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Formation of manganese- and manganese,zinc-bearing ferrites by oxidation of aqueous suspensions and analysis of their cation distributions
JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Formation of manganese- and manganese,zinc-bearing ferrites by oxidation of aqueous suspensions and analysis of their cation distributions Tadao Kanzaki a'*, Koichi Kikuchi a, Mitsuhiko Sato a, Kiyoshi Nagai a, Takashi Oowada a, Hiroaki Onozuka a, Kenzo Kitayama b alwaki Meisei University, lwaki-shi, Fukushima 970, Japan bDepartment of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Received 2 September 1995; accepted 3 November 1995
Abstract
Oxidized manganese-bearing and manganese,zinc-bearing ferrites have been prepared by air oxidation of iron(II) hydroxide suspensions at initial Mn : Fetot mol ratios (rMn) of 0.20 : 2.80 to 1.40 : 1.60 and initial (Mn + Zn) : Fetot mol ratios (rMn+Zn) of 0.20 : 2.80 to 1.00 : 2.00, respectively, at pH 10.0 and 65°C. Mfssbauer spectra at room temperature and chemical analysis of the manganese-bearing ferrites suggest that Mn 2+ ions are incorporated into the tetrahedral sites in the ferrites with manganese compositions of less than 0.74 and into both the tetrahedral and the octahedral sites at compositions of 0.74 to 1.06. In addition, manganese ions in a higher valence state than 3+ are incorporated into the octahedrai sites at manganese compositions above 0.74. M6ssbauer spectra at room temperature of the manganese,zinc-bearing ferrites prepared at rMn+Zn ---- 1.00:2.00 indicate that Mn 2+ ions are incorporated on the lattice points in the ferrites and in both the tetrahedral and the octahedral sites at manganese compositions of 0.33 to 0.51. Mfssbauer spectra and chemical analysis also suggest the incorporation of manganese ions in valence states higher than 3+. M6ssbauer spectra and the lattice constants imply that the octahedral lattice sites in the ferrites with manganese compositions equal to or greater than 0.51 are distorted. M6ssbauer spectra at room temperature of the manganese,zincbearing ferrites prepared at rMn+Zn = 0.20:2.80 to 0.80:2.20 suggest that Mn 2+ ions are incorporated into both the tetrahedral and the octahedral sites in the ferrites, and that the amounts of Mn 2+ ions on the octahedral sites increase with increases in the manganese composition of the ferrites prepared at each rM, + Z, value.
Keywords." Ferrite; Manganese-bearing ferrite; Manganese,zinc-bearing ferrite; M6ssbauer spectrum
I. Introduction Manganese- and manganese,zinc-bearing ferrites have been studied extensively because of their interesting physical properties and their importance * Corresponding author.
for technical application. Kiyama [1] studied the formation by air oxidation (at R(2OH/(Fe2++ Mn2+)) = 1.1 to 3.0 and 70°C) of manganesebearing ferrites MnxFe3_xO4 in an aqueous iron(II) hydroxide suspension containing manganese ions. Kiyama [1] reported that only oxidized manganese-bearing cubic spinel ferrites
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T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
MnxFe3_x04+ 6 formed in the suspension at 0~ 1.4 tetragonally deformed spinel ferrite coexisted with the cubic spinel ferrites. The lattice constants a0 of the manganese-bearing cubic spinel ferrites at x ~< 2.0 increased with increases in x (at x >~ 1, the slope of the straight lines decreased) and approached 0.85 nm. The value for a manganese-bearing ferrite MnFe204, prepared by a solid-state reaction, is 0.8499 nm [2]. Heating the ferrite sample in vacuo at 450°C increased the lattice constants, a result that is in good agreement with those for manganese-bearing ferrites MnxFe3_xO4 with the same composition, prepared by a solid-state reaction (at x ~< 1, the lattice constants increased linearly with increases in x, and at x >~ 1, these values became constant at about 0.85 nm). Lotgering and Van Diepen [3] theorized, from M6ssbauer spectra and saturation moment measurements, that Mn 2+ ions occupy the octahedral sites exclusively in manganese-bearing ferrites MnxFe3_xO 4, at manganese compositions x of up to 0.53, and that at x ~>0.7 the Mn z+ ions switch from the octahedral to the tetrahedral sites. These researchers proposed a cation distribution model in the manganese-bearing ferrites at x>~l. 3
2-
3+
(Mn~+_eFee+)tet [Mn~* Mnx - 1Fe~+ ~_x]octO4
(1)
Here, the amount of divalent ions on the octahedral sites is denoted by e. However, from X-ray diffraction analysis and thermogravimetric measurements, El Guendouzi et al. [4] proposed a cation distribution in the spinel lattice of manganese,zinc-bearing ferrites that can be expressed by the formula
2. Experimental procedure 2.1. Reagents Analytical-grade reagents were used for the present experiment. Sodium hydroxide solutions were prepared using distilled water free of carbon dioxide and oxygen. A 0.8632 mol dm -3 manganese(II) sulfate solution was prepared from Mn(II) SO4.4.96H20.
2.2. Procedure The assays were conducted using a Dewar type reaction vessel (capacity, 2 dm 3) with a glass cover [5]. Distilled water, a 0.5 mol dm -3 sulfuric acid solution, a manganese(II) sulfate solution, zinc sulfate heptahydrate, and sodium sulfate were transferred to the reaction vessel, and nitrogen gas was bubbled into the solution with rapid stirring (600 rev min -1) for 1 h to remove the dissolved CO2 and oxygen. Iron(II) sulfate was then added (total amount of iron, zinc and manganese =0.4316 mol). The pH was adjusted to 10.0 by using both solutions containing either 2 or 5 mol dm -3 sodium hydroxide. The volume of the suspension was brought up to 2.00 dm 3 and the total sulfate ion concentration fixed at 0.375 mol dm -3. The suspension was left to stand under nitrogen for 1 h at 65°C with constant stirring. Air was passed through this suspension, designated the "initial suspension", at a rate of 0.2 dm 3 min -1. During oxidation the pH was maintained at 10.0 by adding a 0.2 mol dm -3 solution of sodium hydroxide, and the oxidation potential was measured. Oxidation stopped 10 min after the abrupt change in this parameter.
2+ 2+ 3+ 3+ 2+ 3+ 2(Mno.8xZny F e l _0.8x-y)tet[Fel +0.6x+yFe1-0.8x-yMno.2x]octO4
The present paper focuses on the formation of manganese and manganese,zinc-bearing ferrites by the air oxidation of aqueous iron(II) hydroxide suspensions containing manganese ions and both manganese and zinc ions, respectively. Cation distribution in the ferrites was also investigated and is discussed here.
(2)
These procedures have been described in detail by Kaneko and Katsura [5]. The final product was obtained by lowering the pH of the suspension to 4.0 with 0.5 mol dm -3 sulfuric acid after air oxidation. At that pH, the hydroxides of manganese(II), zinc, and iron(II) dissolved completely.
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
2.3. X-ray powder diffraction measurements X-ray powder diffraction patterns were obtained using manganese-filtered Fe K a radiation. The 20 angles of the diffraction peaks were calibrated versus standard silicon powder using the Fe Kal radiation.
2.4. M6ssbauer spectroscopic measurements M6ssbauer spectra were obtained at room temperature by applying cobalt-57 radiation with an electromechanical transducer operating in constant-acceleration mode in conjunction with a 512-channel analyzer. The velocity scale was calibrated using the hyperfine spectra of a natural a-iron foil at room temperature, which also served as the isomer shift reference.
223
[6], and manganese (>/ III) was reduced by an excess of iron(II) to manganese(II). The amounts of iron were determined by titration [5] with a 0.005 mol dm -3 standard solution of potassium permanganate and by subtracting the added amounts of iron (determination limit = 7x 10-6 mol, corresponding to 0.01 of iron of the composition 3 - (x + y); that is, MnxZnyFe3_ (x+y)O4 had an error of 3 - (x + y) + 0.01). Minimum amounts of manganese (>~ III) were determined from the amounts of 0.1 mol dm -3 iron(II) ammonium sulfate standard solution consumed. The amounts of manganese and zinc were determined to within 1% by atomic absorption spectrometry. The oxygen content was estimated by subtracting the weights of the manganese, zinc, and/or iron from the total weight of the solid sample.
2.5. Chemical analysis
3. Results and discussion
Solid samples (200 mg) of the products were dissolved in a mixture of 0.1 mol dm -3 iron(II) ammonium sulfate standard solution and 9 mol dm -3 sulfuric acid. Under this procedure, iron(II) was not oxidized by oxygen from the air
3.1. Formation of manganese-bearing ferrites in iron( II) hydroxide suspensions at initial Mn:Fetot mol ratios (rnn) of O.20 : 2.80 to 1.40 : 1.60
0.850
0
0.84e
~o 0-84£
0.844
0.84~
0.2
o14
0'.6
o18
11o
Mn:Fetot mol ratio Fig. 1. Variation of the lattice constant of spinel-type compound obtained from iron hydroxide suspension containing manganese ion with Mn : Fetot tool ratio. Error for the lattice constant -4-0.0001 nm.
