DSpace at VNU: Mossbauer spectroscopic evaluation of chemical and electronic distributions in La(Fe0.81Si0.19)(13)

Journal of Magnetism and Magnetic Materials 269 2004 404–409a Department of Physics, Wichita State University, Wichita, KS 67260, USA b Faculty of Physics, Vietnam National University an

Journal of Magnetism and Magnetic Materials 269 (2004) 404–409

a Department of Physics, Wichita State University, Wichita, KS 67260, USA

b Faculty of Physics, Vietnam National University and International Training Institute for Materials Science, Hanoi, Viet Nam

Received 5 May 2003

Abstract

Substitution of nonmagnetic Si for Fe in La(Fe1
xSix)13lowers the magnetic moment but surprisingly raises the Curie temperature.To provide a basic knowledge of the charge distribution as perturbed by Si, M.ossbauer measurements were made on the compound with x ¼ 0:19 in its paramagnetic state.Detailed analysis of the highly accurate quadrupole splittings thus obtained indicates that Si has a preference to substitute Fe in one of its two non-equivalent sites in the cubic structure, and reduces the Fe magnetic moment through a redistribution of 3d electrons between the spin-up and -down sub-bands

r2003 Elsevier B.V All rights reserved

PACS: 76.80; 61.18.F

Keywords: Paramagnetic M ossbauer spectroscopy; Atomic-site distribution; La(Fe 1
x Si x ) 13

1 Introduction

In materials research and development, one of

the elements in a model compound is often

partially or even totally substituted by others,

thus to induce or enhance a specific property of

interest.Typical examples can be easily found in

magnetic rare earth-transition metal intermetallics,

high-temperature superconductors and giant

mag-netoresistive materials, as well as well-developed

ferrites.Needless to say, such an approach has been quite fruitful in the practical sense.Toward a fundamental understanding of partial substitution, though, there is clearly a basic question as to where the original and the substituting elements are located, whenever there are two or more nonequivalent sites for them in the lattice.Beyond that, their distribution in a given site can be treated statistically.One should consider then the effect on the electronic configuration due to the various nearest-neighbor arrangements.This is indeed the

In the limited concentration range with x

*Corresponding author.Tel.: 9783994; fax:

+1-316-9783350.

E-mail address: hussein.hamdeh@wichita.edu

(H.H Hamdeh).

0304-8853/$ - see front matter r 2003 Elsevier B.V All rights reserved.

doi:10.1016/j.jmmm.2003.07.004

ferromagnetic state at relatively low temperatures.

Through the substitution of nonmagnetic Si for

Fe, the magnetic moment of Fe decreases as

surpris-ingly increases.While this behavior has not been

observed in other intermetallic compounds based

on rare earth-iron, a similar one is known to occur

potential for applications, along with the

occur-rence of an itinerant electron metamagnetic

their magnetic properties, the aforementioned

question about the Fe and Si distribution in the

lattice obviously needs to be addressed.There are

actually two nonequivalent sites of Fe in the cubic

on the Si preference between these two sites.This

spectro-scopy to successfully solve the ambiguity.Just as

important, it clarifies the charge distribution in

such a Si-substituted compound

known to be useful in elucidating the degree of

inversion in ferrites having also two nonequivalent

earth-iron intermetallics having three nonequivalent Fe

hyperfine magnetic fields as deduced from the

spectra obtained in a magnetically ordered state

External magnetic fields are often applied to

delineate the individual sextet components.In

contrast, this report will offer a more conclusive

deter-mined quadrupole splitting (QS) distribution of a

zero-field spectrum obtained in the paramagnetic

2 Experimental

by arc-melting a weighted mixture of La (3N), Fe

(4N) and Si (5N) in a purified argon atmosphere,

M.ossbauer studies in the paramagnetic state at

300 K were made on a thin disk of powder sample The spectrum was obtained in a transmission

a standard constant acceleration spectrometer

3 Results and discussion

asym-metric and off-centered peaks clearly indicate a distribution of the QS values and a strong correlation between QS and the isomer shift (IS)

It is worthwhile to note that, in the absence of magnetic order and applied magnetic field, QS is measured with great accuracy.To obtain the QS distribution, the experimental data are fitted by two different methods

data fitting to a linear relation between QS and IS (relative to pure a-Fe) yields

shows three remarkably distinct peaks, corre-sponding to different Fe environments.The middle one, which is significantly broader than the two side peaks, is believed to be the envelop of

2θ(degree)

Fig.1 X-ray diffraction pattern of the single-phase La(Fe 0.81

-Si 0.19 ) 13 sample with a NaZn 13 structure.

overlapped sub-peaks associated with Fe having

same width of the third peak, this middle peak was

deconvoluted into three sub-peaks.To find the

area under each peak, the QS distribution was

fitted to five Gaussian functions sharing the same

width.All parameters including the shared width are freely adjusted by the fitting routine

In the second method, the original spectrum is fitted to a sum of three independent doublets in

Fig.4, each with its own line width, IS and QS as fitting parameters.The QS and IS values from the

agreement appears to be reasonable, in terms of the comparison between Peak I and Doublet I, Peak II and Doublet II, and Peak III and Doublet III, respectively.It should be noted that these two seemingly independent approaches actually comple-ment each other.The first method forms the basis of the discussion below on Si site preference.The second method relies on the first one to decide the number of fitting components required, 3 doublets in this case Once decided, it yields a better fit to the data

formula units.Fe atoms occupy two different sites designated as 8(b) and 96(i) according to the Wijckoff notation.The ratio of the minority site, 8(b), to the majority site, 96(i), is 1:12.For the

0:92Þ13

from heat-of-formation analysis that the substitu-tion of Si proceeds in a more or less random way

QS (mm/s)

0

1

2

3

4

5

6

Fe I

FeII

FeII

Fig.3 QS distribution obtained by the first method, from the

M ossbauer spectra in Fig.2 The peaks correspond to the Fe

sites as labeled.

