Vale ´rie Carles

Christophe Laurent
email: laurent@iris.ups-tlse.fr

Marc Brieu

Abel Rousset

Synthesis and characterization of Fe/Co/Ni alloys-MgO nanocomposite powders

A mixed oxalate b-Mg 0.896 Fe 0.047 Co 0.034 Ni 0.023 C 2 O 4 •2H 2 O of well controlled size and morphology was prepared by coprecipitation. The corresponding quaternary solid solution between MgO, FeO, CoO and NiO was prepared by thermal decomposition and calcination in a H 2 /H 2 O/N 2 atmosphere. The selective reduction of the solid solution in an H

2 atmosphere was studied by a combination of X-ray diffraction, Mo ¨ssbauer spectroscopy, transmission electron microscopy and associated analysis. This work has brought to light the very high stability of Fe2+, Co2+ and notably Ni2+ when substituted for Mg2+ in the MgO rocksalt lattice. It is necessary to perform the reduction at 1300 °C fully to reduce the transition metal ions. The alloy particles are either distributed as relatively large particles (tens to hundreds of nanometers) at the surface of the MgO grains or as much smaller particles (∏20 nm) probably located inside the matrix grains and epitaxial with it. The composition distribution of the large surface particles is fairly broad when the reduction was performed at 1100 and 1300 °C. In contrast, it is much narrower in the powder prepared by reduction at 1200 °C, although the particles are still low in Ni compared to the target composition. reduction temperature. This temperature is relatively high, in

Introduction

the 800-1300 °C range, in order to reduce oxide solid solutions Nanocomposite materials have been widely studied in recent having a low specific surface area (typically 5-20 m2 g-1) as years owing to their potentially interesting properties.1-4 In required for the dispersion of metal nanoparticles with a small particular, the dispersion of metal nanoparticles in an inorganic average size and a narrow size distribution. Chatterjee et al.17 matrix has aroused great interest. Metal-oxide nanocomposite have claimed that the H 2 reduction of gel pieces at moderate powders have been prepared by direct reactions between the temperatures could produce Fe/Cr alloy nanoparticles disappropriate reagents by several routes such as the sol-gel persed in silica, but they do not give the composition of method,5 mechanical alloying6 and combustion synthesis.7 their particles. These methods generally do not allow a precise control of the The present work aims at extending this chemical process microstructure of the composite powders, notably with respect to the dispersion of ternary alloy nanoparticles in an oxide to the size and composition of the metal particles. In .15,16 In these powders, most of value of the coercive force. 18 The first part of the present the metallic phase is found in the form of nanometric particles paper is devoted to the synthesis of the appropriate quaternary dispersed within the oxide grains or at their surface. It has solid solution between MgO, FeO, CoO and NiO by the been shown that several factors such as the transition metal oxalate precursor route that was used for the preparation of content, the mutual solubility of the parent oxides, the specific binary and ternary solid solutions. 13,14,19 The second part surface area and the crystalline state of the starting oxide solid deals with the reduction behaviour of the oxide solid solution solution can modify the size, the size distribution and the and the formation of the composite powder, notably with location (inside or at the surface of the oxide grains) of the respect to the composition of the metallic phase. metal particles. In addition, it is possible to control, to a certain extent, the composition of the nanoparticles in the case of binary alloys (Fe/Cr, Fe/Ru, Fe/Co and Fe/Ni H 2 flow at either 1100, 1200, or 1300 °C for 1 h, giving rise to the metal-oxide nanocomposite powder. These powders will be hereafter designated R1100, R1200 and R1300, respectively.

b form depending on the precipitation conditions. Pezerat reveal that the decomposition takes place in two steps and confirm the above hypothesis. Indeed, the weight losses for About twenty spot analyses were performed for a given powder using a 4, 7 or 15 nm diameter probe depending on the size the mixed oxalate are equal to the theoretical ones (23.7 and 47.5%) calculated using the experimental contents found for of the metal particles under examination. TEM EDX microanalyses have been performed on metal particles larger than the metallic ions (Mg=89.6, Fe=4.7, Co=3.4, Ni=2.3 cat.% [where cat. means cationic]). The difference between the latter 20 nm in size and located on the edges of the MgO grains. The nature of the iron species present in the oxide solid values and the theoretical ones (Mg=90.0, Fe=4.7, Co=3.1, Ni=2.2 cat.%) are within the limits of the analysis method. In solutions and in the nanocomposite powders was determined by 57Fe Mo ¨ssbauer spectroscopy. Spectra were recorded at contrast, the decomposition of the mixture of oxalates [Fig. 2(c)] gives rise to a more complex pattern, involving room temperature (RT ) with a constant acceleration spectrometer using a 50 mCi 57Co source in a rhodium matrix. more than two steps. Thus, it can be concluded from the above results that the present precipitation process has allowed Absolute velocity calibration was carried out with an iron foil with respect to which center shifts (CS) are reported in the us to prepare a mixed oxalate, the chemical formula of which could be b-Mg 0.896 presence of the same system of peaks, corresponding to the rocksalt lattice. This indicates that OX500 is a monophasic and therefore the formation of large overconcentration zones in transition metal within the oxide powders.

