CONDENSED MATTER

119Sn- AND 57Fe- MÖSSBAUER INVESTIGATION OF xSnO2-[(1-X)[α-Fe2O3] NANOPARTICLE SYSTEM. (I) MAGNETIC HYPERFINE FIELD

DISTRIBUTION IN Sn-LOW DILUTE SYSTEM

S. CONSTANTINESCU, L. DIAMANDESCU, I. BIBICU, D. TARABASANU-MIHAILA, M. FEDER

National Institute of Materials Physics, P.O. Box MG-07, Bucharest, Romania E-mail: sconst@infim.ro

Received January 12, 2010

A series of xSnO2-(1-x)α-Fe2O3 (x=0.0-1.0) was prepared by a hydrothermal route starting with an aqueous mixture of iron (III) chloride hexahydrate, FeCl3

.6H2O, and tin (IV) chloride pentahydrate, SnCl4

.5H2O. The as resulted nanoscaled powders have been investigated by X-ray diffraction (XRD) and 119Sn Mössbauer spectroscopy in transmission geometry and conversion electron. Under x = 0.300, the spectra reveal the appearance of magnetic hyperfine induced structure. At x = 0.300, the spectrum shows a magnetic phase (M) with the percentage of about 17%. The appearance of hyperfine magnetic fields suggests the location of the nonmagnetic Sn4+ ions, surrounded by the Fe3+ ions, in the interstitial and substitutional sites of α-Fe2O3 phase. On the other hand, the dependence of the total spectrum areas, vs. molar concentration x, displays a jump around x ~ 0.45, suggesting a change of the characteristic Debye temperature in the Mössbauer factor. The reported spectra have been fitted and the obtained hyperfine parameters are discussed in terms of reduced dimensional effects, super-transferred hyperfine interactions (STHI) and magnetic hyperfine field distribution (MHFD).

Key words: 119Sn-Mossbauer effect, xSnO2-[(1-x)[α-Fe2O3] nanoparticles, super-transferred hyperfine interactions.

1. INTRODUCTION

During the last few years a special attention has been paid to the synthesis and the investigation of the semi-conducting oxides in order to apply their sensing properties in the detection of the toxic or dangerous gases (like as CO, NO2, Cl2, CH4 [1-3]). Enhanced gas sensing properties of nano-structured semi-conducting oxides are expected due to the great surface activity. The components of xSnO2-(1-x) αFe2O3 oxide family are promising gas sensing materials. Indeed, the recent investigations evidenced their both bulk and different thin film deposition properties are usefully to the detection such mentioned dangerous gases. The literature suggests

Rom. Journ. Phys., Vol. 56, Nos. 5–6, P. 692–707, Bucharest, 2011

2 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

693

the important role of Sn4+ ions in their sensing activity and now a reach literature is dedicated to prepare the above mentioned oxide system at nanometer scale by various method [4-8]. On the other hand this system is very interesting from scientific point of view. So, the deposited films of SnO2 doped with Fe showed a remarkable strong ferromagnetism, suggesting a novel ferromagnetic exchange mechanism, “super-transferred hyperfine magnetic interactions”, STHI, [4, 9-10]. Moreover one reported high temperature ferromagnetism with giant Co-moments in Co-doped SnO2 nano-particle system [11. The investigation of chemically synthesized powders of n- and p- type SnO2 doped with Fe (as dilute magnetic semiconductor, DMS) had become of grate interested [12], suggesting a close relationship between the structure and magnetic properties of Co-doped SnO2 nanoparticles [13].

Continuing the earlier investigations [9, 14-17], this work is presenting the Mössbauer and x - ray diffraction data and analyze of their, in order to obtain new data about the microstructure of xSnO2-(1-x)αFe2O3 nano-particle system and about the effects of reduce dimension and dimensionality on the hyperfine fields.

The most common crystalline structure of the SnO2 is the tetragonal (rutile) type structure (P42/mnm, a = 4.7382(4) Å, c = 3:1871(1) Å). The structure of αFe2O3 (hematite) is based on the oxygen hexagonal closed packed (HCP; R32/c, a = 5.038 Å and c = 13.772 Å). The ferric ions are located in 2/3 octahedral sites. Chains of face-sharing octahedra are directed along the c -axis, and the Fe3+ ions within each chain form pairs separated by an empty interstitial site.

