- Email: [email protected]
Oxygen-deficient brownmillerites: Cation coordination and magnetic properties
1. /‘kw. Chews. Soti& Vol. St. No. I. PP. 79-U. Printed in Great Britain.
0022.369?!5Q S3.M) + 0.00 b 1990 Pcrgamon Pfes plc
1990
OXYGEN-DEFICIENT BROWNMILLERITES: CATION COORDINATION AND MAGNETIC PROPERTIES R. BENLOUCIF,~ N. NGUYEN,? J. M. GRENECHE~ and B. RAVE& tl-aboratoire CRISMAT, ISMRa, Bd du Marichal Juin, 14032 Cacn C&x, France &aboratoire
de Spectromitrie MBssbauer, U.A. 807,Universite du Maine, 72017 Le Mans C&iex, France (Received 5 July 1989; accepted in revised form 7 September 1989)
Abstract-New oxygen-deficient brownmillerites Ca,Fe2_,,M,0,_ r!Zhave been synthesieed and characterized bv X-rav diffraction and chemical analvsis for M = Me. Ni. Cu. Zn. The Mrissbautr study of these phases shows only the presence of Fe(II1) and confirms th;~oxygen.non-stoichiometry; it suggests the existence of pyramidal sites and allows the ordered cationic distribution to be established according to the formula Ca,(Fe$ (Fe”’ , _zrM~),(Fe,~‘)pyOs _ri2. The study of the magnetic susceptibility vs temperature shows a broad an;iferromagnetic transition, with a classical decrease of F, vs x. A particular feature is the higher TN value of the magnesium phase compared with the nickel and copper oxides. This latter behavior is interpreted in terms of size factors influencing the interaction between the octahedral (or pyramidal) and tetrahedral layers. Keywords: BrownmilIerite, oxygen-deficient oxides, oxygen non-stoichiometry, properties.
INTRODUCTION The brownmillerite
structure
was determined
for the
first time by Bertaut et al. [ 11for the oxide Cal Fe20s. It can be described [2] as an ordered oxygen-deficient perovskite in which one layer of FeO, octahedra alternates with one layer of FeO, tetrahedra (Fig. 1). Isotypic compounds have been obtained for Sr,Fe,Os [3,4] and BazFe20, [5] and substituted oxides in which Fe’+ is replaced either by another trivalent cation f6] or by a couple Mn’+-Mz+ [7]. The possibility of introduction of an oxygen excess, has been shown in the case of manganese-iron oxides leading to the non-stoichiometric phase Car Fez _ zVMn,O, + 6 (0 Q x d 0.25) [S]. No oxygen-deficient brownmillerite has been observed for either calcium or for strontium compounds. Only Ba,Fe,O, was found to be stable in the presence of oxygen vacancies, leading to the limiting phase Ba,Fe,O,, [9]. Moreover, the magnetic properties of the materials, and especially the antiferromagnetic order temperature TNcan be affected by the distribution of the cations in the octahedral and tetrahedral sites [lo], and depend also on the exchange integrals which are governed by the C~stalIographic cell, and consequently by the size of the cations. We report here on the study of the magnetic properties of oxygen-deficient brownmillerites Ca2 Fe, _.VM,Os _ r,Z(M = Cu. Ni, Mg, Zn) in connection with the cationic distribution and the oxygen deficiency of these oxides. EXPERIMENTAL
Synthesis and X-ray study The oxtdes Car Fe, _ J M,O, _ .r;t+6 were prepared by heating mixtures of chemically pure CaCO,, FetO, 79
Miissbauer study, magnetic
and divalent oxides MO (M = Cu, Ni, Zn, Mg) at temperatures ranging from 900 to 1200°C. The zinccompound was heated progressively up to 1100°C in order to limit the volatilization of zinc. Repeated thermal treatments were required to obtain homogeneous compounds. To determine the oxygen content, chemical analysis was carried out by redox back titration using an Fe(H) standard solution. For both phases the experimental value S is very small (6 < 0.03). The X-ray powder diffractograms were recorded with a Philipsgoniometer using CuKx radiation. Magnetic study The different substituted materials were characterized by Miissbauer spectrometry: the spectra were recorded in transmission geometry with a constant acceleration spectrometer, using a s7Co source diffused into a Rh-matrix at room temperature. The magnetic susceptibility was measured with a Faraday balance in the range of temperature 300-950 K.