The X-ray powder diffraction patterns of the products obtained from the iron(II) hydroxide suspensions exhibited only peaks for the spinel type of structure, and electron micrographs of the products obtained at rMn = 0.20:2.80 to 1.40:1.60 showed the formation of only a cubic particle of a spinel-type compound. Chemical analysis of the spinel-type compounds showed that the manganese content (Mn : Fetot mol ratio) increased from 0.068 to 0.715:1.00 with increasing rMn in the range from 0.20:2.80 to 1.40:1.60 (Mn : Fetot = 0.071 to 0.875 : 1.00); that is, 96% to 82% of the manganese in the initial hydroxide suspensions was taken into the spinel-type compounds. The lattice constants of the spinel-type compounds increased with increasing manganese contents from 0.068 to 0.449: 1.00, followed by a constant value (Fig. 1) [1]. These results suggest that the spinel-type compounds were manganesebearing ferrites. Fig. 2 shows the chemical compositions of the manganese-bearing ferrites prepared at rMn = 0.20:2.80 to 0.80:2.20 (manganese contents of
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
224
0.068 to 0.329 : 1.00) for the ternary system FeO, FezO3, and MnO. As shown by the figure, the chemical compositions of the manganese-bearing ferrites were located on the oxygen reaction lines (solid lines) and were slightly lower in FeO than those of stoichiometric, manganese-bearing ferrites (broken line). This result implies that oxidized manganese-bearing ferrites are formed by the air oxidation of iron(II) hydroxide suspensions with rMn = 0.20 : 2.80 tO 0.80 : 2.20. The sums of manganese and iron compositions for the manganesebearing ferrites formed at rMn = 1.00:2.00 to 1.40:1.60 ranged from 2.62 to 2.93, i.e. oxidized manganese-bearing ferrites also formed. 3.2. Cation distribution for manganese-bearing ferrites Fig. 3 shows the M6ssbauer spectra at room temperature of the manganese-bearing ferrites. These M6ssbauer spectra were analyzed using a least-squares-fit computer program [7]. The results are shown in the figure by solid lines, and M6ssbauer parameters are listed in Table 1. Area fractions were calculated from the peak area estimated from the least-squares fit by assuming that the nonrecoiled fractions for the different spectra were equal. The M6ssbauer spectra for the ferrites showed two hyperfine splitting patterns, A and B, but resolution began to deteriorate with increasing manganese composition. The M6ssbauer spectra
(a) A,B,
,
,
25 v tO O_
<
-0.8
MnO
',
0 Velocity/cm s-1
0.8
Fig. 3. M6ssbauer spectra at room temperature of manganesehearing ferrites.Manganesecompositions:0.18 (a), 0.74 (b) and 0.81 (c). Solid lines representleast-squares fit; A and B patterns are indicated.
FeO
Fe3Oz,
Fe203
Fig. 2. Chemical composition of manganese-bearing ferrites for the ternary system FeO, Fe203 and MnO: . . . . , value for a solid solution of FeaO4-MnFe204; - - , oxygen reaction lines.
at room temperature for Fe304, showed similar hyperfine splitting patterns A and B, arising from Fe 3+ ions at the tetrahedral sites and from Fe 2+ and Fe 3+ ions at the octahedral sites, between which a rapid electron exchange occurred [8]. For manganese-bearing ferrites, the isomer shift of the hyperfine splitting pattern A equalled the value (0.029 cm s -1) of pattern A for Fe304 within experimental error. Apparently, then, pattern A
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
225
Table 1 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn-bearing ferrites prepared from aqueous suspensions at rMn = 0.20 : 2.80 to 1.20 : 1.80 Chemical composition
M6ssbauer pattern
6 a / c m s -1
A a / c m S-x
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mn0A8Fe2.6504.00
A B
0.031 0.062
0.002 0.000
499 464
0.430 0.570
0.309 0.691
Mn0.36Fe2.4204.00
A B
0.031 0.056
0.001 0.003
499 467
0.404 0.596
0.264 0.736
Mn0,54Fe2.3504.00
A B
0.030 0,049
0.000 0.004
500 469
0.338 0.662
0.196 0.804
Mn0,74Fe2.2304.00
A B
0.031 0.042
0.000 0.001
498 468
0.261 0.739
0.117 0,883
Mn0.sl Fe181O4,00
A B
0.033 0.041
0.000 0.001
494 463
0.287 0.713
0.105 0.895
Mnl.06Fe1.8704.00
A B
0.031 0.039
-0.001 0.001
490 458
0.249 0.751
a Values with respect to natural a-iron foil; error +0.004 c m s - l . b l Oe = 1000/4~r A m - l . c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and s u m of the compositions of M n 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00.
for the manganese-bearing ferrites was caused by Fe 3+ ions at the tetrahedral sites. The isomer shift of the hyperfine splitting pattern B for the manganese-bearing ferrites with a manganese composition of 0.18 was nearly equal to the value (0.067 cm s-1) of pattern B for Fe304, but the isomer shifts of the ferrites with manganese compositions less than or equal to 0.74 decreased with increasing manganese composition. (The values for ferrites with manganese compositions -of 0.74, 0.81 and 1.06 were the same within experimental error.) These values suggest that pattern B was caused by Fe 2÷ and Fe 3+ ions at the octahedral sites, between which a rapid electron exchange occurred, and that the Fe2+:Fe 3+ ion ratios at the octahedral sites decreased with increasing manganese composition up to 0.74; that is, the valence state of the manganese ions incorporated into the lattice sites was 2+. Such a valence state was also suggested by the chemical compositions of the manganese-bearing ferrites with manganese
compositions of 0.18 to 0.74 for the ternary system FeO, FezO 3 and MnO. The area fractions for hyperfine splitting pattern A in the ferrites with manganese compositions of less than 0.74, as estimated from the M6ssbauer spectra, decreased with increasing composition, and those for hyperfine splitting pattern B increased with increasing manganese composition. Such values imply that Mn 2+ ions had been incorporated into the tetrahedral sites in the ferrites with manganese compositions below 0.74. The area fractions of hyperfine splitting patterns A and B were the same for manganese-bearing ferrites with manganese compositions of 0.74, 0.81, and 1.06. Evidently, Mn 2+ ions had been incorporated into both the tetrahedral and the octahedral sites. However, the area fractions for pattern A were lower than those calculated from the chemical compositions under the assumption that Mn 2+ ions had been incorporated into the tetrahedral sites, and the sum of Mn 2+ and Fe 3+ ions in the tetrahedral sites
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T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Table 2 Chemical composition of the Mn,Zn-bearing ferrites Initial suspension
Chemical composition a
(Mn + Zn) : Feto t mol ratio, rMn+ Zn
Mn : Zn mol ratio
Mn
0.20 : 2.80
0.00:0.20 0.05 : 0,15 0.10 : 0.10 0.15 : 0,05 0.20 : 0.00
0.05 0.10 0.14 0.18
0.00 : 0.40 0.10 : 0.30 0.20 : 0.20 0.30:0.10 0.40 : 0.00
0.10 0.19 0.28 0.36
0.00 0.15 0.30 0.45 0.60
: 0.60 : 0.45 : 0.30 : 0.15 : 0.00
0.14 0.28 0.41 0.54
0.00 : 0.80 0.20 : 0.60 0.40 : 0.40 0.60:0.20 0.80 : 0.00
0.18 0.36 0.56 0.74
0.00 : 1,00 0.20 : 0.80 0.40 : 0.60 0.60 : 0.40 0,80:0.20 1.00 : 0.00
0.17 0.33 0.51 0.70 0.81
0.40 : 2.60
0.60 : 2,40
0.80 : 2.20
1.00 : 2.00
Zn
Fe
(Mn + Zn + Fe)
0.19 0.15 0.10 0.05
2.86 2.76 2.73 2.74 2.65
3.05 2.96 2.93 2.93 2.83
0.37 0.29 0.19 0.09
2.55 2.52 2,59 2.48 2.42
2.92 2.91 2.97 2.85 2.78
0.54 0.45 0.29 0.14
2.28 2.39 2.32 2.26 2.35
2.82 2.98 2.89 2.81 2.89
0.72 0.57 0.38 0,19
2.01 2.19 2.20 2.19 2.23
2.73 2.94 2.94 2.94 2.97
0.93 0.75 0.53 0.34 0.19
1.94 2.02 1.89 1.86 1.99 1.81
2.87 2.94 2.75 2.71 2.88 2.62
a For all ferrites the value for oxygen is 4.00.
equalled 1.00. Thus, Mn 2+ ions may have been incorporated not only into the tetrahedral sites but also into the octahedral sites in the manganese-bearing ferrites with manganese compositions less than or equal to 0.74. Lotgering and Van Diepen [3] compared the experimental saturation moments and values calculated for N6el coupling. These researchers suggested that at manganese compositions up to 0.53, the Mn 2+ ion occupies the octahedral sites exclusively, and at manganese compositions greater than or equal to 0.7, the Mn 2+ ions switch from octahedral to tetrahedral sites. The present results, mentioned earlier, are not in agreement with these suggestions.