2. EXPERIMENTAL ASPECTS

A xSnO2-(1-x)[α -Fe2O3] (nominal values x = 0.0–1.0) series was prepared by a hydrothermal route starting with an aqueous mixture of iron (III) chloride hexahydrate, FeCl3⋅6H2O and tin (IV) chloride pentahydrate, SnCl4⋅5H2O with adding precipitation agent of 25%NH3[aq] solution. Suspended solid precipitate was heated in autoclave at 200oC and p = 15 atm. for 4 h. The resulting precipitate was filtered, washed with water until no chloride ions were detected by silver nitrate solution and finally dried at 105 oC. The as resulted nano-scaled powders have been investigated by X-ray diffraction (XRD) and 119Sn – TMS (standard transmission Mössbauer spectroscopy) and – CEMS (conversion electron Mössbauer Spectroscopy).

119Sn -TMS spectra were recorded at 300K (RT) and 77K (LNT), using a CaSnO3 - matrix source and a constant acceleration spectrometer, in the velocity range v ∈ [-14÷+14] mm/s. The sample thickness was about 13 mg/cm2.

S. Constantinescu et al. 3

694

20 25 30 35 40 45 50 55 60 65 700

400

800

1200

1600

x=0.05

2θ [grd]

20 25 30 35 40 45 50 55 60 65 700

400

800

1200

1600x=0.075

20 25 30 35 40 45 50 55 60 65 700

200400600800

1000x=0.125

20 25 30 35 40 45 50 55 60 65 700

200400600800

1000

XR

D In

tens

ity [c

ount

s]

x=0.175

I(2θ)/IMAX

0.940.960.981.00

0 100 200 300 400 500 0 100 200 300 400 500

0.940.960.981.00

0.940.960.981.00

0.940.960.981.00

0.920.940.960.981.00

0.960.970.980.991.00

0.940.960.981.00

0.984

0.992

1.000

0.940.960.981.00

0.9880.9920.9961.000

εEXP(n)

x=1.0

v [mm/s]

x=0.5

-3.11

x=0.9

x=0.4

x=0.8

x=0.3

x=0.7x=0.2

+6.25-3.11-12.4

x=0.6

xx=0.1

-12.4 +6.25

Rel

ativ

e Tr

ansm

issi

on In

tens

ity [u

.a.]

Fig. 1 – XRD- and RT-TMSs of some of the investigated xSnO2(1-x)α-Fe2O3 samples.

Generally, TMS technique gives us the information as an average of the whole thickness of the sample and the scattering one (by internal conversion process, CEMS, Auger or photo, CXMS, emission processes) is restricted to the layer to which electronic or γ-photons can penetrate from the surface. In order to observe and to evaluate the effects of the reduced dimension and dimensionality, the CEMS-spectra have recorded on thin film obtained by the deposition of an homogenous mixture powder - 5% collodium+95% amyl acetate, (CH3COO(CH2)4CH3), and normal dried in an Al-support (<0.3 mm). The estimated total thickness of the film was about of ~0.06 mm.

The XRD and TMSs spectra at RT for different nominal x-values are shown in the Fig. 1. In the Fig. 2 are shown TMS and CEMS 119Sn-spectra of nominal values x < 0.3.

The spectra have been analyzed by a fit-procedure, using the χ2 – criteria:

[ ]2

k k21 2 m

1 1( )

( ) ( )1χ ; ( ) (α ,α ,....α , )v

k

NNi i i

k i i kk iv

E v T vT v p f v

N n E= =

−= =

− ∑ ∑ (1)

4 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

695

where E(v) and T(v) are the normalized intensities of the experimental data and the convolution of Nv weighted (pi) fit-patterns fi (α1

i ,α2i , …αj

i, v), depending of m - hyperfine spectral parameters αk

i. The fit-patterns are corresponding to the hyperfine electric and/or magnetic interactions between Mössbauer isotope and its surrounding. In the Fig. 3 are presented the hyperfine splitting of the 119Sn-Mössbauer isotope levels in the EFG and/or hyperfine magnetic field, Bh, corresponding to fit patterns.

