Dopant effects on the structural, low temperature Ramanscattering and electrical transport properties in SrTi12xFexO3

nanoparticles synthesized by sol-gel method

Nguyen Van Minh • Doan Thi Thuy Phuong

Received: 12 February 2010 / Accepted: 4 May 2010 / Published online: 14 May 2010

� Springer Science+Business Media, LLC 2010

Abstract We investigate effects of Fe dopant concen-

tration on the structure, as well as low temperature Raman

scattering and electrical transport properties in SrTi1-x

FexO3 (x = 0.00, 0.10, 0.20, 0.30, 0.40) nanoparticles

prepared by sol-gel method. The results show an average

particle size of powder is about 30 nm, and the lattice

parameters decrease as increasing the Fe content. In

the Raman spectra, a broad structure in the region

200–500 cm-1 is almost absent and the peaks in the region

600–800 cm-1 show different weights with respect to

SrTiO3, relating to structural changes with increasing

dopant concentration in conjunction with increasing grain

boundary contribution to the impedance. The abrupt

change in Raman peak position as function of temperature

suggests a phase transition in our samples in the range of

110–150 K. These results indicate that the Fe ion has

replaced the site of Ti in unit cell. These results also

demonstrate the feasibility of synthesizing the compound

with low annealing temperature.

Keywords Nanoparticle � Structure � Raman spectra �Impedance spectroscopy

1 Introduction

Nowadays, one of the most active research field in applied

physics is the study of low-dimensional nanostructures

especially semiconductor quantum dots (QDs). It was

generally recognized that the optical and electrical properties

of QDs are strongly sensitive to their sizes, shapes, and

compositions. While electronic materials are widely inves-

tigated at nano-size, this is not so much the case for ionically

as well as mixed materials [1], although they play a signifi-

cant role in a variety of applications such as batteries, fuel

cells, and sensors. Furthermore, the family of perovskite

oxides also has important applications in electronics. There

are various reports on the effects of impurity doping on the

properties of this interesting system [2, 3]. For substitutions

of the Sr site in SrTiO3, the suppression of the quantum

paraelectric state was reported [4]. By substitution of Bi for

Sr in the same compound leads to the occurrence of several

polarization modes and final transition to ferroelectric

relaxor behavior [5], while La doping strongly suppresses the

paraelectric state, without the occurrence of intrinsic polar-

ization modes, except for polarization effects related to

oxygen vacancies [6]. SrTiO3 doped with transition metal,

such as Fe combines the required stability and interesting

transport properties at relatively high temperatures. These

materials have been considered for application as electro-

chemical electrodes and resistive oxygen sensors [7, 8]. For

these applications, special attention was paid to the electrical

transport properties at high temperatures. However, very few

papers were concerned with effects of the dopant concen-

tration and the dopant ionic size. In this paper, we present the

effects of Fe dopant content on the structure, temperature

dependent Raman scattering and impedance properties of

SrTi1-xFexO3 nanoparticles.

2 Experimental section

Nanopowders of SrTi1-xFexO3 were prepared by the

polymeric precursor method (PPM), which is based on the

N. Van Minh (&) � D. T. T. Phuong

Center for Nano Science and Technology, Hanoi National

University of Education, 136 Xuan Thuy Road, Hanoi, Vietnam

e-mail: minhsp@gmail.com; minhnv@hnue.edu.vn

123

J Sol-Gel Sci Technol (2010) 55:255–260

DOI 10.1007/s10971-010-2242-5

chelation of the metal cations by citric acid in a solution of

water and ethylene glycol. The precursor solution was

prepared from a titanium citrate formed by dissolving

titanium isopropoxide in an aqueous solution of citric acid

heated to about 70 �C. A stoichiometric amount of

Sr(NO3)2 was added to the titanium citrate solution, which

was stirred slowly until the reactional mixture became

clear. Fe(NO3)2�6H2O solution (0.2 M) was then added to

the solution slowly. To completely dissolve the Sr(NO3)2

and Fe(NO3)2�6H2O, ammonium hydroxide was added

drop by drop at a time until the pH reached 7–8. The

complete dissolution of the salts resulted in a transparent

solution. After the solution containing Sr and Fe cations

was homogenized, ethylene glycol was added at a 60:40

mass ratio of citric acid/ethylene glycol to promote the

polyesterification reaction. The solution became more

viscous as the temperature was raised to 90 �C, without

showing any visible phase separation. This resin was then

placed in a furnace and heated to 350 �C for 4 h, causing it

to pulverize into powder. This powder was then heated at

300 �C for 20, 30 and 40 h to obtain the disordered phase

of the SrTi1-xFexO3. The crystalline phase was obtained by

heating the powder at 900 �C for 8 h.