RESULTS AND DISCUSSION From the X-ray diffraction study, it is clear that brownmillerite can be synthetized as a pure phase with a maximum substitution of 10% for M = Cu, Ni, Mg (0 d x ;SE0.20) and of 7.5% for M = Zn (0 Q x Q 0.15). The crystal data (Table 1) show that the “a” and “c” parameters of the orthorhombic cell are not really affected by the substitution. On the other hand, the “b” parameter increases with the size of the cations. The chemical analysis shows without any ambiguity that 6 is close to zero in the fo~ulation
R. BENLOCXIF et ai.
80
Fig. 1. Brownmillerite structure. Space group: fnma.
ca2Fe2-.yWA4- .ri2 + 6 (6 < 0.03). consistent with copper, nickef and iron in the low oxidation states, Cu” Ni” and Fe”‘, respectively. The latter result is cbnfirmed by Mtissbauer spectroscopy, which does not detect Fe(W) in any of the samples. Consequently, these oxides can be formulated as Ca, Fe:‘_, MflOS r *. The MGssba&‘spectra recorded at room temperature for several oxides are presented in Fig. 2 and compared with that of Ca,FeZO,. In contrast to the Ca,Fe?O, pattern, which exhibits two well-resolved magnetic contributions, the spectra of the substituted oxides are characterized by several anomalies. One indeed observes that some of the fines are broader and asymmetrical, whereas the outer lines of the external sextet corresponding to the octahedral sites are weakly split. This shows that two types of “octahedral” sites are available for iron. Thus, in a first approach, the magnetic Mtissbauer spectra can be analyzed in terms of one tetrahedral contribution and two octahedral contributions whose hyperfine field values vary from 24 to 33 kG (Table 2). The refinement of the cationic distribution shows that about 55% of iron is located on the tetrahedral sites for x = 0.10 against 57% for x = 0.20. This leads in the limit of the error, to a total occupation of the tetrahedrat sites by Fe”’ (Table 3). Clearly, at1 the divalent cations are preferentially distributed over Table 1. Crystallographic data and antiferromagnetic ordering temperatures for CalFez _c M,O,_,,, compounds M Fe CU
x
Ni
0.1 0.2 8::
Mg
0.1
Zn
0.2 0.1 0.15
a (A)
b (A)
c (A)
TN (W
5.580 (1) 5.579(l) 5.583 (1) 5.589(l) 5.590(l) 5.596 (I) 5.600(l) %590(l) 5.601 (2)
14.752 (2) 14.813(3) 14.823 (2) 14.790(l) 14.802 (2) 14.791 (3) 14.808(2) 14.803(4) 14.621 (3)
5.421 (1) S.410(1) 5.409 (1) 5.410(t) 5.400 (I) 5.420 (t) 5.421 (I) 5.413(i) 5.4W (2)
722 666 658 680 665 720 689 -
A
V(mm.s”J
Fig. 2. Mbssbauer spectra at room temperature of Ca,Fe,.~M&,, tM = Fe, Cu, Ni, Mg) and Ca,Fe,.~, Zn0.A.9z5 phases.