1.00

1.02

1.04

1.06

1.08

1.10

1.12 εEXP(v)x=0.3

v [mm/s]

1.00

1.01

1.02

1.03

x=0.2

REL

ATI

VE IN

TEN

SITY

[a.u

.]

εSAA3(v)

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0

1.000

1.002

1.004

1.006

1.008

1.010 x=0.1

εSAA2(v)

(a)

0.9840.9880.9920.9961.000

0.9880.9920.9961.000

0.9840.9880.9920.9961.000

-15 -10 -5 0 5 10 15

0.9880.9920.9961.000

0.992

0.996

1.000

0.960.970.980.991.00

X=0.150

Rel

ativ

e Tr

ansm

issi

on In

tens

ity

X=0.075

X=0.050

X=0.125

v [mm/s]

X=0.100

v [mm/s]

X=0.300

0.9800.9850.9900.9951.000X=0.175

-15 -10 -5 0 5 10 150.94

0.96

0.98

1.00

X=0.400

(b)

Fig. 2 – The RT-CEMS- (a) and -TMS-lineshapes (b) vs. nominal x.

For the analysis of spectra the modeling constrains of the pattern’ parameters is used:

1. the same line-widths for all resonance of the pattern 2. the theoretical relative area of the pattern’ resonances. The used fit-patterns were: • Single pattern

0

1 2 m 2 (i) 2

Γσ2( ,α ,α ,...α ) ;

( δ) (Γ /2)

A S

S

c f f zf v

v=

− + (1a)

δ = central – shift parameter relative to CaSnO3 – source

S. Constantinescu et al. 5

696

Γ = half line – width; fA, fs = (recoil – free fraction) Mossbauer factors of the absorber and source c = the abundance of the Mossbauer isotope in absorber; z = the absorber – thickness σ0 = the cross – section.

Fig. 3 – The hyperfine splitting of the 119Sn-Mössbauer isotope levels in the EFG, VZZ and/or

hyperfine magnetic field, Bh; θ is the angle between VZZ and Bh.

• Doublet pattern

Q 1 2 m Q Q Q Q

0

Q Q Q2 2Q

( ,α ,α ,...α ) L (v,δ,∆E ,Γ) L (v,δ,∆E ,Γ)Γσ e2L ( ,δ,∆E ,Γ) ;∆

( δ ∆ /2) ( /2) 2the quadrupole splitting parameter

A SZZ

f v

c f f z QVv E

v E

+ −

±

= +

= = =− ± + Γ

=

(1b)

• Sextet-pattern

6 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

697

1 2 m Bj Q h Bj Q h Qj 1,2,3

2Q

0

B1 Q 2 2Q

0

B2 Q 2 2Q

( ,α ,α ,...α ) L (v,δ,ε ,B ,Γ) L (v,δ,ε ,B ,Γ) ;ε

∆E 1 3cos θ2 2

Γ3 σ2( ,δ,∆ ,Γ) ;[ ] T

( δ ε 0.7009 ) ( /2)Γ2 σ2( ,δ,∆ ,Γ) ;[ ]

( δ ε 0.5230 ) ( /2)

B

A S

h

A S

Qh

f v

c f f zL v E B

v B

c f f zL v E E

v B

+ −

=

±

±

= + =

−= ⋅

= =− − + Γ

= ∆− + + Γ

∑

∓

∓ Q

0

B2 Q 2 2Q

, [ε ] mm/s

Γσ2( ,δ,∆ ,Γ) ;[δ] mm/s

( δ ε 0.3425 ) ( /2)

A S

h

c f f zL v E

v B±

=

= =− + + Γ∓

(1c)

The fit of TMS and CEMS spectra are shown and discussed in the following sections.

