J. MATER. CHEM., 1994, 4( 12), 1867-1870 Synthesis, Crystal Structure and Properties of [Sr2Cu(C,0,),(H20),]: Precursor of Sr2Cu0, Oxide M. Insausti: M.K.Urtiaga; R. Cortes: J.L. Mesa,a M.I. Arriortuab and T. Rojo*a a Departamento de Quimica Inorganica, Departamento de Mineralogia- Petrologia, lnstitufo de Sinfesis yEstudio de Materiales (ISEM), Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain [Sr2Cu(C,0,),(H20),] (C204*-=oxalate ion) has been synthesized and characterized. The structure has been determined by X-ray diffraction methods. It crystallizes in the triclinic system, space group P T with a =6.349(2)A, b =10.258(2)A, c =15.737(2)A, x =73.21 (l)", =93.66(2)", y =76.44(2)", V= 944.3(4)A3, Z =2. The structure consists of an intricate network of S?' and Cu2+ ions linked by oxalate and H20 groups.A nonacoordinated arrangement around the two strontium ions and a distorted octahedral geometry for the copper(ii) ions are observed. The thermal decomposition of this complex yields a unique and homogeneous phase at comparatively short reaction times and lower temperatures than the ceramic method. Conductivity measurements of the oxide obtained show semiconductor behaviour. As the existence of the copper oxide plane structure seems to play an important role in mechanisms of high-T, superconduc- tivity,' different studies on ternary copper-oxygen systems have been performed. Most of these investigations have been dedicated to phase equilibria and structural details. Among a large number of ternary compounds the oxides present in the MO-CuO binary phase diagram (M =Ca, Sr, Ba) are not only important for a further understanding of crystal chemistry of high-temperature superconductor-related materials but they also have the potential to become new superconductors if subjected to adequate carrier d~ping.~,~ Therefore S~,CUO,~ is an interesting compound because of its possible modification at high pressures to a superconductor with transition tempera- tures of 70 and 100 K.5 These oxides are traditionally prepared by the ceramic method which does not always yield single-phase products of the required stoichiometry.Alternative strategies which decrease the diffusion pathlengths can improve the final products. In this way, metallo-organic precursors hold much promise,6 as this method not only allows mixing of the different metal species on an aiomic scale but also reduces the diffusion distance to ca.10 A. Moreover, suitable precur- sors would result in significant modifications in the micro- structure of the final products. Carboxylates, which have long been known and studied as binding agents, have the disadvantage of incorporating very large amounts of carbon, which later have to be removed.',1° Furthermore, the high stability of some carbonates increases the temperature at which metal oxides are obtained. Accordingly, mixed oxalates may be considered to be among the best possible cuprate precursors. This paper shows the usefulness of the metallo-organic method for the clean and soft synthesis of pure Sr,CuO,.For this purpose, the complex with the formula Sr,Cu(C,O,),( H20), (C2042-=oxalate) has been prepared and structurally characterized. The decomposition of the compound has been extensively studied by TG and DSC. Taking these data into consideration, the complex was heated at different temperatures to obtain the respective oxide. Experimental Synthesis of CSr,Cu(CzO,),( H20)7] [Sr,Cu(C,O,),( H20),] was synthesized by slowly mixing an