J O U R N A L O F C H E M I S T R Y Materials Communication The synthesis and structure of Tl(Sr1.4La2.6)Ni2O9; a direct structural analogue of a superconducting cuprate Christopher S. Knee and Mark T.Weller* Department of Chemistry, The University of Southampton, Southampton, UK SO17 1BJ. Email: mtw@soton.ac.uk Received 21st October 1998 The nickelate Tl(Sr1.4La2.6)Ni2O9, synthesised by the reac- larger than expected value and this atom was therefore placed on a new 4-fold site (x,y,0) at quarter occupancy.This tion of Tl2O3, La2 O3 and Sr2Ni2O5, is isostructural with the equivalent superconducting cuprate phase, consisting displacement refined steadily with x=y#0.052, and a concomitant reduction in the thermal parameter to a reasonable of layers of apex-sharing, stoichiometric NiO6 octahedra separated by a TlO layer.value. Attention was then focused on the mixed Sr/La sites, in particular the (0,0,0.2) site, which had an unreasonably low temperature factor. The fractional occupancies of the La and The structural chemistry of complex nickelates has recently Sr sites were reciprocally linked and allowed to vary from the regained impetus from its relationship to that of cuprates initial 50550 ratio.The site was found to favour a marked exhibiting high temperature superconductivity. One area of increase in the La content to around 0.8 and the thermal particular interest has been complex nickel oxides adopting parameter now refined to a larger, more sensible, value. The the perovskite and K2NiF4 structures e.g. La2-xSrxMO4±d occupancy of the other Sr/La site (0,0,0.08) was probed in (M=Ni, Cu);1,2 superconductivity has been claimed in the this way however the variation was negligible and the site nickelate but this has never been fully substantiated.2 A few occupancy factors maintained at 0.5/0.5.The final refined other phases exist with structural analogies in copper and structural parameters are summarised in Table 1 and derived nickel chemistry for example Li2MO23,4 and ‘BaMO2’.5,6 These bond lengths given in Table 2.The final fit to the profile is structural analogues derive from the ability of both copper(II ) shown in Fig. 1, and Fig. 2 shows the structure of and nickel(II) to adopt four- (square planar), five- (square Tl(Sr2La2)Ni2O9. pyramidal ) and six-fold (octahedron based) coordinations to The refined stoichiometry of Tl(Sr1.4La2.6)Ni2O9 gives a oxygen though the five- and six-fold geometries are normally nickel oxidation state of +2.2 with full occupancy of the more distorted for copper due to the Jahn–Teller eVect. More oxygen sites; no evidence was found in this work of site recently the synthesis of TlSr2NiO4+d7,8 has further demondeficiencies and all refined oxygen atom temperature factors strated the ability of nickelates to adopt similar structures were reasonable.The nickel oxidation state is also dependent to cuprates, though in this case the oxygen stoichiometries on the La5Sr ratio and the slight strontium deficiency with and distributions diVer and no complete NiO2 layers exist. respect to the starting stoichiometry is consistent with the In this communication we report the synthesis and characterobservation of the small level of Tl2Sr4O7 impurity.A higher isation of a new layered complex nickel oxide, Tl(Sr2La2)- level of lanthanum on the A-sites is also observed in the Ni2O9, containing complete NiO2 layers, which is a direct reported isostructural cuprate, Tl(Ba1.6La2.4)Cu2O9.9 The analogue of a high temperature superconducting phase, eVects of diVerent starting ratios of Tl, La and Sr are currently Tl(Ba2-xLa2+x)Cu2O9, Tc=35 K.9 Tl2O3, La2 O3, and Sr2Ni2O5, synthesised following the literature method,10 were mixed in the ratio 15151 and ground Table 1 Final refined atomic coordinates for Tl(Sr1.4La2.6)Ni2O9 thoroughly. The reactants were pressed into 13 mm diameter (e.s.d.s are given in parentheses) pellets under ca. 10 tonne cm-2 and the pellets so formed sealed inside a gold tube. The capsule was then slowly ramped Atom Sitex y z B/A° 2 n to 900 °C and fired for 5 h and then allowed to furnace cool. Tl 8h 0.0517(2) 0.0517(2) 0.0 1.50(18) 0.25 The resulting black powder was examined using a Siemens La(1) 4e 0.5 0.5 0.0845(1) 1.82(9) 0.5 D5000 diVractometer (CuKa1 radiation) and the pattern Sr(1) 4e 0.5 0.5 0.0845(1) 1.82(9) 0.5 obtained was indexed on a tetragonal unit cell with a#3.8, La(2) 4e 0.5 0.5 0.2032(1) 1.19(7) 0.8 c#30.0 A° .Weak lines (I/I0<5%), that did not index using Sr(2) 4e 0.5 0.5 0.2032(1) 1.19(7) 0.2 this method, were identified as resulting from the impurity Ni 4e 0.0 0.0 0.1466(3) 1.20(13) 1.0 phase Tl2Sr4O7.O(1) 2b 0.5 0.5 0.0 4.1(1) 1.0 The structure of Tl(Sr2La2)Ni2O9 was refined from X-ray O(2) 4e 0.0 0.0 0.0721(8) 3.0(8) 1.0 powder diVraction data using the GSAS package.11 Data were O(3) 8g 0.5 0.0 0.1439(6) 2.3(4) 1.0 O(4) 4e 0.0 0.0 0.2178(7) 0.5(6) 1.0 collected in the 2h range 