Synthesis of nanocrystalline CeAlO3 by solution-combustion route

Materials Chemistry and Physics 119 (2010) 485–489

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Synthesis of nanocrystalline CeAlO3 by solution-combustion route S.T. Aruna a , N.S. Kini b,∗ , Satish Shetty b , K.S. Rajam a a b

Surface Engineering Division, National Aerospace Laboratories, Post Bag No. 1779, Bangalore 560 017, India Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India

a r t i c l e

i n f o

Article history: Received 28 January 2009 Received in revised form 14 July 2009 Accepted 3 October 2009 Keywords: Oxides Chemical synthesis Electron microscopy Magnetic properties

a b s t r a c t CeAlO3 was synthesised by a modified solution-combustion route using a mixture of urea and glycine as fuel. A trivalent oxidation state of cerium was stabilised and high-quality single phase polycrystalline CeAlO3 was obtained by optimising the ratio of fuels. The transmission electron micrography and powder X-ray diffraction investigations showed that the particles were nanocrystalline in nature. Rietveld refinement confirmed the space group of the structure to be I4/mcm with lattice parameters a = 5.3278(1) Å, c = 7.5717(3) Å. Magnetisation measurements indicated that the sample was paramagnetic up to 2 K. The susceptibility data fitted the Curie–Weiss model in the temperature range 100–300 K with p = −40 K. The value of eff = 2.2B was close to that expected for a Ce3+ ion. The magnetic properties were comparable to that reported for single crystals indicating the high quality of CeAlO3 prepared in the present work. The semiconducting band gap as estimated from UV–visible spectroscopy was 3.26 eV. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The material properties of CeAlO3 have not been studied so much as its crystal structure [1–6]. It has long been known to be only a paramagnetic oxide [7,8]. Very few groups have studied its electrical properties [9–11]. Shylekh et al., reported an electrical conductivity, , of 10−7 S cm−1 and a dielectric constant, r , of 3000–10,000 both measured at 1 kHz frequency [9]. Wang et al., reported a  of 10−9 S cm−1 and r of 600 in a frequency range of 10 kHz to 10 MHz [10]. However, recently Feteira et al., reported that the intrinsic dielectric constant of CeAlO3 is much less than r reported earlier [11]. Since intrinsic ferroelectricity as a cause of the observed high dielectric constant is ruled out due to centrosymmetric crystal structure of CeAlO3 , the same was attributed to grain boundary effects. Certainly the compound needs to be studied further to ascertain the nature of observed high r . Bulk synthesis of CeAlO3 by solid state reaction technique is non-trivial. A trivalent oxidation state of cerium could only be stabilised either in an inert atmosphere or under reducing conditions. For example, polycrystalline CeAlO3 has been prepared by heating a mixture of CeO2 and Al2 O3 either in diluted hydrogen flow at 1475 ◦ C for 30 h [5] or in an induction furnace at 1600 ◦ C in helium atmosphere [12,13]. Alternatively, it was also prepared by heating

∗ Corresponding author. Presently at: Centre of Excellence-Materials Science, R. D. Aga Research Technology Innovation Centre, Thermax Limited, D-13, MIDC Industrial Area, Chinchwad, Pune 411019, India. Tel.: +91 20 66122893; fax: +91 20 27475676. E-mail address: [email protected] (N.S. Kini). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.10.001

a mixture of CeO2 and Al2 O3 (wrapped in Ta-foil) at 1200 ◦ C in evacuated and sealed quartz tube followed by a second heating in dynamic vacuum up to 1500 ◦ C and a subsequent arc-melting [6]. Single crystals of CeAlO3 have been prepared from polycrystalline powders by flux method in helium atmosphere at 1014 ◦ C using KF as flux [12,13]. In this work, we have developed a simple synthesis procedure for rapid synthesis of polycrystalline samples of CeAlO3 by solution-combustion through a mixture-of-fuels approach. Solution-combustion synthesis has long been employed as a technique to synthesise oxide materials [14–19]. In this technique a solution of a mixture of an appropriate fuel and a metal salt (typically a nitrate) is made in water and introduced into a preheated furnace, at relatively low temperatures (≈500 ◦ C) to initiate a self-propagating exothermic reaction to yield the required oxide. As the technique involves control of a number of reaction parameters, several modifications of the standard technique are being employed. The main reaction parameters are: type of fuel [20,21], fuel to oxidant ratio [22], additives [23–25], pH of the solution [26], etc. In our previous work on CeO2 –Al2 O3 system using mixture of fuels, we showed that a non-stoichiometric mixture of Ce:Al in 4:6 atomic ratio gives rise to nanocrystalline CeO2 (with Al2 O3 being in amorphous state) when urea alone was used as fuel but gives a nanocomposite of CeO2 –CeAlO3 when a mixture of urea and glycine was used as fuel [27]. In this article, we show that high quality, single phase, nanocrystalline powders of CeAlO3 can be rapidly synthesised by a simple solution-combustion synthesis technique through a mixture-of-fuels approach [28,29] requiring no hydrogen reduction or special gaseous environments. We also present the structural, microstructural, magnetic and optical