3. RESULTS AND DISCUSSIONS

As one can remarks, the change of the pattern shape accompanied with a line broadening vs. increasing x is the main feature of XRD spectra showing in Fig. 1. The characteristic spectra of SnO2 and α-Fe2O3 phases are obtained for the extreme values of x. The continuous broadening of the XRD resonances is given by the increase grain size vs. the decrease of x. That is in according to the reported evolution of XRD pattern shape vs. x evidenced a continuous change for x∈[0.08÷0.86], from the characteristic XRD spectra of SnO2 and α-Fe2O3 [13]. The observed increase in peak broadening of XRDSs versus increasing x is due to decreasing grain size, as shown by particle dimension calculation using Scherrer equation [14]. The plot (Fig. 4a) of particle size dimension vs. tin concentration has shown a good exponential decay fit of data [13]. The mean grain size of xSnO2-(1-x)[α -Fe2O3]-particles decreases from 70 nm to 10 nm vs. x.

For RT-TMS spectra, the dependence of the single line pattern area vs. x reveals a nearly linear decrease to increasing x values and a gap larger two, three times more as the experimental error in range x∈[0.5÷0.4]. So, the observed gap of TMSs’ area suggests a critical point xc, where structural and lattice dynamics changes are inducing by the change of x. Taking into account the sensibility of MS’s area to structural change in investigated sample, we suppose a sudden change of the Mössbauer-Lamb factor f(T, θD ) via the Debye temperature θD(x) associated with the structural change caused the gap of MSs’ area.

S. Constantinescu et al. 7

698

Fig. 4 – The mean grain size determined from XRD spectra (a) and relative area of RT-TMS (b)

of xSnO2-(1-x)[α -Fe2O3] compound series vs. nominal x-values.

Θ (x)DT

2

R

B D D 0

( , ) exp 1 4 d ; Θ ( ) Θ ( ) e 1y

E yTf T x y T RTk x x

= − + = − ∫ (2)

On the other hand all TMS and CEMS spectra are showing the presence of the incipient hyperfine magnetic interaction between tin nucleus and its neighborhood, for x ≤ 0.3 and ≤ 0.2 respectively. So, the critical point xc detected by Mössbauer effect evidences a lattice dynamics change previous to, moreover it can be considered as the upper limit under the hyperfine magnetic interactions could exist else in SnO2: α-Fe2O3. Indeed, as one can see the RT-TMS- and RT-CEMS-spectra evidence a single line spectra above x = 0.4 and respectively 0.3. Under these limits the single line is broadening and complex spectra are consisting by superposition of single and magnetic patterns for 0.125 < x < 0.3 and x < 0.2.

The main feature of the 119Sn spectra of the low concentration of SnO2 in α-Fe2O3 structure are consisting in the presence of magnetic patterns, revealing magnetic interactions between tin nucleus and its surrounding, in spite of diamagnetic properties of Sn4+. Magnetic patterns are only visible at RT for samples x ≤ 0.3. The balance of the magnetic and non-magnetic patterns’ area is showing in Fig. 5 for x ≤ 0.3.

The fit of the spectra evidence a superposition of non-magnetic pattern (which under x = 0.125 is non distinguishable for TMS spectra) and three/four distinct magnetic patterns (with broad resonances line-width), for low x-values (x < 0.175, see Fig. 5). The evidence of the magnetic patterns is in according with the observed and reported patterns at low x values [4, 11, 12, 16].

8 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

699

(b)

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.50.00

0.02

0.04

0.06

0.08

0.10

0.12 CEMS_exp CEMS_exp_ patterns 2 3 4

I CEM

S (v

) [m

m/s

]

v [mm/s]

x=0.3

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.50.000

0.005

0.010

0.015

0.020

0.025

0.030CEM S exp CEM S_exp_A CEM S_exp_f 1 2 3 4

I CEM

S (v

) [a.

u.]

v (mm/s)

x=0.20

(a)

0.980

0.985

0.990

0.995

1.000

εEXP(v) εFIT(v)

x=0.175

0.990

0.995

1.000

x=0.150

-10 -5 0 5 10

0.995

1.000

v [mm/s]

x=0.125

REL

ATI

VE T

RA

NSM

ISSI

ON

INTE

NSI

TY [A

.U.]

-10 -5 0

v [mm/s]

-1 0 .0 -7 .5 -5 .0 -2 .5 0 .0 2 .5 5 .0 7 .