Structural characterization was performed by means of

X-ray diffraction using a D5005 diffractometer with Cu Karadiation. The FE-SEM observation was carried out by

using a S4800 (Hitachi) microscope. Raman measurements

were performed in a back scattering geometry using Jobin

Yvon T 64000 triple spectrometer equipped with a cryo-

genic charge-coupled device (CCD) array detector, and the

514.5 nm line of Ar ion laser. Impedance spectroscopy was

obtained by a Le Croy equipment.

3 Results and discussion

Figure 1 presents the SEM image of the nanopowder cal-

cined at 600 �C for 5 h. For clarity, we show only SEM

images of three samples (x = 0.0, 0.2, 0.4). The average

grain size obtained under this calcination condition is about

30 nm and very homogeneous. This size is in agreement

with that obtained by Scherrer formula (*26 nm) from

(110) and (220) peaks of XRD patterns in Fig. 2.

Figure 2 shows the X-ray diffraction (XRD) of SrTi1-x

FexO3 powders at room temperature. The XRD results

identify that powders are polycrystalline and have a cubic

perovskite structure as previously reported [9]. The change

in the cell parameters (Fig. 3) as well as the shift and

broadening of the peaks (inset of Fig. 3) indicate that the

Fe ions have replaced the Ti ions in the unit cell. This

substitution implies some changes in the structure with

possible appearance of additional phase or some disorder in

the samples.

The optical property and Raman spectra of pure SrTiO3

were investigated in some previously published papers

[10, 11]. Here, we use Raman spectra of SrTi1-xFexO3

powders to investigate the breaking of inversion, and/or

translation symmetries, impurities and defects, doping, that

come from Fe doping. It was known that bulk SrTiO3

crystals have a centrosymmetric structure: cubic at high

temperatures and tetragonal below 105 K. The zone-cen-

tered optical phonons are of odd parity, and consequently

are not Raman active. The results of our SrTi1-xFexO3

samples are very different. The Raman spectra of SrTi1-x

FexO3 samples of different dopant contents measured at

room temperature shown in Fig. 4 are very different from

Fig. 1 SEM images of SrTi1-xFexO3 nanopowders: a x = 0.0,

b x = 0.2, c x = 0.4

256 J Sol-Gel Sci Technol (2010) 55:255–260

123

that of the pure compound, completely due to second order

scattering [12]. In particular, the broad structure in the

region 200–500 cm-1 is observed from x = 0.0 sample is

visibly absent and the x [ 0 cases the peak in the region

600–800 cm-1 shows remarkable broadening and intensity

reduction with increasing x. In addition, a vague new

structure at about 520 cm-1 for x = 0.1 and x = 0.2 which

seems due to the overlapping of two distinct bands, can be

considered as a manifestation of first order scattering,

probably due to activation of the bulk TO4 phonon mode

and of local modes. At higher Fe content, this peak seems

to be disappeared. By comparing with the hyper-Raman

results of bulk single crystals [13], where optical phonons

are active, the following assignments of the peaks

appearing in the spectra for x [ 0 are proposed, namely

the peak at 170 cm-1 as due to the TO2, the weak peak

at 264 cm-1 to the silent TO3, and the strong peak at

545 cm-1 to the TO4 phonons. These peaks can be observed

at room temperature.

The observed line broadening can be attributed to the

doping induced disorder and compositional fluctuation

arising from a random distribution of Ti and Fe cations

over the B-site in ABO3 sublattices. Upon Fe doping, these

ionic species compete for a given site in SrTiO3 and

destroy the translational invariance. Since the Raman mode

frequency is related to the force constant and the reduced

mass, the observed upward shift or downshift of the mode

frequencies in the SrTi1-xFexO3 samples reflects either a

decrease or an increase of partial replacement of more

ionic species. These effects could lead to corresponding

changes in the Raman spectrum, such as appearance of

phonons from other points of Brillouin zone and splitting

of the degenerate B1 ? E silent modes.

Now, we compare the Raman spectra of SrTiO3 doped

with different Fe contents. For Fe doped samples, the

strong peak appears around 720 cm-1 and its position

up-shifts with increasing doping content until x = 0.2.

