Preparation of Cerium Orthophosphate Nanosphere by Coprecipitation Route and Its Structural, Thermal, Optical, and Electrical Characterization

Cerium orthophosphate (CePO 4 ) nanoparticles were synthesized via wet chemical coprecipitation technique using cerium nitrate hexahydrate for Ce ion and ammonium dihydrogen phosphate for PO 4 3− ion source. X-ray diffraction (XRD) analysis suggests that the material belongs to monoclinic crystal system with crystallite size of 49.3 nm. Scanning electron microscope (SEM) and transmission electronmicroscope (TEM) reveal the surfacemorphology of the prepared nanoparticles as nanospheres having grain size in the range of 30–50 nm. The energy dispersive X-ray analysis (EDAX) gives elemental composition present in the grown nanomaterial. Thermogravimetric analysis suggests that the structural phase transition is above 800C. The optical properties include UV-VIS-NIR absorption and the photoluminescence spectrum shows the absorption and emission peaks in the ultraviolet region. The dielectric constant (ε/) and ac conductivity properties were investigated with regard to change in temperature (40 to 500C) and frequency (5 kHz to 1MHz). The dielectric measurement indicates that the transition is diffused and the activation energy values decrease suggesting that the conduction mechanism is due to hopping of the charge carriers from one site to another.


Introduction
Rare earth elements (REEs) known as lithophile are the elements enriched in the earth crust which occur naturally because all are trivalent and form a special group of transition elements in the periodic table. Among these REEs, cerium (Ce) is found in abundance which makes it one of the cheapest rare earth metals. However, cerium is found in several minerals like monazite (Ce, La, Nd, Th) PO 4 , xenotime (Y, Ce) PO 4 , bastnasite (Ce, La) CO 3 (F, OH), and cerite (Ce, La) 9 (Mg, Fe)Si 7 (O, OH, F) 28 . Orthophosphates are substances that are composed of isolated PO 4 tetrahedra analogous to "orthosilicates. " The most common naturally occurring orthophosphates are apatite [Ca 3 (PO 4 ) 3 F, Cl, OH] and monazite (LnPO 4 ) where "Ln" refers to rare earth elements. Rare earth orthophosphates with the general formula RXO 4 (where R = rare earth and X = P, As, or V) belong

Material Preparation.
Cerium orthophosphate nanoparticles were prepared by coprecipitation method using cerium nitrate hexahydrate (Ce(NO 3 ) 3 ⋅6H 2 O) 99.9% pure (Alfa Aesar) and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ) 99.9% pure (Alfa Aesar) as precursor. Figure 1 illustrates the schematic diagram for synthesis of cerium orthophosphate (CePO 4 ) via coprecipitation method. Cerium nitrate hexahydrate of 0.5 molarity (M) was prepared in deionized water under stirring using a mechanical stirrer. Ammonium dihydrogen phosphate of the same molarity was added slowly to the cerium nitrate in the 1 : 1 molar ratio. Ammonium hydroxide (NH 4 OH) was added to the mixture drop by drop to adjust the pH value at 12 for the flocculation of the precipitate and then keeping it under vigorous stirring at 80 ∘ C for half an hour for complete precipitation. A milky white precipitate was obtained in the beaker and is allowed to settle down for 24 hr at room temperature. These precipitates were then filtered and washed with distilled water and ethanol for several times to remove the nitrate ions. The milky white slurry of cerium orthophosphate thus obtained was dried, ground well in an agate mortar, and further calcinated in a furnace at 800 ∘ C for 2 hr under air atmosphere. The following reaction is expected to take place leading to the formation of cerium orthophosphate (CePO 4 ) nanoparticles:

Characterization.
The cerium orthophosphate nanoparticles were characterized using various physicochemical techniques like X-ray diffraction analysis, particle size analyzer, transmission electron microscope, and scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDAX); optical properties include UV-VIS-NIR spectroscopy and photoluminescence spectrophotometer, and electrical properties include dielectric constant, dielectric loss, and conductivity. The phase identification was carried out by X-ray diffraction patterns using PAnalytical X'Pert Pro diffractometer with a CuK radiation ( = 1.54060Å), the intensities of the diffraction beam were obtained in the 2 range between 20 ∘ and 70 ∘ . Debye-Scherer relation was used to determine the crystallite size. The particle size was measured with Zetasizer Nano ZS (Malvern Instruments Ltd., UK). In order to see the microstructure studies scanning electron microscope of JEOL Model JSM-6390 LV supplemented with energy dispersive X-rays analysis (EDAX) of model number JEOL JED-2300 was used. The size and shape of nanoparticles were observed under transmission electron microscope of Tecnai G 2 20 S-Twin working at a 200 kV accelerating voltage. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) curves were recorded simultaneously on the thermal analyzer (Shimadzu make DTG-60) over the temperature range from room temperature to 1000 ∘ C at the heating rate of 10 ∘ C/min at a flow rate of 30 mL/min. Fourier transform infrared (FTIR) spectrum was obtained on IR Prestige-21 (Shimadzu) spectrophotometer in the region from 400 to 3500 cm −1 using KBr pellet. The optical absorption was measured in UV-VIS-NIR region with Varian Carry 5000 spectrophotometer. The photoluminescence (PL) was investigated by Perkin Elmer Luminescence LS50B at room temperature. The electrical characteristics which include behaviour of dielectric constant ( / ), dielectric loss (tan ) with frequency, and temperature were studied using automated impendence analyzer (LF 4192A model) interface with USB GPIB converter 82357 B (Agilent) and further automated by using a computer for data recording, storage, and analysis.