0 .0 0 00 .0 0 10 .0 0 20 .0 0 30 .0 0 40 .0 0 50 .0 0 60 .0 0 70 .0 0 8

C E M S _ e x p _ A A 5 C E M S _ e x p _ A A 5 _ f it 5 p a tte rn s

I CEM

S [a.

u.]

v [m m /s ]

x = 0 .0 5

Fig. 5 – The balance of the magnetic and non-magnetic patterns’ area, observed in RT-TMS (a)

and RT-CEMS (b) spectra, by the fit of those.

The relative areas (weights) and the hyperfine magnetic fields values of the observed patterns are plotted vs. nominal x-values in Fig. 6. The fitted parameters of spectra are given in Table 1. One can remark: (1) the ratio of the single line and magnetic patterns’ areas r = AS/AM decreases as the x value decrease; (2) under x ≤ 0.125 the single resonance is not else distinguished; (3) the average values of the central shifts and quadrupole parameters k Q

,δ ( )δ ( ); εk

k xA x x= =∑

k,

( )ε ( );kk x

A x x= ∑ are very closed values for TMS and respectively CEMS spectra

( δ ( 0.13 0.10)mm/s≈ − ± , Q ε ( 0.03 0.10)mm/s≈ − ± and respectively

Qε ( 0.07 0.12)mm/s≈ − ± ) and correspond in the errors-limits to Sn4+-O bond iconicity in distorted surrounding of tin Mössbauer isotope; (4) three distinct hyperfine magnetic fields distinguish at lower x = 0.2; (5) the highest limits of the fields intensities are ~13.7 T, ~10.2 T and ~6.2 T at x = 0.05; (6) the area of the most intense field’s pattern increases more sharply vs. decreasing x values; (7) the medium hyperfine magnetic fields is the most sensitive to the increasing of x; (8) it “seems” that detected fields in the CEMS spectra are sensible lower as those detected in TMS spectra (see x=0.1); (9) the broad line-widths of the patterns’ resonances (2Γ∈[1.6 ÷ 2.7mm/s]) relative to natural 2Γnat (= 0.642 mm/s);

S. Constantinescu et al. 9

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moreover, the line-widths’ average Γ(x)2 revealed a sudden change in x ∈ [0.1÷0.125] ( ( ) 2Γ2 3ε∆ Γ > ); (10) the broad line-widths of the patterns resonances suggest a distribution of the magnetic fields Ak(Bhf) (MHFD) around 119Sn- Mössbauer isotope; (11) the line-shapes of TMS- and CEMS- spectra at reveal the differences between MHFDs.

0.050 0.075 0.100 0.125 0.150 0.175

0

50

100

150

Are

a(x)

[%]

Bhf

(x) [

10-1T]

x nominal values

0.050 0.075 0.100 0.125 0.150 0.1750

20

40

60

80

Bhf4(x)

Bhf3(x)

Bhf2(x)

Bhf1(x)

Line-guides view of

A4(x)A3(x)A2(x)A1(x)

a(x)a(x)a(x)

Fig. 6 – x-dependence of areas and magnetic hyperfine fields in xSnO2-(1-x)α-Fe2O3.

Table 1

The fit parameters of RT- TMS and – CEMS spectra for x ≤0.3

Nominal x

No. pattern

Bhfk(x) [KGs]

2Γk(x) [mm/s]

Relative Area,Ak

[%]

RT-TMS fit parameters 1 137.03 2.34 79.51 2 97.21 2.27 14.56 0.050 3 62.84 1.21 5.93 1 137.34 2.23 58.62 2 102.22 2.17 20.44 0.075 3 57.29 2.56 20.94 1 130.43 2.07 42.82 2 98.75 2.57 27.4 0.100 3 50.50 3.89 29.78

10 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

701

Tabelul 1 (continuare)