the octahedral sites. Moreover, the presence of only Fe(IIf) from the Miissbauer refinement ctearly estabIishes the existence of additional anionic vacancies (x/2) with respect to the normal brownmillerite according to the formula Caz Fe, _,rM,YO,_ _r/2 ; this implies that some of the cations in the two types of “octahedral” sites in fact exhibit a coordination smaller than six, characterized by the formation of oxygen vacancies in the basal plane {a, c), The value of the hyperfine fiefd of the second “octahedral” site, which is significantly smaller than that of the first one, suggests that it is this site which is not in fact octahedral but characterized by a smaller coordination number, leading to a pyramidat coordination for Fe(III). The ability of Fe(IIEf to take this type of coordination as shown for BaYFeCuQ [l l] suggests strongly this point of view. The consideration of the chemical formula, Ca, Fe!! .YMf’OJ_ x,2, leads then to the formation of x pyramidal sites (Py) against (1 - X) octahedral site (Oc) per formula unit, whereas the tetrahedral site (Te) remains equal to one. The comparison of the iron distribution deduced from the refinement of the intensity of Miissbauer spectra,
Oxygen-deficient brownmillerites Table 2. The hypertine M&batter
x
M
CazFezO,
cu 0.1 0.2 Ni 0.1 0.2 Mg 0.1 0.2 Zn 0.1
0.15
(+O.Ol’~ms-1)
81
parameters at room temperature compounds (fO.O21;nms-I)
for Ca,Fe,_,M,yO,_,,r
Sites
(fO.Ofims-I)
&!LG
0.18 0.36
0.28 0.28
0.72 -0.54
438 513
2 51 49
Te oc
0.14 0.30 0.20 0.13 0.30 0.25
0.42 0.34 0.34 0.42 0.38 0.38
0.70 -0.51 -0.56 0.69 -0.51 -0.53
418 494 466 413 489 461
55 37 8 ::
Te oc Py Te
12
g
0.14 0.31 0.26 0.14 0.31 0.28
0.38 0.32 0.32 0.42 0.31 0.31
0.72 -0.53 -0.52 0.72 -0.52 -0.42
420 495 466 416 493 469
54 40 6 57 29 14
Te oc Py Te Oc Py
0.18 0.35 0.24 0.13 0.30 0.22
0.44 0.34 0.34 0.48 0.34 0.38
0.71 -0.52 -0.61 0.70 -0.51 -0.57
422 498 471 413 489 462
55 38 7 57 33 10
Te Oc Py Te oc Py
0.14 0.30 0.26 0.18 0.34 0.31
0.48 0.38 0.38 0.60 0.38 0.40
0.70 -0.50 -0.44 0.68 -0.49 -0.45
412 485 452 407 482 451
54 39 7 59 32 9
Te oc Py Te Oc Py
t IS: isomer shift relative to metallic iron; AE: quadrupole shift parameter; f: half-height width; Hf: hyperfine field; 1%: intensity of different sites.
with the number of pyramidal sites deduced from the chemical formula (Table 3) shows that the pyramidal sites are fully occupied by Fe(M). One indeed
observes a very good agreement in the limit of error between the experimental and calculated values, corresponding to 0.1 and 0.2 Fe(II1) in pyramidal
Table 3. Fitt;ir i~F;sity M
X
cu 0.1
0.2 Ni 0.1
0.2 Mg 0.1
0.2 Zn 0.1 0.15
Sites
Fe’+ distribution per cell deduced from the refinement
Calculated occupation deduced from the formula Ca, Fe, - .MJ)s - .r,r
Te oc PY Te oc PY
55 37 8 55 33 12
1.04 0.70 0.15 0.99 0.59 0.22
l.OFe 0.8Fe + O.lCu O.lFe l.OFe 0.6Fe + 0.2Cu 0.2Fe
Te oc PY Te oc PY
54 40 6 57 29 14
1.03 0.76 0.11 1.03 0.52 0.25
1.OFe 0.8Fe + O.lNi O.lFe l.OFe 0.6Fe + 0.2Ni 0.2Fe
Te oc Py Te oc Py
55 38 7
rk;