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absorption properties of CeAlO3 prepared by solution-combustion route. 2. Experimental procedure 2.1. Synthesis The mixture-of-fuels approach to synthesis of CeAlO3 was realised by carrying out combustion in air using a series of mixtures of urea (NH2 CONH2 ), (Qualigens Fine Chemicals, 99%), (hereafter abbreviated U), and glycine (NH2 CH2 COOH), (S. D. FineChem Ltd., 99%), (hereafter abbreviated G), in different ratios and metal nitrates of aluminum and cerium. First, a clear solution of metal-nitrate and fuel-mixture taken in a pre-determined proportion was prepared using minimum amount of distilled water as solvent. This solution was then introduced into a muffle furnace pre-heated to 500 ◦ C. Combustion sets in within a few minutes and continues for a couple of minutes before a foamy mass is generated. All reactions were carried out in cylindrical Al2 O3 crucibles. In some reactions, depending on the composition of the fuel mixture, parts of the sample exposed to air were found to oxidise giving rise to yellow colour powder. In such cases the whole sample is ground together before recording an XRD pattern so as to get a quantitative estimate of various phases formed. The molar ratio of ceric ammonium nitrate ((NH4 )2 Ce(NO3 )6 ), (Sigma–Aldrich, 98.5%) (hereafter abbreviated CAN) to aluminum nitrate (Al(NO3 )3 ·9H2 O), (Sigma–Aldrich, 98%) (hereafter abbreviated AN) was maintained at 1:1 in all the mixtures so as to obtain CeAlO3 in the final product. The complete combustion of each metal nitrate with individual fuel can be represented by the following balanced chemical equations: (1)–(4) (NH4 )2 Ce(NO3 )6 + 4NH2 CONH2 → CeO2 + 4CO2 + 8N2 + 12H2 O

(2) (3)

6Al(NO3 )3 · 9H2 O + 10NH2 CH2 COOH → 3Al2 O3 + 20CO2 + 14N2 + 79H2 O

(4)

When a mixture of CAN and AN is taken in order to obtain CeAlO3 , either or both the fuels (in some known proportion) can be taken. Several combinations of metal-nitrates and fuel-mixtures are possible. For example, assuming that the notation (CAN:G) implies that the amount of G (glycine) taken corresponds to complete combustion of CAN (ceric ammonium nitrate), a generic notation to represent all possible combinations of metal-nitrate and fuel-mixture can be written as (CAN: yG + (1 − y)U, AN: xG + (1 − x)U). When x = y = 0, we have (CAN:U, AN:U) and when x = y = 1 we have (CAN:G, AN:G) and so on. It is to be noted that since quantity of fuel corresponds to complete combustion of nitrate, the oxidant/fuel ratio is 1. From our previous study it is known that in a combustion reaction with a mixture of CAN and AN, when only urea was used as fuel, i.e. when x = y = 0, formation of CeO2 was favoured and when a mixture of glycine and urea was used as fuel, such that x = 0, y = 1, a mixture of CeO2 and CeAlO3 was formed [27]. This clearly indicates that for further optimisation of ratio of fuels to obtain a single phase CeAlO3 , x has to be varied in such a way that 0 ≤ x ≤ 1 with y = 1. In other words, the compositions to be explored are: (CAN:G, AN: xG + (1 − x)U, 0 ≤ x ≤ 1). Additionally, x = y = 0 composition, i.e. (CAN:U, AN:U) was also investigated for completeness. For a given mixture of nitrates, amount of each fuel was calculated according to Eqs. (5) and (6)

