J. MATER. CHEM., 1994, 4(11), 1763-1764 MATERIALS CHEMISTRY COMMUNICATIONS Preliminary Crystal Structure of Mixed-valency Sr4Ni309, the Actual Formula of the so-called Sr5Ni401 Francis Abraham,* Sylvie Minaud and Catherine Renard Laboratoire de Cristallochimie et Physicochimie du Solide, URA CNRS 0452, ENSCL, Universite des Sciences et Technologies de Me, B.P. 708, 59652 Villeneuve d'Ascq Cedex, France A crystal structure investigation of the so-called Sr,Ni,O,, from single-crystal X-ray data has shown that the composition of this oxide is in fact close to Sr,Ni,O,. The structure has been solved in the trigonal P321 space group. The final refinement gave an R factor of 0.045 for 51 2 independent reflections. The structure contains NiO, chains with two NiO, octahedra and one NiOG trigonal prism alternating and sharing faces. The chains run along the three-fold axis and are connected by Sr ions.The Ni-0 distances seem to indicate a possible repartition of Ni'" in the octahedral sites and Ni" in the trigonal prisms. In the Sr-Ni-0 system two compounds have been charac- terized for Sr :Ni = 1. SrNiO,, which contains tetravalent nickel, has been synthesized under a high pressure of oxygen (50-2000 atm, 600°C) from Sr(OH),-8H20 and NiO. It adopts a pervoskite-like stzucture with towo-layer hexagonal close-packing [a =5.355( 1)A, c =4.86( 1)A]. Sr,Ni,O,, which contains trivalent nickel, has been obtained above 600 "C and at 1 atm of wet oxygen gas flow;' the >exagonal unit-cell parameters [a = 5.465(1)A, c =4.137(1)A at 600 "C; 1atm) vary with the synthesis conditions. In the course of a study of the Ba, -xSrxNi03-y system, Marcizak and oKatz2 foun$, for x= 1, a hexagonal nine-layer phase (a =5.47 A, c =19.734)accomp?nied by a 'compressed' two-layer phase (a =5.47 A, c=4.05 A) close to Sr,Ni,O,. In fact, Sr,Ni,O, is unstable and different oxygen-deficient compounds are obtained depending on the conditions.Recently, Lee and Holland3 reported the preparation of a new strontium nickel oxide. From microprobe analysis and thermal gravimetric studies under a reducing atmosphere, they deduced an idealized empirical formula Sr,Ni,O which corresponds to trivalent nickel. Tbe hexagonal unit-cell~ parameters [u=9.480(1) A=5.4743 A; c=7.815(4) A] do not seem to correspond to a close-packing layer.Recently, James and Attfield, also obtained Sr,Ni,O,, as an impurity during the investigation of the Yb,-xSr, NiO,-, system. The behaviour of Ni"' in the oxide correlated to its atomic environment and to the linkage of the nickel polyhedra which governs the interactions between metal centres. Evidently, a knowledge of the crystal structures and the relationship between them is of interest. To date no crystal structure of these strontium nickel oxides has been published. This paper deals with a preliminary investigation of the crystal structure of Sr,Ni,O,, which allows us to question the real composition of this compound and the oxidation state of Ni. A single crystal of Sr,Ni401, was obtained following the method of Lee and H01land.~ The X-ray powder spectra of the crushed single crystals obtained is in accord with that of Sr,Ni,O,, (JCPDS No.42:521). The unit-cell parameters were refined to a =9.477( 1) A, c =7.826(4) A. A single crystal was mounted with the greatest dimension of the needle as the rotation axis. Preliminary rotation and Weissenberg photographs indicated 3ml or 31m Laiie symmetry. The intensity data were collected with a Philips PW1100 automated diffractometer (Mo-Ka radiation, ,I= 0.7107 A, 0.