Upon this content, the peak exhibits downshift. Taking into

account the sign of the change in frequencies and the dif-

ference in the atomic weights of Ti (47.88) and Fe (55.85),

such behavior is unusual in terms of what is known for

mixed crystals, where the frequency change is driven

mainly by mass effect (*ffiffiffiffiffiffiffiffiffi

k=mp

, k is force constant, and

m—weight). We therefore suggest that the mass effect may

be too weak to counter the interaction between Ti/Fe ions

and TiO6 octahedra.

For the SrTi1-xFexO3 nanoparticles, a pronounced line

at around 720 cm-1 appears at an iron concentration as low

as x = 0.1. Its peak position shifts to 740 cm-1 for large x.

Since all these peaks are observed with the same fre-

quencies and intensities for all applied excitation wave-

lengths (647.1, 632.8, and 514.5 nm), luminescence effects

are not the origin of these lines. The fact that the frequency

of 720 cm-1 does not split into TO and LO modes and does

not correspond to any of the SrTiO3 first-order phonons can

be seen as an indication that this peak is caused by a

vibration with local character.

The temperature dependence of Raman spectra of

SrTi1-xFexO3 nanoparticles was shown in Fig. 5. For

clarity, we present only samples with x = 0.1 and 0.3

(220

)

(211

)

(210

)

(200

)

(111

)

(110

)

(100

)0.40

0.30

0.20

0.10

0.00

20 30 40 50 60 70

Inte

nsity

(ar

b. u

nits

)

2θ (degree)

Fig. 2 XRD patterns of SrTi1-xFexO3 powders

0.0 0.1 0.2 0.3 0.4

0.389

0.390

Cel

l par

amet

er (

nm)

Fe content

67.6 68.0 68.42θ (degree)

(220

)

0.40

0.30

0.20

0.100.00

Fig. 3 Cell parameters versus Fe content. The inset shows the shift

and splitting peaks around 2h = 78�

300 600 900

0.40.3

0.2

0.1

0.0

Inte

nsity

(A

rb.u

nits

)

Raman shift (cm-1)

LO4

A2g

TO4

LO3

TO3

TO2

LO1

Eg

+B

1g

A1

Fig. 4 Raman spectra of SrTi1-xFexO3 powders (x = 0.0–0.4)

J Sol-Gel Sci Technol (2010) 55:255–260 257

123

(Fig. 5a, b). For comparison, the Raman spectra of all

samples at 77 K are also shown in Fig. 5c. The anomalous

change in Raman peak position as function of temperature

(Fig. 5d) suggests a phase transition in the range of

110–160 K depending on the iron concentration. As above

mentioned, bulk SrTiO3 crystals have a centrosymmetric

structure: cubic at high temperatures and tetragonal below

105 K (at 120 K, for a polycrystalline sample) [14]. It is

worth noticing that the presence of defects or impurities

increases this transition temperature. For instance, in the

mixed Sr0.7Ca0.3TiO3 perovskite the cubic-to-tetragonal

transition occurs at 230 K [15]. The presence and intensity

behavior of the tetragonal first-order features in our Raman

spectra seem to indicate an increasing tetragonal distortion

of the ‘‘average’’ cubic cells (possibly due to short-range

cation ordering), rather than a complete phase transition for

large x. The observation is consistent with the behavior of

the classical cubic second-order Raman modes [12], and

tends to weaken for increasing Fe content. A possible

scenario would be the smearing and continuous increasing

of the critical temperature with x, which could lead to some

phase coexistence at room temperature, with predominance

of the cubic phase.

To elucidate the role of Fe doped on the conductivity,

we have measured the impedance of the samples. The

impedance spectroscopy is usually used to characterize

bulk grain, grain boundary and electrode interface contri-

butions by exhibiting successive semicircles Cole–Cole

plot (often with some distortion) with its imaginary part

plotted against its real part in the complex plane [16, 17].

A high frequency semicircle originates from the bulk

conduction and dielectric processes; a low-frequency

semicircle is associated with ion and electron transfers at

the contact surface between the sample and the elec-

trode, while an intermediate-frequency semicircle provides

information on the grain boundary and/or impurity-phase

impedance. All these contributions vary with temperature,

and, for a given frequency range of measurement, they may

not all be detected.