 MU = WU

MCAN × (|VCAN |/|VG |) WCAN





+ WG

MAN × (1 − x) × (|VAN |/|VU |) WAN



MAN × x × (|VAN |/|VG |) WAN



(5)

(6)

where M, W and V are mass, molecular weight and valency (reducing or oxidising) respectively with the subscripts U, G, CAN and AN referring to urea, glycine, cericammonium-nitrate and aluminum-nitrate, respectively. The oxidising valency of an oxidant and the reducing valency of a fuel is calculated based on principles of propellent chemistry [30,31]. Valencies of N, H, C, and O are 0, +1, +4, −2, respectively and valency of a metal ion in a salt is equal to the charge on it. Hence the valencies of CAN, AN, G and U are −24, −15, 9 and 6, respectively. Oxidant to fuel ratio is given by Eq. (7) z=

Mo Wf |Vo | Mf Wo |Vf |

MGCAN /(g)

MAN /(g)

x

MGAN,x /(g)

MU

21.92 21.92 21.92 21.92 21.92 21.92 21.92 21.92

8.004 8.004 8.004 8.004 8.004 8.004 8.004 8.004

15 15 15 15 15 15 15 15

0.40 0.50 0.60 0.70 0.80 0.90 0.95 1.00

2.001 2.501 3.001 3.502 4.002 4.502 4.753 5.002

3.602 3.002 2.401 1.801 1.201 0.600 0.300 0.000

/(g)

MGTotal /(g) 10.005 10.505 11.005 11.506 12.006 12.506 12.756 13.006

patterns were obtained over a duration of 12 h in the 2 range: 10–120 ◦ with a 2 step width of 0.02 ◦ . A Rietveld analysis program package FullProf [32,33] was used to analyse the powder X-ray diffraction patterns. The mass fraction of a phase of interest in a mixture of phases was estimated by quantitative multiphase analysis using the Rietveld program [32,33]. The program estimates the mass fraction, Wp , of a given phase, p, using Eq. (8) Wp =

Sp × (ZMV )p

n 

(8)

Si × (ZMV )i

i=1

2Al(NO3 )3 · 9H2 O + 5NH2 CONH2 → Al2 O3 + 5CO2 + 8N2 + 28H2 O



AN,(1−x)

MCAN /(g)

(1)

3(NH4 )2 Ce(NO3 )6 + 8NH2 CH2 COOH → 3CeO2 + 16CO2 + 16N2 + 32H2 O

MG = WG

Table 1 Fuel compositions and quantity of reactants used in combustion synthesis of CeAlO3 . M represents mass and subscripts and superscripts represent reactants as explained in Section 2.1.

(7)

where subscripts ‘o’ and ‘f’ refer to oxidant and fuel respectively and rest of the notations are same as in Eqs. (5) and (6). The actual mass of the various nitrates and the fractions of glycine and urea in a given mixture of fuels taken in our experiments are listed in Table 1. The oxidant/fuel ratio, i.e. the value of z is equal to 1 in all the compositions. 2.2. Characterisation Powder X-ray diffraction patterns were obtained on a diffractometer (D8Discover, Bruker-AXS) with Cu K˛ radiation. Rietveld quality X-ray diffraction

where S is the scale factor for a given phase obtained by Rietveld analysis. Z, M and V are the number of formula units per unit cell, mass of the formula unit and volume of the unit-cell, respectively. The index i refers to the ordinal number of the phase under consideration, e.g., p. Microstructures of as-synthesised powders were studied using a scanning electron microscope (SEM) (S440i, Leica). The size and nature of crystallites of powders were characterised by a transmission electron microscope (TEM) (JEM-3010, Jeol). The electron diffraction (ED) patterns were indexed using the computer program ProcessDiffraction [34]. Diffuse reflection spectrograph (DRS) in the UV–visible range was obtained using a UV/VIS/NIR spectrometer (Lambda900, Perkin Elmer) on powder samples. Magnetisation measurements were carried out using a commercial magnetometer (MPMS XL-5, Quantum Design) in the temperature range: 2–300 K and in applied magnetic fields up to 5 T.

