Mossbauer Spectroscopic Characterization Xa9949649 Of Ferrite


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MOSSBAUER SPECTROSCOPIC CHARACTERIZATION OF FERRITE CERAMICS

XA9949649

S. MUSIC, M. RISTIC Rudjer Boskovic Institute, Zagreb, Croatia

Abstract

The principle of Mossbauer effect and the nature of hyperfine interactions were presented. The discovery of the Mossbauer effect was the basis of a new spectroscopic technique, called Mossbauer spectroscopy, which has already made important contribution to research in physics, chemistry, metallurgy, mineralogy and biochemistry. In the present work the selected ferrites such as spinel ferrite, NiFe2C>4, and some rare earth orthoferrites and garnets were investigated using Mossbauer spectroscopy. X-ray powder diffraction and Fourier transform infrared spectroscopy were used as complementary techniques. The formation of NiFe2C>4 was monitored during the thermal decomposition of mixed salt (Ni(NO3)2+2Fe(NC>3)3)nH2O. The ferritization of Ni^+ ions was observed at 500 °C and after heating at 1300 °C the stoichiometric NiFe2C>4 was produced. The Mossbauer parameters obtained for NiFe2C>4, dpe = 0.36 mm s"* and HMF = 528 kOe, can be ascribed to Fe^+ ions in the octahedral sublattice, while parameters dpe = 0.28 mm s~* and HMF = 494 kOe can be ascribed to F e 3 + ions in the tetrahedral lattice. The effect of ball-milling of NiFe2C>4 was monitored. The formation of oxide phases and their properties in the systems Nd2O3-Fe2C>3, Sm2O3-Fe2O3, Gd2O3-Fe2C>3, Eu2O3-Fe2C>3 and Er2O3-Fe2C>3 were also investigated. Kvantitative distributions of oxide phases, a-Fe2C>3, R2O3, R3Fe5Oi2 and RFeC>3, R = Gd or Eu, were determined for the systems *Gd2O3+(l-;c)Fe2O3 and xEu2C>3+(l-x)Fe2O3. The samples, prepared by chemical coprecipitation in the system xEu2O3+(l-x) Fe2C>3, 0
1. Mossbauer spectroscopy

1.1. The Mossbauer effect

The emission of light from excited atoms and the resonant absorption of the emitted light by the atoms of the same element is a well known process. It might be expected that the same phenomenon should occur for nuclear y-rays. However, this is generaly not so because in the case of nuclear y-resonance, the recoil energy and the line-width become very important factors. The recoil energy, R, is described by the expression:

E2

R = 2Mc2

0)

where E is the photon energy, M is the mass of the emitting atom and c is the velocity of light. The energy, R, may be about 10"11 eV for an optical photon, but for a y-ray it is about 10"1
eV. The width of the resonance is given by the reciprocal of the lifetime of the excited state and may be about 10~8 for an optical photon, and about the same or somewhat broader for
y-rays. Thus, in the case of the resonant absorption of light, the recoil energy can be neglected,
whereas in the other case, it is large enough to destroy the resonance condition completely.

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Rudolf Mossbauer recognized the problems of the degradation of y-rays energies by nuclear recoil and the thermal energy constraints. The essence of his solution of the problem of nuclear y-resonance was to eliminate these destructive factors by incorporation of the emitting and absorbing nuclei in a solid lattice for which the lowest (vibrational) excitation energy is greater than the nuclear recoil energy. Under these circumstances, Mossbauer was able to show that a fraction of the y-rays, f, which are emitted, is exempt from the effects of recoil energy and Doppler broadening, and therefore, the fraction, f, could be resonantly absorbed by other identical nuclide. The fraction of y-ray, f, usually called the Lamb-Mossbauer fraction, is given by expression:

f=exp(-£2)

(2)

where k=2nl\, 1 is the wavelength of the y-ray and is the mean square displacement of the atom. In 1957, Rudolf Mossbauer discovered the recoil-free absorption of the 129-keV y-ray by 191Ir in iridium metal [1]. The y-ray source was 191Os. The transmission of the y-ray decreased unexpectedly as the temperature was lowered from 370 to 90 K. For this discovery, Rudolf Mossbauer received the Nobel Prize for Physics in 1961. After the discovery of the Mossbauer effect, many other nuclides, 57Fe among them, were found to show the resonant absorption of y-ray.
The discovery of the Mossbauer effect was the basis of the development of a new spectroscopic technique called Mossbauer spectroscopy. In the literature, the alternative name nuclear gamma resonance (NGR) spectroscopy can also be found. Mossbauer spectroscopy has already made important contributions to research in physics, chemistry, metallurgy, minerology and biochemistry. In this section, only the fundamentals of Mossbauer spectroscopy, which are important for the reader of this publication, will be discussed.

