Variable-temperature Raman scattering and X-ray diffraction

studies of Bi3.25Nd0.75Ti3O12 ceramics

Y.H. Wang a,b,*, C.P. Huang b, Y.Y. Zhu b

a Department of Physics, China University of Mining and Technology, Jiangsu, Xuzhou 221008, Chinab National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, China

Received 8 October 2005; received in revised form 25 December 2005; accepted 6 March 2006 by T.T.M. Palstra

Available online 23 March 2006

Abstract

Neodymium-substituted bismuth titanate (Bi3.25Nd0.75Ti3O12, BNT0.75) ceramics was prepared by chemical co-precipitation along with

calcinations. The lattice instability has been investigated by variable-temperature Raman scattering and X-ray diffraction. The results showed that

there was an orthorhombic to pseudo-tetragonal phase transition at about 695 K, in terms of the evolution of temperature dependence of Raman

scattering frequencies. Some changes at about 695 K in the XRD lines, the lattice parameters (a, b, and c) as well as the orthorhombic distortion

b/a have been detected in the high temperature X-ray diffraction, which confirmed the conclusion that the BNT0.75 ceramics undergoes a

ferroelectric to paraelectric phase transition at about 695 K.

q 2006 Elsevier Ltd. All rights reserved.

PACS: 77.80.Bh; 61.10.Eq; 63.20.Dj

Keywords: B. Laser processing; C. X-ray scattering; D. Phase transitions; D. Phonons

1. Introduction

Bismuth layered perovskite ferroelectric materials, with the

characteristics of fast switching speed, high fatigue resistance,

and good retention [1], have attracted much attention due to

their potential applications in several important areas,

including ferroelectric, piezoelectric, microelectromechanic,

electric and photoelectronic devices. lanthanide-modified

bismuth titanate (Bi4KxRxTi3O12, RZLa, Nd, Sm, etc. [2–5])

are currently regarded as one of important candidate materials

for the nonvolatile memory applications and have been widely

studied due to its high remnant polarization, low processing

temperature, and good fatigue-free properties [2]. Although

this modifying effect is one of the best routes of stabilizing the

improved properties superior to those of bismuth titanate

(Bi4Ti3O12, BTO) phase, a detailed understanding of doping

effect on structure and structural instability are still lacking.

Optical phonons, particularly those accessible by Raman

0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2006.03.006

* Corresponding author. Address: Department of Physics, China University

of Mining and Technology, Jiangsu, Xuzhou 221008, China Tel.: C86 516

83995213; fax: C86 516 83591508.

E-mail address: wangyh274@sohu.com (Y.H. Wang).

scattering, are sensitive to variations in interatomic potentials

and local site coordinations with atomic substitutions. For this

reason, Raman scattering can be used to investigate the

structure and structural instability of materials through the

observation of frequency shifts, linewidths or intensities of

Raman-active phonons. The influence of composition on

structural instability in lanthanides-modified bismuth titanate

systems have been performed in several materials [6,7],

however, less effort has been directed to the temperature-

dependence phase transitions in these materials up to now.

In the present work, BNT0.75 ceramics were prepared by

chemical co-precipitation along with calcinations. The vari-

able-temperature Raman scattering and X-ray diffraction were

used to investigate the temperature-dependence phase tran-

sition in this material.

2. Experimental

Bi(NO3)3$5H2O, Nd2O3, and Ti(C4H9O)4 were used as

starting materials. First, Nd2O3 and Bi(NO3)3$5H2O were

dissolved into nitric acid at pH!2. Then Ti(C4H9O)4 was

added in proportion corresponding to stoichiometric BNT0.75

composition. Precipitate began to form when dripping the

mixture solution slowly into concentrated ammoniated water

under magnetic stirring. During the whole process, the basicity

Solid State Communications 138 (2006) 229–233

www.elsevier.com/locate/ssc

Y.H. Wang et al. / Solid State Communications 138 (2006) 229–233230

was kept above pH 9. The resulting precipitate was washed for

several times with deionized water and ethanol before dried at

90 8C. The as-prepared powder was preheated at 650 8C for 2 h,

and pressed into a pellet. Then calcination was performed for

4 h at 850 8C.

A Rigaku X-ray diffractometer was used for X-ray

measurement. Variable-temperature XRD was operated with

an accuracy of G1 K using a platinum–rhodium thermocouple

directly attached to the sample holder. Raman spectra were

recorded in backscattering geometry using JY-T6400 triple

monochromator. The 488 nm light from an ArC laser was

focused onto the sample surface. The temperature stability of

the sample was controlled within 0.1 K (THMS600/HFS91).

The scattered signal from the sample was detected by a charge-

coupled device detection system.

Fig. 2. Raman spectra of BNT0.75 ceramics at 295 K.