:: 10
0.13 1.03 0.59 0.18
1.OFe 0.8Fe + 0.1 Mg O.lFe 1.OFe 0.6Fe + 0.2Mg 0.2Fe
:t: 7 59 32 9
I .03 0.74 0.13 1.09 0.59 0.17
l.OFe 0.8Fe + O.lZn O.lFe 1.OFe 0.7Fe + 0.15Zn 0.15Fe
Te oc Py Te oc Py
82
R. BENLOUCIFeral.
Fig. 3. Temperature dependence of reciprocal molar magnetic susceptibility of Ca, Fe, _I M,O, _.,,*phases (M = Fe, Cu, Ni, Mg, Zn). coordination, respectively. Thus, the MGssbauer study confirms the oxygen deficiency observed by chemical analysis, and allows the cationic distribution to be determined according to the formula Ca, (Fe? ),(FeY! r.rM.VL(Fe.t” )@OS- .riz. The variation of the reciprocal molar susceptibility X;’ vs temperature (Fig. 3) shows that the substi-
p:Zb/m
I
dso
Fig. 4. Diagram representing tures upon the ratio p = 2b/
I
, T(K) 7sO
1tempera-
tuted oxides exhibit a rather broad antiferromagnetic transition compared with Ca,Fe,O,. The NCel temperature that we observe for this latter oxide is close to that observed by other authors (725 K). Nevertheless, our values for the susceptibility are slightly higher than those observed by Grenier [12]. The number of Bohr magnetons and the temperature 0,, could not be determined with any accuracy for the substituted compounds owing to the fact that our measurements are limited to a temperature of 950 K. The relatively high magnetic susceptibility observed for the zinc substituted phases, measured below 400 K, shows the formation of a ferrimagnetic impurity. Nevertheless, the very small amount of this phase did not allow us to detect its presence either by X-ray diffraction or by Miissbauer spectroscopy. The other substituted brownmillerites exhibit a classical decrease of X, and of TN as x increases (Table 1). However, a remarkable feature is that the NCel temperature of the magnesium phases is higher than those of the copper and nickel oxides in spite of the fact that M$+ is diamagnetic, contrary to Cu*+ and Niz+. Such a phenomenon has previously been observed in the series Ca,Fe,,jMO,,O, (M = Mn’+, Al’+, Gal+, SC’+, Mn4+/Zn2+) [7, 131. It was indeed shown that the decrease of TN for the manganese substituted brownmillerite with respect to the other phases was the consequence of the weaker interaction J, between the octahedral and tetrahedral layers, which was due to the Jahn-Teller effect for Mn3+ along 6. Clearly, the ratio p =26/.,/m of 3.89 observed for the Mn’+ phase, much greater than
Oxygen-deficient brownmillerites
those observed for Al’+ (3.79) and Mn’+/Zn*+ (3.81) oxides, can be considered as responsible for such behavior. Although not so dramatic, the evolution of TN vs the ratio p = 26/d= in the oxides Ca,Fe,_.KM,05_,,/r is in agreement with this interpretation, as shown from the decrease of TN as p increases (Fig. 4). Thus this magnetic study reinforces the concept according to which this size effect, i.e. the interactions between the octahedral (or pyramidal) and tetrahedral layers, plays a predominant role in the magnetic properties of the brownmillerite phases.
REFERENCES I. Bertaut E. F.. Blum P. and Sagnieres A., Acta crystallogr. 12, 149 (1959).
83
2. Raveau B., Proc. Indian natn. Sci. Acad. 52A, 61(1986). 3. Gallagher P. K.. Mac Chesney J. B. and Buchanan D. N. E., J. them. Phys. 41, 2429 (1964). 4. Greaves C., Jacobson A. J., Tofield B. C. and Fender B. E. F.. Acta crystallogr. 8381, 641 (1975). 5. Gallaaher P. K.. Mac Chesnev J. B. and Buchanan D. N-E.. J. chetk Phys. 43, 5i6 (1965). Grant R. W., Wiedersich H., Geller S.. Gonser U. and Espinosa G. P., /. appl. Phys. 38, 1455 (1967). Goates R. V. and McMillan J. W., J. appl. Chem. 14, 346 (1964). Akiyama T.. Mater. Res. Bud/. 16, 469 (1981). Neu P., Zanne M., Gerardin R. and Gleizer C., Ann. Chim. Fr. 6, 525 (1981).
10. Grenier J. C., Pouchard M. and Hagenmuller P., J. Solid St. Chem. 13. 92 (1975). 11. Er-Rakho L., Miche1.C.. Lacorre Ph. and Raveau B., J. Solid St. Chem. 73, 531 (1988). 12. Grenier J. C.. These. Bordeaux. France (1976). 13. Bando Y.. Takada T: and Akiyama T.. Buk. init. Chem. Res. Kyoto Univ. 49, 342 (1971).
