The Mn2+ions were detected in strontium deficiency Pr0.7Sr0.3−xMnO3 manganites by nuclear magnetic resonance spectroscopy, but the location of the ions was not determined.12To explain th
Trang 1Divalent manganese in A -position of perovskite cell: X-ray absorption finite structure
study of La 0.6 Sr 0.4 − x Mn Ti x O 3 manganites
A N Ulyanov, D S Yang, N Chau, S C Yu, and S I Yoo
Citation: Journal of Applied Physics 103, 07F722 (2008); doi: 10.1063/1.2839318
View online: http://dx.doi.org/10.1063/1.2839318
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Trang 2Divalent manganese in A-position of perovskite cell: X-ray absorption finite
structure study of La0.6Sr0.4−xMnTixO3 manganites
A N Ulyanov,1,a兲,b兲 D S Yang,2N Chau,3S C Yu,4and S I Yoo1,a兲,c兲
1
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
2
Physics Division, School of Science Education, Chungbuk National University, Cheongju 361-763, Republic of Korea
3
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
4
Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
共Presented on 8 November 2007; received 12 September 2007; accepted 10 December 2007;
published online 28 February 2008兲
Local structure and magnetic properties of Ti doped A-site deficient La0.6Sr0.4−xMnTixO3+␦
manganites共0.15艌x艌0兲 have been studied The compositions belong to rhombohedral R3¯c phase.
Segregation of 共La0.6Sr0.4−xMny兲共Mn1−y−zTix兲O3+ ␦ phase and fallout of 共z/3兲Mn3O4 oxide was
observed with x increase Some amount 共y兲 of Mn, being in divalent valence state, occupies the
A 共=La,Sr兲-position of perovskite cell Samples with x=0 and 0.05 are ferromagnetic with Curie
temperature T C = 350 and 172 K, respectively Samples with x = 0.1 and 0.15 are in
spin共cluster兲-glass states at low temperatures © 2008 American Institute of Physics
关DOI:10.1063/1.2839318兴
In the last decades, properties of perovskite such as
Ln1−⌬1R⌬1Mn1−⌬2M⌬2O3 lanthanum manganites have
at-tracted a growing attention because of their interesting
prop-erties and rich phase diagram, especially the colossal
magne-toresistivity effect observed共Ln is a trivalent rare earth, Y; R
is a divalent alkali earth, Sn and Pb; and M is a transition
metal兲.1
The parent LaMnO3compound is antiferromagnetic
insulator Substitution of trivalent Ln3+ ion by divalent R2+
ion gives rise to a coexistence of Mn3+and Mn4+ions and, at
some hole doping level共⌬1兲, manganites become a
conduc-tive ferromagnetic materials According to double exchange
共DE兲 model,2
transfer of itinerant e g electrons between neighboring Mn3+and Mn4+ions through O2−ions results in
ferromagnetic interaction due to the on site Hund’s coupling
Ion size mismatch was introduced to explain the dependence
of Curie temperature 共T C 兲 on average ionic radius in A
共=Ln,R兲-position of ABO3perovskite cell.3B 共=Mn,M兲-site
doping by transition metals damages the traveling path of
itinerant e g electrons and changes both magnetic B – O – B
interaction, and B–O distances and B – O – B angles, thus,
af-fecting the properties of perovskite manganites 共e.g., see
Refs.4 6and references therein兲 The effect depends on size
and electron configuration of dopants Deficiency of La
and/or Mn ions 共or the oxygen excess ␦兲 in LaMnO3
com-position also causes the appearance of Mn3+– Mn4+
mixed-valence state.711 Such, the self-doped manganites exhibit
both ferromagnetic-paramagnetic and metal-insulator
