New preparation method of Lan+1NinO3n+1-d (n=2, 3) Maria Deus Carvalho,*a Fernanda Madalena A. Costa,a Isabel da Silva Pereira,a AlainWattiaux,*b Jean Marc Bassat,b Jean Claude Grenierb and Michel Pouchardb aDepartamento de Quý�mica da Faculdade de Cie�ncias da Universidade de L isboa, Rua Ernesto de Vasconcelos, C1–5°, 1700 L isboa, Portugal bInstitut de Chimie de laMatie`re Condense�e de Bordeaux (ICMCB), CNRS, Avenue du Docteur Albert Schweitzer, 33 608 Pessac Cedex, France Samples Lan+1NinO3n+1-d (n=2,3) have been prepared by two diVerent methods, which lead to diVerent oxygen stoichiometry values.Materials obtained by the citrate route always show higher content of Ni3+, which can be rationalized by the high reactivity and the original morphology of the precursors obtained by this method.The ternary nickel oxides with the general formula 2 Nitrate route Lan+1NinO3n+1, similar to the Ruddlesden–Popper series, show Stoichiometric amounts of NiO (99.99%) and La2O3 (99.999%, dried at 1273 K prior to use) were dissolved in nitric acid a multilayered crystal structure which can be described by the solution (ca. 1/3 HNO3, 2/3 H2O). stacking along the c axis of n finite LaNiO3 perovskite layers This solution was fired to dryness and the resulting nitrates separated by LaO rocksalt-like layers.1,2 slowly decomposed on a hot plate.The dark gray products Generally, it has been diYcult to obtain pure phases of the were ground and heated in air at 1173 K for 2 h. The black Lan+1NinO3n+1 (n=2,3) series and often the referred phases powder was annealed in air at 1423 and 1353 K in order to exhibit intergrowth problems influencing their physical obtain La3Ni2O7-d and La4Ni3O10-d, respectively. Several properties.3–5 intermediate grindings were necessary and single-phase mate- Recently, Zhang et al.prepared the n=2 member6,7 starting rials were obtained after heating for ca. 3 days for La3Ni2O7-d from an organic precursor and obtained a single-phase oxygenand 14 days for La4Ni3O10-d.deficient La3Ni2O6.92. The stoichiometric phase with respect to the oxygen content was made by heating the as-prepared 3 Citrate route sample at high oxygen pressure. These authors had also prepared the stoichiometric n=3 member.8 Stoichiometric amounts of NiO (99.99%) and La2O3 (99.999%, dried at 1273 K prior to use) were dissolved in a slight excess Sreedhar et al.4 have observed that intergrowths occur only of nitric acid solution followed by the addition of an equivalent as isolated line defects for n=2 and n=3 members.These molar proportion of citric acid with respect to NiO and La2O3. results have definitively shown the strong influence of the The solution was heated to dryness and the formation of a synthesis method on the stacking ordering as well as on the green gel was observed during this thermal treatment.Then, oxygen stoichiometry, which results in diVerent Ni3+ contents the product was gently heated on a sand bath which induces and consequently in various physical and chemical properties auto-combustion. Final thermal treatment was the same as for (for the stoichiometric phases the mean oxidation state of the nitrate route to allow a comparative study. A drastic nickel is 2.50 (n=2) and 2.67 (n=3), respectively).decrease in reaction times could be observed by this route, It is well established that decomposition of precursors with especially for La4Ni3O10-d for which only 2 days were necessmall particle size and high surface area often allows one to sary to obtain a single phase.prepare homogeneous pure phases inaccessible by conventional solid-state reactions. The samples were characterized by powder X-ray diVraction The aim of this work was the preparation of the n=2 and using a Philips diVractometer with Cu-Ka radiation. Highn= 3 members of this series, without intergrowth phenomena, resolution transmission electron microscopy (HRTEM) was using diVerent methods.performed with a JEOL 2000FX electron microscope. SEM A careful chemical analysis (lanthanum, nickel and oxygen images were obtained with a JEOL JSM 35C and EDS analysis contents) was also carried out in order to determine their exact carried out with a Noran-Voyager apparatus. Nickel and formulation.lanthanum contents were measured by atomic absorption with a Unicam 929 AA spectrometer and using a Radiometer ionselective electrode (ISE25F), respectively. The oxygen non-stoichiometry content (d) of all the samples Experimental was deduced from chemical analyses of trivalent nickel (t) by The samples were prepared as follows. a well known titration method [d=(n-1-t)/2] according to the formulation Lan+1Ni2+(n-t)Ni3+ tO3n+1-d.Electrical resistivity measurements were carried out in 1 Solid state route the range 4.2–300 K on sintered pellets using a standard four-probe technique.9 Stoichiometric quantities of La2O3 (99.999%, dried at 1273 K prior to use) and NiO (99.99%) were mixed and heated in air at 1423 and 1353 K for the preparation of La3Ni2O7-d and Results and Discussion La4Ni3O10-d , respectively. 