aqueous solution of 1 mmol (0.354 g) potassium bis(oxa1ate)- copper(rr) dihydrate, prepared as previously," with an aqueous solution of 2 mmol (0.534 g) strontium chloride hexahydrate, contained separately in a diffusion device at room temperature.After 2 days, blue crystals were obtained, which were filtered off and washed with ether. Powder X-ray diffraction data were obtained with a STOE diffractometer, equipped with a germanium monochromator, at temperatures of 293 & 1 K. Physical Measurements Infrared spectra (KBr pellets) were obtained with a Nicolet FTIR 740 spectrophotometer in the range 4000-400 cm-l. TG and DSC measurements were carried out with a Perkin- Elmer System-7 DSC-TGA unit. Crucibles containing 20 mg of sample were heated at 2 'C min-' under dry air. A Bruker ESP 300 spectrometer, operating at X and Q bands, was used to record the EPR powder spectra. The temperature was stabilized by an Oxford Instruments (ITC4) regulator. The magnetic field was measured with a Bruker BKM 200 gaussmeter, and the frequency was determined with an HP5352B-microwave frequency counter.Scanning clectron microscopic (SEM) observations were also carried out to give some indication of the compactness of the oxide using a JEOL JSM-6400. The conductivity was measured using the four- point method', and was calculated by measuring the intensity/ voltage ratio between the points in both directions of the current, in order to minimize the asymmetry effect between the contacts. X-Ray Crystal Structure Determination Preliminary oscillation and Weissenberg photographs for the compound showed triclinic symmetry. A well formed prismatic crystal with dimensions 0.8 mm x 0.4 mm x 0.2 min was mounted on a glass fibre and transferred to an Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Mo-Ka radiation.Final unit-cell dimensions, calculated from a least-squares treatment of 25 accurately centred reflections (8"<8< 12") are given in Table 1, with other crystal data. 5490 unique reflections were collected using o scans, of which 3753 were considered observed [I> 3a(l)]. These were cor- rected for Lorentz and polarization effects. Scattering factors were taken from the International Tables for X-ray Crysta110graphy.l~ The structure was solved using the automatic Patterson interpretation routine in SHELXS-86.14 A Fourier map, calculated with phases based on these ions, revealed the atoms of the complexes. Several cycles of anisotropic refinement were carried out.On application of 1868 Table 1 Data collection and structure refinement of [Sr2Cu(C204)3(H@), 1 compound formula Sr,CuC,H 14019 molecular weight crystal system sp!ce group a14 bl+ c/A a/degrees Pldegrees ))/degrees cell volume/A3 Z 646.78 t riclinic 6.349( 2) 10.258( 2) 15.737( 2) 73.21(1) 93.66( 2) 76.44( 2) 944.3(4) 2 pi Dobslg cm -Dcalck cm -p (Mo-Kx)/cm-F(000) 2.22(4) 2.27 59.6 554 measurements: i (Mo-Ka) 6 rangeldegrees no. of measured reflections 0.71069 5490 1-30 interval h, k, 1 f8,+ 14,+22 refinements: no. of variables 265 selection criterion I>30(I) no. of unique reflections 3753 weighting scheme: W= 1.0/[0~1F~(+p(F~)~]p= R=(CI IFol-IFcl I)/(ClFol) Rw= CWlFol-I~c/)2/~wI~01211'2 0.013325 0.055 0.060 a weighting scheme, the model converged at R=0.055, R, =0.060.The geometric calculations were performed with PARST15 and BONDLA,16 and the molecular illustrations were drawn with SCHAKAL.17 Further details have been deposited with the Cambridge Crystallographic Data Centre. Results and Discussion