transi-tions Properties of A- and B-site substituted manganites
have been accurately studied and characterized in literature
At the same time, the self-doped compositions are less
care-fully investigated and their description contains some
vagueness.10,12,13 The problem is in the complexity of the
self-doped manganites from the crystallochemistry point of view; how the structure accommodates the nonstoichiometry and vacancies An early structural study of LaMnO3+␦
man-ganites showed no excess oxygen in the interstitial positions
of the perovskite cell.14Instead, there were found appropriate amounts of vacancies in both La and Mn sites, which indi-cated the cation deficient origin of the entire structure scele-ton Recent magnetic and structural study of La1−⌬1MnO3 共0.3艌⌬1艌0兲 showed a fallout of Mn3O4 oxide and segre-gation of vacancy-doped La0.9MnO3 phase with ⌬1 increase.7 The phase segregation explains the composition independent magnetic properties of La1−⌬1MnO3observed in the wide, 0.3艌⌬1艌0.1, range According to Refs.8 and9, the La1−⌬1MnO3 can accommodate vacancies up to ⌬1
= 0.125 and 0.13, respectively Recently, in the crystal-lochemical characterization of vacancy-doped LaMnO3 samples with different La/Mn ratios by neutron diffraction,
it was suggested that the occurrence of Mn ions at the La site
be at La/Mn⬍1.10
At the same time, authors noted that the samples’ local structure can be quite satisfactory refined in any共with and without Mn in La sublattice兲 distribution mod-els and without supporting additional evidence, it is impos-sible to choose the proper one The Mn2+ions were detected
in strontium deficiency Pr0.7Sr0.3−xMnO3 manganites by nuclear magnetic resonance spectroscopy, but the location of the ions was not determined.12To explain the properties of Nd-deficient Nd0.9−xCaxMnO3 compositions, it was hypoth-esized that a part of Nd ions can be substituted by Mn ions.13
To elucidate these peculiarities, we present the careful local structure analysis of La0.6Sr0.4−xMnTixO3+␦ manganites To
this end, we employed the x-ray absorption fine structure 共XAFS兲 analysis, which gives the information for both the neighborhood of XAFS atoms and their valence states
Samples were synthesized and characterized as in Ref
15 La0.6Sr0.4−xMnTixO3+␦manganites共x=0.0, 0.05, 0.1, and
0.15兲 were prepared by conventional solid state reaction
a兲Authors to whom correspondence should be addressed.
b兲Electronic mail: aគnគulyanov@yahoo.com.
c兲Electronic mail: siyoo@snu.ac.kr.
JOURNAL OF APPLIED PHYSICS 103, 07F722共2008兲
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Trang 3method According to powder x-ray Cu K␣ analysis 关x-ray
diffraction 共XRD兲兴 the samples belong to rhombohedral
共R3¯c兲 phase and contain 共at x⫽0兲 small amount of Mn3O4
impurity oxide On the basis of the XRD data, the oxide
amount is estimated to be less than 5 wt % and no anomalies
in the magnetization data could be attributed to the impurity
phase
Magnetization measurements were carried out with the
superconducting quantum interference device共Quantum
De-sign MPMSXL兲 magnetometer Curie temperature 共T C兲,
de-termined as an inflection point on temperature dependence of
magnetization, decreases dramatically from 350 to 172 K
with x increase from 0 to 0.05 共see inset of Fig 1兲 It is
believed that compounds with higher x content are in
spin共cluster兲-glass-like state at low temperatures and