3. Results and discussion Fig. 1 shows the powder X-ray diffraction (XRD) patterns of asprepared samples. Fig. 1(a) shows the XRD pattern of the sample obtained using only urea as fuel. When only a single fuel, U, was used for both the nitrates (CAN:U, AN:U), the major phase formed is CeO2 . In our previous work, when a single fuel viz., urea was used only CeO2 was formed Al2 O3 being in the amorphous state, where the atomic ratio of Ce:Al was maintained at 4:6 [27]. The same trend is seen in this work even when the atomic ratio of Ce:Al is 1:1. As we see no peaks corresponding to Al2 O3 in Fig. 1(a), we assume that it must be in an amorphous state. The powder XRD patterns of the samples obtained for fuel compositions CAN:G, AN: xG +(1 − x)U, 0 ≤ x ≤ 1 are shown in Fig. 1(b–f). All of them appear to contain a mixture of CeO2 and CeAlO3 . For comparison, the diffraction peaks of CeO2 (◦) and CeAlO3 (*) are marked in Fig. 1(a) and (d), respectively. Most of the peaks of CeAlO3 either overlap on or are very close to those of CeO2 , except the (1 0 0) and (1 1 1) peaks of CeAlO3 that appear at 2 = 23.5◦ and 2  = 41.4◦ , respectively. As the glycine content in the fuel mixture increases, CeAlO3 peaks become prominent and intensities of CeO2 peaks decrease. The fraction of CeAlO3 is maximum when x = 0.8, and with further increase in x, CeO2 peak intensities increase. The mass fraction of CeAlO3 in the sample obtained with x = 0.8, was estimated to be >0.90. It is interesting to note that in all the XRD patterns, the width of the diffraction peaks which is an indication of the crystallite size is different for the two phases. CeAlO3 peaks appear to be sharp in contrast with those of CeO2 which are broad.

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Fig. 2. SEM image of as-prepared powder of CeAlO3 .

Fig. 1. Powder X-ray diffraction patterns of as-prepared powders obtained using different fuel-mixtures. x and y indicate the glycine fraction according to CAN: yG + (1 − y)U, AN: xG + (1 − x)U (see text). CeO2 and CeAlO3 peaks are marked with open circles (a) and asterisks (d), respectively.

In order to study the product obtained using the fuel composition x = 0.80 which gives maximum amount of CeAlO3 , the as prepared powder was examined in detail. The portion of the product inside the alumina crucible was uniformly green in colour and that exposed to air was yellow. The small fraction of CeO2 (less than 10%) present in the product could be due to surface oxidation. To ascertain this, the experiment was repeated by scaling up the quantity of reactant mixture. The as prepared powder that is exposed to atmosphere looks yellow in colour. The sample collected from that portion which is not exposed to atmosphere was uniformly green in colour. XRD pattern obtained on this uniformly green sample showed peaks of only CeAlO3 with not even traces of CeO2 impurity. Therefore the small fraction of CeO2 found in the as-prepared powder must indeed be due to surface oxidation. It is also noteworthy that the peaks of CeAlO3 are very sharp indicating a high degree of crystallinity. All further characterisations were carried out on this uniformly green powder of CeAlO3 which is highly crystalline and single phase. In what follows, CeAlO3 refers to this green-coloured single phase sample. The SEM image of as prepared CeAlO3 powder is shown in Fig. 2. The microstructure appears to be highly porous which is expected of a combustion-synthesised product. This is due to evolution of large quantities of gases during synthesis as mentioned in Section 2.1. In order to further elucidate the nature of powder particles, the CeAlO3 powders were characterised by TEM. Fig. 3(a) shows the TEM image of the powder. The highly crystalline nature of particles is also evident from the selected area electron diffraction (SAED) pattern shown in Fig. 3(b). In fact, peaks in our electron diffraction pattern are much sharper than those reported by Feteira et al. [11]. The nature of crystalline phase of the CeAlO3 powder was further studied by indexing the SAED pattern.

Fig. 3. (a) TEM image of as-prepared powder of CeAlO3 . (b) Indexed SAED pattern obtained from the same powder. The pattern is indexed with a pseudocubic lattice parameter, a = 3.78(1) Å.