1 127.01 2.4 36.94 2 93.01 2.4 24.93 3 49.01 2.4 21.97 0.125

4 0 2.22 16.16 1 130.09 2.33 32.49 2 99.45 2.55 21.61 3 55.61 2.7 24.47 0.150

4 0 2.4 21.43 1 105.29 3.07 30.00 2 54.96 2.66 19.14 3 45.43 1.56 10.68 0.175

4 3.56 1.88 40.18

RT-CEMS fit parameters 1 70.23 1.24 5.54 4 35.18 1.23 12.91 2 15.00 1.23 12.91

0.300

3 0.00 1.25 68.63 1 113.47 1.69 19.35 2 0.46 2.78 43.76 3 50.78 1.85 16.82

0.200

4 84.76 1.70 20.06 1 119.80 1.38 48.58 2 91.62 1.20 12.70 3 89.98 1.51 30.31 4 57.01 1.22 5.60

0.100

5 37.01 1.22 2.81

Errors, ε ±3.35 ±0.36 ±2.10 *∆Q=2εQ

Taking into account the above mentioned remarks, a refined fit, consisting in the fit-deconvolution of the observed magnetic patterns (K=1, 2, 3), have considered in two steps. The detected MHFD is a convolution of several (β=1, 2 or 3) components, called elementary magnetic patterns, grouped in each of the three magnetic patterns. In the first step, the main constrains are the same estimated values of the central-, quadrupole shift and line-width for each of the three deconvoluted magnetic patterns (see the Table 2). The glance of the table permits to observe: (a) the smaller range of the line-widths 2Γ/2Γnat ∈ [1.5÷2.7] relative to the corresponding initially fitted values; the limits are touched for x=0.1 and x=0.05 of RT_CEMS and respectively RT_TMS; (b) the three observed magnetic patterns are characterized by the range [1.7T÷7.2T], [7.7T÷11.8T] and [11.6T÷14.6T] of average hfKB values; the limits are touched for x=0.1 and x=0.05

S. Constantinescu et al. 11

702

of RT_CEMS and respectively LNT_TMS; (c) the balance of the weights between the magnetic patterns’ areas, 1 2 3 K Kβ

β

A :A :A where A A= ∑ , is similarly to that

observed in the initial deconvolution; the weight of the high hf3B value patterns increases on T value decreases; (d) the central and quadrupole shifts of the magnetic patterns are characteristic to Sn4+ in more or less distorted neighborhood; it “seems” a tendency to increase the ionicity of tin vs. Bhf of powder RT spectra and opposite one to Bhf for LNT-TMS spectrum; the central shift of the observed patterns in the RT-CEMS spectrum reveals a decrease of the tin-ionicity accompanied with a less distorted surrounding of 119Sn isotope; generally the less distorted 119Sn surrounding is accompanied to the more intense magnetic fields and the medium intense fields are associated to the more distorted neighborhood of Mössbauer isotope.

The second step of refined deconvolution is consisting in no-constraints on the spectrum parameters. The fitted RT - and LNT - spectra of powder and film samples for x=0.100 and x=0.050 are plotted in Figs. 7 and 8a. The plot of Ak (Bhf) suggested a three modale distribution of the hyperfine magnetic fields.

(a) (b)

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.00.988

0.990

0.992

0.994

0.996

0.998

1.000

RT εEXP(v) εFIT(v)) εk(v) k=1...8

Rel

ativ

e T

rans

mis

sion

Inte

nsity

[a.u

.]

v [mm/s]

IEXP (v) IFIT (v) Ik (v) k=1,2,...9

-10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.00.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

I CEM

S [a.u

.]

v [mm/s] Fig. 7 – The refined deconvolution of the RT-TMS (a) and RT-CEMS spectra (b).

Observed magnetic hyperfine fields in xSnO2-(1-x)α-Fe2O3 of x < 0.2,

respectively a deconvolution of MHFD in three superposed Gauss functions (see Figs. 8b, 8c, 8d and Table 3). The multiple Gauss fit of AK(Bhf) of MHFD permitted to determine the correct of centers Bohf, the characteristic widths FMHW and the relative areas AK of magnetic patterns. The balance between the weights of the three magnetic patterns and the single line resonance is plotted in Fig. 8. One remarks the increase of magnetic field intensities and their probabilities (spectral areas) vs. decreasing nominal x-values.

