Temperature Dependence of the Magnetic and Electric Properties of Ca2Fe2O5

doi:10.4236/msa.2011.29183

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Materials Sciences and Applicatio ns, 2011, 2, 1349-1353

doi:10.4236/msa.2011.29183 Published Online September 2011 (http://www.SciRP.org/journal/msa)

1349

Temperature Dependence of the Magnetic and

Electric Properties of Ca2Fe2O5

Cléber Candido da Silva1,2*, Antonio S. B. Sombra1

1Telecommunications and Materials Science and Engineering Laboratory (LOCEM), Department of Physics, Federal University of

Ceará, Campus do Pici, Fortaleza, Ceará, Brazil; 2Universidade Federal do Maranhão (UFMA), Centro de Ciências Sociais, Saúde e

Tecnologia (CCSST), Departamento de Engenharia de Alimentos, Campus II, Imperatriz, MA, Brazil.

Email: *[email protected].br, [email protected]

Received April 6th, 2011; revised April 25th, accepted July 5th, 2011.

ABSTRACT

Ca2Fe2O5 powder sample, were prepared to investigate the origin of the weak ferromagnetic component reported in

literature for calcium ferrite single crystals. In this work, the calcination method was used to produce nanocrystalline

powders of Ca2Fe2O5. XRD measurement has shown the presence of Fe3O4 magnetite and CaO as impurity phases. The

ferrimagnetic phase deeply influences the magnetic behavior with features very similar to those reported in literature

for Ca2Fe2O5, both powders and single crystals. Our results support the hypothesis that the weak ferromagnetic com-

ponent observed in Ca2Fe2O5 can be also due to the presence of magnetite impurity traces in the samples. The powders

were submitted to calcination processes at 50 0˚C for 2 hours and 950 ˚C for 16 hours. The sintered sample was submit-

ted at 1050˚C for 6 hours and characterized by X-Ray Powder diffraction (XRD), dielectric measurements, Magnetiza-

tion and Scanning Electron Microscopy (SEM) analysis.

Keywords: Ca2Fe2O5, Dielectric Measurements, Magnetization

1. Introduction

CF (Ca2Fe2O5) is a member of the family of com- pounds

with general formula A2B2O5 (A = Ca, Sr; B = Fe, Al),

and it finds application in the field of catalysis, when

obtained from mechano-chemical synthesis [1]. Its struc-

ture is the one known for the Srebrodolskite mineral [2,3]

and it is related to the perovskite (ABO3) structure by the

introduction of an ordered array of oxygen vacancies and

the creation of an alternate layer structure of octahedrally

and tetrahedrally coordinated transition metal cations.

The iron end-member CF [4,5] (mineral name srebro-

dolskite) adopts space group Pnma at ambient conditions

(a = 5.4, b = 14.8, c = 5.6 Ǻ). Their magnetic structures

and properties have been investigated by many authors

[6-12]. Physical properties of CF, such as electronic and

oxygen-ionic transport [13] and catalytic [14] and photo

catalytic [15] behavior, have been studied. Usage as

catalyst for the combustion of volatile organic com-

pounds [16,17] and for direct decomposition of NOx in

exhaust streams [18,19] has been examined. Numerous

entries in patent data bases also highlight a strong inter-

esting brownmillerites for catalytic applications. Brown-

millerite type structures exhibit two different layers, al-

ternately stacked: 1) perovskite-like sheets of octahe-

drally co-ordinated B cations and 2) layers of BO4 tetra-

hedra, which are corner-linked to form parallel zweier

single chains. Mixed occupations are observed frequently.

Two phase transitions are known for the iron end-mem-

ber CF: the loss of the antiferromagnetic order at the

Neel temperature at 720K [7-9,20] and a structural phase

transition [12,20-22] at 950K. The high-temperature

phases of the end-members Ca2Fe2O5 and Ca2Al2O5

turned out to be isotypic modulated structures [6,23],

with an aperiodic sequence of tetrahedral chains. These

structures are described using the (3+1)-dimensional su-

per space approach [24]. Their super space group is

Imma(00 γ)s00. The main purpose of the work is to pre-

pare CF ceramic and study the effect of the analyses of

the magnetic momentum versus magnetic field (H) and

dielectric comportment. X-Ray diffraction and Scanning

Electron Microscopy (SEM) analysis were also done to

characterize such ceramic.

2. Experimental

CF crystalline powders were prepared by stoichiomet-

ric quantities of CaCO3 (Aldrich 99%) and Fe2O3 (Al-

Temperature Dependence of the Magnetic and Electric Properties of CaFe O

1350 22 5

drich 98%) were the mixtures were submitted to heat-

treatment at 500˚C during 2 h and 950˚C during 16 h

both with a heating rate of 3˚C/min. Equation (1) repre-

sents the expected chemical reaction:

323 223

2CaCOFeO CaFeO2CO  (1)

X-Ray Diffraction

The X-ray diffraction (XRD) patterns data were ob-

tained at room temperature using powder samples in an

X’Pert MPD Philips difractometer (with Kα radiation, λ =

1.54056 Å) at 40 KV and 30 mA. Intensity data were

collected by the step counting method (step 0.02˚ and a

time per step of 1s) between 20˚ and 60˚ (2θ). The analy-

sis of the crystallite size (Lc) of the Ca2Fe2O5 phase has

been done using the Scherrer’s equation [A]:

cos







 (2)

where k is the shape coefficient (k = 1 was chosen, con-

sidering that the shape of this point is spherical), λ the

wave length, β the full width at half maximum (FWHM)

of the peak of each phase and θ the diffract- tion angle.