The isomer shifts of hyperfine splitting pattern B for manganese-bearing ferrites with manganese compositions greater than 0.74 were constant, however, suggesting that manganese ions in a valence state higher than 3+, and also Mn 2+ ions, had been incorporated into the octahedral sites. Chemical analysis of the ferrites with manganese compositions of 0.81 and 1.06 by the addition of an iron(II) ammonium sulfate solution showed the incorporation of manganese ions in a valence state higher than 3+, i.e. greater than 3, and more than 21% of the manganese composition as the Mn 3+ ion, respectively. Lotgering and Van Diepen [3] proposed the incorporation of Mn 2+ ions into
T. Kanzaki et al./ Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
0.00
Manganese composition 0.05 0.10 0.15
0.2( 0.843
0.842
0-849
0.848
~/~
o
E
~/~)
0.841 ~o
0-847
0.840 0-8/,(
o.o
o:2
0:4
0:6
o:8
Manganese composition
0.839
~.o
Fig. 4. Variation of the lattice constant of manganese,zincbearing ferrites with manganese composition. Curves (a) and (b) represent the variations for the ferrites obtained at initial (Mn + Zn) : Fetot mol ratios (rM,+Zn) of 1.00:2.00 and 0.20 : 2.80, respectively.
the tetrahedral and octahedral sites in manganesebearing ferrites MnxFe3_xO4 (x >/1) prepared by a solid-state reaction and the presence of Mn 3+ ions at the octahedral sites. The present theory of cation distribution for manganese-bearing ferrites with manganese compositions of more than 0.74 is in good agreement with that of Lotgering and Van Diepen.
3.3. Formation of manganese,zinc-bearing ferrites in iron(II) hydroxide suspensions with initial (Mn + Zn) : Feto t mol ratios (rMn+Zn) of 0.20 : 2.80 to 1.00 : 2.00 The X-ray powder diffraction patterns and electron micrographs of the products obtained from iron(II) hydroxide suspensions only exhibited peaks for a spinel type of structure and cubic particles of a spinel-type compound. Chemical analysis of the spinel-type compounds showed that more than 84% manganese and more than 92% zinc in the suspension had been taken into the spinel-type compounds. Such facts suggest that
227
the spinel-type compounds were manganese,zincbearing ferrites. The chemical compositions of the manganese,zinc-bearing ferrites are listed in Table 2. These chemical compositions show that the sums of manganese, zinc, and iron compositions were in the range of 2.71 to 2.97; i.e. oxidized manganese,zinc-beating ferrites had formed. In addition, chemical analysis of the manganese,zincbearing ferrites obtained at rMn+Zn = 1.00:2.00 with the addition of an iron(II) ammonium sulfate solution showed that manganese ions in a valence state higher than 3+ had been incorporated into the ferrites (>2% to 8% manganese as Mn3+). The lattice constants a0 of the manganese,zincbearing ferrites obtained from the iron(II) hydroxide suspensions at rMn+Zn = 1.00 : 2.00 are plotted versus the manganese compositions of the ferrites in Fig. 4, curve (a). The lattice constants were invariant with manganese composition in the range 0-0.33 and exhibited an abrupt increase above 0.51. This variation in the lattice constants with the manganese composition of the ferrites cannot be explained by an ideal solid solution of MnFe204-ZnFe204 [7]. However, the lattice constants a0 of the manganese,zinc-bearing ferrites obtained from iron(II) hydroxide suspensions at rMn+Zn =0.20:2.80 to 0.80:2.20 increased rapidly with increasing manganese compositions, then slowed to a gradual increase, and finally increased rapidly again. (Fig. 4, curve (b) shows the variation for the ferrites obtained at rMn+Zn = 0.20:2.20.) The region of gradual increase for the lattice constants became unclear as the rMn+Zn values in the suspensions increased.
3.4. Cation distribution for the manganese,zincbearing ferrites Fig. 5 shows the M6ssbauer spectra at room temperature of manganese,zinc-bearing ferrites obtained from the iron(II) hydroxide suspensions at rMn+Z, = 1.00 : 2.00. The results, analyzed using a least-squares-fit computer program, are shown by solid lines, and Mtssbauer parameters and area fractions are listed in Table 3. The Mtssbauer spectra show hyperfine splitting patterns and a paramagnetic pattern, E. The hyperfine splitting patterns for the ferrites with
228
T. ICanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
(a) A,Br"
I
!
I
I
8,r-7
IVl;
(b) l
J
i
D
cO ",,T_, Q.
(c)
"
Y 7'
i
-0.8
I
I
1
I
0 Veloci ty/cm s-1
I
I
I
0.8
Fig. 5. M6ssbauer spectra at room temperature of manganese,zinc-bearing ferrites obtained at initial (Mn + Zn) : Feto t tool ratio (rMn +Zn) of 1.00:2.00. Manganese compositions: 0.17 (a), 0.33 (b) and 0.70 (c). Solid lines represent least-squares fits; A, B and B' patterns are indicated.
manganese compositions greater than or equal to 0.33 consisted of two types, A and B. The area fractions of the paramagnetic pattern B' increased with increasing zinc composition, and the isomer shifts of pattern B' showed a value arising from the Fe 3+ ion. These results suggest that pattern B' was caused by Fe 3+ ions at the octahedral sites in the zinc-bearing ferrite ZnFe204. The M6ssbauer spectra at room temperature for zinc-bearing
ferrites, Z n x F e 3 - x O 4 (1.0 > x >-0.6), exhibited a paramagnetic pattern attributable to ZnFe204 [8,9]. This pattern can be explained by considering that any value of x results in different fractions of octahedral cations surrounded by different fractions of near-neighbor tetrahedral cations that a r e F e 3+ because of the random distribution of Zn 2+ and Fe 3+ ions among the tetrahedral sites [9]. The quadrupole splitting A of the
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
229
Table 3 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn,Zn-bearing ferrites prepared from aqueous suspensions at rMn+Zn = 1.00 : 2.00 Chemical composition
M6ssbauer pattern
6 a / c m s -1
A a / c m s -1
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mno.lvZnovsFez0204.00
A+ B B~
0.057 0.039
0.000 0.048
373 -
0.705 0.295
Mn0.33Zn0.53 Fel.8904.oo
A B B'
0.036 0.048 0.039
0.004 0.001 0.049
429 374 -
0.149 0.721 ~ 0.130 1
0.074 0.926
Mno.51Zno.34Fel.8604.o0
A B B~
0.037 0.046 0.039
0.000 0.002 0.053
453 400 -
0.262 0.675 ]. 0.063 J
0.081 0.919
Mno.70ZnoA9Fe1.9904.00
A
0.034 0.046 0.040
0.000 0.000 0.069
478 440 -
0.224 0.739 } 0.037
0.055 0,935
B'
a Values with respect to natural a-iron foil; error +0.004 cm s -1 . b 1 0 e = 1000/47r A m -1. c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and sum of the compositions of M n 2+, Zn 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00.
