# Structural and Raman Vibrational Studies ofCeO2-Bi2O3 Oxide System

Authors: L. Bourja; B. Bakiz; A. Benlhachemi; M. Ezahri; J. C. Valmalette; S. Villain; J. R. Gavarri
Journal: Advances in Materials Science and Engineering
 (2009)

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## Abstract

A series of ceramics samples belonging to theCeO2-Bi2O3 phase system have been prepared via a coprecipitation route. The crystallized phases were obtained by heating the solid precursors at 600∘C for 6 hours, then quenching the samples. X-ray diffraction analyses show that for x<0.20 a solid solution Ce1−xBixO2−x/2 with fluorine structure is formed. For x ranging between 0.25 and 0.7, a tetragonal β′ phase coexisting with the FCC solid solution is observed. For x ranging between 0.8 and 0.9, a new tetragonal β phase appears. The β′ phase is postulated to be a superstructure of the β phase. Finally, close to x=1, the classical monoclinic α Bi2O3 structure is observed. Raman spectroscopy confirms the existence of the phase changes as x varies between 0 and 1.


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## Body

## 1. Introduction

In the past, several systems based on cerium dioxide CeO2 (ceria) were extensively investigated for their electrochemical, conduction, or catalytic properties [1–15]. Nanostructured powders of pure and doped ceria can be obtained in various ways [16, 17]. In the present work we deal with the bismuth cerium oxide system CeO2-Bi2O3. This system might be of a high interest for catalytic applications and integration in gas sensors.  At present, the cerium bismuth oxide phase diagram (CeO2-Bi2O3) is not well known. For low Bi fractions, it was clearly established that a solid solution was formed. The substituted phase Ce1-x4+Bi3+xO2-x/2(V)x/2 with x<0.20 (where oxygen vacancies are noted V) is cubic and its cell parameter increases with x because of size of Bi3+ ionic radius: r(Bi3+)=0.117 nm and r(Ce4+)=0.097 nm [18, 19]. However, above the composition x=0.20, the nature of phases is not well known. In the present work, we describe a new series of observed phases prepared via a coprecipitation route and after heating at 600°C.


## 2. Experimental

Fourteenth polycrystalline samples were prepared by mixing bismuth and cerium nitrates solutions (Bi(NO3)3, 5H2O + Ce(NO3)3, 6H2O) and adding NH4OH [20, 21] to obtain precipitation of NH4NO3 and bismuth cerium hydroxides.  Bismuth compositions ranged from 0% Bi to 100% Bi. The solid obtained by coprecipitation was then heated under air at 600°C for 6 hours. Experiments carried out at intermediate heating times showed that the observed crystallized phases appear as being stable above heating times of 2 hours.


## 3. Results

The polycrystalline samples were analyzed by X-ray diffraction, using a D5000 Siemens-Bruker diffractometer, equipped with a copper X-ray source (wavelengthλ=1.5410-10 m; tension V=45 kV, intensity I=35 mA), and with a monochromator eliminating Kβ radiation. The analyses were carried out using the classical θ-2θ configuration, with 2θ angle steps of 0.02° and counting times of 19 s per step. Raman spectroscopy was used to characterize the observed various phases. A micro-Raman system Horiba. Jobin-Yvon Labram HR 800 equipped with argon laser source (Raman wavelength λ=514.5 nm) was used to observe the various vibrational spectra. All spectra were acquired with a recording time of 30 seconds.