3. Results and discussion

Fig. 1 shows the XRD pattern of BNT0.75 ceramics at room

temperature. All the diffraction lines are assigned to

orthorhombic perovskite phase. The sharp XRD peaks suggest

that the as-prepared ceramics is well crystallized after sintering

at 850 8C for 4 h and no evidence of preferred orientation or

secondary phases. According to the XRD analysis, BNT0.75

ceramics has orthorhombic crystal structure with the lattice

parameters: aZ5.4147 A, bZ5.4260 A, cZ32.9162 A at room

temperature. There are small changes in the lattice parameters

with the substitution of neodymium for bismuth in BTO, which

possesses an orthorhombic structure with aZ5.411 A, bZ5.448 A, cZ32.83 A at room temperature [8]. The Raman

spectrum of BNT0.75 ceramics at room temperature is showed

in Fig. 2. The Raman selection rules allow 24 Raman active

modes for orthorhombic BTO [9]. However, as shown in Fig. 2,

only 11 Raman modes are observed at room temperature,

which is partially due to the possible overlap of the same

symmetry vibrations or the weak features of some Raman

Fig. 1. XRD pattern of the BNT0.75 ceramics at room temperature.

bands [10]. According to the assignments of bulk BTO [11,12],

the Raman modes at about 270, 344, 555, 623, and 852 cmK1

are attributed to the internal vibration modes of TiO6

octahedron. The sharp and intense mode at 62 cmK1 is the

so-called rigid-layer (RL) mode, which originates from the

movement of layer like a rigid unit in layer-structured crystals

[12]. The appearance of vibration modes of TiO6 octahedra and

the RL mode indicated that the layered perovskite structure has

well formed in the as-prepared BNT0.75 ceramics, which is

quite consistent with the results of XRD measurements.

Fig. 3 showed the low temperature Raman spectra of

BNT0.75 ceramics from 8 to 1000 cmK1. The broad mode at

270 cmK1 clearly splits into two modes centered at 250 and

270 cmK1 at 220 K, and the intense mode at 62 cmK1 becomes

evident asymmetric and also clearly splits into two modes at 47

Fig. 3. Raman spectra of the BNT0.75 ceramics below 295 K.

Fig. 5. Temperature dependences of the square of the peak frequency of the

modes at 31 cmK1.

Y.H. Wang et al. / Solid State Communications 138 (2006) 229–233 231

and 62 cmK1 at 200 K. When the temperature decreases

further, several modes were discerned gradually and there were

23 modes can be observed at 80 K. These changes of the

Raman spectra at low temperature may be due to the

sharpening of the modes as temperature decreasing. Moreover,

as shown in Fig. 3, the low-frequency modes below 200 cmK1

shifted upwards slightly while the higher frequency modes

showed no substantial change with the decrease of temperature.

Fig. 4 shows the Raman spectra of BNT0.75 ceramics in the

temperature range 295–870 K. The lowest mode at 26 cmK1

weakens and softens to 17 cmK1 with increasing the

temperature from room temperature to 870 K, and the rigid-

layer mode at 62 cmK1 shifts to 48 cmK1 at the same

temperature range, while its intensity increases. The triplet

bands at 31, 90, and 123 cmK1 at room temperature, which are

indicated by arrows and assigned to the modes of Bi ions (A

site) in the perovskite slab [6], show weakening and softening

with increasing temperature and disappear in the background

above 695 K. The internal modes above 200 cmK1 hardly shift,

though their intensities gradually decrease as the temperature

increases. This intensity change can be attributed to the

decrease of the optical penetration depth caused by the increase

of the conductivity [12] and to the fact that the deformation of

TiO6 octahedra decreases towards the transition temperature

[13]. It is noticeable that the frequency of the mode at 154 cmK

1 remains unchangeable, while its intensity increases with the

increasing of temperature, which needs detailed investigation.

Fig. 5 shows the temperature dependence of the square of

the peak frequency of the mode at 31 cmK1. The square of the

mode frequency decreases linearly with temperature and

becomes zero as temperature approaching 695 K. This

behavior strongly suggests that the BNT0.75 ceramics under-

goes an orthorhombic to pseudo-tetragonal phase transition at

695 K. Therefore, it is concluded that this mode at 31 cmK1 is

the soft mode responsible for the phase transition [13].