paramagnetic pattern B' for the manganese,zincbearing ferrites with manganese compositions greater than or equal to 0.51 increased with increasing manganese composition, an indication that the octahedral lattice sites in the ferrites had been distorted by increases in the manganese composition. This result is in good agreement with the abrupt increase of the lattice constant shown in Fig. 4, curve (a). Such distortion of the lattice sites, caused by the incorporation of manganese ions, was also proposed by Lotgering and Van Diepen [3]. The isomer shifts of hyperfine splitting pattern A were in good agreement with that (0.029 cm s-1) of the pattern caused by Fe 3+ ions at tetrahedral sites in Fe304, i.e. pattern A was attributable to Fe 3+ ions at the tetrahedral sites. The isomer shifts of hyperfine splitting pattern B were lower than that (0.067 cm s-l) of the pattern produced by Fe 2+ and Fe 3÷ ions at octahedral sites for Fe304, between which a rapid electron exchange occurred. In this case, Mn 2+ and Zn 2+ ions seem to have been incorporated onto the lattice points in the ferrites. The values of the isomer shift, however, were slightly higher than that caused by Fe 3+ ions;
that is, Fe 2+ ions were present at the octahedral sites. Chemical analysis of a zinc-bearing ferrite prepared from an iron(II) hydroxide suspension at an initial Zn:Fetot tool ratio of 1.00:2.00 showed that small amounts of Fe z+ ions were contained in the ferrite. Conversely, chemical analysis of the manganese,zinc-bearing ferrites showed the presence of manganese ions in a valence state higher than 3+ (>2% to 8% as Mn 3+ ions). Part of the Fe 2+ ions at the octahedral sites therefore may have resulted from manganese ions in a valence state higher than 3+. The area fractions of hyperfine splitting pattern A increased with increasing manganese composition from 0.33 to 0.51, and the area fractions for the sum of hyperfine splitting pattern B and paramagnetic pattern B~ decreased. The area fractions of pattern A attributable to Fe 3+ ions at the tetrahedral sites, and of patterns B and B' caused by Fe z+ and Fe 3+ ions at the octahedral sites, were calculated from the chemical composition of the ferrites by assuming that manganese ions had been incorporated into the tetrahedral sites and that the sum of manganese, zinc, and
230
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Table 4 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn,Zn-bearing ferrites prepared from aqueous suspensions at rMn+Zn = 0,20 : 2.80 tO 0.80:2.20 Chemical composition
M6ssbauer pattern
6 a / c m S-1
A a / c m S-l
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mno.05Zno j5 Fe2.7604.00
A B B'
0.032 0.059 0.035
0.004 -0.001 -
493 450 -
0.378 0.610 ~ 0.012 J
0.290 0.710
Mno.loZn0.1oFe2.7304.o0
A B
0.034 0.061
0.002 -0.001
495 456
0.401 0.599
0.293 0.707
Mno.14Zno.o5Fe2.7404.~
A B
0.034 0.061
0.004 -0.003
497 457
0.455 0.545
0.296 0.704
Mno. 1oZno.29Fe2.5204.00
A B B'
0.027 0.056 0.034
0.005 0.001 -
484 436 -
0.189 0.742 ] 0.069 J
0.242 0.758
Mno.19Zno.19Fe2.5904.00
A B B'
0.032 0.055 0.034
0.003 0.001 0.049
491 447 -
0.239 0.737 ~ 0.024 J
0.239 0.761
Mno.2sZno.o9Fe2.4s O4.oo
A B B'
0.032 0.057 0.029
0.001 0.002 -
495 454 -
0.338 0.650 ~ 0.012 J
0.254 0.746
Mno.14Zno.45Fe2.3904.00
A B
0.000 0.002 0.048
469 404 -
0.209 0.725 ~ 0.066 J
0.172 0.828
B'
0.029 0.055 0.034
Mno.28Zno.29Fe2,3204.00
A B B~
0.031 0.048 0.035
0.001 -0.003 0.049
479 429 -
0.230 0.749 ~ 0.021 J
0.185 0.815
Mno.41Zno. 14Fe2.2604,00
A B B ~d
0.031 0.052 0.017
-0.001 0.004 -
487 453 -
0.299 0.661 ~ 0.040 J
0.199 0.801
Mno.lsZn0.57Fe2.1904.0o
A+ B B'
0.042 0.034
0.001 0.049
412 -
0.846 0.154
Mno.36Zno.38Fe2.2oO4.oo
A B B~
0.030 0.050 0.035
-0.001 0.004 0.049
469 416 -
0.220 0.701 ], 0,079 J
0.118 0.882
Mno.56Zno.19Fe2.19O4.oo
A B B' d
0.031 0.045 0.026
0.001 -0.001 0.062
484 442 -
0,269 0.716 ~ 0.015 J
0.114 0.886
a Values with respect to natural a-iron foil; error +0.004 cm s -~. b 1 0 e = 1000/47r A m -l. c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and sum of the compositions of M n 2+, Z n 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00. d Resolution o f the pattern had deteriorated.
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Fe 3+ ions in the tetrahedral sites equalled 1.00. As shown in Table 3, if all the manganese ions had been incorporated into the tetrahedral sites, then the area fractions of the patterns would have been independent of manganese composition, and the area fractions estimated from the M6ssbauer spectra would have been higher than those calculated from the chemical composition. The results mentioned earlier suggest that manganese ions had been incorporated into both the tetrahedral and the octahedral sites in the manganese,zinc-bearing ferrites. M6ssbauer spectra at room temperature of the manganese,zinc-bearing ferrites obtained from iron(II) hydroxide suspensions at rMn+Zn = 0.20 : 2.80 showed two hyperfine splitting patterns, arising from Fe 3+ ions on the tetrahedral sites and from the sum of Fe 2+ and Fe 3+ ions on the octahedral sites. In addition, a small paramagnetic pattern was analyzed in the spectra of the ferrite with a manganese composition of 0.05 (zinc composition, 0.15). Ferrites obtained from the suspensions at rMn+Z n ----0.40:2.60 to 0.80:2.20 gave paramagnetic patterns with two areas of hyperfine splitting. (The hyperfine splitting for the ferrite with a manganese composition of 0.18 prepared at rMn+Z n = 0 . 8 0 : 2 . 2 0 ( M n : Z n mol ratio = 0.20 : 0.60) was not analyzed in terms of patterns.) The M6ssbauer parameters estimated from the spectra of ferrites obtained from suspensions at rMn+Z n = 0.20:2.80 to 0.80:2.20 are listed in Table 4. These values of isomer shift 6 for hyperfine splitting pattern A are in good agreement with that (0.029 cm s -1) for Fe 3+ ions on the tetrahedral sites in Fe30 4. The 6 values for hyperfine splitting pattern B were lower than that (0.067 cm s -1) for Fe 2+ and Fe 3+ ions on the octahedral sites in Fe304, between which a rapid electron exchange occurred, and the values decreased slightly with increases in the sum of manganese and zinc compositions. This phenomenon suggests that the Fe 2+ : Fe 3+ ion ratios on the octahedral sites decreased with increases in the sum of manganese and zinc compositions and that Mn 2+ ions had been incorporated onto the lattice points in the ferrites. The values of isomer shift 6, and quadrupole splitting A, for paramagnetic pattern B' were in good agreement with the values (6 = 0.035 cm s -1, A = 0.045 cm s -1) [8] of the zinc-bearing ferrite
231
Zn0.97Fe2.1004.00 prepared from an iron(II) hydroxide suspension at an initial Zn" Fetot mol ratio of 1.00 : 2.00, and the area fractions for pattern B' estimated from the M6ssbauer spectra increased with increasing zinc composition. These results indicate that pattern B' was caused by Fe 3+ ions on the octahedral sites in the ferrites [9]. The area fractions of hyperfine splitting pattern A estimated from the M6ssbauer spectra increased with increasing manganese composition of the ferrites obtained at each rMn + Zn value. The values were larger than those calculated from the chemical composition of the ferrites on the assumption that the Mn 2+ ions had been incorporated into the tetrahedral sites and that the sum of the cations on the tetrahedral sites equalled 1.00 (except for ferrites obtained from suspensions at rMn+Zn ---0.40:2.60 ( M n : Z n mol ratios of 0.10:0.30 and 0.20"0.20)), and the differences increased with increasing manganese composition of the ferrites obtained at each run+zn value (Table 4). Apparently, Mn 2+ ions had been incorporated into both the tetrahedral and the octahedral sites in the manganese,zinc-bearing ferrites, and the amount of Mn 2+ ions on the octahedral sites increased with the increasing manganese composition of the ferrites obtained at each rMn+Z n value. As shown in Table 3, the area fraction of hyperfine splitting pattern A for the ferrite (with a manganese composition of 0.70) obtained from the suspension at rMn+Zn ----1.00:2.00 ( M n : Z n mol ratio = 0.80:0.20) did not increase with increasing manganese composition, and this result does not agree with the results for ferrites obtained at rMn+Zn = 0.20 : 2.80 to 0.80 : 2.20, as described earlier. However, the A values of paramagnetic pattern B' for the ferrites obtained at rMn+Zn ~- 0.20 : 2.80 to 0.80 : 2.20 did not increase with increases in the manganese composition, and variations in the lattice constants with manganese composition are not explainable by the distortion of the octahedral lattice sites in the ferrites. These disagreements in the M6ssbauer parameters and the variation of the lattice constants with manganese composition between the rMn+Zn values can only be explained by more detailed study of the cation distribution and the lattice structure.
232
T. Kanzaki et al./Journal o f Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Acknowledgments We thank Professor Utaka Tamaura of the Tokyo Institute of Technology for help in the measurement of the M6ssbauer spectra, Associate Professor Kazuo Shimokoshi of the same institute for use of the finite inpulse response operator program for the computer analysis of the M6ssbauer spectra and Mr. Rokur6 Ooki of the same institute for measurement of the electron micrograph.
References [1] M. Kiyama, Bull. Chem. Soc. Jpn., 51(1) (1978) 134.
[2] X-Ray Data Card no. 10-319, Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [3] F.K. Lotgering and A.M. Van Diepen, J. Phys. Chem. Solids, 34 (1973) 1369. [4] M. E1 Guendouzi, K. Sbai, P. Perriat and B. Gillot, Mater. Chem. Phys., 25 (1990) 429. [5] K. Kaneko and T. Katsura, Bull. Chem. Soc. Jpn., 52(3) (1979) 747. [6] T. Katsura, Zikken Kagaku Koza, The Chemical Society of Japan, Maruzen, No. 15, Vol. 2, 1958, pp. 272-275. [7] T. Katsura, M. Wakihara, S. Hara and T. Sugihara, J. Solid State Chem., 13 (1975) 107. [8] T. Kanzaki, K. Kitayama and K. Shimokoshi, J. Am. Ceram. Soc., 76(6) (1993) 1491. [9] D.C. Dobson, J.W. Linnett and M.M. Raiiman, J. Phys. Chem. Solids, 31(12) (1970) 2727.