Results and discussion

The OX800 powder was also studied by RT 57Fe Mo ¨ssbauer 57Fe Mo ¨ssbauer spectroscopy. The RT 57Fe Mo ¨ssbauer spectra of the R1100, R1200 and R1300 composites and the corresponding Mo ¨ssbauer parameters are reported in Fig. 7 and Table 1, respectively. The spectra have been fitted with a doublet representing the Fe2+ ions in substitution of Mg2+ in the MgO lattice and one or more sextets accounting for the ferromagnetic Fe/Co/Ni alloy(s). It should be noted that in the case of R1300 the fit gives a fairly poor description of the spectrum. It was not attempted to improve the fits, notably by using hyperfine-field distributions, because there are several populations for the composition of the Fe/Co/Ni particles as discussed later. In line with earlier results on Fe-MgO,13 Fe3+ totally reduced to the divalent or metallic state at a temperature lower than or equal to 1100 °C. In contrast, Fe3+ ions were still detected in a Fe 0.65 Ni 0.35 -MgO powder reduced at confirm the homogeneity of the oxide solid solution, the local 1100 °C,14 which indicated that the presence of nickel was contents of Fe, Co and Ni (4.9, 3.5 and 2.3 cat.%, respectively) detrimental to the reduction of iron. Thus, the present results being close to their respective global contents. may indicate that cobalt favours the reduction of iron and that its effect is strong enough to offset that of Ni. The Nanocomposite powders proportion of Fe2+ ions sharply decreases from 44 to 18% upon increasing the reduction temperature from 1100 to XRD pattern analysis. XRD pattern analysis of the nanocomposite powders reveals the diffraction peaks of the 1200 °C. For the R1300 specimen, lines 3 and 4 of the sextet have a very similar intensity, showing that the proportion of MgO matrix and the peaks corresponding to one or several metallic phases, depending on the reduction temperature Fe2+ ions is very small. Despite the fact that the spectrum of the latter specimen, and therefore the corresponding fit, is (Fig. 6). The interplanar distance measured for the R1100 powder (0.2025 nm) is close to that of the (110) planes of the fairly poor and should be considered with utmost caution, this result is in acceptable agreement with the TEM EDX micro-body centred cubic (bcc) a-Fe lattice (d 110 =0.2027 nm). A similar distance is also measured for the R1200 powder analyses present in a following section showing that the proportion of residual iron ions in the matrix is very low (ca. (0.2022 nm). Both these peaks could also account for a Fe- TEM observations. TEM observations of the nanocomposite powders have revealed that the size of the MgO matrix grains is of the order of a few hundreds of nanometers. The metallic particles dispersed in the MgO matrix have a bimodal size distribution (Fig. 8). The size of the metal particles in the two populations (noted P 1 and P 2 ) are presented in Table 2. As 344 kG in R1100 and 352 and 329 kG in R1200. In both cases, R1300 90<d<380, <d>=180 d∏20