transi-tion to paramagnetic state is observed at 120 and 100 K for
the x = 0.1 and 0.15 samples, respectively Spin
共cluster兲-glass-like behavior of La0.6Sr0.4−xMnTixO3+␦ manganites
with x = 0.1 was also reported in Ref. 16 Topfer and
Goodenough17 also pointed out that lanthanum manganites
with small content of cation vacancies exhibit spin-glass
be-havior below the Curie point Detailed discussion of these
features lie beyond the scope of present report and will be
published elsewhere
XAFS experiments were performed at the 3C extended
x-ray absorption fine structure 共EXAFS兲 beam line of
Po-hang Light Source 共PLS兲 in Korea PLS operates with
elec-tron energy of 2.5 GeV and maximum current of 230 mA
X-rays were monochromatized by Si共111兲 double-crystal
monochromator with energy resolution, ⌬E/E=2⫻10−4 Higher harmonics were removed by a 15% detuning of the
crystal XAFS spectra were obtained near the Mn K edge
共6540 eV兲 in a fluorescence mode at room temperature XAFS represents EXAFS, and x-ray absorption near edge structure 共XANES兲 analysis EXAFS gives information about the local structure around central atoms Electronic configuration 共valence兲 of the core Mn cations can be de-duced with the XANES spectra, obtained directly by the nor-malization of absorption spectra.18
XANES共Fig.1兲, EXAFS 共Fig.2兲, and Fourier transform
of EXAFS spectra共Fig.3兲 show continuous change with x XANES spectra shift to lower energy and essentially
broaden with x It is important to emphasize that XANES
spectra of La1−xCaxMnO3 compositions19,20 showed almost
the same shape with x and only shifted parallel to each other
with increasing of Ca contents The shift of the absorption
edge from the lower to higher energy with x was caused by
the change of average Mn valence from 3+ 共in LaMnO3兲 to 4+ 共in CaMnO3兲 The main absorption for the Mn3+ion 共in
FIG 1 共Color online兲 共a兲 XANES spectra and temperature dependencies of
magnetization 共on the inset兲 for La 0.6 Sr0.4−xMnTixO3+␦manganites共x=0.0,
0.05, 0.1, and 0.15, from the right to the left 兲 共b兲 XANES spectra for
La0.6Sr0.4MnO3共x=0兲 phase 共solid line兲, and for linear combination
共Lin-Comb, dotted line 兲 for 0.9La 0.6 Sr0.4MnO3and 共0.1/3兲Mn 3 O4 The x = 0 and
LinComb lines almost coincide Figure also shows the XANES spectra for
La0.6Sr0.3MnTi0.1O3共x=0.1兲, LaMnO3 , and Mn3O4and MnO oxides.
FIG 2 共Color online兲 EXAFS spectra for La 0.6 Sr0.4−xMnTixO3+␦manganites
共x=0.0, 0.05, 0.1, and 0.15兲, and linear combination 共LinComb, dotted line兲
for 0.9La0.6Sr0.4MnO3and 共0.1/3兲Mn 3 O4 The x = 0 and LinComb lines
al-most coincide.
FIG 3 共Color online兲 Fourier transform of EXAFS spectra for
La0.6Sr0.4−xMnTixO3+␦compositions共x=0.0, 0.05, 0.1, and 0.15兲, and linear
combination 共LinComb, dotted line兲 for 0.9La 0.6 Sr0.4MnO3 and 共0.1/3兲Mn 3 O4 The x = 0 and LinComb lines almost coincide.
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Trang 4LaMnO3兲 was observed at the interval from
6550 to 6556 eV
A very different picture has been observed in our
La0.6Sr0.4−xMnTixO3+␦, MnO, and Mn3O4are presented兲 The
spectrum for the La0.6Sr0.4MnO3 共x=0兲 composition shows
almost the same shape and position as those in
La0.7Ca0.3MnO3 共Ref 19兲 and even small amount of Ti 共x
= 0.05兲 causes considerable changes in XANES spectra The
spectra become broader because of increase of absorption at
low energy 共E⬍6.55 keV兲—the low energy “tail” appears.