~26' scans). A total of 3774 measured reflections (0 range 2-30", -13<1.1<13, -13<k<13,0<1<10) yielded 512 independent reflections with I >341) used in the structure determination (merging R factor =0.055 on I).From extinc- tion conditions, the possible space groups are P3rn1, P31m, P31m, Phi, P321 and P312. Analysis of our single crystals by EDS [Sr, 56.6(5); Ni, 43.2(5); K, 0.2(2)%] gave an Sr :Ni ratio >5 :4 and ~ose -1 to 5.25 :4 or 4 :3. Then the weight loss for the reduction process (obs. loss, 11.8%) suggests the idealized formula Sr,Ni,O, (calc. loss, 11.9%); this formula was confirmed by a successful structural determination. The density measured from a few single crystals [p =5.4(1)g cm-,] is in fair agreemcnt with the theoretical density (5.49 g cmP3) assuming three formulae units in the unit cell. Then absorption corrections were applied using the analytical method of De Meulenaer and rompa5 (p=343 cm-l, transmission factor range 0.020-0.064 1.The structure was successfully solved in the space group P321. The positional parameters for the strontium and nickel atoms were determined from the SHELXS program, with the oxygen atoms being found from a difference Fourier map. Refinement was carried out by the full-matrix least-squares method. In a preliminary stage the positional parameters and isotropic factors were refined. Then a new difference Fourier synthesis showed maxima at the vertices of triangles centred on Ni(4) and Ni(5); these atoms were delocalized onto the centres [Ni(4) and Ni(5) sites in Table 11 and the apices [Ni(4)' and Ni(5)' sites in Table 11 of the triangles. Owing to the high correlations between occupancy and temperature Table 1 Atomic coordinates for Sr,Ni,O, atom site x Y Z B/A2 occupancy 0.0233( 3) 0.3276(4) 0.6918(3) 0 0.2476( 3) 112 0.63 (4) 0.97(5) 1 1 0.3603( 3) 113 0 213 0 0.1086(9) 0.77 (6) 0.93(9) 1 1 113 213 0.4217( 6) 0.58(9) 1 0 0 0.3383( 7) 0.36(8) 1 213 113 0.237( 1) 0.3(2) 0.58 0 0 0 1.5(3) 0.655 0.610(2) 0.924( 4) 0.819( 2) 0.1 58 (3) 0.172( 3) 0.671 (2) 0.846( 3) 0.273(2) 0 O.SOO( 2) 0.007 (2) 0.519(3) 0.177( 2) 0 0.241 (2) 0 0.038(2) 0.190( 3) 0.263(2) 0.445( 2) 112 0.6( 3) 1.4( 7) 0.3(2) 1.8(3) 1.0( 3) 1.8(3) 0.6(4) 0.14 0.115 1 1 1 1 1 J.MATER. CHEM., 1994, VOL. 4 The structure of Sr,Ni,09 consists of NiO, chains running along the three-fold axis and linked together by Sr-0 bonds \ a Fig.1 Projection of the Sr,Ni,O, structure along the [OOl] direction +-NiIr tNiJv Fig. 2 The NiO, chain in Sr,Ni,O, Tableo2 Nickel-oxygen distances in A (esds are between 0.01 and 0.02 A) Ni(l)O(l)(3x) 1.90 Ni(4) 0(1)(3x)2.17 Ni(4)’ 0(1)(1x) 2.06 0(3)(3x) 1.90 0(4)(3x) 2.22 O(1)(1 x) 2.08 O(1)( 1 x) 2.61 Ni(2)0(3)(3x) 1.92 Ni(5) 0(2)(6x) 2.09 0(4)( 1 x) 2.06 0(4)(3x) 1.83 0(4)( 1 x) 2.08 Ni(5)’ 0(2)(2x) 1.94 0(4)( 1 x) 2.60 Ni(3) 0(2)(3 x) 1.87 0(2)(2x) 1.97 0(5)(3x) 1.93 0(2)(2x) 2.64 factors, simultaneous refinement was difficult and occupancy rates were varied step by step to minimize reliability factors. Finally, in the last cycles, the refinement of anisotropic displacement parameters for strontium and non-delocalized nickel atoms led to R=0.045 and Ro=0.045 with o=l for all reflections.The better results are reported in Table 1. The principal interatomic distances are given in Table 2. The structure determination confirmed a chemical formula close to Sr4Ni,09, which corresponds to a mean