understand the thermal decomposition pathway of Sr2Cu(OH)6 in oxygen, we have collected XRD data in situ from 25°C to 550°C. As shown in Fig. 4, the three phases are easily identified from their powder patterns. The pelletized sample was heated at a rate of 2 ° C/rain from room temperature to 150°C and then at 0.2°C/min from 150°C to 550°C. Differences between the temperatures of the various events reported here and in the TGA studies are related to the different heating rates, the lower temperatures here consistent with the slower heating rate. Thus, we believe the in situ results are more representative of the events under equilibrium conditions. X-ray patterns were taken serially on an inhouse rotating anode diffractometer equipped with a high-temperature furnace, each scan covering approximately a 5 °C temperature window. The salient results of the experiment are shown in Fig. 5. Consistent with the TGA experiments, the first dehydration step occurs between 220°C and 250°C, converting SrECu(OH)6 to a mixed phase material. .........

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Fig. 5. Thermal decompositionof Sr2Cu(OH)6 monitored by insitu X-ray diffraction. Symbolsmark distinctive peaks in each phase and trace their evolution with temperature. Legend: Sr2Cu(OH)6 ("), Sr( OH )2 (A), tetragonal Sr2CuO~+~(0), orthorhombic Sr2CuO3 (*). The Pt line arises from the sample holder. The final panel (600~C) is from a separate experiment (see text ). The sharp lines in the 248 °C pattern correspond to Sr(OH)2, a decomposition product reported by lvanov-Emin [ 9 ]. However, there is clearly evidence of a substantial amount of amorphous material, presumably a copper-rich phase (s). No identification of this copper-rich material has been possible. At approximately 350°C, the low pressure tetragonal phase is clearly forming, as found evidence for by sharp diffraction lines in the pattern at 356 ° C. The tetragonal material continues to form with increasing temperature, and at 399°C a single-phase tetragonal Sr2CuO3+.~ material is obtained. Above 399°C, SrO becomes evident by its lines at 20~ 30 ° and 50 °. Further heating of the sample results in the tetragonalto-orthorhombic transformation, beginning around 450°C and complete by about 550°C. The final panel in Fig. 5 is that of a separate sample annealed in oxygen at 600°C for 12 h. Comparison of this pattern to the 545°C pattern demonstrates that the final product of the in-situ experiment is indeed the orthorhombic "low-pressure" phase of Sr2CuO3. Examination of the pellet after the experiment revealed a whitish area where the beam had been positioned on the sample. XRD of the undamaged area showed no SrO peaks, only orthorhombic Sr2CuO3. Furthermore, EDS analysis of a damaged region of the pellet

J.F. Mitchell et al. / Physica C 227 (1994) 279-284

showed areas containing only SrO. Similar analysis of undamaged areas revealed only orthorhombic material with a composition Sr:Cu=2.0: 1. Thus, it is clear that the SrO is formed by damage from the Xray beam. The in-situ experiment clearly demonstrates that the formation of the tetragonal phase occurs in a twostep process. Perhaps the intimate mixing of the Sr(OH)2 with the amorphous copper-rich phase facilitates the formation of the tetragonal compound. Such intimate contact between reactants and the associated improvement in reaction kinetics provides a distinct advantage to single-source precursors, such as Sr2Cu (OH)6, over traditional oxide precursors.

283

sized pellets, typically 1-10 Mf~ at room temperature, indicates a highly insulating material. Unfortunately, preliminary annealing experiments on Sr2CuO3+x synthesized from the hydroxometallate precursor have thus far failed to produce a superconductor. Another intriguing possibility is that metalion nonstoichiometry precludes superconductivity. Such a prospect is suggested by the Sr deficiency noted in the EDS analysis of our specimens and the persistence of SrO in the XRD of our tetragonal material. A subtle difference in the Sr sublattice might have only minor effects on the crystal structure yet dramatic consequences on the electronic properties.