1.2. Measurement of the Mossbauer effect

In principle, to arrange a Mossbauer experiment the following basic components are needed: a source of y-ray, an absorber of y-ray, a driving mechanism to produce a slight change in the y-ray energy and a detector with a multichannel analyzer.

(270 dqyS)

EC (99.8%)

2

136.4 keV (89 ns)

3 2
1 v.
57Fe

14.4 keV (99.3 ns) 0

FIG. 1. Decay scheme of $7Co.

\2

Although many nuclides can be used to obtain the Mossbauer effect, to date, the most popular nuclide is 57Fe. It is fortunate that iron, containing 57Fe in its natural nuclidic composition, is
an element of great scientific and technological importance. The standard source for the observation of the 57Fe Mossbauer effect is 57Co, which decays by electron capture into the 136.4 keV level of 57Fe. After 89 ns, this level emits a y-ray of 122 keV. The 14.4 keV level of 57Fe decays after 99.3 ns to the ground state. This transition is generally used to observe the Mossbauer effect in 57Fe. The decay sheme of 57Co is shown in Figure 1. The radionuclide 57Co is formed by the proton irradiation of 5^Ni. The irradiated material is refined by ion-
exchange procedures to the high standard of chemical purity essential for Mossbauer
applications.
Several metals, such as Cr, Cu, Pd, Pt and Rh, are suitable matrices for 57Co because they
fulfill the basic requirements regarding the structure, magnetic properties and recoil-free
fraction. Other cubic metals do not offer an advantage as matrices or are unsuitable for practical reasons. Stainless steel, popular in early 57Fe Mossbauer works, is no longer applied
because self-resonance together with an unresolved isomer shift and quadrupole splittings
cause substantial line-broadening. Rhodium possesses the most favorable combination of properties, and is regarded as the best all-round matrix. After 57Fe, 119Sn ranks the second in
Mossbauer studies.
As absorbers, synthetic compounds or natural samples containing
Mossbauer active nuclides, can be used. In many cases, it is important to perform the
measurements at very low temperatures. The Mossbauer source and/or absorber can be cooled
using different types of cryostats. The Mossbauer effect can also be studied at high
temperatures using a specially designed furnace.
An essential part of the Mossbauer spectrometer is a driving unit capable
of producing slight energy changes of the y-ray generated in recoil-free emission. This slight
energy change in the y-ray is produced by importing a velocity, v, to the source or absorber,
and due to the Doppler effect the corrected energy E is given by the expression:

E=Er(l±^-)

(3)

where c denotes the velocity of light. In this way, the extra velocity will either reduce or increase the y-ray energy just enough to match the energy of the level exactly and allow resonant absorption to take place. For practical reasons the function of the velocity is better to use than the function of the energy.
Different types of detectors can be used in measurements of the Mossbauer effect: scintillation detectors, proportional counters, and Li-drifted Ge and Si detectors. The y-ray detector selected depends on the type of Mossbauer experiment. Efficiency, resolution and the other properties of the detectors must be considered separately for different Mossbauer sources.
The usual way of monitoring the Mossbauer effect is to display it as a spectrum on a multichannel analyzer used in the multiscaling mode, whereby the number of counts per channel in a fixed time of a few microseconds is shown on a selected number of channels, usually 512 or 1024 chanels. The channel "start" and "advance" signals are synchronized with a signal generator that also drives the Doppler source motion to and from the absorber.