To measure the impedance spectroscopy, the SrTi1-x

FexO3 powders were pressed in cylinders and then annealed

at 1,100 �C for 6 h. For clarity, in Fig. 6 we show the

Z00–Z0 plots of the impedance of three samples (x = 0.00,

0.20 and 0.40) together with the simulations results as most

impedance investigations on polycrystalline samples with

perovskite structure and other dielectric compounds, a

series of parallel RC elements are used for the numerical

simulation. Figure 6a shows the plot for sample with

x = 0.00 together with the simulated spectrum displaying a

good fit. The spectrum of sample with x = 0.20 in Fig. 6b

shows two clearly separated semicircles with both of them

is fitted with a simulated curve. The semicircle attributed to

the grain boundaries is relatively large. The sample with a

relatively high Fe content might lead to high grain

boundary resistivity. Figure 6c shows the results for sam-

ple with x = 0.40. Here, the high-frequency semicircle

Inte

nsity

(ar

b. u

nits

)(b)

(a)

250 500 750 1000

205 K180 K

155 K130 K120 K110 K100 K

90 K

77 K

Raman shift (cm -1)

230 K205 K

180 K

155 K

130 K120 K

110 K

100 K

90 K78 K

(d)

(c)

80 120 160 200690

695

700

705

710

130

110

110160

Pea

k ar

ound

700

cm

-1

Temperature (K)

x=0.1 x=0.2 x=0.3 x=0.4

250 500 750 1000

77 K

0.40.30.20.1

0.0

Inte

nsity

(ar

b. u

nits

)

Raman shift (cm-1

)

Fig. 5 Temperature dependent

Raman spectra of samples

SrTi1-xFexO3 with a x = 0.1,

b x = 0.3, c with various

content of x at 77 K, and

d peak around 720 cm-1

versus temperature

258 J Sol-Gel Sci Technol (2010) 55:255–260

123

seems to disappear. We attribute the semicircle at low

frequencies tentatively to grain boundaries. We note that in

all cases, the simulated results fit the experimental data

quite well. The relatively small spreads of the data could be

related to inhomogeneity (microstructural, compositional,

etc.) of the polycrystalline materials.

It can be seen from Fig. 6, that there are two effects

pertaining to microstructural inhomogeneity—grain and

grain boundary. Impedance spectroscopy allows the sepa-

ration of the resistance related to grains (bulk) and grain

boundaries because each of them has different relaxation

times resulting in separate semicircles in the complex

impedance plane. For sample with x = 0.4, the high fre-

quency semicircle originates from the bulk conduction

and dielectric processes does not detect. Therefore, the

spectrum reveals a relatively large grain boundary contri-

bution to the total impedance [16, 17]. The reason may be

that close to the grain boundaries, the transport properties

of the material are controlled by imperfections, expected to

be present in higher concentration than in grains, leading to

an additional contribution to the intergrain (grain bound-

ary) impedance. The internal space charge created at the

grain boundaries may lead to a significant increase in the

concentration of mobile effects. Fig. 6 also shows the fitted

and experimental curve at room temperature. In the present

case equivalent circuit modeling is done using the equiv-

alent circuit simulation software of LabVIEW.

4 Conclusions

In summary, a number of nanocrystalline SrTi1-xFexO3

samples with x = 0.0, 0.1, 0.2, 0.3 and 0.4 have been

synthesized by means of polymeric precursor method for

the study of doping effects on the structure, Raman spectra

and impedance spectroscopy. XRD and Micro-Raman

results reveal a cubic perovskite structure for SrTi1-xFexO3

nanoparticles in all Fe-doping contents with no evidence of

any secondary phase. Upon Fe doping increases, the cell

parameter is decreased. We also find evidence for the

disappearance of the region 200–500 cm-1 in the Raman

spectra of sample with x [ 0, and the region 600–

800 cm-1 shows remarkable broadening and intensity

reduction with increasing x, suggesting that there may be

some symmetry breaking structural distortions in these

samples. The abrupt change in Raman peak position as

function of temperature suggests a phase transition in the

range of 110–160 K depending on the iron concentration.

The role of grain boundary was changed with Fe-doping

concentration: the Fe impurity increases the grain boundary

and/or impurity-phase impedance and decreases bulk grain

impedance. An exciting result based on the impedance

spectroscopy measurement is that at high Fe doping con-

tent, only a low-frequency semicircle corresponding to the

grain boundary is observed. Further progress on the Fe

doped SrTiO3 will be reported in forth coming publica-

tions, including the effect of grain boundary dopant, oxy-

gen partial pressure, and temperature.

Acknowledgments This work was supported by National Founda-

tion for Science and Technology Development (NAFOSTED) of

Vietnam (Code 103.06.14).

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0.5

1.0

1.5

Data Fit

Data Fit

Data Fit

-Z"

(KΩ

)

Z' (KΩ)

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0.3

0.6

0.9

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