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Table 2 Structural parameters for CeAlO3 in P4/mmm space group with a = 3.7676(1) Å, c = 3.7860(2) Å. Atoms Ce Al O1 O2 O3

Position

x

y

z

∗ Foccu

∗ Biso /Å2

1d 1a 2f 2f 1b

1 2

1 2

1 2

0 1 2

0 0

0 0

0 0 0

0

1.0 1.0 0.5 0.5 1.0

0.50 0.50 0.75 0.75 0.75

1 2

1 2 ∗

Rwp = 10.4, Rp = 12.4,  = 3.57, Foccu = Occupancy, Not refined. 2

A large number of different structures are reported for CeAlO3 . The first structural study on the system assumed a tetragonal unit cell with lattice parameters a = 3.760(4) Å and c = 3.787(4) Å [1]. Subsequently, a rhombohedral cell with a = 5.327 Å, and ˛ = 60.25◦ [2], a primitive tetragonal cell with a = 3.763 Å and c = 3.792 Å and an orthorhombic cell with a = 5.316(1) Å, b = 5.314(1) Å and c = 7.576(1) Å [35] have also been reported. Citations to reports of hexagonal, trigonal and cubic structures for CeAlO3 can also be seen in literature [6]. These structural variations arise due to high symmetry of the structure and a small distortion due to tilting of AlO6 octahedra [5]. The two most widely accepted structures are: a primitive tetragonal cell with space group P4/mmm and lattice parameters aP = 3.7669(9) Å and cP = 3.7967(7) Å [3] and a body centered tetragonal cell with space group I4/mcm and lattice parameters aI = 5.32489(6) Å and cI = 7.58976(10) Å [5]. Due to the high symmetry of the structure, a pseudocubic cell can be conceived from each of the above mentioned structures with anaverage lattice parameter (aA ), such that aA ≈ (aP + cP )/2 ≈ aI / (2) ≈ cI /2 ≈ 3.78(1) Å. The subscripts P and I refer to primitive and body-centered cells, respectively. As a first approximation, we have indexed our SAED pattern based on a pseudocubic unit cell with an average a-cell parameter of 3.78(1) Å as shown in Fig. 3(b). For studying the crystal structure in detail, Rietveld refinements were carried out with the two structures mentioned above. The profile was fit with a pseudo-voigt function. Total number of refined parameters in both the cases were 42 including the background points. The results of the refinement of CeAlO3 structure with the two space groups P4/mmm and I4/mcm are given in Tables 2 and 3, respectively. It can be seen from the tables that the R-factors in the two cases are not much different rendering any discrimination based on R-factors alone difficult. The Rietveld plot for the structure refinement with I4/mcm space group is given in Fig. 4. The two peaks at 39.62 ◦ and 52.79◦ shown by arrows in Fig. 4 (and in the inset of Fig. 4) are exclusive to I4/mcm space group, which are attributed to superlattice reflections [5]. Based on the presence of these two feeble peaks in our data, we infer that the I4/mcm should be the correct space group for this structure. Further, to estimate the size of the crystallites in the sample, the XRD peak of CeAlO3 with highest intensity, occurring at 2  = 33.55◦ corresponding to (2 0 0) planes was chosen. The (2 0 0) peak of a standard microcrystalline CeO2 occurring at 2  = 33.32◦ was chosen as standard. To determine the full-width-at-half-maximum () accurately, both the peaks were fitted independently with a

Fig. 4. Rietveld plot for CeAlO3 structure refined with space group I4/mcm, lattice parameters a = 5.3278(1) Å, c = 7.5717(3) Å. The peaks indicated by arrows are exclusive to I4/mcm space group which are not present in P4/mmm. The inset shows the distinguishing peaks magnified.

pseudo-Voigt function using the computer program WinPLOTR [36]. Pseudo-Voigt function is of the form pV () = L + (1 − )G, where L and G represent the fractions of Lorentzian and Gaussian contribution, respectively. The refined value of  in both the cases was ≈0.62. Assuming that the crystallites are free of strain, the average crystallite size (d) was estimated using Scherrer [37] formula given by Eq. (9)



d=2

ln2 180

0 cos 

where 0 is in degrees and equal to

(9)