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Formation of manganese- and manganese,zinc-bearing ferrites by oxidation of aqueous suspensions and analysis of their cation distributions Tadao Kanzaki a'*, Koichi Kikuchi a, Mitsuhiko Sato a, Kiyoshi Nagai a, Takashi Oowada a, Hiroaki Onozuka a, Kenzo Kitayama b alwaki Meisei University, lwaki-shi, Fukushima 970, Japan bDepartment of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Received 2 September 1995; accepted 3 November 1995
Abstract
Oxidized manganese-bearing and manganese,zinc-bearing ferrites have been prepared by air oxidation of iron(II) hydroxide suspensions at initial Mn : Fetot mol ratios (rMn) of 0.20 : 2.80 to 1.40 : 1.60 and initial (Mn + Zn) : Fetot mol ratios (rMn+Zn) of 0.20 : 2.80 to 1.00 : 2.00, respectively, at pH 10.0 and 65°C. Mfssbauer spectra at room temperature and chemical analysis of the manganese-bearing ferrites suggest that Mn 2+ ions are incorporated into the tetrahedral sites in the ferrites with manganese compositions of less than 0.74 and into both the tetrahedral and the octahedral sites at compositions of 0.74 to 1.06. In addition, manganese ions in a higher valence state than 3+ are incorporated into the octahedrai sites at manganese compositions above 0.74. M6ssbauer spectra at room temperature of the manganese,zinc-bearing ferrites prepared at rMn+Zn ---- 1.00:2.00 indicate that Mn 2+ ions are incorporated on the lattice points in the ferrites and in both the tetrahedral and the octahedral sites at manganese compositions of 0.33 to 0.51. Mfssbauer spectra and chemical analysis also suggest the incorporation of manganese ions in valence states higher than 3+. M6ssbauer spectra and the lattice constants imply that the octahedral lattice sites in the ferrites with manganese compositions equal to or greater than 0.51 are distorted. M6ssbauer spectra at room temperature of the manganese,zincbearing ferrites prepared at rMn+Zn = 0.20:2.80 to 0.80:2.20 suggest that Mn 2+ ions are incorporated into both the tetrahedral and the octahedral sites in the ferrites, and that the amounts of Mn 2+ ions on the octahedral sites increase with increases in the manganese composition of the ferrites prepared at each rM, + Z, value.
Keywords." Ferrite; Manganese-bearing ferrite; Manganese,zinc-bearing ferrite; M6ssbauer spectrum
I. Introduction Manganese- and manganese,zinc-bearing ferrites have been studied extensively because of their interesting physical properties and their importance * Corresponding author.
for technical application. Kiyama [1] studied the formation by air oxidation (at R(2OH/(Fe2++ Mn2+)) = 1.1 to 3.0 and 70°C) of manganesebearing ferrites MnxFe3_xO4 in an aqueous iron(II) hydroxide suspension containing manganese ions. Kiyama [1] reported that only oxidized manganese-bearing cubic spinel ferrites
0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0368-2048(95)02554-5
222
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
MnxFe3_x04+ 6 formed in the suspension at 0~
2-
3+
(Mn~+_eFee+)tet [Mn~* Mnx - 1Fe~+ ~_x]octO4
(1)
Here, the amount of divalent ions on the octahedral sites is denoted by e. However, from X-ray diffraction analysis and thermogravimetric measurements, El Guendouzi et al. [4] proposed a cation distribution in the spinel lattice of manganese,zinc-bearing ferrites that can be expressed by the formula
2. Experimental procedure 2.1. Reagents Analytical-grade reagents were used for the present experiment. Sodium hydroxide solutions were prepared using distilled water free of carbon dioxide and oxygen. A 0.8632 mol dm -3 manganese(II) sulfate solution was prepared from Mn(II) SO4.4.96H20.
2.2. Procedure The assays were conducted using a Dewar type reaction vessel (capacity, 2 dm 3) with a glass cover [5]. Distilled water, a 0.5 mol dm -3 sulfuric acid solution, a manganese(II) sulfate solution, zinc sulfate heptahydrate, and sodium sulfate were transferred to the reaction vessel, and nitrogen gas was bubbled into the solution with rapid stirring (600 rev min -1) for 1 h to remove the dissolved CO2 and oxygen. Iron(II) sulfate was then added (total amount of iron, zinc and manganese =0.4316 mol). The pH was adjusted to 10.0 by using both solutions containing either 2 or 5 mol dm -3 sodium hydroxide. The volume of the suspension was brought up to 2.00 dm 3 and the total sulfate ion concentration fixed at 0.375 mol dm -3. The suspension was left to stand under nitrogen for 1 h at 65°C with constant stirring. Air was passed through this suspension, designated the "initial suspension", at a rate of 0.2 dm 3 min -1. During oxidation the pH was maintained at 10.0 by adding a 0.2 mol dm -3 solution of sodium hydroxide, and the oxidation potential was measured. Oxidation stopped 10 min after the abrupt change in this parameter.
2+ 2+ 3+ 3+ 2+ 3+ 2(Mno.8xZny F e l _0.8x-y)tet[Fel +0.6x+yFe1-0.8x-yMno.2x]octO4
The present paper focuses on the formation of manganese and manganese,zinc-bearing ferrites by the air oxidation of aqueous iron(II) hydroxide suspensions containing manganese ions and both manganese and zinc ions, respectively. Cation distribution in the ferrites was also investigated and is discussed here.
(2)
These procedures have been described in detail by Kaneko and Katsura [5]. The final product was obtained by lowering the pH of the suspension to 4.0 with 0.5 mol dm -3 sulfuric acid after air oxidation. At that pH, the hydroxides of manganese(II), zinc, and iron(II) dissolved completely.
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
2.3. X-ray powder diffraction measurements X-ray powder diffraction patterns were obtained using manganese-filtered Fe K a radiation. The 20 angles of the diffraction peaks were calibrated versus standard silicon powder using the Fe Kal radiation.
2.4. M6ssbauer spectroscopic measurements M6ssbauer spectra were obtained at room temperature by applying cobalt-57 radiation with an electromechanical transducer operating in constant-acceleration mode in conjunction with a 512-channel analyzer. The velocity scale was calibrated using the hyperfine spectra of a natural a-iron foil at room temperature, which also served as the isomer shift reference.
223
[6], and manganese (>/ III) was reduced by an excess of iron(II) to manganese(II). The amounts of iron were determined by titration [5] with a 0.005 mol dm -3 standard solution of potassium permanganate and by subtracting the added amounts of iron (determination limit = 7x 10-6 mol, corresponding to 0.01 of iron of the composition 3 - (x + y); that is, MnxZnyFe3_ (x+y)O4 had an error of 3 - (x + y) + 0.01). Minimum amounts of manganese (>~ III) were determined from the amounts of 0.1 mol dm -3 iron(II) ammonium sulfate standard solution consumed. The amounts of manganese and zinc were determined to within 1% by atomic absorption spectrometry. The oxygen content was estimated by subtracting the weights of the manganese, zinc, and/or iron from the total weight of the solid sample.
2.5. Chemical analysis
3. Results and discussion
Solid samples (200 mg) of the products were dissolved in a mixture of 0.1 mol dm -3 iron(II) ammonium sulfate standard solution and 9 mol dm -3 sulfuric acid. Under this procedure, iron(II) was not oxidized by oxygen from the air
3.1. Formation of manganese-bearing ferrites in iron( II) hydroxide suspensions at initial Mn:Fetot mol ratios (rnn) of O.20 : 2.80 to 1.40 : 1.60
0.850
0
0.84e
~o 0-84£
0.844
0.84~
0.2
o14
0'.6
o18
11o
Mn:Fetot mol ratio Fig. 1. Variation of the lattice constant of spinel-type compound obtained from iron hydroxide suspension containing manganese ion with Mn : Fetot tool ratio. Error for the lattice constant -4-0.0001 nm.