### 3.1. Structural Studies

X-ray diffraction shows that a strong evolution occurs in the phase system as bismuth atom fraction increases. Figures1(a), 1(b), 1(c), and 1(d) show the X-ray diffraction patterns for samples noted (1-x)CeO2, x/2Bi2O3 with x varying between 0 and 1. The cell parameters of substituted samples Ce1-xBixO2-x/2(V)x/2 noted as a(x) were refined. From x=0 to x=0.25, the cell parameters linearly vary with x: a(x=0)=0.5409±0.0001 nm; a(x=0.05)=0.5413±0.0001; a(x=0.10)=0.5417±0.0004; a(x=0.15)=0.5419±0.0003; a(x=0.20)=0.5421±0.0002.XRD patterns (λCuKα1=1.54·10-10m) of pure samples (1-x) CeO2, x/2 Bi2O3heated at 600°C. (a) XRD patterns for 0<x<0.20; (b): XRD patterns for 0.25<x<0.70 biphasic system;  (c) x=0.8 and 0.9; (d) x=1   α- Bi2O3.
(a)(b)(c)(d)Above the compositionx=0.20, a multiphase system is evidenced and the ceria-based phase presents a constant cell parameter a=0.5421nm: the two new additional phases are identified as being tetragonal and closely related to bismuth oxide structural varieties:  their cell parameters were refined. In the composition range from 0.3 to 0.7, a tetragonal β′ phase is observed with refined cell parameters: a=1.5542±0.0003nm; c=0.5645±0.0001nm. It is a superstructure of the tetragonal β phase observed for compositions 0.7<x<0.9, with refined cell parameters: a=0.7742±0.0001nm; c=0.5633±0.0001nm. These substituted phases were never observed, and testing structural models are in progress to better describe these phases.


### 3.2. Vibrational Studies

raman spectroscopy data are reported on Figures2(a) and 2(b): in Figure 2(a), the solid solution (0<x<0.25) is characterized by a main vibrational band at 460–465 cm-1 with complementary small bands at 520–590 cm-1 associated with the presence of Bi3+ and oxygen vacancies in the cubic lattice. In Figure 2(b) the Raman spectra of other samples are represented for x compositions ranging between 0.3 and 1. The vibration bands are increasingly more complex as Bi composition increases. The cubic phase of CeO2 is well characterized by the 465 cm-1 Raman band. In the composition range from x=0.05 to 0.20 the bands located at 462–516–595 cm-1 might be associated with the solid solution  Ce1-xBixO2-x/2(V)x/2. The additional bands are underlined and should be linked to presence of Bi3+ ions and vacancies (clusters Bi3+-V-Bi3+). In the range x=0.30 to 0.70, the Raman bands 460, 520, 590, 94, 126, 316, 530 (in cm-1) might be related to the biphasic system: cubic solid solution + tetragonal superstructure β’. In the range x=0.80 to 0.90, a new biphasic system associated with the bands 95, 120, 315, 450, 538 (tetragonal phase) and 70, 85, 140, 152, 184, 212, 285, 418, 630 (monoclinic lattice) is observed: these vibration bands could characterize the system  “Tetragonal β+ Monoclinic α.” Finally for the Bi2O3 sample, the standard α monoclinic structure is observed.Raman spectra(λ=514.5nm) of bismuth cerium oxide phases, (1-x) CeO2, x/2. Bi2O3; (a) solid solution for x=0 to 0.25; (b) multiphase system for x=0.30 to 1. Raman shift island are in cm-1, x values from 0 to 1. The bands at 520 and 590 cm-1 are linked to structural defects.
(a)(b)


## 3.1. Structural Studies

X-ray diffraction shows that a strong evolution occurs in the phase system as bismuth atom fraction increases. Figures1(a), 1(b), 1(c), and 1(d) show the X-ray diffraction patterns for samples noted (1-x)CeO2, x/2Bi2O3 with x varying between 0 and 1. The cell parameters of substituted samples Ce1-xBixO2-x/2(V)x/2 noted as a(x) were refined. From x=0 to x=0.25, the cell parameters linearly vary with x: a(x=0)=0.5409±0.0001 nm; a(x=0.05)=0.5413±0.0001; a(x=0.10)=0.5417±0.0004; a(x=0.15)=0.5419±0.0003; a(x=0.20)=0.5421±0.0002.XRD patterns (λCuKα1=1.54·10-10m) of pure samples (1-x) CeO2, x/2 Bi2O3heated at 600°C. (a) XRD patterns for 0<x<0.20; (b): XRD patterns for 0.25<x<0.70 biphasic system;  (c) x=0.8 and 0.9; (d) x=1   α- Bi2O3.
(a)(b)(c)(d)Above the compositionx=0.20, a multiphase system is evidenced and the ceria-based phase presents a constant cell parameter a=0.5421nm: the two new additional phases are identified as being tetragonal and closely related to bismuth oxide structural varieties:  their cell parameters were refined. In the composition range from 0.3 to 0.7, a tetragonal β′ phase is observed with refined cell parameters: a=1.5542±0.0003nm; c=0.5645±0.0001nm. It is a superstructure of the tetragonal β phase observed for compositions 0.7<x<0.9, with refined cell parameters: a=0.7742±0.0001nm; c=0.5633±0.0001nm. These substituted phases were never observed, and testing structural models are in progress to better describe these phases.