12 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

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The revealing hyperfine magnetic interaction, in xSnO2-(1-x)αFe2O3, can be explained by the mechanism of the super-transferred hyperfine magnetic interactions (STHI) [4, 11]. MHFD suggest different cationic surroundings (Fe3+/Sn4+) or/and vacancies of 119Sn in the αFe2O3 lattice – disorder effect. We believe that several elementary patterns, which have observed in 119Sn spectra, should correspond to the several possible tin site paths. Generally the possible Sn-site paths are O-2 - Fe3+- O2-- Sn4+- O-2 - Fe3+ - O-2, O-2 - Fe3+- O2-- Sn4+- - Fe3+ - O-2, O-2 - Fe3+- - Sn4+- O-2 - Fe3+ - O-2, O-2 – Sn4+- O2-- Sn4+- O-2 - Fe3+ - O-2, O-2 - Sn4+- O2-- Sn4+- O-2 – Sn4+ - O-2, - Fe3+- O2-- Sn4+- O-2 - Fe3+ - O-2 , O-2 - Fe3+- O2-- Sn4+- O-2 - Fe3+ - , etc., where the tin ion has: a) substitutional site and b) interstitial one. Thus, there are some tin ions located close to oxygen vacancies and others that are not and/or iron ions. On the other hand, the observed increase in peak broadening of XRDSs versus increasing x suggests the reduced dimension effect on the evolution of three observed MHFD in MS versus x. Indeed, generally for particles of reduced dimension, the MS has two relevant contributions - volume and surface contribution - of Mossbauer probes in side of grains and respectively on the grain boundaries (GBs). The two contributions are distinguished by the difference between the hyperfine spectral parameters.

In our case, it is attempted than each of the three observed MHFDs contains a superposition’s of the two contributions and 119Sn on GB are feeling low hyperfine magnetic fields as Mössbauer isotopes in side of grains.

So, in the range x ∈ (0.175 ÷ 0.125) the reduced dimension and the disorder effects compete to the spectrum pattern shape’s dependence of x and it is very difficult to distinguish between them. We consider the balance of MHFDs’ areas is mainly due to the reduced dimension effect in this range of x > 0.125, where average grain size is smaller as 40 nm [13]. In the range of x < 0.125, where the particle dimension is grater as 40 nm, the volume contribution is mainly contribution in the spectrum and the disorder effect is preponderant in MHFD. So, taking into account the two possible effects (reduced dimension and Sn-vicinity effects) and following [11, 12], we suppose the three modal observed MHFD in RT- and LNT-spectra of x < 0.2 can be associated by (1) Fe3+-O2--bSn4+-O2-- Fe3+ (d Sn-6O=2.11Å; d Sn-6Fe=3.08Å); (2) Fe3+-O2-- bSn4+-O2-- aSn4+ - O2- Fe3+ by the tin location in interstitial (b) / substitutional (a) site; bd Sn-6O=2.11Å, ad Sn-6O=2.02Å, d Sn-3Fe=3.08Å, d Sn4+-3Sn4+ =3.08Å; (3) Fe3+- O2-- aSn4+- O2-- □ - Fe3+; dSn-6O= 2.02Å; d Sn4-3Fe=3.07Å; d Sn-3Fe=3.43Å. Taking into account the weight balance between the single line and magnetic patterns (which one observed in spectra of x > 0.100) corresponds the most probably to the Fe3+- O2-- aSn4+- O2-– Fe3+ and more less to Fe3+- - aSn4+- - Fe3+ , Sn4+- O2-- Sn4+- O2—Sn4+.

S. Constantinescu et al. 13

704

14 119Sn- and 57Fe- Mössbauer investigation of xSnO2-[(1-x)[α-Fe2O3] nanoparticle system

705

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0

0.9800

0.9825

0.9850

0.9875

0.9900

0.9925

0.9950

0.9975

1.0000

Rel

ativ

e T

rans

mis

sion

Inte

nsity

[a.u

.]

v [mm/s]

LNT εEXP(v) εFIT(v)) εk(v) k=1...9

(a)

0 20 40 60 80 100 120 140 1600

2

4

6

8

10

12

14

16

0.125SnO2 0.875α-Fe2O3

AEXPp(Hhf) AFIT(Hhf); Multi-Gaussian Peaks' Fit χ2 = 2.43472; yo=0 G1(Hhf) G2(Hhf) G3(Hhf)