these particular values could reflect the most intense peaks in the reduction temperature is increased from 1100 to 1300 °C both MgO and these metal nanoparticles reveals an alignment according to several directions of the diffraction spots corre-the proportion of the large P 1 particles increases to the detriment of the small P 2 particles. The large metal particles sponding to (hkl ) MgO and (h∞k∞l∞) c-alloy planes. The corresponding dark field image [Fig. 9(b)] using a (220) c-alloy (P 1 ) are approximately spherical and are located at the surface of the MgO grains (Fig. 8). The size distribution and the spot as operative reflection reveals that all the nanoparticles are in contrast. The relationships of common orientation average size of the metal particles were derived from the measurement of the size of about 90 particles pictured on the between MgO and the metal particles observed on the electron diffraction pattern for the different powders are reported in original TEM negatives. The increase of the average size of the P 1 particles from 40 to 180 nm with the reduction tempera-Table 3. These results show that there is a perfect cube/cube orientation relationship between MgO and the metal particles ture is attributed to an increased proportion of metallic phase on the one hand and to the coalescence of the metal particles in the present R1100, R1200 and R1300 powders. The cube/cube orientation has been observed in Fe 0.65 Ni 0.35 -MgO on the other hand. Indeed, increasing the reduction temperature (notably from 1100 to 1200 °C ) provokes a widening of nanocomposite powders.14 The present results as well as a comparison with earlier studies on Fe-Al 2 O 3 nanocomposite the size distribution and the formation of much larger particles, several hundreds of nanometers in size. Compared to previous powders12,31 could indicate that the P 2 nanoparticles grow epitaxially inside the grains of the MgO matrix. This could studies,13,14 it appears that the evolution of the present materials is more similar to that observed for Fe-MgO than for account for the moderate growth observed upon the increase in reduction temperature. Fe 0.65 Ni 0.35 -MgO. Indeed, in the latter specimen, the major size and size distribution increases occur between 1200 and 1300 °C. In line with the Mo ¨ssbauer spectroscopy results, this TEM EDX microanalyses. TEM EDX microanalyses performed on metal particles larger than 20 nm in size and could indicate that the presence of cobalt offsets the hampering effect of nickel on the reduction of the iron species and thus located on the edges of the MgO grains (P 1 particles) have revealed the presence of Fe/Co/Ni alloy particles with different favours the formation of Fe/Co/Ni alloy particles and their growth at lower temperatures than in the case of Fe/Ni alloys.

compositions ( Table 4). Three populations of metal particles (denoted A, B and C hereafter) have been observed in the The P 2 metal particles are much smaller (∏20 nm) and are either spherical, cubic or parallelepipedic in shape. The electron R1100 specimen. Particles in population A, which account for ca. 70% of the investigated particles, have an iron content in diffraction pattern [Fig. 9 ). It is noteworthy that only one population of metal particles (D) is observed in the R1200 powder. The contents of Fe, Co and Ni are in the 53-56, 30-32 and 13-16 at.% ranges, respectively, which fall in between those measured for populations A and B. The cobalt content is the desired one, but those of Fe and Ni are too high and too low, respectively. In contrast, powder R1300 presents a large proportion (ca. 65%) of alloy particles with a composition ( E) very close to the desired one. However, two more populations (F and G) are observed in small proportions and present an iron content lower and a nickel temperature could allow one to prepare alloy particles with both the desired composition and a narrow composition content higher than in the target composition. Although no correlation could be made between the composition and the distribution. The microstructure of the present Fe/Co/Ni-MgO composite powders is fairly complex, but it is anticipated that size of a particle, it is probable that the formation of populations different in size and composition are related. In dense materials prepared from these powders could present interesting physical and mechanical properties. addition, spot analyses were performed for R1300 on areas of the matrix appearing devoid of metal particles. Very small contents of residual iron (ca. 4.7), cobalt (ca.

Fig. 1

 1 Fig. 1 XRD patterns of b-MgC 2 O 4 •2H 2 O (a) and of the mixed

  et al.21 claimed that both forms are monoclinic, differing from A total reduction of the transition metal ions should yield a 10 wt.% Fe 0.47 Co 0.31 Ni 0.22 -MgO composite. This particular each other by the presence of stacking faults in the b form. In contrast, Deyrieux and Peneloux22 claimed that the a and b metal content was chosen because it is an acceptable compromise between lower amounts, which generally give rise to forms are distinct crystallographic structures differing from each other by the relative positions of the metal-oxalate smaller particles but increase the characterization difficulties, and higher amounts which result in much larger metal particles. chains. The pattern of the (Mg, Fe, Co, Ni)-oxalate [Fig. 1(b)] is similar to the former one, no other crystallized phase being Phase detection and identification was carried out by X-ray diffraction ( XRD) pattern analysis (l CoKa =0.17902 nm). Some detected. This could indicate that the precipitation product indeed is a mixed oxalate rather than a mixture between the patterns (not shown) have been recorded using NaCl as an internal standard. The total metal contents in the OX800 oxide simple MC 2 O 4 •2H 2 O (M=Mg, Fe, Co or Ni) oxalates, which are known to be isomorphous and crystallize in the were determined by plasma emission spectroscopy with a relative error of about 1 wt.%. The oxalate and oxide powders monoclinic system. The DTA and TGA curves relative to the thermal were observed by scanning electron microscopy (SEM ). The specific surface areas of the oxide powders were measured by decomposition of both oxalates are shown in Fig. 2(a) and (b). The DTA curves are similar to each other and present the Brunauer-Emmett-Teller (BET ) method using N 2 adsorption at liquid N 2 temperature. The nanocomposite powders two endothermic peaks, the first one of which (ca. 200 °C ) could be related to the loss of the 2 constitutional H 2 O were observed by transmission electron microscopy (TEM ). The local metal contents in OX800 and the composition of molecules. The second peak (ca. 465 °C ) could correspond to the decomposition of the oxalate group. The TGA curves also the alloy particles in the nanocomposite powders were investigated by TEM energy dispersive X-ray (EDX ) microanalysis.