Crystal Structure The structure can be described as a complex three-dimensional network of Sr2+ and Cu2+ ions coordinated by oxalate and water molecules, indicating the non-existence of discrete mol- ecules. The asymmetric unit is formed by two strontium atoms, two copper atoms situated in special positions (occu- pation factor OS), three oxalate ligands and seven water molecules. A perspective drawing of [Sr,Cu(C,O,), (H20),] as well as the labelling of the atoms is shown in Fig.1. Atomic coordinates and some selected bond distances and angles are given in Tables 2 and 3, respectively. The coordination polyhedra around the Cu" ions can be described as distorted octahedra, with four coplanar bonds, [0(1)-O(2)-O( 1 p-0(2)iv] for Cu(1) and [0(5)-O(6)-O( 5)"'-0( 6)"'] for Cu(2), of nearly equivalent IengJh (ca. 2 A), and two longer tetragonal bonds around 2.5 A (Table 3). The low octahedral distortions around the copper ion, A=O.Ol and A=0.02 for Cu(1) and Cu(2), respectively, were calculated by quantification of the Muetterties and Guggenberger description." The strontium ions are nonacoordinated, and the coordi- nation polyhedra for both ions were analysed using the Cavas and Kepert l9 description, considering the monocapped square antiprism and the tricapped trigonal prism configurations as ideal geometries.The topology for the Sr(1) atom is similar to a monocapped square antiprism, with the atoms 0(3), 0(7), O(11) and oO(10)' [the maximum distance from this plane is 0.203(6)A for O(ll)] and Ow(l), 0(4), O(7)" and J. MATER. CHEM... 1994, VOL. 4 Fig. 1 View of the [S~,CU(C,O,)~(H~O),] molecule with atom numering [i=l+x, y, z; ii=-x, -y, 1-z; iii=l-u, -l-y, 1-z; iv= -9, 1-y, z] (SCHAKAL 88, Ketler, 1988) Table 2 Atomic coordinates (x 104)LSr and Cu x ( 1u5)] and equival- ent isotropic temperature factors (/A2) for [Sr2Cu(C204)3( H20),] atom X Y Z Be, 1346(7) 7780(5) 35040( 3) 1.98(1) 30596(8) -26746( 5) 26373( 4) 2.84( 2) 0 50000 0 6.98( 6) 50000 -50000 50000 3.06( 3) -1166( 13) 3361(7) 447( 4) 6.5(2) 1464( 12) 4469(6) 1217(4) 5.8( 2) -531(15) 2762(7) 1282(5) 4.1(2) 987( 12) 3404( 7) 1725r 5) 3.5(2) -1089(11) 1736( 6) 1750(4) 5.0(2) 1576(8) 2926( 5) 25361 3) 3.5( 1) 3079(8) -3116(4) 44961 3) 2.6( 1) 4346(7) -4854(4) 6155(3) 2.5(1) 2434(9) -2614( 5) 5107(4) 2.3(1) 3152(9) -3629( 5) 6076( 4) 2.2(1) 1310(7) -1390( 4) 5003(3) 2.4(1) 2536(8) -3240( 4) 6714( 3) 3.0( 1) -2997( 7) -2483( 5) 2732(4) 4.1(1) -6222( 6) -558(4) 3102(3) 3.2( 1) -.4308(9) -457(6) 3212(4) 2.6( 1) -2487( 9) -1506( 6) 2934( 4) 2.3( 1) -3688( 7) 390( 6) 3538(4) 5.4(2) -623 (6) -1288(4) 2940( 3) 2.7( 1) 3162(8) 1386(5) 4448( 3) 3.8( 1) 770(8) -4531(5) 30701 4) 4.9( 2) 4788( 8) -4564( 5) 1830( 4) 3.9(1 ) 3351( 17) -760( 10) 1155(6) 9.0(4) 198( 14) -2228 (9) 1114( 6) 7.9(3)1835( 11) -6042( 7) 4864(6) 6.8( 2) 3002( 18) 3435( 10) -470( 7) 10.0(4) ~ Be,=8/3 7t2 CiCj Uijai* aj* aiaj.O(8)" forming parallel planes (Fig. 2). The environment of the Sr(2) atom deviates greatly from the ideal geometries, so it cannot be described as a characteristic polyhedron. The O(12) and O(l0)' atoms act as bridges between the Sr(1) and Sr(2) atoms, forming a qutsiplanar ring, with a maximum deviation of 0.074(5)A for O(10)' J. MATER. CHEM., 1994, VOL. 4 Table 3 Some selected bond distances (/A)and angles (/degrees) with esds in parentheses for [Sr,Cu(C,O,),( H20)7] Sr( 1)-O( 12) Sr( 1)-O( lo)' Sr(1)-0(3) Sr( 1 )-0(4) Sr(1)-O(7) Sr( 1)-O( 11 ) Sr( 1)-Ow( 1) Sr( 1)-O(7)" Sr(1)-O( 8)" CU( 1)-O( 1) Cu( 