0022.369?!5Q S3.M) + 0.00 b 1990 Pcrgamon Pfes plc
1990
OXYGEN-DEFICIENT BROWNMILLERITES: CATION COORDINATION AND MAGNETIC PROPERTIES R. BENLOUCIF,~ N. NGUYEN,? J. M. GRENECHE~ and B. RAVE& tl-aboratoire CRISMAT, ISMRa, Bd du Marichal Juin, 14032 Cacn C&x, France &aboratoire
de Spectromitrie MBssbauer, U.A. 807,Universite du Maine, 72017 Le Mans C&iex, France (Received 5 July 1989; accepted in revised form 7 September 1989)
Abstract-New oxygen-deficient brownmillerites Ca,Fe2_,,M,0,_ r!Zhave been synthesieed and characterized bv X-rav diffraction and chemical analvsis for M = Me. Ni. Cu. Zn. The Mrissbautr study of these phases shows only the presence of Fe(II1) and confirms th;~oxygen.non-stoichiometry; it suggests the existence of pyramidal sites and allows the ordered cationic distribution to be established according to the formula Ca,(Fe$ (Fe”’ , _zrM~),(Fe,~‘)pyOs _ri2. The study of the magnetic susceptibility vs temperature shows a broad an;iferromagnetic transition, with a classical decrease of F, vs x. A particular feature is the higher TN value of the magnesium phase compared with the nickel and copper oxides. This latter behavior is interpreted in terms of size factors influencing the interaction between the octahedral (or pyramidal) and tetrahedral layers. Keywords: BrownmilIerite, oxygen-deficient oxides, oxygen non-stoichiometry, properties.
INTRODUCTION The brownmillerite
structure
was determined
for the
first time by Bertaut et al. [ 11for the oxide Cal Fe20s. It can be described [2] as an ordered oxygen-deficient perovskite in which one layer of FeO, octahedra alternates with one layer of FeO, tetrahedra (Fig. 1). Isotypic compounds have been obtained for Sr,Fe,Os [3,4] and BazFe20, [5] and substituted oxides in which Fe’+ is replaced either by another trivalent cation f6] or by a couple Mn’+-Mz+ [7]. The possibility of introduction of an oxygen excess, has been shown in the case of manganese-iron oxides leading to the non-stoichiometric phase Car Fez _ zVMn,O, + 6 (0 Q x d 0.25) [S]. No oxygen-deficient brownmillerite has been observed for either calcium or for strontium compounds. Only Ba,Fe,O, was found to be stable in the presence of oxygen vacancies, leading to the limiting phase Ba,Fe,O,, [9]. Moreover, the magnetic properties of the materials, and especially the antiferromagnetic order temperature TNcan be affected by the distribution of the cations in the octahedral and tetrahedral sites [lo], and depend also on the exchange integrals which are governed by the C~stalIographic cell, and consequently by the size of the cations. We report here on the study of the magnetic properties of oxygen-deficient brownmillerites Ca2 Fe, _.VM,Os _ r,Z(M = Cu. Ni, Mg, Zn) in connection with the cationic distribution and the oxygen deficiency of these oxides. EXPERIMENTAL
Synthesis and X-ray study The oxtdes Car Fe, _ J M,O, _ .r;t+6 were prepared by heating mixtures of chemically pure CaCO,, FetO, 79
Miissbauer study, magnetic
and divalent oxides MO (M = Cu, Ni, Zn, Mg) at temperatures ranging from 900 to 1200°C. The zinccompound was heated progressively up to 1100°C in order to limit the volatilization of zinc. Repeated thermal treatments were required to obtain homogeneous compounds. To determine the oxygen content, chemical analysis was carried out by redox back titration using an Fe(H) standard solution. For both phases the experimental value S is very small (6 < 0.03). The X-ray powder diffractograms were recorded with a Philipsgoniometer using CuKx radiation. Magnetic study The different substituted materials were characterized by Miissbauer spectrometry: the spectra were recorded in transmission geometry with a constant acceleration spectrometer, using a s7Co source diffused into a Rh-matrix at room temperature. The magnetic susceptibility was measured with a Faraday balance in the range of temperature 300-950 K.
