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Mechanically induced cation redistribution in ZnFe2O4 and its thermal stability
ELSEVIER
Physica B 234-236 (1997) 617-619
Mechanically induced cation redistribution in ZnFe204 and its thermal stability V. Sepel~ik a'*, K . Tk~i~ov/t a, V.V. B o l d y r e v b, S. W i g m a n n c, K . D . B e c k e r c "Institute of Geotechnics, Slovak Academy of Sciences, 04353 Ko~ice, Slovakia bInstitute of Solid State Chemistry, Russian Academy of Sciences, 630091 Novosibirsk, Russian Federation Institute of Physical and Theoretical Chemistry, TU Braunschweig, 38106 Braunschweig, Germany
Abstract The changes in zinc ferrite c~sed by high-energy ball-milling are investigated. Formation of spin arrangement in the ball-milled ZnFe204 is caused by the onset of the exchange interaction of the Fe 3+(A)-O z --Fe 3+ [B] type, taking place due to the mechanically induced inversion as well as by the onset of the interaction of the Fe 3÷ [B] - O 2--Fe 3+[B] type with deformed bond angle. Structural metastability of the milled ZnFe20 4 is manifested by the gradual recrystallization terminating at 900 K by a total recovery of the structure. Keywords: Disordered materials; Ferrimagnetism; M~Sssbauer effect; Powder diffraction
1. Introduction The unusual properties exhibited by new metastable mechanically treated (high-energy ball-milled) and nanoscaled materials have opened, in the last few years, new frontiers in basic science and increasing perspectives for new engineering applications [1]. Due to the physical flexibility of the structure of spinel-ferrites [2], they have been considered as very convenient model systems for the investigation of processes induced by mechanical action [3]. The present work focuses on the explanation for an origin of mechanically induced changes in magnetic properties of the ball-milled ZnFe204 and on the description of the structural response of the
* Corresponding author.
metastable ball-milled ZnFezO4 to changes in temperature.
2. Experimental Zinc ferrite was prepared in polycrystalline form by a solid-state reaction from 0t-Fe203 and ZnO (products of Merck). The ball-milling was performed in a planetary ball mill EI 2 x 150 (Institute of Solid State Chemistry, Novosibirsk). Ceramiccovered vials and balls made of the ot-A1203 ceramics were used. The ball-to-powder mass ratio was 50: 1. The milling was done in air. X-ray diffraction (XRD) measurements were carried out using a Siemens-D500 diffractometer. An XRD technique [4] was used for determination of cation distribution in tetrahedral (A) and octahedral [B] sites of the milled ZnFe204. The Mrssbauer spectra were recorded using a conventional constant-acceleration spectrometer with a 57Co source in a Rh matrix.
0921-4526/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved PII S092 1-4526(96)0 106 1-7
K ZSepel&ket al. / Physica B 234-236 (1997) 617-619
618
3. Results and discussion
The XRD patterns of ZnFe20, taken after various milling times are shown in Fig. 1. It has been found that the inversion degree 6, defined as the fraction of (A) sites occupied by Fe a÷ cations, monotonically increases with increasing milling time from the 0 to 0.94, i.e. the normal spinel is, after 24 min of milling, almost completely converted into the inverse one. Determination of the unit cell dimension has revealed that milling of ZnFe20, leads to the contraction of the crystal lattice from 0.84432nm (for the as-prepared sample) to 0.84136 nm (for the sample milled for 24 min). It follows from our previous work that mechanically induced contraction of the crystal lattice of ZnFe20, is accompanied by deformation of the octa/cation-anion-octa/cation bond angle [-5, 6]. The M6ssbauer spectrum of the as-prepared sample at 77 K consists of a paramagnetic doublet
1.10 -4 [cps]
11
i I
I~
15
i
2O
,9080, .. /f
~5~~ ~ :/:~'' c .":;i 1 9 :':.0 / a9 0~
99
~
~
[
~
d
Fig. 2. M6ssbauer spectrum of the as-prepared sample (a) and of samples milled for 5 min (b), 12 min (c), and 24 min (d). Spectra taken at 77 K. The velocity scale calibrated relative to a-Fe.