The X-ray powder diffraction patterns of the products obtained from the iron(II) hydroxide suspensions exhibited only peaks for the spinel type of structure, and electron micrographs of the products obtained at rMn = 0.20:2.80 to 1.40:1.60 showed the formation of only a cubic particle of a spinel-type compound. Chemical analysis of the spinel-type compounds showed that the manganese content (Mn : Fetot mol ratio) increased from 0.068 to 0.715:1.00 with increasing rMn in the range from 0.20:2.80 to 1.40:1.60 (Mn : Fetot = 0.071 to 0.875 : 1.00); that is, 96% to 82% of the manganese in the initial hydroxide suspensions was taken into the spinel-type compounds. The lattice constants of the spinel-type compounds increased with increasing manganese contents from 0.068 to 0.449: 1.00, followed by a constant value (Fig. 1) [1]. These results suggest that the spinel-type compounds were manganesebearing ferrites. Fig. 2 shows the chemical compositions of the manganese-bearing ferrites prepared at rMn = 0.20:2.80 to 0.80:2.20 (manganese contents of
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
224
0.068 to 0.329 : 1.00) for the ternary system FeO, FezO3, and MnO. As shown by the figure, the chemical compositions of the manganese-bearing ferrites were located on the oxygen reaction lines (solid lines) and were slightly lower in FeO than those of stoichiometric, manganese-bearing ferrites (broken line). This result implies that oxidized manganese-bearing ferrites are formed by the air oxidation of iron(II) hydroxide suspensions with rMn = 0.20 : 2.80 tO 0.80 : 2.20. The sums of manganese and iron compositions for the manganesebearing ferrites formed at rMn = 1.00:2.00 to 1.40:1.60 ranged from 2.62 to 2.93, i.e. oxidized manganese-bearing ferrites also formed. 3.2. Cation distribution for manganese-bearing ferrites Fig. 3 shows the M6ssbauer spectra at room temperature of the manganese-bearing ferrites. These M6ssbauer spectra were analyzed using a least-squares-fit computer program [7]. The results are shown in the figure by solid lines, and M6ssbauer parameters are listed in Table 1. Area fractions were calculated from the peak area estimated from the least-squares fit by assuming that the nonrecoiled fractions for the different spectra were equal. The M6ssbauer spectra for the ferrites showed two hyperfine splitting patterns, A and B, but resolution began to deteriorate with increasing manganese composition. The M6ssbauer spectra
(a) A,B,
,
,
25 v tO O_
<
-0.8
MnO
',
0 Velocity/cm s-1
0.8
Fig. 3. M6ssbauer spectra at room temperature of manganesehearing ferrites.Manganesecompositions:0.18 (a), 0.74 (b) and 0.81 (c). Solid lines representleast-squares fit; A and B patterns are indicated.
FeO
Fe3Oz,
Fe203
Fig. 2. Chemical composition of manganese-bearing ferrites for the ternary system FeO, Fe203 and MnO: . . . . , value for a solid solution of FeaO4-MnFe204; - - , oxygen reaction lines.
at room temperature for Fe304, showed similar hyperfine splitting patterns A and B, arising from Fe 3+ ions at the tetrahedral sites and from Fe 2+ and Fe 3+ ions at the octahedral sites, between which a rapid electron exchange occurred [8]. For manganese-bearing ferrites, the isomer shift of the hyperfine splitting pattern A equalled the value (0.029 cm s -1) of pattern A for Fe304 within experimental error. Apparently, then, pattern A
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
225
Table 1 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn-bearing ferrites prepared from aqueous suspensions at rMn = 0.20 : 2.80 to 1.20 : 1.80 Chemical composition
M6ssbauer pattern
6 a / c m s -1
A a / c m S-x
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mn0A8Fe2.6504.00
A B
0.031 0.062
0.002 0.000
499 464
0.430 0.570
0.309 0.691
Mn0.36Fe2.4204.00
A B
0.031 0.056
0.001 0.003
499 467
0.404 0.596
0.264 0.736
Mn0,54Fe2.3504.00
A B
0.030 0,049
0.000 0.004
500 469
0.338 0.662
0.196 0.804
Mn0,74Fe2.2304.00
A B
0.031 0.042
0.000 0.001
498 468
0.261 0.739
0.117 0,883
Mn0.sl Fe181O4,00
A B
0.033 0.041
0.000 0.001
494 463
0.287 0.713
0.105 0.895
Mnl.06Fe1.8704.00
A B
0.031 0.039
-0.001 0.001
490 458
0.249 0.751
a Values with respect to natural a-iron foil; error +0.004 c m s - l . b l Oe = 1000/4~r A m - l . c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and s u m of the compositions of M n 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00.
for the manganese-bearing ferrites was caused by Fe 3+ ions at the tetrahedral sites. The isomer shift of the hyperfine splitting pattern B for the manganese-bearing ferrites with a manganese composition of 0.18 was nearly equal to the value (0.067 cm s-1) of pattern B for Fe304, but the isomer shifts of the ferrites with manganese compositions less than or equal to 0.74 decreased with increasing manganese composition. (The values for ferrites with manganese compositions -of 0.74, 0.81 and 1.06 were the same within experimental error.) These values suggest that pattern B was caused by Fe 2÷ and Fe 3+ ions at the octahedral sites, between which a rapid electron exchange occurred, and that the Fe2+:Fe 3+ ion ratios at the octahedral sites decreased with increasing manganese composition up to 0.74; that is, the valence state of the manganese ions incorporated into the lattice sites was 2+. Such a valence state was also suggested by the chemical compositions of the manganese-bearing ferrites with manganese
compositions of 0.18 to 0.74 for the ternary system FeO, FezO 3 and MnO. The area fractions for hyperfine splitting pattern A in the ferrites with manganese compositions of less than 0.74, as estimated from the M6ssbauer spectra, decreased with increasing composition, and those for hyperfine splitting pattern B increased with increasing manganese composition. Such values imply that Mn 2+ ions had been incorporated into the tetrahedral sites in the ferrites with manganese compositions below 0.74. The area fractions of hyperfine splitting patterns A and B were the same for manganese-bearing ferrites with manganese compositions of 0.74, 0.81, and 1.06. Evidently, Mn 2+ ions had been incorporated into both the tetrahedral and the octahedral sites. However, the area fractions for pattern A were lower than those calculated from the chemical compositions under the assumption that Mn 2+ ions had been incorporated into the tetrahedral sites, and the sum of Mn 2+ and Fe 3+ ions in the tetrahedral sites
226
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Table 2 Chemical composition of the Mn,Zn-bearing ferrites Initial suspension
Chemical composition a
(Mn + Zn) : Feto t mol ratio, rMn+ Zn
Mn : Zn mol ratio
Mn
0.20 : 2.80
0.00:0.20 0.05 : 0,15 0.10 : 0.10 0.15 : 0,05 0.20 : 0.00
0.05 0.10 0.14 0.18
0.00 : 0.40 0.10 : 0.30 0.20 : 0.20 0.30:0.10 0.40 : 0.00
0.10 0.19 0.28 0.36
0.00 0.15 0.30 0.45 0.60
: 0.60 : 0.45 : 0.30 : 0.15 : 0.00
0.14 0.28 0.41 0.54
0.00 : 0.80 0.20 : 0.60 0.40 : 0.40 0.60:0.20 0.80 : 0.00
0.18 0.36 0.56 0.74
0.00 : 1,00 0.20 : 0.80 0.40 : 0.60 0.60 : 0.40 0,80:0.20 1.00 : 0.00
0.17 0.33 0.51 0.70 0.81
0.40 : 2.60
0.60 : 2,40
0.80 : 2.20
1.00 : 2.00
Zn
Fe
(Mn + Zn + Fe)
0.19 0.15 0.10 0.05
2.86 2.76 2.73 2.74 2.65
3.05 2.96 2.93 2.93 2.83
0.37 0.29 0.19 0.09
2.55 2.52 2,59 2.48 2.42
2.92 2.91 2.97 2.85 2.78
0.54 0.45 0.29 0.14
2.28 2.39 2.32 2.26 2.35
2.82 2.98 2.89 2.81 2.89
0.72 0.57 0.38 0,19
2.01 2.19 2.20 2.19 2.23
2.73 2.94 2.94 2.94 2.97
0.93 0.75 0.53 0.34 0.19
1.94 2.02 1.89 1.86 1.99 1.81
2.87 2.94 2.75 2.71 2.88 2.62
a For all ferrites the value for oxygen is 4.00.
equalled 1.00. Thus, Mn 2+ ions may have been incorporated not only into the tetrahedral sites but also into the octahedral sites in the manganese-bearing ferrites with manganese compositions less than or equal to 0.74. Lotgering and Van Diepen [3] compared the experimental saturation moments and values calculated for N6el coupling. These researchers suggested that at manganese compositions up to 0.53, the Mn 2+ ion occupies the octahedral sites exclusively, and at manganese compositions greater than or equal to 0.7, the Mn 2+ ions switch from octahedral to tetrahedral sites. The present results, mentioned earlier, are not in agreement with these suggestions.