Results and Discussion
3.1. Structural Characterization. The phase, purity, and crystallinity of the prepared cerium orthophosphate nanoparticle were firstly characterized by powder X-ray diffraction as shown in Figure 2. The graph shows that the pattern consists of peak at specific Bragg's angle depicting the crystalline nature of the material. The X-ray data was compared with JCPDS card number 32-0199 which suggests that material belongs to monoclinic phase of the monazite structure with lattice parameters = 6.79(6)Å, = 7.03(8)Å, = 6.43(6)Å, and = 103 ∘ 28 / and space group P2 1/ . In the XRD pattern, no additional peak of other phases was observed, thereby indicating the confirmation of the composition. Fang et al. [17] demonstrated that the rare earth orthophosphates (RPO 4 ) have structural transition from the hexagonal to the monoclinic by calcinations at high temperature. Obviously, monoclinic is a high temperature phase and the hexagonal where " " is the crystallite size (nm), " " is the wavelength of monochromatic X-ray beam (Å) ( = 1.54184Å for CuK radiation), " " is the full width at half maximum for the diffraction peak under consideration (rad.), " " is the diffraction angle ( ∘ ), and " " is a constant varying with crystal habit and chosen to be 0.9. The crystallite size comes out to be 49.3 nm. Figure 3 is the graph showing the particle size (nm) by Zeta particle size analyzer. From the particle size analyzer graph and XRD results, it may be concluded that the synthesized cerium orthophosphate is in the nanoscale range. are composed of agglomerated spherical particles. The possible growth mechanism for the formation of agglomerated spherical particles requires the surface potential to be low so that the attractive interaction between the grain boundaries dominates over the repulsive interaction forces. So the magnitude of the attractive interaction depends on the various factors like ionic strength, the dielectric constant of the solvent, and its viscosity which affects the kinetic agglomeration [18][19][20]. Further insight into the morphology and the microstructure of the prepared material was investigated under transmission electron microscope (TEM) and selected area electron diffraction pattern (SAED). Figure 5(a) clearly depicts the nanosphere with partial agglomeration having grain size in the range of 30-50 nm which is comparable to the crystallite size calculated by Scherer formula and from zeta particle size analyzer. The three-dimensional porous network structure of dried powder results in the formation of interspace between the particles, which inhibits the growth of particles at all directions and hence the particle shape is spherical instead of any other shape. Therefore, the coprecipitation process in the present investigation leads to homogenous powder with small spherical particles with a narrow size distribution. The corresponding selected area electron diffraction (SAED) pattern in the inset of Figure 5(b) reveals that the samples are consistent with the high crystallinity, and diffraction spots can be indexed as the phase formed is purely monoclinic in monazite structure which has also been confirmed from the powder XRD results. The spectrum obtained from energy dispersive X-ray analysis (EDAX) is shown in Figure 6. The EDAX spectrum shows peaks corresponding to all the major elements present in the grown composition as should be expected in case of cerium orthophosphates. The spectra show peaks corresponding to all major elements like cerium, oxygen, and phosphorous. No such impurity was found in the grown material. The weight % and atomic % as observed in the experiment are given in Table 2.

Thermal
Analysis. Thermogravimetric and differential thermal analyses (TGA/DTA) were used to identify different phase transition and to assess the stability of a prepared material. Figure 7 shows the recorded thermograph for cerium orthophosphate dried at 100 ∘ C. TGA/DTA was carried out between 14 ∘ C and 1000 ∘ C at uniform heating rate of 10 ∘ C/min and the mass was found to lose continuously as a function of temperature. The material starts to decompose at 21 ∘ C and decomposition went up to 335 ∘ C; thereafter the material is thermally stable up to 1000 ∘ C. The total weight loss observed is 15.81%. From the graph, it is clear that the thermal decomposition can be divided into three stages. The first stage of decomposition begins at 21 ∘ C and terminates at 127 ∘ C. During this temperature range, the mass loss observed is 9.31%. In this first stage of decomposition, the endothermic peak is observed at 54 ∘ C which is attributed to the loss of physisorbed water. The second stage of decomposition starts at around 127 ∘ C and is completed at around 255 ∘ C and net weight loss observed is 3.92%. Second stage of decomposition involves conversion of hydrated hexagonal cerium orthophosphate into anhydrous in nature as indicated in the following equation [21]: The third stage of decomposition occurs in the temperature range from 255 to 335 ∘ C. The weight loss in this stage is 2.58%. During this decomposition stage, this weight loss is accompanied by an exothermic peak at 285 ∘ C which is attributed to the transformation of cerium orthophosphate from the amorphous state to the crystalline phase. After the third decomposition stage (255-335 ∘ C), the material is thermally stable up to 1000 ∘ C. A weak exothermic peak at 817 ∘ C may be due to the crystallization of monoclinic monazite, cerium orthophosphate from hexagonal form ((CePO 4 ) Hexagonal → (CePO 4 ) Monoclinic ). From the thermal analysis of the system, we can therefore confirm that the product formed is in the monoclinic phase above 800 ∘ C which support the results obtained by powder XRD.