oxidation state of nickel equal to 3.33. Iodometric titration, using the method described by James and Attfield,4 gave, assuming the formula Sr4Ni,0g, an experimental average oxidation state of 3.28. (Fig. 1). The Ni03 chains are built from alternating face- sharing Ni06 octahedra and NiOs trigonal prisms with the sequence two octahedra-one trigonal prism (Fig. 2). The Ni06 octahedra are quit? regular with Ni-0 distances rangiag from 1.83 to 1.93 A; the mean values [1.90, 1.88 and 1.90A for Ni(l), Ni(2) and Ni(3)] are in agreement with Ni4+ located in these sites.6 Within the trigonal prisms, the actual location of the nickel atoms is still doubtful.It is unusual for nickel atoms to occupy an oxygen trigonal prismatic site; moreover, the distances between the centre of the prism and the six vertices are too long, even for the Ni2+ cation. The nickel atoms are certainly displaced from the centre towards one of the rectangular faces of the trigonal prism, giving rise to a pseudo-planar coordi!ation with four Ni-0 distances being short [average: 2.07 A for Ni(4)’ and 1.96 A for Ni(5)’] and the remain@ two Ni-0 distances considerably longer C2.60 and 2.64 A for Ni(4)’ and Ni( 5)’].The electron density at the centre of the trigonal prism would result from the overlapping of thermal vibration ellipsoids elongated perpendicularly to the pseudo-square. The Ni- 0 distances and the chemical formula are in favour of Ni2+ in these sites. However, the combination of Ni4+ and square Ni2+ would be diamagnetic, yet the compound is paramag-neti~.~in view of the disorder in the structure and the large errors on the Ni-0 distances, the possibility that Ni3+ is present on the octahedral site cannot be totally excluded. It is interesting to compare this mixed-valency oxide to Sr,C~Pt0~,~9’which contains chains of alternating Pt4+06 octahedra and Cu2+04 ‘squares’. In this compound the dis- placement of the copper atom from the centre of the trigonal prism generates a slight deformation of the unit cell, which becomes monoclinic.For Sr4Ni,09, such a distortion and a twinning of the crystals would explain the difficulties of the location of the nickel atoms in the trigonal prism. In Sr4Ni,09 there are two kinds of NiO, chains with slightly different nickel-nickel distances: Ni (3)-Ni( 3)-Ni( 5) located at (0,O) and Ni(l)-Ni(2)-Ni(4) at (2/3,1/3) and (1/3,2/3) in the ab plane. The results of this preliminary crystal structure determi- nation which leads to a different chemical formula from that reported previously and indicates a mixed-valency oxide should allow the interpretation of the electrical and magnetic behaviour of this compound. It would be also interesting to study the substitution, for example, of Cu2+ for Ni2+. References Y. Takeda, T. Hashino, H. Miyamoto, F. Kanamuru, S. Kume and M. Koizumi, J. Inorg. Nucl. Chem., 1972,34, 1599. R. J. Marcizak and L. Katz, J. Solid State Chem., 1978,34, 295. J. Lee and G. F. Holland, J. Solid State Chem., 1991,93,267. M. James and J. P. Attfield, J. Muter. Chem., 1994.4, 575. J. De Meulenaer and H. Tompa, Acfa Crystallogr., 1965, 19, 1014. R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 721. A. P. Wilkinson, A. K. Cheetham, W. Kunnman and A. Kvick, Eur. J. Solid State Inorg. Chem., 1991,28,453. J. L. Hodeau, H. Y. Tu, P. Bordet, T. Fournier, P. Strobel, M. Marezio and G. V. Chandrashekhar, Acta Crj.stallogr., Sect. B, 1992,48,1. Communication 4/04929B; Received 1 1th August, 1994