Fig. 4. Temperature dependence of Raman spectra of BNT0.75 ceramics above

295 K.

It is well known that the phase transition of BTO occurs at

948 K, but in the present work, the phase transition temperature

of BNT0.75 is much lower. The vanishing of the soft mode

frequency is usually the result of a balance between the short-

range repulsive force and the long-rang Coulombic force in

ionic crystals. The Nd3C ion has the same valence as the Bi3C

ion, but with a larger ionic radius. The replacement of Bi3C

ions by isovalent, larger, and nonpolarizable Nd3C ions is

expected to change mainly the short-range force constant,

which may suppress the orthorhombic distortion and lower the

phase transition temperature [6]. In the present work, the phase

transition temperature of BNT0.75 is 695 K, which can be

explained by the substitution effect that the replacement of the

Bi3C by Nd3C ions reduced the orthorhombic distortion

causing a decrease of TC. This effect was also found in other

bismuth layered perovskite ferroelectric materials, and the

phase transition temperature decreases further with increasing

the quantity of the doped element [14].

As mentioned above, the BNT0.75 ceramics is well crystal-

lized into an orthorhombic perovskite phase after sintered at

850 8C for 4 h. However, it undergoes an orthorhombic to

pseudo-tetragonal phase transition at about 695 K, in terms of

the evolution of temperature dependence of Raman scattering

frequencies. In order to further confirm this conclusion, we

investigated the structure instability of the BNT0.75 ceramics

by variable temperature XRD in the range of 295–950 K. The

results showed that all the reflections shift downwards due to

the lattice expansion as the temperature increases. The changes

of the lattice parameters are plotted in Fig. 6. The orthorhombic

distortion b/a is illustrated as well. All the lattice parameters a,

b, and c of the orthorhombic structure increases linearly with

the increase of temperature. The a parameter expands at a

faster rate than b, so that the orthorhombic distortion (b/a)

decreases with increasing temperature below 695 K. At about

695 K, a parameter undergoes a sudden expansion and b

parameter contracts suddenly, and then they increase again.

While c parameter shows no dramatic change and increases

Fig. 6. Lattice parameters and orthorhombic distortion of the BNT0.75 ceramics

at different temperatures.

Fig. 8. The FWHM of reflects (200) and (020).

Y.H. Wang et al. / Solid State Communications 138 (2006) 229–233232

continuously but with a different thermal expansivity from that

of below 695 K. The orthorhombic distortion b/a also exhibits

a sudden change around the same temperature, but increases

linearly with the increasing of temperature above 695 K. An

equality aZb is not achieved at 695 K, which indicates that the

symmetry of the BNT0.75 ceramics does not change into

tetragonal at 695 K. But the orthorhombic distortion b/a is

1.0006 at 695 K, which indicated that the symmetry of BNT0.75

ceramics transformed into pseudo-tetragonal within the

validity a0zb0zffiffiffiffiffiffiffiffi

2aTp

, aTZ3.86 A [14].

Fig. 7. Temperature evolution of several reflections.

Although there is neither appearance nor disappearance of

XRD lines in the present work, it is noticeable that several

reflections are temperature dependent. Fig. 7 presents the

temperature evolution of some reflections. Both the reflection

(0012) and (200) and (020) shift to a lower 2q side with

increasing temperature, but the reflection (0012) shifts faster

than the (200) and (020) does, which makes the two reflections

become more resolved and more separative. The reflections

(0212), (220), and (1115) also shift downwards as the

increasing of temperature. The two reflections (0212) and

(1115) shift more significantly than the reflection (220) does, so

the (0212) reflection becomes more resolved and more

separative with the (220) reflection while the reflection

(1115) merges with the reflection (220) and is undistinguish-

able. The phenomena can be interpreted by anisotropic thermal

expansivities. The thermal expansivities of a-, b- and c-axis can

be determined to be 4.7384!10K6 KK1, 1.2303!10K6 KK1,

and 9.8577!10K6 KK1 below 695 K, and 8.1206!10K6 KK1,

1.3830!10K5 KK1, and 1.3841!10K5 KK1 above 695 K,

respectively, which are consistent with the results obtained by

Subbarao and Hirata [15,16].

The change of reflects (200) and (020) is very interesting. It

looks like two reflects at room temperature (shown in Fig. 8),

but merges into one reflect at about 695 K and then seems to

split again above 815 K. Its FWHM decreases to the minimum

at 695 K as increase the temperature and then increase again,

which indicated that the structure of the BNT0.75 is more like

tetragonal at 695 K.

4. Conclusion

In conclusion, the BNT0.75 ceramics was prepared by

chemical co-precipitation along with calcinations. The lattice

dynamical properties of BNT0.75 have been studied by

temperature dependence Raman scattering. According to the

evolution of temperature dependence of Raman scattering

frequencies, it can be concluded that there was a ferroelectric to

Y.H. Wang et al. / Solid State Communications 138 (2006) 229–233 233

paraelectric phase transition at about 695 K. The results of

variable-temperature X-ray diffraction are in good agreement

with what deduced from high temperature Raman scattering.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (Grant no. 60378017) and by a grant from

the State Key Program for Basic Research of China.

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