For this purpose, we chose the avarege of peak within the

pattern and according to Pnma space group of Ca2Fe2O5.

This peak corresponded to hkl = 141, both along the c

crystallographic axis.

VSM

The magnetization (M/H) was measured using an Ox-

ford Instruments VSM (Vibrating Sample Magne- tome-

ter) between 1.6 and 300 K, on a field-cooled sample,

under an applied field of 100 Oe.

SEM

The morphological analysis of the sample structure

was performed using the scanning electron microscopy

(SEM), Philips XL-30, operating with bunches of pri-

mary electrons ranging from 12 to 20 keV.

3. Results and Discussions

Figure 1 present XRD pattern of the CF calcinated at

950˚C. Brownmillerite or srebrodolskite (Ca2Fe2O5) [25]

was identified. This structure (Ca2Fe2O5) can be seen like

a perovskite deficient in oxygen, where as brownmillerite

(A2B2O5) is a kind of oxygen-deficient perovskite struc-

ture that is composed of perovskite-like three-dimen-

sional framework of corner-sharing BO6 octahedra alter-

nating with slabs containing rows of corner-sharing BO4

tetrahedra which are formed by the deficiency of oxygen

during the formation of the structure [26]. The little im-

purity peaks () and () indicate the formation of

Fe3O4 [25] and CaO [25] phases respectively were de-

tected in the XRD of the sample (Figure 1). In addition,

investigations of magnetic resonance are of special in-

terest, since Ca2Fe2O5 is a many-sublattice system with

Figure 1. XRD pattern of the sample. Ca2Fe2O5 (), Fe3O4

() and CaO () [25].

a nontrivial magnetic layer structure [27]. The Figure 2

exhibits the variation of

 with frequency for CF at

different temperatures. A well-defined relaxation mecha-

nism is observed in the temperature range of 303 - 353 K.

The relaxation peaks shift towards higher fre- quencies

with increasing on temperature. For sample, a single

peak is observed. The presence of such relaxation peaks

in the



 plots indicates that the samples are ionic

conductors [28]. The nature of the variation of



dc (10

KHz) vs. 1000/T and fmax (peak maximum, Figure 2) vs.

1000/T with temperature follows the Arrhenius relation 3

and 4 respectively:

exp a

ff kT













(3)

exp a















(4)

where f0 and 0 is a pre-exponential factor, Ea is the

activation energy; k is the Boltzmann constant; and T

the absolute temperature [29].

The activation energy calculated from the modulus

spectrum (0.20 eV) is also comparable to the value

Figure 2. The temperature dependence of the of the Imagi-

nary Modulus of the CF sample, from 303 to 353 K.

Temperature Dependence of the Magnetic and Electric Properties of CaFe O1351

22 5

obtained from the conductivity (0.18 eV) (Figure 3). The

comparable values of the activation energy of both con-

ductivity and modulus spectra indicate that the relaxation

and conductivity process may be attributed to the same

type of charge carries [30]. Figure 4 shows the magneti-

zation as a function of temperature. There is a decrease

of the magnetization with the increase in temperature

characteristic of the brownmillerite. The Ca2Fe2O5 struc-

ture is a weak antiferromagnet directed along the c-axis

[27]. This observation shows that the magnetocrystalline

anisotropy in the a-c plane is small. The influence of

Fe3O4 on the magnetic behavior not is observed. The

micrographics (Figure 5(a) and (b)) showed particles of

the sample CF, where the micro-structures demonstrate

average grain size of 0.61 μm, while the crystallites for

that sample obtained by the diffraction of x-ray range

from 70.92 ± 3.38 nm indicating the presence of large

agglomerates of particles due to the sintering process.

The reason for this morphology depends to the sintering

Figure 3. dc (10 KHz) vs. 1000/T () and fmax (peak

maximum in Imaginary Modulos of CF) vs. 1000/T ().

Figure 4. Temperature Dependence of the Magnetization

for the CF sample, from 1.6 to 300 K.

(a)

(b)

Figure 5. SEM of the CF sample with 20.000X ((a) and (b)).

effect, where, probably, the formation of Fe3O4 phase

changes the grain size in sample.

4. Conclusions

Ca2Fe2O5 (brownmillerite) phase was obtained with the

presence of impurity phases (Fe3O4 and CaO) probably

due heat-treated at 950˚C. The comparable values of the

activation energy of both conductivity and modulus

spectra indicate that the relaxation and conductivity

process may be attributed to the same type of charge car-

ries. The VSM analysis show that the Ca2Fe2O5 structure

is a weak antiferromagnet directed along the c-axis. Be-

tween 75 and 150 K we have a decrease of the magneti-

zation with the temperature characteristic of the brown-

millerite [27]. This observation shows that the magneto-

crystalline anisotropy in the a-c plane is small. The crys-

tallites for that sample obtained by the diffraction of

x-ray (70.92 ± 3.38 nm) indicating the presence of large

agglomerates of particles due to the sintering process

Temperature Dependence of the Magnetic and Electric Properties of CaFe O

1352 22 5

where the formation of Fe3O4 phase changes the grain

size in sample.

5. Acknoledgements

The authors thank this work to the Telecommunications

and Materials Science and Engineering Laboratory

(LOCEM) Physics Department, Federal, University of

Ceará-Brazil.

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