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1. 3. Hyperfine interactions
Nuclear y-resonance is highly sensitive to the variations in the energy of the y-quanta as a consequence of their extreme resolution (TTEy ~ 10~13). This is the basis for the measurement of very small energy shifts and the splittings of nuclear levels caused by hyperfine interactions, i. e., interactions of the atomic nucleus with electric and magnetic fields. The three main hyperfine interactions usually measured by Mossbauer spectroscopy are: the isomer shift, electric quadrupole interaction and magnetic dipole interaction. The numerical values of these interactions and their temperature dependence can yield a large amount of information, which is of importance in studies of the lattice, electronic and magnetic properties of the matter in solid state. Additional information can be deduced from line-width data, the sign of the quadrupole interaction and the influence of the relaxation times on the hyperfine parameters.
1. 3. 1. Isomer shift
If the Mossbauer nuclei of the both source and absorber are in an identical environment, the minimum of the single Mossbauer line is centered at about zero velocity. The position of the centroid of a Mossbauer spectrum is a function of the difference in the electronic densities at the nucleus in the source and absorber. Any chemical change that influences the electron density at the nucleus of either the source or the absorber will shift the position of the centroid of the spectrum. This shift is usually called the isomer shift (or chemical shift) and is given by expression:
5 =5^-Ze2
where Z is the atomic number, e is the unit charge, Rg is the radius of the nucleus in the excited state, Rg is the radius in the ground state, and I \j/ (o) I2 is the wave function of the electron charge at the nucleus of the absorber and source, respectively. The isomeric term,
(Re — R g ) , is a physical property characteristic of each Mossbauer nuclide. On the other
hand, the term Jl^. ( 0 ) | 2 _ ^ ( o ) | 2 | *s of particular interest in the chemical applications of the Mossbauer effect. The s-electrons primarily contribute to the electronic charge density at the nucleus. The p- and ^/-electrons with little or no charge density at the nucleus contribute indirectly through screening effects on the s-electrons. For example, the spectrum of the Fe3+ ion is shifted in a positive direction relative to the metallic iron because of a decrease in the electronic charge density at its nucleus following the loss of the 4s-electrons. The spectrum of the Fe2+ ions is shifted to even higher positive velocities because of of the screening effect of the additional 3 14

1. 3. 2. Quadrupole splitting
Quadrupole splitting is induced by the electrostatic interaction between the electric field gradient (EFG) at the nucleus and the nuclear quadrupole moment (Q). The degeneracy of the nuclear states is partially lifted in the EFG for a nuclear spin I>l/2. For a nuclear level with a spin of 1=3/2, which is frequently exploited in Mossbauer spectroscopy, the eigenvalues are given by the expression:
( (5)

where q represents the principal component of the EFG and r\ the asymmetry parameter, y-transitions occur between the substates of the excited and ground levels. The resultant spectrum is known as a quadrupole doublet. Generally, the contribution of the electric field gradient at the Mossbauer absorber arises from an asymmetric distribution of the surrounding atoms or ligands. The intensities, angular variation and temperature dependence of the quadrupole interaction yield valuable information about the electronic and crystallographic structure, and particularly about the structural symmetry of various chemical compounds.

1. 3. 3. Magnetic splitting

Magnetic splitting arises from the interaction of the nuclear magnetic dipole moment with a magnetic field, H, created by the atom's own electrons. The energy for each of the magnetic sublevels is given by the expression:

E = -rnHmj/I

(6)

1 r
-10 -8 -6 -4

0 24 6

VELOCITY ,mm/s

10 12

FIG. 2. 57Fe Mossbauer spectrum (RT) of a cervice corrosion product from an isothermal (288 °C) capsule (Inconel 600).

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TABLE I. 57Fe MOSSBAUER PARAMETERS CALCULATED FOR THE SPECTRUM SHOWN IN FIG. 2.