## 3.2. Vibrational Studies

raman spectroscopy data are reported on Figures2(a) and 2(b): in Figure 2(a), the solid solution (0<x<0.25) is characterized by a main vibrational band at 460–465 cm-1 with complementary small bands at 520–590 cm-1 associated with the presence of Bi3+ and oxygen vacancies in the cubic lattice. In Figure 2(b) the Raman spectra of other samples are represented for x compositions ranging between 0.3 and 1. The vibration bands are increasingly more complex as Bi composition increases. The cubic phase of CeO2 is well characterized by the 465 cm-1 Raman band. In the composition range from x=0.05 to 0.20 the bands located at 462–516–595 cm-1 might be associated with the solid solution  Ce1-xBixO2-x/2(V)x/2. The additional bands are underlined and should be linked to presence of Bi3+ ions and vacancies (clusters Bi3+-V-Bi3+). In the range x=0.30 to 0.70, the Raman bands 460, 520, 590, 94, 126, 316, 530 (in cm-1) might be related to the biphasic system: cubic solid solution + tetragonal superstructure β’. In the range x=0.80 to 0.90, a new biphasic system associated with the bands 95, 120, 315, 450, 538 (tetragonal phase) and 70, 85, 140, 152, 184, 212, 285, 418, 630 (monoclinic lattice) is observed: these vibration bands could characterize the system  “Tetragonal β+ Monoclinic α.” Finally for the Bi2O3 sample, the standard α monoclinic structure is observed.Raman spectra(λ=514.5nm) of bismuth cerium oxide phases, (1-x) CeO2, x/2. Bi2O3; (a) solid solution for x=0 to 0.25; (b) multiphase system for x=0.30 to 1. Raman shift island are in cm-1, x values from 0 to 1. The bands at 520 and 590 cm-1 are linked to structural defects.
(a)(b)


## 4. Conclusions

New correlations between XRD data and Raman spectroscopy have been established for the system CeO2-Bi2O3. From samples prepared at 600°C, a partial phase diagram is proposed with the probable existence of at least 4 domains. The X-ray diffraction and Raman spectroscopy analyses clearly show that phase changes occur at 600°C, with at least (i) a solid solution domain (cubic phase), (ii) a biphasic domain (tetragonal phase β′ rich in bismuth coexisting with the cubic phase), (iii) a biphasic system with coexistence of two β and β′ tetragonal phases, the β phase being highly rich in bismuth), and finally (iv) a biphasic domain in which monoclinic and tetragonal β phases coexist. The solid solution can be represented from the basic CeO2 face-centered cubic lattice. The tetragonal phase β(x>0.8) can be represented by a cell built on the ceria fcc structure, with lattice vectors (a√2,a√2,a): this structure was previously observed in the literature as a tetragonal variety of pure or non stoichiometric Bi2O3 phase [22, 23]. The Bi rich phase (0.25<x<0.70) having the superstructure noted β′ can be represented by a cell built on lattice vectors (2a√2,2a√2,a). The observed pure Bi2O3 phase is monoclinic. The effective compositions of the β and β′ new cerium bismuth phases are not clearly known and new studies using transmission electron microscopy analyses are in progress.



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*Source: 502437-2010-02-17.xml*