1 Characterization of the precursors However, this conventional solid-state reaction always resulted in the formation of a phase mixture, even after SEM images of the precursors revealed significant diVerences in their morphology, depending on the preparation. SEM prolonged heating as previously reported.6 J. Mater. Chem., 1997, 7(10), 2107–2111 2107images of the precursors obtained after auto-combustion (citrate route) and after decomposition on a hot plate (nitrate route) for n=2 and n=3 samples are shown in Fig. 1 and 2. It is clear that, for both phases, the morphologies of the precursors obtained by the citrate and nitrate routes are completely diVerent. The materials obtained by the citrate route are extremely thin, ‘fly-wing’ like particles without grains and agglomerates [Fig. 1, 2], in contrast to those prepared by the nitrate route.After heating for 2 h at 1173 K the SEM characteristics show similar diVerences for the two samples. However, the grain size of the powders obtained by the nitrate route is obviously bigger and agglomerates can be seen, which was not observed when using the citrate method.XRD studies of these precursors after annealing 2 h at 1173 K revealed that the citrate route leads to a mixture of diVerent members of the series Lan+1NinO3n+1 for n=2 (nominal composition), but a single phase for n=3 [Fig. 3, 4]. Conversely, by the nitrate route, both precursors seem to be a mixture of diVerent members of the series, La2O3 and NiO [Fig. 3, 4]. These results show the higher reactivity observed when preparing the samples by the citrate route; this is due to the original morphology of the precursors obtained by this rapid thermal procedure.10 2 Lan+1NinO3n+1 (n=2,3) compounds The change in the X-ray pattern of each sample with time of reaction revealed that the kinetics in the citrate route is faster than in the nitrate route.After a heat treatment of 12 h at the annealing temperature (T=1423 K for n=2, T=1353 K for n=3), the X-ray data revealed single phases for the samples obtained with the citrate precursors, while by the nitrate route, Fig. 2 SEM images of the precursor of La4Ni3O10-d they showed a mixture of diVerent phases. This feature was observed for both prepared phases (n=2 and n=3).The XRD patterns for the final materials prepared by both routes are identical as can be seen in Fig. 5 and 6, with only Fig. 3 X-Ray powder diVraction patterns of the precursor of La3Ni2O7-d after heating for 2 h at 1173 K Fig. 4 X-Ray powder diVraction patterns of the precursor of La4Ni3O10-d after heating for 2 h at 1173 K Fig. 1 SEM images of the precursor of La3Ni2O7-d 2108 J.Mater. Chem., 1997, 7(10), 2107–2111Fig. 5 X-Ray powder diVraction patterns of the final material La3Ni2O7-d Fig. 7 High resolution electron microscopy image of La3Ni2O7-d (citrate route) ([110] zone axis) Fig. 6 X-Ray powder diVraction patterns of the final material La4Ni3O10-d the reaction times diVering (Table 1). The compounds are pure and well crystallized. The observed XRD data were used for least-squares refinement of the unit-cell parameterse Fmmm space group.The refined unit-cell parameters are given in Table 1; they are in good agreement with those previously reported for the n=2 phase3,5,6,11 and also for the n=3 member.3,8,12,13 However, a careful examination using high-resolution electron microscopy revealed a more complex situation.For n=2, one can still observe some disordered intergrowths even in the samples obtained by the citrate route. Fig. 7 shows stacking defects, which is confirmed by streaking observed in the electron diVraction pattern. For n=3, the citrate route leads to a well ordered structure (Fig. 8); only a few defects were observed, which was not the case for the sample prepared by the nitrate route, which showed a disordered intergrowth. Electron microscopy seems to reveal that these phases do not crystallize in the Fmmm space group. 3 Chemical analysis Quantitative chemical analyses of lanthanum and of nickel Fig. 8 High resolution electron microscopy image of La4Ni3O10-d (citrate route) ([110] zone axis) indicate a La/Ni ratio very close to 1.5 and 1.3 for La3Ni2O7-d Table 1 Cell parameters and non-stoichiometry content of the prepared compounds reaction dd t route time/h aa/nm ba/nm cb/nm V c/nm3 [(n-1-t)/2] (% Ni3+) formulation La3Ni2O7-d nitrate 72 0.5393 0.5451 2.054 0.604 +0.07 43 La3Ni2O6.93 citrate 48 0.5400 0.5452 2.052 0.604 -0.03 53 La3Ni2O7.03 La4Ni3O10-d nitrate 336 0.5413 0.5468 2.795 0.827 +0.25 50 La4Ni3O9.75 citrate 28 0.5415 0.5467 2.797 0.828 -0.02 68 La4Ni3O10.02 a±0.0002 nm. b±0.001 nm.c±0.001 nm3. d±0.02 J. Mater. Chem., 1997, 7(10), 2107–2111 2109Fig. 10 Temperature dependence of the electrical resistivity for Fig. 9 Temperature dependence of the electrical resistivity for La4Ni3O10-d