Lines

8* (mm s"1)

A or AEq (mm s"1)

H** (kOe)

Oi 0.375

0.589

0.339

0.119

491

0.724

0.105

461

* **Errors:±0.005 mm s"1 and ±1 kOe

r
(mm s"1)
0.929 0.295 0.315

Sextet Area (mm s'1)
0.064 0.472 0.795

A (%)
4.81 34.42 59.77

Identification
FeOOH Fe3O4 Fei?O4

where m is the nuclear magnetic dipole moment, H is the magnetic field, mj is the nuclear magnetic quantum number and I is the nuclear spin quantum number. The energy transitions are governed by the selection rule, Amj = 0, ±1. Since 57Fe has a ground state of I =1/2 mi = ±1/2 and the first excited state of I = 3/2, mi = ± 1/2, + 3/2, the magnetic field will generate six transitions at this nuclide. In this case, a six-line Mossbauer spectrum is obtained. For a thin absorber in which the magnetic moments are randomized, for example in a powder sample, there is ratio of relative peak intensities of approximately 3:2:1:1:2:3. The source of the magnetic field may be either "internal", as in the case of ferromagnetic and antiferromagnetic materials, or it may be an externally applied magnetic field. When the absorber is magnetized perpendicular to the y-ray direction, the peak intensities ratio is 3:4:1.
In practice, the Mossbauer spectra very often reflect a combination of magnetic and electric quadrupole interactions. The energy levels depend in a complex way on the angles between the principal axes of the EFG tensor and the axes of the magnetic field. Fortunately, symmetry considerations reduce these angles to zero or ninety degrees in a number of important compounds and the expression for the energy levels simplifies. Mossbauer measurements of hyperfine magnetic interactions yield extremly important information about the origin and properties of the magnetism in various materials containing Mossbauer nuclides.
As an illustration for readers, Figure 2 shows the complex Mossbauer spectrum, where different 57Fe contributions to the total Mossbauer effect are present [2]. After the mathematical deconvolution of this spectrum, the values of the 57Fe Mossbauer parameters are obtained (Table I), which serve as the basis for the chemical and physical conclusions. The magnetic splitting components, M^ and M2, are ascribed to stoichiometric magnetite, FeFe2C»4, which possesses the structure of spinel ferrite.
2. The ferrites
Ferrites is the general name for a group of mixed metal oxides containing iron ions, which are characterized by specific electric and magnetic properties. They have an appreciably higher (by six orders of magnitude) electric resistance and a lower saturation magnetization than those of silicon steels and permalloys. This reduces eddy-current losses and makes the ferrites sutable for making cores (toroids) which have application in the production of AF and HF transformers, as well as chokes. Ferrites have also found applications as magnetic recording media, wave-guides in microwave techniques, and in magneto-optical
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devices. Some type of ferrites have found application as permanent magnets. These important applications of ferrites are a consequence of their unique structural, chemical and physical properties.
Ferrites can generally be considered as the reaction products between Fe2C>3 and other metal oxides. In general [3], their formula is (Me2+O^~)m/2(Fe2+O32~)n,
where Me is a metal cation with valence k and m and n are full numbers. In addition to ferrites containing oxygen anions, there are ferrites in which oxygen anions are replaced by anions of flourine, chlorine, sulphur, selenium or tellurium. On the basis of their structural properties, ferrites can be divided in four subgroups: -spinel ferrites with the structure of the mineral spinel, MgAl2C>4, -hexagonal ferrites with the structure of the mineral magnetoplumbite, PbFe7.5Mn3.5Alo.5Tio.5O19, -orthoferrites with the structure of the mineral perovskite, CaTiO3 , and -garnet-type ferrites with the structure of the mineral grossular,