Hhf1=115.92±2.61 Hhf2=44.76±2.20 Hhf3=14.18±1.53w1=67.97±11.10 w2=27.02±8.95 w3=17.43±5.53A1=974.47±115.42 A2=241.11±76.44 A3=137.63±37.81

Rel

ativ

e A

reas

Bhf [10-1T]

(c)

40 60 80 100 120 140 160 1800

10

20

30Qua

drup

olar

Shi

ft[m

m/s

]R

elat

ive

Are

as[%

]

Bhf 10-1T

-0.6

-0.4

-0.2

0.0

-0.6

-0.4

-0.2

0.0

Isom

er S

hift

[mm

/s]

(b)

0 20 40 60 80 100 120 140 1600

2

4

6

8

10

12

14

16

Rel

ativ

e A

reas

Bhf [10-1T]

0.150SnO2 0.850α-Fe2O3

AEXP(Hhf); AFIT(Hhf) Multi-Gaussian Peaks' Fit χ2 =1.8506 yo=0 G1(Hhf) G2(Hhf) G3(Hhf)) G4(Hhf))

Hhf1=127.49±1.50 Hhf2=65.995±1.39 Hhf3=33.85±0.52 Hhf4=1.58±1.13w1=43.90±4.99 w2=24.02±5.76 w3=16.58±3.91 w4=12.39±7.05

A1=677.51±64.09 A2=255.74±45.73 A3=163.82±39.72 A4=139.71±65.11(e)

0 20 40 60 80 100 120 1400

5

10

15

20

25

30

Bhf [10-1T]

Rel

ativ

e A

reas

[a.u

.]

Aexp(Hhf) Afit(Hhf) Multi-Gaussian peaks fit

χ2 = 4.15471;y0=0. G1 G2

xc1=131.6±4.5 xc2=107.8±6.4w1=45±10 w2=30.±13A1=514±100 A2=215±100

G3 G4

xc3=46.3±3.2 xc4=0w3=41±10 w4=3A3=751±100 A4=129

(d)

Fig. 8 – The second step fit of LNT-TMS spectrum for x=0.05 (a) and the corresponding patterns’ hyperfine parameters vs. Bhf (b). The balance between the single line – and the tri-modale MHFD areas

for x= 0.125 (c), x=0.150, (d) and x= 0.175 (e).

S. Constantinescu et al. 15

706

Table 3

The fit parameters of three-modale distribution of MHFD for the considered nominal x -values

Nominal x Bohf [T]

FMHW [T]

Relative Area [%]

5.51 1.28 3.74 9.91 2.50 24.46

0.050+

14.51 2.16 71.80 1.42 1.74 10.17 4.48 2.70 17.82

0.125

11.59 6.97 72.01 0.16 1.27 11.30 3.38 1.66 13.25 6.60 2.40 20.68

0.150 12.75 4.39 54.78 0.00 0.30 8.02 4.63 4.12 46.67

10.78 3.00 13.36

0.175 13.16 4.50 31.95

Errors ±0.23 ±0.76 ±3.14 +LNT

4. CONCLUSION

A structural transition, implying the change of lattice dynamics characteristics, have been detected in xSnO2-(1-x)α-Fe2O3 system for 0.4 < x < 0.5, in XRD spectra and MS. The pattern shape of MS is very strong dependent of x. Under x ≈ 0.3 a superposed of three MHFDs is revealed in MS. That can be explained by STHI only. The effect of the grain size and the near-neighbor interactions compete to MHFD area’s dependence of x in the range [0.125÷0.3]. Under x = 0.125, the mainly contribution to aMHFD(x) is given by the near-neighbor interactions and local disorder. MHFDs, at low x values, have associated with different local vicinity of 119Sn in α-Fe2O3. The results of the 119Sn-Mössbauer investigation for 0.050 < x < 0.125 at RT are in agreement to the literature [4, 11-13].

Acknowledgements. The authors acknowledge to ANSTI-project: PN09-450102/09, for

financial support.

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