The isomer shifts of hyperfine splitting pattern B for manganese-bearing ferrites with manganese compositions greater than 0.74 were constant, however, suggesting that manganese ions in a valence state higher than 3+, and also Mn 2+ ions, had been incorporated into the octahedral sites. Chemical analysis of the ferrites with manganese compositions of 0.81 and 1.06 by the addition of an iron(II) ammonium sulfate solution showed the incorporation of manganese ions in a valence state higher than 3+, i.e. greater than 3, and more than 21% of the manganese composition as the Mn 3+ ion, respectively. Lotgering and Van Diepen [3] proposed the incorporation of Mn 2+ ions into
T. Kanzaki et al./ Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
0.00
Manganese composition 0.05 0.10 0.15
0.2( 0.843
0.842
0-849
0.848
~/~
o
E
~/~)
0.841 ~o
0-847
0.840 0-8/,(
o.o
o:2
0:4
0:6
o:8
Manganese composition
0.839
~.o
Fig. 4. Variation of the lattice constant of manganese,zincbearing ferrites with manganese composition. Curves (a) and (b) represent the variations for the ferrites obtained at initial (Mn + Zn) : Fetot mol ratios (rM,+Zn) of 1.00:2.00 and 0.20 : 2.80, respectively.
the tetrahedral and octahedral sites in manganesebearing ferrites MnxFe3_xO4 (x >/1) prepared by a solid-state reaction and the presence of Mn 3+ ions at the octahedral sites. The present theory of cation distribution for manganese-bearing ferrites with manganese compositions of more than 0.74 is in good agreement with that of Lotgering and Van Diepen.
3.3. Formation of manganese,zinc-bearing ferrites in iron(II) hydroxide suspensions with initial (Mn + Zn) : Feto t mol ratios (rMn+Zn) of 0.20 : 2.80 to 1.00 : 2.00 The X-ray powder diffraction patterns and electron micrographs of the products obtained from iron(II) hydroxide suspensions only exhibited peaks for a spinel type of structure and cubic particles of a spinel-type compound. Chemical analysis of the spinel-type compounds showed that more than 84% manganese and more than 92% zinc in the suspension had been taken into the spinel-type compounds. Such facts suggest that
227
the spinel-type compounds were manganese,zincbearing ferrites. The chemical compositions of the manganese,zinc-bearing ferrites are listed in Table 2. These chemical compositions show that the sums of manganese, zinc, and iron compositions were in the range of 2.71 to 2.97; i.e. oxidized manganese,zinc-beating ferrites had formed. In addition, chemical analysis of the manganese,zincbearing ferrites obtained at rMn+Zn = 1.00:2.00 with the addition of an iron(II) ammonium sulfate solution showed that manganese ions in a valence state higher than 3+ had been incorporated into the ferrites (>2% to 8% manganese as Mn3+). The lattice constants a0 of the manganese,zincbearing ferrites obtained from the iron(II) hydroxide suspensions at rMn+Zn = 1.00 : 2.00 are plotted versus the manganese compositions of the ferrites in Fig. 4, curve (a). The lattice constants were invariant with manganese composition in the range 0-0.33 and exhibited an abrupt increase above 0.51. This variation in the lattice constants with the manganese composition of the ferrites cannot be explained by an ideal solid solution of MnFe204-ZnFe204 [7]. However, the lattice constants a0 of the manganese,zinc-bearing ferrites obtained from iron(II) hydroxide suspensions at rMn+Zn =0.20:2.80 to 0.80:2.20 increased rapidly with increasing manganese compositions, then slowed to a gradual increase, and finally increased rapidly again. (Fig. 4, curve (b) shows the variation for the ferrites obtained at rMn+Zn = 0.20:2.20.) The region of gradual increase for the lattice constants became unclear as the rMn+Zn values in the suspensions increased.
3.4. Cation distribution for the manganese,zincbearing ferrites Fig. 5 shows the M6ssbauer spectra at room temperature of manganese,zinc-bearing ferrites obtained from the iron(II) hydroxide suspensions at rMn+Z, = 1.00 : 2.00. The results, analyzed using a least-squares-fit computer program, are shown by solid lines, and Mtssbauer parameters and area fractions are listed in Table 3. The Mtssbauer spectra show hyperfine splitting patterns and a paramagnetic pattern, E. The hyperfine splitting patterns for the ferrites with
228
T. ICanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
(a) A,Br"
I
!
I
I
8,r-7
IVl;
(b) l
J
i
D
cO ",,T_, Q.
(c)
"
Y 7'
i
-0.8
I
I
1
I
0 Veloci ty/cm s-1
I
I
I
0.8
Fig. 5. M6ssbauer spectra at room temperature of manganese,zinc-bearing ferrites obtained at initial (Mn + Zn) : Feto t tool ratio (rMn +Zn) of 1.00:2.00. Manganese compositions: 0.17 (a), 0.33 (b) and 0.70 (c). Solid lines represent least-squares fits; A, B and B' patterns are indicated.
manganese compositions greater than or equal to 0.33 consisted of two types, A and B. The area fractions of the paramagnetic pattern B' increased with increasing zinc composition, and the isomer shifts of pattern B' showed a value arising from the Fe 3+ ion. These results suggest that pattern B' was caused by Fe 3+ ions at the octahedral sites in the zinc-bearing ferrite ZnFe204. The M6ssbauer spectra at room temperature for zinc-bearing
ferrites, Z n x F e 3 - x O 4 (1.0 > x >-0.6), exhibited a paramagnetic pattern attributable to ZnFe204 [8,9]. This pattern can be explained by considering that any value of x results in different fractions of octahedral cations surrounded by different fractions of near-neighbor tetrahedral cations that a r e F e 3+ because of the random distribution of Zn 2+ and Fe 3+ ions among the tetrahedral sites [9]. The quadrupole splitting A of the
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
229
Table 3 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn,Zn-bearing ferrites prepared from aqueous suspensions at rMn+Zn = 1.00 : 2.00 Chemical composition
M6ssbauer pattern
6 a / c m s -1
A a / c m s -1
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mno.lvZnovsFez0204.00
A+ B B~
0.057 0.039
0.000 0.048
373 -
0.705 0.295
Mn0.33Zn0.53 Fel.8904.oo
A B B'
0.036 0.048 0.039
0.004 0.001 0.049
429 374 -
0.149 0.721 ~ 0.130 1
0.074 0.926
Mno.51Zno.34Fel.8604.o0
A B B~
0.037 0.046 0.039
0.000 0.002 0.053
453 400 -
0.262 0.675 ]. 0.063 J
0.081 0.919
Mno.70ZnoA9Fe1.9904.00
A
0.034 0.046 0.040
0.000 0.000 0.069
478 440 -
0.224 0.739 } 0.037
0.055 0,935
B'
a Values with respect to natural a-iron foil; error +0.004 cm s -1 . b 1 0 e = 1000/47r A m -1. c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and sum of the compositions of M n 2+, Zn 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00.