Mossbauer spectroscopy is an exellent tool for studying the chemical composition, phase transitions and magnetic properties of the ferrites. In this report, we shall present the capabilities of Mossbauer spectroscopy in the characterization of oxide phases formed during the synthesis of different ferrites, as well as in the investigation of the structural and magnetic properties of these materials. These applications of Mossbauer spectroscopy are not only important from an academic standpoint but are also of great importance for researchers and engineers involved in the commercial production of ferrite ceramics. Selected ferrites, such as spinel ferrite, NiFe2O4, some rare earth orthoferrites and garnets, will be the subject of discussion.
2. 1. Nickel ferrite
Nickel ferrite, NiFe2O4, is an inverse spinel in which the tetrahedral or A sites are occupied by Fe3+ ions, and the octahedral or B sites by Fe3+ and Ni2+ ions. This compound is described with the formula (Fe3+)i\(Ni2+Fe3+)g04. The magnetic structure of NiFe2C>4 is of the Neel collinear type in which all the spins at the A and B sites are parallel, while the A and B sublattice magnetizations are antiparallel.
Nickel ferrite and substituted nickel ferrites were the subject of many investigations due to their specific magnetic and microwave energy absorption properties. Different methods were used in the synthesis of NiFe2C>4. Many researchers have observed that the chemical and physical properties of NiFe2O4 are dependent on the conditions of its synthesis.
Linnett and Rahman [4] performed one of the earliest Mossbauer studies with nickel ferrites, NixFe3_xO4, 0 Morrish and Haneda [5] investigated the magnetic structure of small NiFe2O4 particles. Three kinds of NiFe2O4 particles having particle sizes of 250±50, 800±200 and 1300±200 A, and crystallite sizes of 250, 400 and 500 A, respectively, were investigated. 57Fe Mossbauer spectra, recorded with a longitudinal magnetic field, indicated that a non-collinear magnetic structure exists. Further, the magnetic moment at low temperatures was appreciably lower than the value reported for bulk material. A model was proposed in which the NiFe2O4 particles consist of a core with the usual spin arrangement and
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a surface layer with atomic moments inclined to the direction of the net magnetization. The existence of a non-collinear magnetic structure in the surface layers of y-Fe2C>3 and CrC>2 was also found [6]. It was concluded that Mossbauer spectroscopy in conjuction with large magnetic fields is a powerful tool in the study of non-collinear magnetic structures of ferromagnetic and ferrimagnetic fine particles. However, this investigative approch was not convenient for antiferromagnets.
Nickel ferrite has been studied by Mossbauer spectroscopy in the temperature range of 134 850 K
857.5 K
862 K-
-12 -8 -4 0 4 8 12 VELOCITY (mm/s)
FIG. 3. Typical Mossbauer spectra ofNiFe2C>4 at various temperatures. The solid curves are the best two-site fits to the spectra [7].
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The reaction of spinel ironsand with NiO in an air inert atmosphere up to 1200 °C was compared with the analogous reactions of synthetic hematite or magnetite with NiO [8]. All three iron oxide phases in reaction with NiO produced NiFe2O4. Under inert conditions, the reaction products showed larger lattice parameters, thus indicating the presence of Fe2*. Mossbauer spectra suggested that the Fe2+ ions in the ferrite spinels were located preferentially at the octahedral sites. Lattice constant calculations suggested that NiFe2O4 formed in air was not fully inverse, as usually assumed, but contained varying amounts of tetrahedral Ni2+. In the case of ironsand-derived spinels, Ti4+ ions were located at the octahedral sites.
For practical reasons, it is very important to understand the early stages of Ni2+ ferritization. This is not only of interest for those involved in the commercial production of ferrites but also for corrosion scientists and engineers. Morozumi et al. [9]
investigated the hydrothermal synthesis of NiFe2O4. The formation and growth of nickel ferrite particles were completed within 24 h at temperatures between 100 and 250 °C. Mossbauer spectroscopy gave clear evidence of nickel ferrite formation at all temperatures above 100 °C. Raw [10] applied Mossbauer spectroscopy to study the formation of nickel ferrite as the corrosion product. The principal corrosion product formed in the stainless steel primary cooling circuit of a PWR (Pressurized Water Reactor) is non-stoichiometric nickel