.
s
VELOCITY [mm/s] -10 0 10
i
I) I
:i
#7
( J',-~.~-~;'~i./Jt'. . i b
I
~v., / 4
j tI I I M
t I
/
fs
p
,,
3"o
2 THETA [degree] Fig. 1. X-ray diffraction patterns of the as-prepared sample (a) and of samples milled for 5 min (b), 12 min (c), and 24 min (d) (Mo K~ radiation).
with quadrupole splitting QS=0.346mms -1, linewidth F = 0.356 mm s- 1 and isomer shift I S = 0.327mms -I (Fig. 2(a)). With increasing milling time the doublet in the spectrum disappears and is gradually replaced by a sextet typical of the magnetically ordered state of the structure (Fig. 2). To interpret the formation of a magnetic hyperfine sextet in the spectrum of the milled ZnFe204, it should be taken into consideration that the presence of cations with non-zero magnetic moment on (A) and [B] sites of spinel-ferrite brings about the formation of an exchange interaction of the (A)-O =--[B] type. Thus, the mechanically induced redistribution of Zn 2+ and Fe 3+ cations in ZnFe20, brings about the onset of the Fe a +(A)-Oa--Fe a +[B] interaction. The second factor determining the modified magnetic properties of the milled ZnFe204 is the mechanically induced alteration of
v. ~Sepelitk et al. / Physica B 234-236 (1997) 617-619
the octa/cation-anion-octa/cation bond angle leading to the onset of the Fe 3 + EB]-O 2 - - F e 3 + [-B] interaction with deformed bond angle different from 90 ° (typical for the as-prepared sample). High-temperature X R D analysis and measurements of temperature dependence of the integral intensities of diffraction lines have revealed that in the temperature range 600-900 K the mechanically induced inversion and deformation of the structure of the milled Z n F e 2 0 4 disappear. This work was supported by the Slovak G r a n t Agency for Science (Grant 3/1369/96).
619
References [1] C. Suryanarayana, Bibliography on Mechanical Alloying and Milling (Cambridge International Science Publishing, Cambridge, 1995). [2] A. Goldman, Modern Ferrite Technology (Van Nostrand Reinhold, New York, 1990). E3] K. Tk~ov/t, V. ~epelhk, N. Stevulovh and V.V. Boldyrev, J. Solid State Chem. 123 (1996) 100. [4] H. Schmalzried, Z. Phys. Chem. Neue Folge 28 (1961) 203. [5] V. Sepel~k, K. Tk/~ovh and A.I. Rykov, Crystal Res. Technol. 28 (1993) 53. [6] V. Sepelhk, K. Tkh~ovh, V.V. Boldyrev and U. Steinike, Mater. Sci. Forum, 228-231 (1996) 783.
Physica B 234-236 (1997) 617-619
Mechanically induced cation redistribution in ZnFe204 and its thermal stability V. Sepel~ik a'*, K . Tk~i~ov/t a, V.V. B o l d y r e v b, S. W i g m a n n c, K . D . B e c k e r c "Institute of Geotechnics, Slovak Academy of Sciences, 04353 Ko~ice, Slovakia bInstitute of Solid State Chemistry, Russian Academy of Sciences, 630091 Novosibirsk, Russian Federation Institute of Physical and Theoretical Chemistry, TU Braunschweig, 38106 Braunschweig, Germany
Abstract The changes in zinc ferrite c~sed by high-energy ball-milling are investigated. Formation of spin arrangement in the ball-milled ZnFe204 is caused by the onset of the exchange interaction of the Fe 3+(A)-O z --Fe 3+ [B] type, taking place due to the mechanically induced inversion as well as by the onset of the interaction of the Fe 3÷ [B] - O 2--Fe 3+[B] type with deformed bond angle. Structural metastability of the milled ZnFe20 4 is manifested by the gradual recrystallization terminating at 900 K by a total recovery of the structure. Keywords: Disordered materials; Ferrimagnetism; M~Sssbauer effect; Powder diffraction
1. Introduction The unusual properties exhibited by new metastable mechanically treated (high-energy ball-milled) and nanoscaled materials have opened, in the last few years, new frontiers in basic science and increasing perspectives for new engineering applications [1]. Due to the physical flexibility of the structure of spinel-ferrites [2], they have been considered as very convenient model systems for the investigation of processes induced by mechanical action [3]. The present work focuses on the explanation for an origin of mechanically induced changes in magnetic properties of the ball-milled ZnFe204 and on the description of the structural response of the
* Corresponding author.
metastable ball-milled ZnFezO4 to changes in temperature.
2. Experimental Zinc ferrite was prepared in polycrystalline form by a solid-state reaction from 0t-Fe203 and ZnO (products of Merck). The ball-milling was performed in a planetary ball mill EI 2 x 150 (Institute of Solid State Chemistry, Novosibirsk). Ceramiccovered vials and balls made of the ot-A1203 ceramics were used. The ball-to-powder mass ratio was 50: 1. The milling was done in air. X-ray diffraction (XRD) measurements were carried out using a Siemens-D500 diffractometer. An XRD technique [4] was used for determination of cation distribution in tetrahedral (A) and octahedral [B] sites of the milled ZnFe204. The Mrssbauer spectra were recorded using a conventional constant-acceleration spectrometer with a 57Co source in a Rh matrix.