RESULTS AND DISCUSSION From the X-ray diffraction study, it is clear that brownmillerite can be synthetized as a pure phase with a maximum substitution of 10% for M = Cu, Ni, Mg (0 d x ;SE0.20) and of 7.5% for M = Zn (0 Q x Q 0.15). The crystal data (Table 1) show that the “a” and “c” parameters of the orthorhombic cell are not really affected by the substitution. On the other hand, the “b” parameter increases with the size of the cations. The chemical analysis shows without any ambiguity that 6 is close to zero in the fo~ulation
R. BENLOCXIF et ai.
80
Fig. 1. Brownmillerite structure. Space group: fnma.
ca2Fe2-.yWA4- .ri2 + 6 (6 < 0.03). consistent with copper, nickef and iron in the low oxidation states, Cu” Ni” and Fe”‘, respectively. The latter result is cbnfirmed by Mtissbauer spectroscopy, which does not detect Fe(W) in any of the samples. Consequently, these oxides can be formulated as Ca, Fe:‘_, MflOS r *. The MGssba&‘spectra recorded at room temperature for several oxides are presented in Fig. 2 and compared with that of Ca,FeZO,. In contrast to the Ca,Fe?O, pattern, which exhibits two well-resolved magnetic contributions, the spectra of the substituted oxides are characterized by several anomalies. One indeed observes that some of the fines are broader and asymmetrical, whereas the outer lines of the external sextet corresponding to the octahedral sites are weakly split. This shows that two types of “octahedral” sites are available for iron. Thus, in a first approach, the magnetic Mtissbauer spectra can be analyzed in terms of one tetrahedral contribution and two octahedral contributions whose hyperfine field values vary from 24 to 33 kG (Table 2). The refinement of the cationic distribution shows that about 55% of iron is located on the tetrahedral sites for x = 0.10 against 57% for x = 0.20. This leads in the limit of the error, to a total occupation of the tetrahedrat sites by Fe”’ (Table 3). Clearly, at1 the divalent cations are preferentially distributed over Table 1. Crystallographic data and antiferromagnetic ordering temperatures for CalFez _c M,O,_,,, compounds M Fe CU
x
Ni
0.1 0.2 8::
Mg
0.1
Zn
0.2 0.1 0.15
a (A)
b (A)
c (A)
TN (W
5.580 (1) 5.579(l) 5.583 (1) 5.589(l) 5.590(l) 5.596 (I) 5.600(l) %590(l) 5.601 (2)
14.752 (2) 14.813(3) 14.823 (2) 14.790(l) 14.802 (2) 14.791 (3) 14.808(2) 14.803(4) 14.621 (3)
5.421 (1) S.410(1) 5.409 (1) 5.410(t) 5.400 (I) 5.420 (t) 5.421 (I) 5.413(i) 5.4W (2)
722 666 658 680 665 720 689 -
A
V(mm.s”J
Fig. 2. Mbssbauer spectra at room temperature of Ca,Fe,.~M&,, tM = Fe, Cu, Ni, Mg) and Ca,Fe,.~, Zn0.A.9z5 phases.