1)-O(2) CU( 1)-0~(7) 2.650(5) 2.613(4) 2.644( 5) 2.683(5) 2.652(4) 2.551(5) 2.650( 6) 2.759( 5) 2.605(4) 1.958(8) 1.940( 6) 2.487(11) Sr(2)-O(12) Sr(2)-O(10)' Sr( 2)-Ow( 2) Sr(2)-0w(3) Sr(2)-0w(4) Sr(2)-0w(5) Sr( 2)-O( 9)' Sr( 2)-O(5) Sr(2)-O( 6),,' Cu(2)-0(5) CU(2)-O( 6) Cu(2)-0~(6) 2.587(4) 2.607( 5) 2.623 (6) 2.678 (6) 2.629(8j 2.770( 9) 2.555( 5) 2.827( 5) 2.759( 3) 1.933(4) 1.930(5) 2.51 2( 8) O(12)-Sr( 1)-O( lo? 71.4( 1) O(12)-Sr(2)-0( 10)' 723 1) O(I)-C( 1 I-0(3) O(10)-C( 5)-O( 11) O(5)-C( 3 f-O(7) O(1j-Cu( 1)-0(2) 125.0(9) 126.5(6) 126.5(5) 85.3(3) 0(2)-C(2)-0(4) 0(9)-C(6)-0( 12) O(6)-C( 4)-O( 8) 0(5)-C~(2)-0(6) 124.1(8) 125.5(6) 124.1 (5) 86.5(2) symmetry code: i=l+x, y, z; ii=-x, -y, 1-z; iii=l-x, -l-y, 1 -z.0 (12 1 n 0(1( 0 (41 Fig. 2 A ball and stick view of the coordination polyhedron of Sr( 1) in CSr2Cu(C,O'l), (H20171 (see Fig. 1). The ring formed by the atoms Sr( l)-Sr(2)-0( 5)-C(3)-0(7)-Sr( 1)is also quibte planar, with a maximum deviation from plane of 0.144( 5) A for the O(7) atom. The standard notation for bridging carboxylates has been used. According to the description presented by Porai- Koshits," the classification for the oxalates is the following: a-2-a for C(l),C(2) and C(4), s-3-sa for C(3), s-2-a for C(5) and sa-3-a for C( 6). Thermal Treatment The decomposition of the starting material was studied by thermogravimetry in the temperature range 30-850 "C (Fig.3). The data show the occurrence of three consecutive steps: dehydration (40-230 "C), ligand pyrolysis (280-500 "C) and inorganic residue evolution (520-850 "C).The loss of the molecules of water is registered in three endothermic steps with the ratio 2 :2 :3 and is complete at 230 "C. Dehydration supposes a lowering of the coordination number of the alkaline-earth-metal ions, which probably results in a rearrangement of the ligands between the metallic centres. The dehydration enthalpy, calculated from the calorimetric measurements, gives a value of 54 kJ mol-', in good agree- ment with related compounds.' Decomposition of the anhydrous compounds follows immediately after dehydration in the temperature range 280-500 'C, with a total weight loss of 23.2%.In this process 1 1 1 100 300 500 700 TIT Fig. 3 TG(-) and TGD (-.-) curves of [Sr,Cu(C,O,),( H20),] alkaline oxalates and CuO are formed, according to the weight losses and the results appearing for other mixed oxalates.21 In the control of the pyrolysis results, the stability of the metal carbonates may be one of the main factors which induces the formation of mixed oxides. The following reaction schemes are the most reasonable to describe the decompo- sition process. MM'(C,O,)(s)+MCO,(s)+ M'CO,(s) +R(g) (1) MM'(C,O,)(s)+MO(s)+ M'CO,(s)+ R(g) (2) MM'(C,O~)(S)+MO(S)+M'O(s)+R(g) (3) Depending on the stability of the metal Carbonates, reac- tions (l),(2) or (3) can occur.In our case, as alkaline-earth-metal carbonates are stable, the Iigand pyrolysis can be described by reaction (2), which is in good agreement with the theoretical weight losses accompanying the degradation and with X-ray diffraction data. A stable stage is observed between 520 and 790 "C, where strontium carbonate and CuO are formed. At higher temperatures the weight losses continue and the phase Sr,CuO, is formed, as observed bq X-ray diffraction. Nevertheless, SrCO, and CuO phases itre still present. Other experiments performed in a nitrogen atmos- phere yielded similar results. Taking into account this result, different thermal tre, <itments