The tail becomes wider and more intensive with x The
changes of XANES spectra probably originate from the
oc-currence of divalent Mn ions, which is manifested by the
appearance of x-ray absorption at energies lower than that
for the LaMnO3, where the Mn is only in trivalent state关see
Fig 1共a兲and 1共b兲兴 Sharp increase of absorption for the x
= 0.1 and 0.15 compositions begins at the same energy as that
for Mn2+ ion in MnO oxide
Essential changes of EXAFS spectra共Fig.2兲 and Fourier
transform of EXAFS spectra共Fig.3兲 are also observed The
changes can be attributed to nonuniform distribution of Mn
ions—partial occupation of A-position by the Mn ions
Re-ally, it is well established18 that the共i兲 regularity in
appear-ance of high intensity peaks of Fourier transform of EXAFS
spectra, as for the x = 0 samples, evidences for uniform
dis-tribution of Mn atoms in lattice, and, vice versa, a complete
disappearance of third and forth peaks with x, as for the x
艌0.10 compositions, is an evidence of nonuniform
distribu-tion of Mn ions in perovskite cell and共ii兲 smoothing of
EX-AFS spectra also confirms the nonuniform distribution of
XAFS atoms in compositions studied We suppose that the
Mn2+ occupy the A-position, and Mn3+,4+ ions, as usually,
occupy the B共=Mn,Ti兲-site By normalizing the number of
atoms in B-site to unit the La0.6Sr0.4−xMnTixO3+␦
composi-tions can be presented as self-doped A-site deficient
compo-sitions of La0.6/共1+x兲Sr共0.4−x兲/共1+x兲Mn1/共1+x兲Tix /共1+x兲O3 The
A-position is occupied by La3+and Sr2+ions with ionic radii
1.216 and 1.31 Å, respectively共all ionic radii are taken
ac-cording to Shannon21兲 The most preferable ions, which can
occupy the A-position 共to accommodate the vacancies兲
among the Mn2+ 共=0.83 Å兲, Mn3+ 共=0.645 Å兲, and Mn4+
共=0.53 Å兲 are the Mn2+ ion as the largest one We have to
note that if the Ti4+共=0.605 Å兲 ions occupy the A-position
there will not be strong change in the EXAFS and Fourier
spectra There will be the only a weak change in intensity of
second peak, which is caused by the backscattering of
elec-trons by the atoms, located in the A-position共e.g., see results
for La0.7Ca0.3−xBaxMnO3 manganites22兲 Thus, it is finally
possible to describe the compositions as 共La0.6Sr0.4−xMny兲
⫻共Mn1−y−zTix兲O3+ ␦ 1+共z/3兲Mn3O4, where y and z depend on
x The atoms in first and second brackets occupy the A- and
B-positions, respectively Similar La0.9MnO3+共z/3兲Mn3O4
segregation in the range 0.9艌La/Mn艌0.7 was reported7
when studying the La1−⌬1MnO3 compositions To be sure
that the change in XAFS spectra, observed with x, are not
caused by the fall out of the parasitic Mn3O4 phase, the
simulation of the spectra was done We fitted the XANES,
EXAFS and Fourier transform of EXAFS spectra by linear combination
LinComb=共1 −兲共La0.6Sr0.4MnO3兲 +
3共Mn3O4兲 共1兲
of spectra for La0.6Sr0.4MnO3 phase and Mn3O4 共similar to fitting presented in Ref.7兲 Only very weak changes of the spectra 共for = 0.1兲 were obtained It confirms that the changes observed in XAFS spectra are caused by the internal change of local structure of La0.6Sr0.4−xMnTixO3+␦ with Ti
content
Change in Curie temperature for the B-site substituted
manganites mainly originates from the weakening of the DE
interaction due to the breaking of the pathway for itinerant e g
electrons caused by the difference in electron configurations between the Mn3+, Mn4+ ions, and transition metal ions
共E-factor兲 and by the structural S-factor: change of 具Mn–O典
bond distances and具Mn–O–Mn典 bond angles because of the difference in Mn and dopant size ionic radii共see, e.g., Ref.6 and references therein兲 The stronger TC decrease in
La0.6Sr0.4−xMnTixO3+␦ than that in La0.7Ca0.3Mn1−xTixO3 共Ref 4兲 and La0.7Sr0.3Mn1−xTixO3 共Ref 5兲 is obviously caused by the occurrence of Mn2+ ions in A-position of
per-ovskite cell and deficiency of atoms in above position in
addition to the E- and S-factors.
In conclusion, the segregation of 共La0.6Sr0.4−xMny兲
⫻共Mn1−y−zTix兲O3+ ␦ 1phase and fallout of 共z/3兲Mn3O4oxide
with x increase was observed The x increase causes the
Mn2+ions appearance and deficiency of atoms in A-position, which together with the substitution of Ti for Mn in B-site
causes the strong decrease in Curie temperature and changes the character of low temperature magnetic state of samples
with high x value.
The research was supported by BK21 Materials Educa-tion and Research Division
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