  the contrary, the Mo ¨ssbauer spectra are not converted with respect to natural Fe, the CS value of which is given in Analysis of the XRD patterns of MgO [Fig. 3(a)] and of the decomposition product OX500 [Fig. 3(b)] reveals the the corresponding figure caption.

Fig. 3

 3 Fig. 3 XRD patterns of pure MgO (a), of the oxide prepared by thermal decomposition at 500 °C (b), and of the oxide calcined at 800 °C (c).

Fig. 2

 2 Fig. 2 TGA and DTA curves showing the thermal decomposition of MgC 2 O 4 •2H 2 O (a), of the mixed oxalate (b) and of the mixture of oxalates (c).

Fig. 4

 4 Fig. 4 RT 57Fe Mo ¨ssbauer spectrum of the oxide calcined at 800 °C;

Fig. 6

 6 Fig. 6 XRD patterns of the Fe/Co/Ni-MgO nanocomposite powders prepared by reduction at 1100 (a), 1200 (b) or 1300 °C (c).

Fig. 5

 5 Fig. 5 SEM images of the the mixed oxalate (a), of the oxide prepared by thermal decomposition at 500 °C (b) and of the oxide calcined at

Fig. 8

 8 Fig. 8 TEM image of the Fe/Co/Ni-MgO nanocomposite powder prepared by reduction at 1100 °C. P 1 =Population of large metal particles; P 2 =population of small metal particles.

Fig. 7

 7 Fig. 7 RT 57Fe Mo ¨ssbauer spectra of the Fe/Co/Ni-MgO nano-Table 2 Size (d±10%/nm) of the metal alloy particles present in the composite powders prepared by reduction at 1100 (a), 1200 (b) or two populations (P 1 , P 2 ) observed in the Fe/Co/Ni-MgO nanocom-1300 °C (c); the CS value of natural Fe is equal to -0.15 mm s-1. posite powders prepared by reduction at 1100, 1200 or 1300 °C; <d> denotes the average size Nanocomposite P 1 P 2

Fig. 9

 9 Fig. 9 (a) Electron diffraction pattern recorded on an area containing R1100 A, great 57-63 25-29 8-15 both MgO and the small metal particles (P 2 population) in the powder B, small 50-54 27-35 15-19 prepared by reduction at 1100 °C. The inset shows diffraction spots C, very small 43 33 24 corresponding to (hkl ) MgO [#] and (h∞k∞l∞) c-alloys [×] planes (the R1200 D 53-56 30-32 13-16 other spots are attributed to double diffraction, zone axis [11 :3]). (b) R1300 E, great 44-48 31-33 21-24 The corresponding TEM image (in the dark-field mode with 220 alloy F, very small 43.5 27 29.5 spot as operative reflection) of the c alloy (Fe/Ni and/or Fe/Co/Ni) G, small 31-37 25-33 36-38 particles dispersed inside the MgO matrix.