in tubular furnaces were performed in order to obtain the mixed oxide as a unique phase at relatively low temperatures.In this way, the complex [Sr,Cu(C,O,),( H,0),] was fired first at 500°C to remove the organic part then treated at 800°C for 24 h. X-Ray diffraction of the final product was performed and indexation was performed by FULLPROF (pattern-matching analysis)22 in the range 28 =10-70". The diffraction pattern showed the existence of a unique phase of S~,CUO,,~~ which was indexed onothe basis of an >ortho- rhombic cel) Imrnrn, a= 12.704(4) A, b =3.912(2) ,I and c =3.499(2) A. This structure contains planar CuO, squares, which share corners to form single chains, so it has one-dimensional 180"Cu-0-Cu interactions. According to the literat~re,~~calcination at 850 "C with intermittent grinding for 8 days was required to obtain the Sr,Cu03 phase using the ceramic method.Considering the above results, it can be concluded that better results for isolating the mixed oxide have been obtained from the metallo-organic method. In order to detect the presence of carbonates in this final oxide, IR experiments were performed. The Sr,CuO com-pound shows very weak bands at 1445 and 855 cm-l. corre- sponding to impurities of SrCO,, in good agreement nith the elemental analysis (C, 0.4%). The absence of bands at lower frequencies implies that the carbonate ion is not coordinated in the structure, as it is in related corn pound^.^^ EPR experiments on powder samples of the mixed oxide were performed and no signals were observed in the X-band region of the freshly prepared compound, in accord with a high antiferromagnetic ordering.26 Nevertheless, EPR signals appear when the samples are left in air.27 A similar result was observed for MCu02 oxides (M =Ca, Sr, Ba).28 Measurements of electrical resistance from room tempera- ture to 120 K indicate semiconducting behaviour, displaying a large temperature dependence (the resistivity at 125 K is ca.lo6 cm and at room temperature it is lo3 fi cm) (Fig. 4). The activation energy calculated for this range is 0.15 eV. Below 120 K the conductivity becomes so weak that accurate data could not be obtained using our equipment. These data are in accord with those in literat~re.’~ Scanning electron microscopy was also performed for the Sr2Cu03 sample.SEM examinations of portions of the com- pound using backscatter imaging revealed a homogeneous material with no additional phases. SEM photographs of secondary electrons show polycrystalline aggregates formed by small prismatic particles (Fig. 5), the size of which was around 1 pm. Concluding Remarks A new oxalate compound [Sr,Cu(C,O,)(H,O),] has been synthesized and structurally characterized by X-ray diffraction methods. The structure can be described as a complex three- dimensional network of Sr2 + (nonacoordinated) and Cu2+ (hexacoordinated ) ions surrounded by oxalate and water 120 165 210 255 300 T/K Fig. 4 Variation of resistance with temperature for Sr,CuO, -10l.lm Fig. 5 SEM photograph of Sr,CuO, from [Sr2C~(C204)3(H20)7] J.MATER. CHEM., 1994, VOL. 4 molecules, indicating the non-existence of discrete molecules. 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Yamauchi, M. Motokawa, M. Azuma and M. Takano, J. Phys. Soc. Jpn., 1992,61,3370. 28 T. Rojo, M. Insausti, L. Lezama. R. Cortes and M. I. Arriortua, Solid State Commun., submitted. 29 R. C. Lobo, F. J. Berry and C. Greaves, J. Solid Stare Chem., 1990, 88, 513. Paper 4/03738C; Receiced 20th June, 1994