the octahedral sites. Moreover, the presence of only Fe(IIf) from the Miissbauer refinement ctearly estabIishes the existence of additional anionic vacancies (x/2) with respect to the normal brownmillerite according to the formula Caz Fe, _,rM,YO,_ _r/2 ; this implies that some of the cations in the two types of “octahedral” sites in fact exhibit a coordination smaller than six, characterized by the formation of oxygen vacancies in the basal plane {a, c), The value of the hyperfine fiefd of the second “octahedral” site, which is significantly smaller than that of the first one, suggests that it is this site which is not in fact octahedral but characterized by a smaller coordination number, leading to a pyramidat coordination for Fe(III). The ability of Fe(IIEf to take this type of coordination as shown for BaYFeCuQ [l l] suggests strongly this point of view. The consideration of the chemical formula, Ca, Fe!! .YMf’OJ_ x,2, leads then to the formation of x pyramidal sites (Py) against (1 - X) octahedral site (Oc) per formula unit, whereas the tetrahedral site (Te) remains equal to one. The comparison of the iron distribution deduced from the refinement of the intensity of Miissbauer spectra,
Oxygen-deficient brownmillerites Table 2. The hypertine M&batter
x
M
CazFezO,
cu 0.1 0.2 Ni 0.1 0.2 Mg 0.1 0.2 Zn 0.1
0.15
(+O.Ol’~ms-1)
81
parameters at room temperature compounds (fO.O21;nms-I)
for Ca,Fe,_,M,yO,_,,r
Sites
(fO.Ofims-I)
&!LG
0.18 0.36
0.28 0.28
0.72 -0.54
438 513
2 51 49
Te oc
0.14 0.30 0.20 0.13 0.30 0.25
0.42 0.34 0.34 0.42 0.38 0.38
0.70 -0.51 -0.56 0.69 -0.51 -0.53
418 494 466 413 489 461
55 37 8 ::
Te oc Py Te
12
g
0.14 0.31 0.26 0.14 0.31 0.28
0.38 0.32 0.32 0.42 0.31 0.31
0.72 -0.53 -0.52 0.72 -0.52 -0.42
420 495 466 416 493 469
54 40 6 57 29 14
Te oc Py Te Oc Py
0.18 0.35 0.24 0.13 0.30 0.22
0.44 0.34 0.34 0.48 0.34 0.38
0.71 -0.52 -0.61 0.70 -0.51 -0.57
422 498 471 413 489 462
55 38 7 57 33 10
Te Oc Py Te oc Py
0.14 0.30 0.26 0.18 0.34 0.31
0.48 0.38 0.38 0.60 0.38 0.40
0.70 -0.50 -0.44 0.68 -0.49 -0.45
412 485 452 407 482 451
54 39 7 59 32 9
Te oc Py Te Oc Py
t IS: isomer shift relative to metallic iron; AE: quadrupole shift parameter; f: half-height width; Hf: hyperfine field; 1%: intensity of different sites.
with the number of pyramidal sites deduced from the chemical formula (Table 3) shows that the pyramidal sites are fully occupied by Fe(M). One indeed
observes a very good agreement in the limit of error between the experimental and calculated values, corresponding to 0.1 and 0.2 Fe(II1) in pyramidal
Table 3. Fitt;ir i~F;sity M
X
cu 0.1
0.2 Ni 0.1
0.2 Mg 0.1
0.2 Zn 0.1 0.15
Sites
Fe’+ distribution per cell deduced from the refinement
Calculated occupation deduced from the formula Ca, Fe, - .MJ)s - .r,r
Te oc PY Te oc PY
55 37 8 55 33 12
1.04 0.70 0.15 0.99 0.59 0.22
l.OFe 0.8Fe + O.lCu O.lFe l.OFe 0.6Fe + 0.2Cu 0.2Fe
Te oc PY Te oc PY
54 40 6 57 29 14
1.03 0.76 0.11 1.03 0.52 0.25
1.OFe 0.8Fe + O.lNi O.lFe l.OFe 0.6Fe + 0.2Ni 0.2Fe
Te oc Py Te oc Py
55 38 7
rk;