5. Conclusion 4. Discussion

The decomposition of Sr2Cu(OH)6 to tetragonal Heating the tetragonal Sr2CuO 3+x above ~ 450 ° C converts it to the orthorhombic material. This transformation is irreversible in 1 atm of oxygen, an observation consistent with those of Lobo et al. [4 ], who found that moderately high oxygen pressure converts the orthorhombic phase to a nonsuperconducting tetragonal phase with a diffraction pattern identical to that of Fig. 3. This indicates that the "high-pressure" tetragonal form is stabilized with an excess of oxygen, consistent with our TGA results, and suggests that the tetragonal material may be formulated as Sr2CuO3 +x with x ~ 0.29. This is to be compared with the value x = 0.1 reported by Hiroi et al. [ 1 ]. A value of x,.~ 0.29 gives Cu a formal oxidation state of 2.58 (assuming stoichiometric Sr). This value is more plausible than the value of 3.8 implied by Lobo et al. [4]. Importantly, our experiments have shown that the tetragonal form of Sr2CuO3+x can be formed as a metastable phase in 1 arm of oxygen below 450°C, and that high pressure routes are not required to form this material. As measured by AC susceptibility, these materials are not superconducting. This is particularly interesting in light of the fact that our sample is apparently structurally the same as the Hiroi superconducting material. It is possible that the oxygen content of the present samples is too high, leading to an overdoped material. Such a possibility seems unlikely, however, as the overdoped cuprate superconductors are typically metallic, while the resistance of our as-synthe-

Sr2CuO3+ x ( x ~ 0.29 ) provides the first low-pressure

synthesis of the n = 1 member of the Srn+ ~CunO2n. t family. It is intriguing that this material, while sharing the same structure as the superconducting phase, is not superconducting and cannot be made so under routine annealing conditions. Our TGA experiments indicate that the compound may be slightly oxygenrich compared with Hiroi's superconducting material, but its high resistance challenges the conclusion that our compound is overdoped. Based on Rietveld refinement of neutron-diffraction data, Lobo et al. [ 4 ] suggest that their samples possess both Sr and Cu deficiency. From our XRD evidence of persistent SrO in our materials and EDS measurements indicating Sr: Cu < 2.0: l, the possibility arises that Sr deficiencies may play a role in preventing superconductivity in our tetragonal material. Finally, the low superconducting volume fractions typically found in superconducting samples of Sr2CuOa+x suggest that perhaps some other, as yet unidentified phase might be responsible for superconductivity. Importantly, whatever the superconducting phase, it contains only two cations: Sr and Cu. This fact alone reinforces the importance of studying these materials to determine what factors render two such structurally similar materials different in their superconducting behavior. Low-temperature, single-source precursors open a new avenue for synthesizing large quantities of these materials without the need for a high-pressure apparatus. Work is currently in progress on other hydrox-

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ZF. Mitchell et aL /Physica C 227 (1994) 279-284

ometallate precursors appropriate for the synthesis of the infinite layer and Srn+ ~CunOzn+~ families in an attempt to understand their chemistry a n d to u n a m biguously identify the superconducting phase in these classes of compounds.

Acknowledgements This research was supported in part by a appointment to the US D e p a r t m e n t of Energy Distinguished Postdoctoral Research Program sponsored by the US D e p a r t m e n t of Energy, Office of Science Education and Technical Information, and administered by the Oak Ridge Institute for Science and Education ( J F M ) , by the US D e p a r t m e n t of Energy, BES Materials Science u n d e r contract # W - 3 1 - 1 0 9 - E N G - 3 8 ( D G H ) , and by the National Science F o u n d a t i o n ( D M R 91-20000) through the Science and Technology Center for Superconductivity (JLW).

References [ 1] Z. Hiroi, M. Takano, M. Azuma and Y. Takeda, Nature (London) 364 (1993) 315. [2 ] M.T. Wellerand D.R. Lines,J. SolidState Chem. 82 ( 1989 ) 21. [ 3 ] C.L. Teskeand H. Miiller-Buschbaum,Z. Anorg.Allg.Chem. 372 (1969) 325. [4] R.C. Lobo, F.J. Berry and C. Greaves, J. Solid State Chem. 88 (1990) 513. [5 ] R. Scholder, R. Felstein and A. Apel, Z. Anorg. Allg.Chem. 216 (1933) 138. [ 6 ] T.N. Nadezhina, E.A. Pobedimskaya and N.V. Belov, Soy. Phys. Dokl. 25 (1980) 73. [ 7] E. Dubler, P. Korber and H.R. Oswald, Acta Crystallogr. B 29 (1973) 1929. [8]A.F. Wells, Structural Inorganic ChemistD' IOxlbrd University Press, Oxford, t 975 ). [9 ] B.N. lvanov-Emin,L.P. Petrishcheva, B.E. Zaitsev and A.S. izmailovich, Russ. J. Inorg. Chem. 29 (1984) 860. [10] X-ray Rietveld refinement was accomplished with the LHPM3 code of Hill and Howard. No attempt to model the supercell was attempted. [ 11 ] Y. Shimakawa, J.D. Jorgensen, Z. Hiroi, M. Takano, J.F. Mitchell and D.G. Hinks, manuscript in preparation.