0921-4526/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved PII S092 1-4526(96)0 106 1-7
K ZSepel&ket al. / Physica B 234-236 (1997) 617-619
618
3. Results and discussion
The XRD patterns of ZnFe20, taken after various milling times are shown in Fig. 1. It has been found that the inversion degree 6, defined as the fraction of (A) sites occupied by Fe a÷ cations, monotonically increases with increasing milling time from the 0 to 0.94, i.e. the normal spinel is, after 24 min of milling, almost completely converted into the inverse one. Determination of the unit cell dimension has revealed that milling of ZnFe20, leads to the contraction of the crystal lattice from 0.84432nm (for the as-prepared sample) to 0.84136 nm (for the sample milled for 24 min). It follows from our previous work that mechanically induced contraction of the crystal lattice of ZnFe20, is accompanied by deformation of the octa/cation-anion-octa/cation bond angle [-5, 6]. The M6ssbauer spectrum of the as-prepared sample at 77 K consists of a paramagnetic doublet
1.10 -4 [cps]
11
i I
I~
15
i
2O
,9080, .. /f
~5~~ ~ :/:~'' c .":;i 1 9 :':.0 / a9 0~
99
~
~
[
~
d
Fig. 2. M6ssbauer spectrum of the as-prepared sample (a) and of samples milled for 5 min (b), 12 min (c), and 24 min (d). Spectra taken at 77 K. The velocity scale calibrated relative to a-Fe.
.
s
VELOCITY [mm/s] -10 0 10
i
I) I
:i
#7
( J',-~.~-~;'~i./Jt'. . i b
I
~v., / 4
j tI I I M
t I
/
fs
p
,,
3"o
2 THETA [degree] Fig. 1. X-ray diffraction patterns of the as-prepared sample (a) and of samples milled for 5 min (b), 12 min (c), and 24 min (d) (Mo K~ radiation).
with quadrupole splitting QS=0.346mms -1, linewidth F = 0.356 mm s- 1 and isomer shift I S = 0.327mms -I (Fig. 2(a)). With increasing milling time the doublet in the spectrum disappears and is gradually replaced by a sextet typical of the magnetically ordered state of the structure (Fig. 2). To interpret the formation of a magnetic hyperfine sextet in the spectrum of the milled ZnFe204, it should be taken into consideration that the presence of cations with non-zero magnetic moment on (A) and [B] sites of spinel-ferrite brings about the formation of an exchange interaction of the (A)-O =--[B] type. Thus, the mechanically induced redistribution of Zn 2+ and Fe 3+ cations in ZnFe20, brings about the onset of the Fe a +(A)-Oa--Fe a +[B] interaction. The second factor determining the modified magnetic properties of the milled ZnFe204 is the mechanically induced alteration of
v. ~Sepelitk et al. / Physica B 234-236 (1997) 617-619
the octa/cation-anion-octa/cation bond angle leading to the onset of the Fe 3 + EB]-O 2 - - F e 3 + [-B] interaction with deformed bond angle different from 90 ° (typical for the as-prepared sample). High-temperature X R D analysis and measurements of temperature dependence of the integral intensities of diffraction lines have revealed that in the temperature range 600-900 K the mechanically induced inversion and deformation of the structure of the milled Z n F e 2 0 4 disappear. This work was supported by the Slovak G r a n t Agency for Science (Grant 3/1369/96).
619
References [1] C. Suryanarayana, Bibliography on Mechanical Alloying and Milling (Cambridge International Science Publishing, Cambridge, 1995). [2] A. Goldman, Modern Ferrite Technology (Van Nostrand Reinhold, New York, 1990). E3] K. Tk~ov/t, V. ~epelhk, N. Stevulovh and V.V. Boldyrev, J. Solid State Chem. 123 (1996) 100. [4] H. Schmalzried, Z. Phys. Chem. Neue Folge 28 (1961) 203. [5] V. Sepel~k, K. Tk/~ovh and A.I. Rykov, Crystal Res. Technol. 28 (1993) 53. [6] V. Sepelhk, K. Tkh~ovh, V.V. Boldyrev and U. Steinike, Mater. Sci. Forum, 228-231 (1996) 783.