:: 10
0.13 1.03 0.59 0.18
1.OFe 0.8Fe + 0.1 Mg O.lFe 1.OFe 0.6Fe + 0.2Mg 0.2Fe
:t: 7 59 32 9
I .03 0.74 0.13 1.09 0.59 0.17
l.OFe 0.8Fe + O.lZn O.lFe 1.OFe 0.7Fe + 0.15Zn 0.15Fe
Te oc Py Te oc Py
82
R. BENLOUCIFeral.
Fig. 3. Temperature dependence of reciprocal molar magnetic susceptibility of Ca, Fe, _I M,O, _.,,*phases (M = Fe, Cu, Ni, Mg, Zn). coordination, respectively. Thus, the MGssbauer study confirms the oxygen deficiency observed by chemical analysis, and allows the cationic distribution to be determined according to the formula Ca, (Fe? ),(FeY! r.rM.VL(Fe.t” )@OS- .riz. The variation of the reciprocal molar susceptibility X;’ vs temperature (Fig. 3) shows that the substi-
p:Zb/m
I
dso
Fig. 4. Diagram representing tures upon the ratio p = 2b/
I
, T(K) 7sO
1tempera-
tuted oxides exhibit a rather broad antiferromagnetic transition compared with Ca,Fe,O,. The NCel temperature that we observe for this latter oxide is close to that observed by other authors (725 K). Nevertheless, our values for the susceptibility are slightly higher than those observed by Grenier [12]. The number of Bohr magnetons and the temperature 0,, could not be determined with any accuracy for the substituted compounds owing to the fact that our measurements are limited to a temperature of 950 K. The relatively high magnetic susceptibility observed for the zinc substituted phases, measured below 400 K, shows the formation of a ferrimagnetic impurity. Nevertheless, the very small amount of this phase did not allow us to detect its presence either by X-ray diffraction or by Miissbauer spectroscopy. The other substituted brownmillerites exhibit a classical decrease of X, and of TN as x increases (Table 1). However, a remarkable feature is that the NCel temperature of the magnesium phases is higher than those of the copper and nickel oxides in spite of the fact that M$+ is diamagnetic, contrary to Cu*+ and Niz+. Such a phenomenon has previously been observed in the series Ca,Fe,,jMO,,O, (M = Mn’+, Al’+, Gal+, SC’+, Mn4+/Zn2+) [7, 131. It was indeed shown that the decrease of TN for the manganese substituted brownmillerite with respect to the other phases was the consequence of the weaker interaction J, between the octahedral and tetrahedral layers, which was due to the Jahn-Teller effect for Mn3+ along 6. Clearly, the ratio p =26/.,/m of 3.89 observed for the Mn’+ phase, much greater than
Oxygen-deficient brownmillerites
those observed for Al’+ (3.79) and Mn’+/Zn*+ (3.81) oxides, can be considered as responsible for such behavior. Although not so dramatic, the evolution of TN vs the ratio p = 26/d= in the oxides Ca,Fe,_.KM,05_,,/r is in agreement with this interpretation, as shown from the decrease of TN as p increases (Fig. 4). Thus this magnetic study reinforces the concept according to which this size effect, i.e. the interactions between the octahedral (or pyramidal) and tetrahedral layers, plays a predominant role in the magnetic properties of the brownmillerite phases.
REFERENCES I. Bertaut E. F.. Blum P. and Sagnieres A., Acta crystallogr. 12, 149 (1959).
83
2. Raveau B., Proc. Indian natn. Sci. Acad. 52A, 61(1986). 3. Gallagher P. K.. Mac Chesney J. B. and Buchanan D. N. E., J. them. Phys. 41, 2429 (1964). 4. Greaves C., Jacobson A. J., Tofield B. C. and Fender B. E. F.. Acta crystallogr. 8381, 641 (1975). 5. Gallaaher P. K.. Mac Chesnev J. B. and Buchanan D. N-E.. J. chetk Phys. 43, 5i6 (1965). Grant R. W., Wiedersich H., Geller S.. Gonser U. and Espinosa G. P., /. appl. Phys. 38, 1455 (1967). Goates R. V. and McMillan J. W., J. appl. Chem. 14, 346 (1964). Akiyama T.. Mater. Res. Bud/. 16, 469 (1981). Neu P., Zanne M., Gerardin R. and Gleizer C., Ann. Chim. Fr. 6, 525 (1981).
10. Grenier J. C., Pouchard M. and Hagenmuller P., J. Solid St. Chem. 13. 92 (1975). 11. Er-Rakho L., Miche1.C.. Lacorre Ph. and Raveau B., J. Solid St. Chem. 73, 531 (1988). 12. Grenier J. C.. These. Bordeaux. France (1976). 13. Bando Y.. Takada T: and Akiyama T.. Buk. init. Chem. Res. Kyoto Univ. 49, 342 (1971).