  ).[10][11][12]14,16

	H 2 Ni) compounds were also prepared and a mechanical mixture O 2.14 mol l-1]. The other MC 2 O 4 O (M=Fe, Co or •2H 2
	of all four simple oxalates in the same proportion of the
	desired mixed oxalate was made for the sake of comparing
	the thermal decomposition behaviours. The thermal decompo-
	sition of the oxalates was followed in Ar by thermogravimetric
	( TGA) and differential thermal (DTA) analysis because the
	apparatus available in the laboratory does not allow one to
	work in a H 2 The oxide powders were prepared by thermal decomposition /H 2 atmosphere. O/N 2
	at the appropriate temperature (500 °C ), as determined by
	thermogravimetric analyses. The magnesium oxalate was
	whereas the specimen containing the transition metal cations was decomposed in a flowing decomposed in flowing N 2
	controlled H 2 2.0 mol.%, N 2 the MgO-based (rocksalt) structure.20 This latter specimen will /H 2 O/N 2 atmosphere (H 2 O= =4.6 mol.%, H 2 =93.4 mol.%) to avoid phase partitioning from
	be hereafter denoted by OX500. It was further calcined at
	800 °C for 1 h in a controlled H 2 1.5, N =98.5 mol.%) in order to improve the crystallization O/N 2 atmosphere (H O= 2 2 state and lower the specific surface area (specimen denoted by
	These studies have notably thrown light on the high stability	Experimental
	upon reduction of the iron ions substituting in the lattices of Al 2 O 3 , Cr 2 O 3 , MgO and MgAl 2 . However, the presence of O 4 other transition metal ions that are more easily reduced to the metallic state has been found to enhance the reducibility of	The appropriate amounts of Mg(NO 3 FeCl 2 •4H 2 O (0.26 mol l-1), NiCl 2 •6H 2 ) 2 O (0.12 mol. l-1) and O (5.00 mol l-1), •6H 2 CoCl 2 O (0.17 mol l-1) were dissolved in deionized water. •6H 2 Hydrochloric acid (35 vol.%) was added to prevent oxidation
	the iron ions. Both ruthenium and nickel favour the reducibility	of the divalent iron ions and hydroxide precipitation. The
	of Fe() substituting for Al() ions in Al 2 enhances the reducibility of Fe() substituting for Mg() ions O .11 Cobalt 3	mixture, heated at 50 °C, was added while stirring to an aqueous solution of (NH 4 ) 2 C 2 O 4 •H 2 O also maintained at
		) 2	C 2	O 4 •

50 °C (2.38 mol l-1; in stoichiometric quantity with the metal in MgAl 2 O 4 .16 Similarly, the presence of iron favours the salts). The oxalate so obtained was quickly cooled to ambient reduction of Cr() ions substituting for Al() ions in Al 2 O 3 .10 temperature and stirred for 1 h at 20 °C. After filtering, alcohol The reduction temperature is the main parameter that controls washing and drying overnight at 50 °C, the oxalate was manuthe composition of the alloy particles for a given composition ally ground and passed through a 250 mm sieve. For the sake of the starting oxide. The nanoparticles are progressively of comparison, MgC 2 O 4 •2 H 2 O was prepared using the same enriched in the less reducible metal as the selective reduction of the oxide solid solution progresses upon the increase in conditions [Mg(NO 3 ) 2 •6H 2 O 5.00 mol l-1 and (NH 4

Table 1

 1 RT 57Fe Mo ¨ssbauer parameters of the different iron species present in the Fe/Co/Ni-MgO nanocomposite powders prepared by reduction at 1100, 1200 or 1300 °C; H=hyperfine field/kG; CS= centre shift/mm s-1; D EQ =quadrupole splitting/mm s-1; C=half width at half height/mm s-1; P=proportion (%); ferro=ferromagnetic Fe/Ni alloys.16,25-29 However, the present values are significantly higher the peaks reported for c-Fe/Ni-MgAl 2 O 4 nanocomposites (300 and 240 kG),16 which could reflect that the alloy particles also contain some cobalt. Indeed, a RT hyperfine field equal to 365 kG has been measured for Fe 0.65 Co 0.35 -MgAl 2 O 4 nanocomposite powders16 and Johnson et al.30 have reported a nearly constant value (365 kG) in the Fe 0.80 Co 0.20 -Fe 0.65 Co 0.35 composition range. It is also noteworthy that a singlet corresponding to a paramagnetic c-Fe/Ni phase is not detected, in contrast to previous results on Fe 0.65 This could also point to the presence of cobalt in the alloy particles.

			Mo ¨ssbauer parameters
	Nanocomposite Iron species	H	CS	D EQ	C	P
	R1100	Fe2+	-	1.05 0.33 0.22	44
		ferro Fe/Co/Ni 362 0.04 -	0.19	25
		alloy(s)	344 0.04 -	0.24	31
	R1200	Fe2+	-	0.99 0.19 0.21	18
		ferro Fe/Co/Ni 352 0.04 -	0.22	57
		alloy(s)	329 0.06 -	0.20	25
		ferro Fe/Co/Ni 326 0.03 -	0.25 100
	R1300	alloy(s)
	Ni 0.35 powders.16	-MgO14 and Fe 0.65	Ni 0.35	-MgAl 2	O 4	nanocomposite

the hyperfine field distribution which is known to exist in c-

  (a)] recorded on an area containing the 57-63 at.% range, a cobalt content slightly lower than the target one (ca. 31 at.%) and a nickel content markedly lower than the desired one (ca. 22 at.%). Comparing population B, which represents a smaller proportion of particles, to A shows that the iron content is slightly lower (50-54 at.%) whereas those of Co and Ni are higher B. The same evolutions are observed from populations B to C, which represent a still smaller proportion of particles. Thus, C particles have a composition close to the desired one (Fe 0.47