2sample − 2standard and is

the wavelength of radiation used for diffraction (taken to be equal to 0.15418 nm). The value of d so calculated was 50 nm clearly indicating that the crystallites are nanometer in size. Our magnetic measurements show that CeAlO3 is paramagnetic up to 2 K. Fig. 5 shows the variation of magnetic susceptibility,  (measured at an applied magnetic field of 5 T), and its inverse −1 with temperature. The data fits to Curie–Weiss model given by Eq. (10) very well in a wide temperature range: 100–300 K. =

C T − p

(10)

The −1 (T ) data in the same temperature range is a straight line. The values of eff and p obtained by fitting were 2.33B and −50 K,

Table 3 Structural parameters for CeAlO3 in space group: I4/mcm with a = 5.3278(1) Å, c = 7.5717(3) Å. Atoms Ce Al O1 O2

Position 4b 4c 4a 8h

x

y

z

∗ Foccu

∗ Biso /Å2

0 0 0 0.2768

1 2

1 4

0 0 x+

0

1.0 1.0 1.0 1.0

0.50 0.50 0.75 0.75

1 2

1 4

0

Rwp = 10.5, Rp = 12.6, 2 = 3.36, Foccu = Occupancy,∗ Not refined.

Fig. 5. Magnetic susceptibility of CeAlO3 powder measured in an applied magnetic field of 5 T. Inset shows the magnetisation data for the same at 2 K.

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obtain single phase CeAlO3 . The powder is nanocrystalline in nature as confirmed by TEM studies. Structural studies by XRD and Rietveld analysis confirmed the space group of CeAlO3 structure to be I4/mcm. Magnetic measurements showed that the sample is paramagnetic up to 2 K. The electronic band-gap estimated from UV–visible spectroscopy is 3.26 eV. All the physical measurements on CeAlO3 confirm the very high quality of the sample prepared using mixture-of-fuels approach. Acknowledgements The authors STA and KSR thank Director, NAL for permitting to publish this work. The help received from Miss. Divya Swamy, N. Balaji and Dr. Ramachandra Rao of NAL is duly acknowledged. NSK is grateful to JNCASR for the fellowship and research facilities. References

Fig. 6. UV–visible spectrum of CeAlO3 powder obtained in the diffuse reflectance (DR) mode. The band-gap of 3.26 eV was estimated from the onset (380 nm) of increase in reflectance.

respectively. The value of eff is slightly less than that expected for a Ce3+ ion, (2.54 B ) but matches very well with the only reported data on polycrystalline sample, 2.34 B [7]. The reciprocal susceptibility −1 deviated from the Curie–Weiss model below 100 K with a curvature concave towards the temperature axis. Shishido et al., who measured the magnetic properties of CeAlO3 reported a small hump around 80 K for polycrystalline sample which was absent in their single crystal samples [7]. Our data is very similar to the single crystal data reported by them indicating the high quality of our sample. The deviation observed at 100 K was suspected to be due to magnetic ordering [7,8]. Our measurements of M(H) at 2 K clearly showed a linear behaviour passing through the origin (shown in the inset of Fig. 5) indicating a paramagnetic nature at that temperature. Therefore, the observed deviation from the Curie–Weiss behaviour can be attributed to crystal field effects in CeAlO3 . UV–visible spectrum obtained on powder CeAlO3 sample in DR mode is shown in Fig. 6. Reflectance curve shows an onset of increase in reflectivity at 380 nm and flattens-off at higher wavelengths. The energy band gap estimated from the onset of increase in reflectivity was 3.26 eV which agrees very well with the value 3.29 eV reported by Wang et al. [10]. All the physical properties of our samples are as-good-as and in some cases better than those reported earlier. Thus the rapid synthesis technique by solutioncombustion through our novel mixture-of-fuels approach yields high quality single phase powders of CeAlO3 . Therefore we hope that this work will not only facilitate but also accelerate the work on some issues such as the observed high dielectric constant in this compound. 4. Conclusions A mixture-of-fuels approach has been employed to rapidly synthesise high quality single phase polycrystalline powders of CeAlO3 using a mixture of glycine and urea as fuel in a solution-combustion technique. The ratio of glycine to urea has been optimised to

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