TABLE II. EXPERIMENTAL CONDITIONS FOR THE PREPARATION OF SAMPLES S2 TO S9 (HOURS OF HEATING AT GIVEN TEMPERATURES).

Temperature / °C

Sample 200 300 400 500 700

900

1100

1300

S2

1

S3

1

1

S4

1

11

s5

1

111

S5

1

111

1

s7

1

111

1

1

Sg

1

111

1

1

1

So

1

111

1

1

1

TABLE III. THE RESULTS OF X-RAY DIFFRACTION PHASE ANALYSIS OF SAMPLES TOS9.

Sample
Si
s2
S3
s4
S5
s6
S7
s8
SQ

Phase composition
Ni(NO3)2 6H2O + Fe(NO3)3 nH2O (?) a-Fe2O3 + Ni(NO3)2 6H2O + ... NiO + a-Fe2O3 NiO + a-Fe^O^
NiO + a-Fe2O3 + Fe2O4 NiFe2O4 + NiO NiFe2O4 NiFe2O4 NiFe?O4

Remarks increased broadening of diffraction lines of a -Fe2O3 and NiO
sharpening of NiFe2O4 diffraction lines and no shifts of diffraction lines

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ferrite, NiyFe3_j;O4 , 0.45 The thermal decomposition products of the coprecipitated salts Fe(NH4)2(SC>4)2*6H2O and Ni(NH4)2(SO4)2»6H2O were analyzed by Mossbauer spectroscopy [14]. The optimum temperature for NiFe2C>4 preparation was 900 °C.
Music et al. [15] studied the oxide phases generated during the synthesis of NiFe2C>4 using X-ray diffraction, FT-IR and 57Fe Mossbauer spectroscopy. An aqueous solution of Ni(NC>3)2/(Fe(NO3)3 of the molar ratio NiO:Fe2C>3 = 1:1 was prepared. The excess water was evaporated using an infrared lamp. The solid residue (sample S\) was heated at the conditions given in Table II. The results of the X-ray diffraction phase analysis of the prepared samples are given in Table III. The observed phases, a-Fe2O3, NiO, NiFe2O4 and Ni(lSfO3)2» 6H2O, were identified to the JCPDS-PDF [16] card numbers 13-534, 4-835, 10-325 and 25-577, respectively. Since the lattice constant of NiFe2C>4 is almost twice as large as that of NiO, there are no diffraction lines of NiO independent of the ones of NiFe2O4. The presence of NiO was proved [17] by comparison of the intensities of the overlapped diffraction lines belonging to both NiO and NiFe2O4. Besides Ni(NO3)2« 6H2O, sample S\ contained an additional phase, which may be considered as an iron nitrate hydrate; however, its positive identification by the JCPDS-PDF data was not possible with certainity.
The presence of oc-Fe203 in sample S\ could not be recognized with certainity on the basis of the FT-IR spectrum because a very strong band at 590 cm"1, with shoulders at 690 and 480 cm"1, was the superposition of several IR bands of different origins. Heating of sample S\ at 200 °C for one hour caused changes in the corresponding FT-IR spectrum. In the FT-IR spectrum of sample S2, two new bands at 1532 and 978 cm"1 were visible, and also a band at 1324 cm"1 instead of the shoulder at 1355 cm"1. This spectrum also showed a separation of bands at 708, 579 and 477 cm"1, and the appearance of a weak band at 389 cm"1. On the basis of the FT-IR spectra of samples, S\ and S2 , no conclusion about the presence of ct-FeOOH in these samples can be made. Additional heating of sample S2 at 300 °C for one hour significantly suppressed the relative intensities of the IR bands corresponding to the H2O molecules and nitrate groups. The FT-IR spectrum of sample S3 gave more evidence for the presence of a-Fe2O3 , but the band at 460 cm"1 was an overlap of a band of a-Fe2O3 and a band of NiO. The FT-IR spectrum of sample S4 (additional heating at 400 °C) showed a sharp peak at 1385 cm"1 indicating traces of nitrates, while the FT-IR spectrum of sample S5 (additional heating at 500 °C) indicated that all the nitrates were decomposed. The spectrum of sample S5 also showed two dominant bands at 589 and 423 cm"1 with shoulders at 560 and 454 cm"1. The bands at 584 and 423 cm"1 indicated ferritization in sample S5. The FT-IR spectrum of sample Sg (additional heating at 700 °C) showed the presence of nickel ferrite on the basis of bands at 600 and 417 cm"1. With further increasing of the heating temperature (900 °C for sample S7 ; 1100 °C for sample Sg; 1350 °C for sample S9), the band at 600 cm"1 shifted to 604 cm"1, while the band at 417 cm"1 shifted to 400 °C. These IR shifts can be ascribed to the crystal lattice ordering and/or to an approach to the foil stoichiometry of nickel ferrite.
Figures 4 to 7 and Table IV summarize the results obtained by Mossbauer spectroscopy. The Mossbauer spectra of samples S\, S2 and S3 were interpreted as the superposition of two sextets, M] and M2, while the spectrum of sample S4 was characterized by one sextet, M\.
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Mossbauer Spectroscopic Characterization Xa9949649 Of Ferrite