	Co 0.31 Ni 0.22

Table 3

 3 Cube/cube epitaxial relations between the alloy nanoparticles

  , because of the clustering of iron ions and vacancies 9 Ch. Laurent, A. Rousset, M. Verelst, K. R. Kannan, A. R. Raju that occurs in the MgO-based solid solution. Cobalt has been and C. N. R. Rao, J. Mater. Chem., 1993, 3, 513. 10 M. Verelst, K. R. Kannan, G. N. Subbanna, C. N. R. Rao,shown to be reduced to the metallic state at a temperature M.Brieu and A. Rousset, Mater. Res. Bull., 1993, 28, 293. about 100 °C lower than iron and nickel in a MgAl 2 O 4 matrix 11 Ch. Laurent, J. J. Demai, A. Rousset, K. R. Kannan and and has been found to promote the reduction of the Fe2+ C. N. R. Rao, J. Mater. Res., 1994, 9, 229. ions.16 It is proposed that cobalt has the same role in MgO, 12 Ch. Laurent, Ch. Blaszczyk, M. Brieu and A. Rousset, but its influence on the reduction of the Ni2+ ions is unknown. NanoStruct. Mater., 1995, 6, 317. 13 V. Carles, M. Brieu, J. J. Demai and A. Rousset, in Fourth Euro-Ceramics Vol. 1, ed. C. Galassi, Gruppo Editoriale Faenza Editrice S. p. A., Faenza, Italy, 1995, p. 323. Que ´nard, Ch. Laurent, M. Brieu and A. Rousset, NanoStruct. well controlled size (ca. 1 mm) and morphology was obtained Mater., 1996, 7, 497. by coprecipitation. Thermal decomposition at 500 °C in the 16 O. Que ´nard, E. De Grave, Ch. Laurent and A. Rousset, J. Mater. CoO and NiO. Further calcination at 800 °C decreases the Phys. Lett., 1990, 57, 1360. specific surface area but does not provoke any phase partitionspecimen the metal particles are too rich in Fe compared to 29 I. Ortalli, A. Vera, G. Fratucello and F. Ronconi, Hyperfine half the studied particles have the desired composition, 30 C. E. Johnson, M. S. Ridout and T. E. Cranshaw, Proc. Phys. but the other particles are too rich in Ni. In contrast, the Soc., 1963, 81, 1079. composition distribution is much narrower in the powder 31 X. Devaux, Ch. Laurent, M. Brieu and A. Rousset, J. All. Compd., prepared by reduction at 1200 °C, although the particles are 1992, 188, 179. still too poor in Ni. It is proposed that properly adjusting the transition metal contents in the oxide powder and the reduction

				1) and nickel
	MgO matrix, contrary to what is observed in a MgAl 2	O 4	1993, 2, 339.
	A mixed oxalate b-Mg 0.896 Fe 0.047 15 O. appropriate H 2 Co 0.034 Ni 0.023 C 2 O 4 O of 8, 529. •2H 2 /H 2 O/N 2 atmosphere allowed us to prepare the Chem., 1997, 7, 2457. corresponding quaternary solid solution between MgO, FeO,
	the target composition (Fe 0.47	Co 0.31	Ni 0.22	). In the latter more	Interact., 1986, 28, 1025.

matrixConclusion 14 V. Carles, M. Brieu and A. Rousset, NanoStruct. Mater., 1997, 17 A. Chatterjee, D. Das, D. Chakravorty and K. Choudhury, Appl.

than

Acknowledgements

(ca. 4%) ions have been detected. The present results, in line

The authors would like to thank J. Sarrias for SEM with those obtained for Fe/Ni-MgO powders,14 seem to observations of the oxalate and oxide powders and J. J. of specimen R1100. Another explanation could be that the Mater., 1994, 4, 603.

Fe2+ ions are more easily reduced than the Ni2+ ions in the 8 X. Devaux, Ch. Laurent and A. Rousset, NanoStruct. Mater.,