La3Ni2O7-d preparation. From our XRD study, no structural change was and La4Ni3O10-d , respectively; these results are confirmed by evidenced at low temperature (77<T298 K).We suggest SEM/EDS analysis. that the nature of this transition is electronic. Thus, according Values of d are reported in Table 1. For the citrate route to Taniguchi et al.,11 the properties of these compounds can stoichiometric compounds (d=0) were obtained, while the be interpreted by a model of charge ordering in the NiO2 nitrate route samples are oxygen deficient (d>0). This result planes induced by oxygen vacancy ordering, although it does is still not very well understood since the nitrate route should not explain the observed anomaly.be more oxidative than the citrate one. This confirms the For La4Ni3O10-d, metallic behavior is always observed with strong influence of the morphology of the precursors on the a small variation in the resistivity values. These resistivity preparation of these phases.However, it seems that the occurvalues are one order of magnitude smaller for n=3 compared rence of some oxygen non-stoichiometry, d, does not aVect the to those for n=2. Moreover, the inflexion point seen at ca. cell parameters, which is in accordance with the results of 140 K in the n=3 sample (Fig. 10) is quite diVerent from the Zhang et al.6 metal–insulator transition seen at ca. 120–140 K in the n=2 Occasionally, it has been shown that some samples prepared sample (Fig. 9). One should point out that all the previous via the citrate route showed a small excess of oxygen, which data for this compound present some controversy. The most corresponds to an oxygen over-stoichiometry likely resulting recent work of Zhang et al.8 do not reveal any resistivity from the occurrence of interstitial oxygen atoms in the La2O2 anomaly but Shreedhar et al.4 report an inflexion point in the layers, as was previously described for La2NiO4+d .14,15 curve at 140 K, but with thermal behavior diVering from ours, while Tkalich et al.13 report a very marked anomaly in 4 Electrical resistivity their work.Additional investigations are necessary in order to give an Fig. 9 and 10 show the thermal dependence of the electrical resistivity, r(T ), for the La3Ni2O7-d and La4Ni3O10-d samples, interpretation of the observed anomalies. respectively. For both phases, resistivity values are always higher for the Conclusion nitrate route samples than for the citrate ones.Thus, it seems that resistivity decreases with increased Ni3+ content, in agree- The n=2 and n=3 members of the series Lan+1NinO3n+1-d, have been successfully synthesized using, for the first time, the ment with previous work.3,6,11 La3Ni2O7-d (Fig. 9) clearly shows a diVerent electrical citrate method. This process leads to powders which exhibit small particle sizes, which is responsible for the high reactivity behavior depending on the preparation method, which results from the diVerent oxygen stoichiometry of the compounds and of the precursors.This work clearly shows that the preparation method has a the value of the carrier concentration is directly correlated to the Ni3+/Ni2+ ratio. Above 120 K, the metallic behavior of strong influence on the oxygen stoichiometry of these compounds.It is very important to carefully measure the Ni3+ the citrate material agrees well with the electrical resistivity data published earlier by Moham-Ram et al.3 and by Zhang content, and consequently, the total oxygen content in each sample. et al.6 for samples with similar composition. However, there is some discrepancy between these results and those of Sreedhar The samples obtained by this new method are always more oxidized than those prepared by other methods and have a et al.4 who found a non-metallic behavior for this phase, but no data were given about the oxygen content of their sample.higher Ni3+ content. These observations are very important since they lead to These results clearly show that the electrical resistivity is strongly dependent on d as previously reported by Taniguchi variations in the physico-chemical properties of the diVerent samples as has been shown by electrical resistivity behavior.et al.11 However, our results are somewhat diVerent with respect to those of these authors. In this work, a marked These studies will be discussed in future work.inflexion is visible for both samples (d=0.0 and 0.1). These inflexions occur at diVerent temperatures: 120 K for the sample The authors are grateful to ‘Embaixada de Franc�a em Portugal’ and ‘JNICT’ (Portugal) for financial support. obtained by the citrate preparation and 140 K for the nitrate 2110 J. Mater. Chem., 1997, 7(10), 2107–211110 P.Barboux, P. Griesmar, F. Ribot and L. Mazerolles, J. Solid State References Chem., 1995, 117, 343. 1 P. Lacorre, J. Solid State Chem., 1992, 97, 495. 11 S. Taniguchi, T. Nishikawa, Y. 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