paramagnetic pattern B' for the manganese,zincbearing ferrites with manganese compositions greater than or equal to 0.51 increased with increasing manganese composition, an indication that the octahedral lattice sites in the ferrites had been distorted by increases in the manganese composition. This result is in good agreement with the abrupt increase of the lattice constant shown in Fig. 4, curve (a). Such distortion of the lattice sites, caused by the incorporation of manganese ions, was also proposed by Lotgering and Van Diepen [3]. The isomer shifts of hyperfine splitting pattern A were in good agreement with that (0.029 cm s-1) of the pattern caused by Fe 3+ ions at tetrahedral sites in Fe304, i.e. pattern A was attributable to Fe 3+ ions at the tetrahedral sites. The isomer shifts of hyperfine splitting pattern B were lower than that (0.067 cm s-l) of the pattern produced by Fe 2+ and Fe 3÷ ions at octahedral sites for Fe304, between which a rapid electron exchange occurred. In this case, Mn 2+ and Zn 2+ ions seem to have been incorporated onto the lattice points in the ferrites. The values of the isomer shift, however, were slightly higher than that caused by Fe 3+ ions;
that is, Fe 2+ ions were present at the octahedral sites. Chemical analysis of a zinc-bearing ferrite prepared from an iron(II) hydroxide suspension at an initial Zn:Fetot tool ratio of 1.00:2.00 showed that small amounts of Fe z+ ions were contained in the ferrite. Conversely, chemical analysis of the manganese,zinc-bearing ferrites showed the presence of manganese ions in a valence state higher than 3+ (>2% to 8% as Mn 3+ ions). Part of the Fe 2+ ions at the octahedral sites therefore may have resulted from manganese ions in a valence state higher than 3+. The area fractions of hyperfine splitting pattern A increased with increasing manganese composition from 0.33 to 0.51, and the area fractions for the sum of hyperfine splitting pattern B and paramagnetic pattern B~ decreased. The area fractions of pattern A attributable to Fe 3+ ions at the tetrahedral sites, and of patterns B and B' caused by Fe z+ and Fe 3+ ions at the octahedral sites, were calculated from the chemical composition of the ferrites by assuming that manganese ions had been incorporated into the tetrahedral sites and that the sum of manganese, zinc, and
230
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Table 4 Parameters and area fractions estimated from the room-temperature M6ssbauer spectra of Mn,Zn-bearing ferrites prepared from aqueous suspensions at rMn+Zn = 0,20 : 2.80 tO 0.80:2.20 Chemical composition
M6ssbauer pattern
6 a / c m S-1
A a / c m S-l
Internal fieldb/ kOe
Area fraction M6ssbauer spectra
Chemical composition c
Mno.05Zno j5 Fe2.7604.00
A B B'
0.032 0.059 0.035
0.004 -0.001 -
493 450 -
0.378 0.610 ~ 0.012 J
0.290 0.710
Mno.loZn0.1oFe2.7304.o0
A B
0.034 0.061
0.002 -0.001
495 456
0.401 0.599
0.293 0.707
Mno.14Zno.o5Fe2.7404.~
A B
0.034 0.061
0.004 -0.003
497 457
0.455 0.545
0.296 0.704
Mno. 1oZno.29Fe2.5204.00
A B B'
0.027 0.056 0.034
0.005 0.001 -
484 436 -
0.189 0.742 ] 0.069 J
0.242 0.758
Mno.19Zno.19Fe2.5904.00
A B B'
0.032 0.055 0.034
0.003 0.001 0.049
491 447 -
0.239 0.737 ~ 0.024 J
0.239 0.761
Mno.2sZno.o9Fe2.4s O4.oo
A B B'
0.032 0.057 0.029
0.001 0.002 -
495 454 -
0.338 0.650 ~ 0.012 J
0.254 0.746
Mno.14Zno.45Fe2.3904.00
A B
0.000 0.002 0.048
469 404 -
0.209 0.725 ~ 0.066 J
0.172 0.828
B'
0.029 0.055 0.034
Mno.28Zno.29Fe2,3204.00
A B B~
0.031 0.048 0.035
0.001 -0.003 0.049
479 429 -
0.230 0.749 ~ 0.021 J
0.185 0.815
Mno.41Zno. 14Fe2.2604,00
A B B ~d
0.031 0.052 0.017
-0.001 0.004 -
487 453 -
0.299 0.661 ~ 0.040 J
0.199 0.801
Mno.lsZn0.57Fe2.1904.0o
A+ B B'
0.042 0.034
0.001 0.049
412 -
0.846 0.154
Mno.36Zno.38Fe2.2oO4.oo
A B B~
0.030 0.050 0.035
-0.001 0.004 0.049
469 416 -
0.220 0.701 ], 0,079 J
0.118 0.882
Mno.56Zno.19Fe2.19O4.oo
A B B' d
0.031 0.045 0.026
0.001 -0.001 0.062
484 442 -
0,269 0.716 ~ 0.015 J
0.114 0.886
a Values with respect to natural a-iron foil; error +0.004 cm s -~. b 1 0 e = 1000/47r A m -l. c Estimated from chemical composition with assumption that M n 2+ ions are incorporated into the tetrahedral sites and sum of the compositions of M n 2+, Z n 2+ and Fe 3+ ions in the tetrahedral sites is equal to 1.00. d Resolution o f the pattern had deteriorated.
T. Kanzaki et al./Journal of Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Fe 3+ ions in the tetrahedral sites equalled 1.00. As shown in Table 3, if all the manganese ions had been incorporated into the tetrahedral sites, then the area fractions of the patterns would have been independent of manganese composition, and the area fractions estimated from the M6ssbauer spectra would have been higher than those calculated from the chemical composition. The results mentioned earlier suggest that manganese ions had been incorporated into both the tetrahedral and the octahedral sites in the manganese,zinc-bearing ferrites. M6ssbauer spectra at room temperature of the manganese,zinc-bearing ferrites obtained from iron(II) hydroxide suspensions at rMn+Zn = 0.20 : 2.80 showed two hyperfine splitting patterns, arising from Fe 3+ ions on the tetrahedral sites and from the sum of Fe 2+ and Fe 3+ ions on the octahedral sites. In addition, a small paramagnetic pattern was analyzed in the spectra of the ferrite with a manganese composition of 0.05 (zinc composition, 0.15). Ferrites obtained from the suspensions at rMn+Z n ----0.40:2.60 to 0.80:2.20 gave paramagnetic patterns with two areas of hyperfine splitting. (The hyperfine splitting for the ferrite with a manganese composition of 0.18 prepared at rMn+Z n = 0 . 8 0 : 2 . 2 0 ( M n : Z n mol ratio = 0.20 : 0.60) was not analyzed in terms of patterns.) The M6ssbauer parameters estimated from the spectra of ferrites obtained from suspensions at rMn+Z n = 0.20:2.80 to 0.80:2.20 are listed in Table 4. These values of isomer shift 6 for hyperfine splitting pattern A are in good agreement with that (0.029 cm s -1) for Fe 3+ ions on the tetrahedral sites in Fe30 4. The 6 values for hyperfine splitting pattern B were lower than that (0.067 cm s -1) for Fe 2+ and Fe 3+ ions on the octahedral sites in Fe304, between which a rapid electron exchange occurred, and the values decreased slightly with increases in the sum of manganese and zinc compositions. This phenomenon suggests that the Fe 2+ : Fe 3+ ion ratios on the octahedral sites decreased with increases in the sum of manganese and zinc compositions and that Mn 2+ ions had been incorporated onto the lattice points in the ferrites. The values of isomer shift 6, and quadrupole splitting A, for paramagnetic pattern B' were in good agreement with the values (6 = 0.035 cm s -1, A = 0.045 cm s -1) [8] of the zinc-bearing ferrite
231
Zn0.97Fe2.1004.00 prepared from an iron(II) hydroxide suspension at an initial Zn" Fetot mol ratio of 1.00 : 2.00, and the area fractions for pattern B' estimated from the M6ssbauer spectra increased with increasing zinc composition. These results indicate that pattern B' was caused by Fe 3+ ions on the octahedral sites in the ferrites [9]. The area fractions of hyperfine splitting pattern A estimated from the M6ssbauer spectra increased with increasing manganese composition of the ferrites obtained at each rMn + Zn value. The values were larger than those calculated from the chemical composition of the ferrites on the assumption that the Mn 2+ ions had been incorporated into the tetrahedral sites and that the sum of the cations on the tetrahedral sites equalled 1.00 (except for ferrites obtained from suspensions at rMn+Zn ---0.40:2.60 ( M n : Z n mol ratios of 0.10:0.30 and 0.20"0.20)), and the differences increased with increasing manganese composition of the ferrites obtained at each run+zn value (Table 4). Apparently, Mn 2+ ions had been incorporated into both the tetrahedral and the octahedral sites in the manganese,zinc-bearing ferrites, and the amount of Mn 2+ ions on the octahedral sites increased with the increasing manganese composition of the ferrites obtained at each rMn+Z n value. As shown in Table 3, the area fraction of hyperfine splitting pattern A for the ferrite (with a manganese composition of 0.70) obtained from the suspension at rMn+Zn ----1.00:2.00 ( M n : Z n mol ratio = 0.80:0.20) did not increase with increasing manganese composition, and this result does not agree with the results for ferrites obtained at rMn+Zn = 0.20 : 2.80 to 0.80 : 2.20, as described earlier. However, the A values of paramagnetic pattern B' for the ferrites obtained at rMn+Zn ~- 0.20 : 2.80 to 0.80 : 2.20 did not increase with increases in the manganese composition, and variations in the lattice constants with manganese composition are not explainable by the distortion of the octahedral lattice sites in the ferrites. These disagreements in the M6ssbauer parameters and the variation of the lattice constants with manganese composition between the rMn+Zn values can only be explained by more detailed study of the cation distribution and the lattice structure.
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T. Kanzaki et al./Journal o f Electron Spectroscopy and Related Phenomena 77 (1996) 221-232
Acknowledgments We thank Professor Utaka Tamaura of the Tokyo Institute of Technology for help in the measurement of the M6ssbauer spectra, Associate Professor Kazuo Shimokoshi of the same institute for use of the finite inpulse response operator program for the computer analysis of the M6ssbauer spectra and Mr. Rokur6 Ooki of the same institute for measurement of the electron micrograph.
References [1] M. Kiyama, Bull. Chem. Soc. Jpn., 51(1) (1978) 134.
[2] X-Ray Data Card no. 10-319, Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [3] F.K. Lotgering and A.M. Van Diepen, J. Phys. Chem. Solids, 34 (1973) 1369. [4] M. E1 Guendouzi, K. Sbai, P. Perriat and B. Gillot, Mater. Chem. Phys., 25 (1990) 429. [5] K. Kaneko and T. Katsura, Bull. Chem. Soc. Jpn., 52(3) (1979) 747. [6] T. Katsura, Zikken Kagaku Koza, The Chemical Society of Japan, Maruzen, No. 15, Vol. 2, 1958, pp. 272-275. [7] T. Katsura, M. Wakihara, S. Hara and T. Sugihara, J. Solid State Chem., 13 (1975) 107. [8] T. Kanzaki, K. Kitayama and K. Shimokoshi, J. Am. Ceram. Soc., 76(6) (1993) 1491. [9] D.C. Dobson, J.W. Linnett and M.M. Raiiman, J. Phys. Chem. Solids, 31(12) (1970) 2727.