17–117° over a 17 h period with a step size of 0.02°. The starting structural model was taken Space group I4/mmm; a=3.8062(1), c=30.0854(6) A° ; RF=8.50%, from that of Tl(Ba2-xLa2+x)Cu2O9,9 in the space group x2=1.76. I4/mmm.Initial stages of the refinement placed all atoms on these special sites at full occupancy and proceeded with the variation of global parameters such as background and peak Table 2 Selected derived bond lengths profile coeYcients and the lattice constants.The contribution Tl–O(1) 2.970(9) La(2)–O(3)×4 2.610(1) from the Tl2Sr4O7 impurity was introduced as an additional Tl–O(1)×2 2.706(1) La(2)–O(4)×5 2.727(4) phase using literature data,12 and fitted with the refinement of Tl–O(1) 2.413(9) La(2)–O(4)×1 2.377(8) cell constants and phase fraction only. The isotropic tempera- Tl–O(2)×2 2.188(3) Ni–O(3)×4 1.905(1) ture factors of the atoms in the main phase were then La(1)–O(1) 2.543(4) Ni–O(2) 2.24(3) introduced, and, along with the refinement of atomic positions, La(1)–O(2)×4 2.717(4) Ni–O(4) 2.14(2) resulted in the expected improvement in the least-squares fit.La(1)–O(3)×4 2.610(2) The temperature factor for the thallium atom refined to a J. Mater. Chem., 1998, 8, 2585–2586 2585with an in-plane distance of 1.905(1) A° and two apical bonds of 2.24(3) and 2.14(2) A° .These units are linked by sharing equatorial apices to form infinite layers in the ab-plane of stoichiometry NiO2. There is no evidence of oxygen vacancies in these planes, in contrast to the structurally related 1201 phase TlSr2NiO4+d7,8 in which the asymmetric distribution of oxide vacancies leads to an orthorhombic structure.A TlO layer separates the NiO6 octahedra. The thallium coordination is complicated by the disorder of the thallium atom, which is a common feature of such single layer thallium materials.13 The average thallium environment may be most simply viewed as distorted octahedral. The Sr/La sites exhibit 9-fold coordination and the bond lengths are similar to those found in the (La,Sr)2NiO4 phases with the K2NiF4 type structure.Comparison of the nickel and copper coordination in the materials Tl(Sr1.4La2.6)Ni2O9 and Tl(Ba1.6La2.4)Cu2O9 reveal Fig. 1 Final profile fit for Tl(Sr1.4La2.6)Ni2O9. Crosses mark observed near identical M–O in-plane distances of 1.905(1) and intensities, upper continuous line the calculated profile, lower continu- 1.906(1) A° respectively.The apical metal–oxygen environment ous line the diVerence. Reflections are shown with tick marks for if more distorted for the cuprate, with two quite diVerent Tl(Sr1.4La2.6)Ni2O9 ( lower) and Tl2Sr4O7 (upper). apical distances of 2.63(3) and 2.25(4) A° compared to the more regular 2.24(3) and 2.14(2) A° observed for the nickelate, which may be attributed to Jahn–Teller eVects.The extension of the series of compounds Tl(Sr2-xLn2+x)Ni2O9, to Ln=Nd–Gd, is being undertaken. The materials show the expected reduction in lattice parameters, i.e. for Tl(Sr2Gd2)Ni2O9, a=3.7681(1), c= 29.4012(9) A° . In conclusion the end member of a new family of layered nickel oxides Tl(Sr1.4Ln2.6)Ni2O9, Ln=La has been synthesised and characterised using powder X-ray diVraction. The electronic and magnetic properties of this material and the structural characterisation of the full range of lanthanide derivatives will be reported in a full paper.We thank the EPSRC for a studentship for C.S.K. Notes and references 1 M. James and J. P. Attfield, J. Mater. Chem., 1996, 6, 57. 2 Z. Kakol, J. Spalek and J. M.Honig, J. Solid State Chem., 1989, 79, 288. 3 H. Rieck and R. Hoppe, Z. Anorg. Allg. Chem., 1972, 392, 193. 4 W. Losert and R. Hoppe, Z. Anorg. Allg. Chem., 1970, 379, 234. 5 M. A. G. Aranda and J. P. Attfield, Angew. Chem., Int. Ed. Engl., 1993, 32, 1454. 6 R. Gottscall and R. Scollhorn, Solid State Ionics, 1993, 59, 93. 7 C. S. Knee and M. T.Weller, J. Mater. Chem., 1996, 6, 1449. 8 C. S. Knee and M. T. Weller, The structure of TlSr2NiO4+d by high-resolution powder neutron diVraction, J. Solid State Chem., submitted. 9 C. Martin, A. Maignan, M. Huve, M. Hervieu, C. Michel and B. Raveau, Physica C, 1991, 179, 1. 10 Y. Takeda, T. Hashino, H. Miyamoto, F. Kanamaru, S. Kume and M. Koizumi, J. Inorg. Nucl. Chem. Lett., 1972, 34, 1599. 11 A. C. Larson and R. B. Von Dreele, MS-H805, Los Alamos Fig. 2 Structure of Tl(Sr2La2)Ni2O9. The nickel coordination is shown National Laboratory, Los Alamos, NM, 87545. as octahedra, thallium ions are shown as small dark spheres, Sr/La 12 R. von Schenck and H. Mueller-Buschbaum, Z. Anorg. Allg. ions as large dark spheres and oxygen as medium light spheres. Chem., 1973, 396, 113. 13 Morosin, E. L. Venturini and D. S. Ginley, Physica C, 1991, being probed but absolute phase purity is diYcult to achieve 183, 90. due to loss of thallium from the reactant mixture. The nickel coordination is an elongated NiO6 octahedron Communication 8/08182D 2586 J. Mater. Chem., 1998, 8, 2585–2586