Structural and Raman Vibrational Studies of CeO 2-Bi 2 O 3 Oxide System

A series of ceramics samples belonging to the CeO2-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 αBi2O3 structure is observed. Raman spectroscopy confirms the existence of the phase changes as x varies between 0 and 1.


Introduction
In the past, several systems based on cerium dioxide CeO 2 (ceria) were extensively investigated for their electrochemical, conduction, or catalytic properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][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 CeO 2 -Bi 2 O 3 .This system might be of a high interest for catalytic applications and integration in gas sensors.At present, the cerium bismuth oxide phase diagram (CeO 2 -Bi 2 O 3 ) is not well known.For low Bi fractions, it was clearly established that a solid solution was formed.The substituted phase Ce 4+  1−x Bi 3+ x O 2−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 Bi 3+ ionic radius: r(Bi 3+ ) = 0.117 nm and r(Ce 4+ ) = 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.

Experimental
Fourteenth polycrystalline samples were prepared by mixing bismuth and cerium nitrates solutions (Bi(NO 3 ) 3 , 5H 2 O + Ce(NO 3 ) 3 , 6H 2 O) and adding NH 4 OH [20,21] to obtain precipitation of NH 4 NO 3 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.

Results
The polycrystalline samples were analyzed by X-ray diffraction, using a D5000 Siemens-Bruker diffractometer, equipped with a copper X-ray source (wavelength λ = 1.54 10 −10 m; tension V = 45 kV, intensity I = 35 mA), and with a monochromator eliminating K β 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.

Structural
Studies.X-ray diffraction shows that a strong evolution occurs in the phase system as bismuth atom fraction increases.Figures 1(a), 1(b), 1(c), and 1(d) show the X-ray diffraction patterns for samples noted (1−x)CeO 2 , x/2Bi 2 O 3 with x varying between 0 and 1.The cell parameters of substituted samples Ce 1−x Bi x O 2−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.Above the composition x = 0.20, a multiphase system is evidenced and the ceria-based phase presents a constant cell parameter a = 0.5421 nm: 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 ± 0.0003 nm; c = 0.5645 ± 0.0001 nm.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.

Conclusions
New correlations between XRD data and Raman spectroscopy have been established for the system CeO 2 -Bi 2 O 3 .
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 CeO 2 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
Studies.raman spectroscopy data are reported on Figures2(a) and 2(b): in Figure2(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 Bi 3+ and oxygen vacancies in the cubic lattice.In Figure2(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 CeO 2 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 Ce 1−x Bi x O 2−x/2 (V ) x/2 .The additional bands are underlined and should be linked to presence of Bi 3+ ions and vacancies (clusters Bi 3+ -V-Bi 3+ ).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 Bi 2 O 3 sample, the standard α monoclinic structure is observed.

√ 2 , a √ 2 ,√ 2 ,
a): this structure was previously observed in the literature as a tetragonal variety of pure or non stoichiometric Bi 2 O 3 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 √ a).The observed pure Bi 2 O 3 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.