Velocity (mm/s)

Fig.4 Second method of data fitting yields three doublets.The solid lines represent the three doublets and their sum.

Velocity (mm/s)

Fig.2.Paramagnetic M ossbauer spectrum at 300 K, showing

an asymmetric doublet.The solid line is calculated from the QS

distribution of Fig.3

there is apparently no preference of Fe or Si for the

8(b) site.On the other hand, based on a neutron

concluded that there is a very strong preference for

the Al atoms to occupy the 96i site, with no Al

occupancies at the 8b site

crystal-lographic information and knowing that QS is

zero in a cubic symmetry, Peak I (Doublet I)

Furthermore, the approximately 10% area

frac-tion, based on the average of Peak I and Doublet I

in Table 1, suggests that the minority sites are

in a modified formula of the partially

conclusion contradicts with that of Palstra et al

ternary rather than a pseudo-binary compound as

The next step is to consider the nature of Peak

II, along with its three sub-peaks, and Peak III

Most likely they represent a consequence of

atoms in the lattice.After all, QS results from the

interaction of the Fe nuclear quadrupole moment

with the electric field gradient (EFG).In other

words, variations of QS could reflect the

distribution of Si atoms on the majority sites as

given by the binomial function:

then normalized as P to a total of 90%, leaving

I or Doublet I.The first three populations should correspond to the three sub-peaks, II-1, II-2 and

producing the easily resolvable Peak III and Doublet III.It can be seen that the theoretically

agreement with the area ratio between Doublet II and Doublet III (63.3% versus 25.9%) The not-so-consistent area ratio between Peak II and Peak III (51.2% versus 39.6%) is likely caused by the inadequacy of the linear relation between IS and

QS in Eq.(1) The IS–QS relation in the second method is non-linear.Even so, the result can still

be used as a basis to discuss the effect on electronic configuration due to Si-substitution

The two fundamental sources for QS-inducing EFG at a Fe nucleus are the electrons of the atom itself and the charges on neighboring atoms having

a lower than cubic symmetry.Since the EFG due

to neighboring atoms is anti-shielded by the electrons on the Fe atom, electrons in the partially filled and non-spherical 3d shell create a greater EFG at the nucleus.Basically, the lattice provides the crystalline electric field that lifts the degeneracy

of 3d states, which in turn causes EFG at the

Table 1

IS, QS, and integrated area of individual components of the M ossbauer spectrum delineated through two different analytical methods

51.2

nucleus.Therefore, the observed increase in QS

increase in 3d electrons charge and/or distortions

different number of Si as nearest neighbors

Considering the relation between IS and the Fe

suggests that Si atoms reduce the net charge

density of s-electrons at the Fe nucleus.This could

be caused either directly by changes in the density

of 4s electrons at the Fe nucleus or indirectly by

changes in the 3d states.The influence of the latter

on IS is minor and takes place through screening

effects on core s-electrons.Accordingly, charge

transfers of 4s and 3d electrons have opposing

contributions, as non-local and local components,

to the increase in IS.The non-local component

was attributed to Si atoms beyond the nearest

neighboring shell.The chemical electronegativity

of Si (1.90) is greater than that of Fe (1.83), and

acts to deplete the 4s electrons at the nucleus, thus

enhancing IS.Here, we make use of this finding

and the lack of complications from polarization,

to further our understanding of the effect of Si

atoms on the 3d states.The IS value here as well as

perturbation to IS by nearest neighboring Si atoms

to either the loss of Fe-3d polarization or the loss

of Fe-3d charge.Although a reduction in Fe-3d

charge alone should make IS less positive, but the authors prefer the loss of Fe-3d charge based on a chemical trend for IS from 3d and 4d transition metal solutes.The relationship between IS and the

changes in 3d electrons of +0.001, +0.002, and

2, and 3, respectively.These changes, by them-selves, are not significant to justify the reduction of

increase goes to the 3d spin down band.The behavior of QS, IS and magnetic moment, how-ever, can all be explained by the effects of Fe-3d and Si-sp states hybridization.In this context, it is useful to recall and utilize the previously reported

of the Fe-3d band, the minority-spin down of the Fe-3d band strongly overlaps with the minority-spin down of the Si-sp band, which apparently enhances the screening of the s-like electrons from the nucleus.Also, the interaction between the two overlapping bands may have lowered their en-ergies with respect to that of the majority-spin up

of the Fe 3d-band.Consequently, electrons from the Fe-3d spin-up band drop to the lower energy band of Fe-3d and Si-sp spin-down, thus causing the observed significant reduction in the Fe magnetic moment

comprehen-sive QS and IS values, from which the distribution

of Fe and Si atoms between the two nonequivalent

The charge distribution is then correlated to the number of nearest neighboring Si atoms to a given

Fe atom and discussed in terms of the Fe-3d and Si-sp hybridization.The trends of the local isomer shift, the quadrupole splitting and the Fe magnetic moment are best attributed to the redistribution of the Fe-3d electrons between the spin-up and -down sub-bands

Acknowledgements This work is partially supported by the Vietnam National University, Hanoi, under the Research Grant No.QGTD-00-01

Table 2

Calculated probability P for different Nnn(Si) based on Eq.(2)

64.8

After being normalized by a factor of 0.9, the P values are

comparable to the area percentages for Doublet II and Doublet

III (or Peak II, along with its three sub-peaks, and Peak III,

even though to a less degree in agreement) in Table 1 , revealing

the Si distribution and the origin of the various peaks of the

M ossbauer spectrum.

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