Fourier Transform Infrared Spectroscopy (FTIR).
In order to identify and characterize the inorganic species and analyse the chemical bonds, functional groups, and internal structure of molecules present in the grown nanomaterial, Fourier transform infrared spectroscopy (FTIR) was used. Figure 8 shows the FTIR spectra of cerium orthophosphate (CePO 4 ). Fundamentals IR frequencies observed in other phosphate compounds were also found in the present case, which confirms the presence of orthophosphate group in the grown material. The band observed in the range 2885.51-2341.58 cm −1 shows the presence of absorption of water vibration in addition to the wide bands associated with the OH group and another peak at 1635.64 is due to the scissor bending mode of associated water molecule [21]. The bands around wave numbers 1043.49, 623.01, and 543.93 cm −1 correspond to P-O stretching, O=P-O bending, and O-P-O bending mode of vibration, respectively. The bands from orthophosphate (PO 4 3− ) functional group appeared at 493.78 to 395.41 cm −1 in the lanthanide phosphates of monoclinic (monazite structure) and agree well with the literature data [22]. Table 3 gives complete information about the type of    orthophosphate is shown in Figure 9. It can be seen that there are two absorption bands at 208 and 274 nm in the ultraviolet range which is assigned to transitions from ground state 2 F 5/2 to the excited state, namely, 2 D 5/2 and 2 D 3/2 energy level. The cerium absorption band shows a weak extension from 300 to 450 nm, which is caused by the presence of a small amount of Ce 4+ and a small peak is observed at 1452 and 1939 nm due to the presence of phosphate in the NIR region. The obtained results are consistent with the data for transition of Ce 3+ where the transition in case of Ce 3+ is assigned from 4f ground state ( 2 F 5/2 ) to the 5d energy level split ( 2 D 5/2 , 2 D 3/2 ) within monoclinic crystal field [17,23]. UV-Visible absorption spectra also give the optical band gap of the material. The energy band gap is related to the absorption coefficient " " by the Tauc relation, ℎ] = (ℎ] − ) [24], where " " is constant, "ℎ]" is the photon energy (] = / ), " " is the band gap energy of the material, and = 1/2 for an allowed direct transition. For allowed direct transition, the band gap energy can be obtained from extrapolation of the straight line ( ℎ]) 2 versus ℎ] as shown in Figure 10. From the intersection of the extrapolated linear portion, the optical band gap energy ( ) of the material was found to be 1.07 eV; it means the absorption of photon with energy is greater than the optical band gap energy for direct transition from the occupied state in the valence band to unoccupied states in the conduction band. From the optical results, one can conclude that the cerium orthophosphate shows the properties of direct band gap material. In direct band gap material the electron wave vector remains the same (there is no phonon interaction); that is, if the momentum of electrons and holes is the same in both the conduction band and valence band, an electron can directly emit a photon.

3.4.2.
Photoluminescence. In addition, optical property of the material was also analyzed by photoluminescence (PL) emission spectrum using the excitation wavelength of 330 nm as shown in Figure 11. The emission spectra of Ce 3+ show a strong ultraviolet broad band at 400 nm which is caused by the 5d-4f emission. The inset spectra ( Figure 11) show Ce 3+ emission band ranging from 388 to 418 nm which consists of the expected doublet peak arising from the 2 D 3/2 excited state to the ground state 2 F 7/2 and 2 F 5/2 . The shape of emission band at shorter wavelength 2 D 3/2 -2 F 5/2 overlapped with 2 D 3/2 -2 F 7/2 emission bands depends on the Ce/P ratio [25]. From the emission spectra, it can be found that the value Journal of Nanoparticles of the luminescence intensity is very high approximately 30,000. This high value of emission intensity shows less impurity and defects present on the surface of the grown material which is important pathway for nonradiative relaxation in nanomaterials. The defects in materials can form the quenching centres which lead to nonradiative recombination and luminescence quenching [8].

Electrical
Properties. The investigation of electrical property is an elegant tool for understanding the dielectric behaviour of the material in response to an applied electric field. The study includes finding dependence of dielectric constant ( / ) and dielectric loss (tan ) on temperature and frequency. In order to carry out these investigations, the agglomerated nanoparticle was mixed with 1-2% PVA (polyvinyl alcohol) which acts as a binder and then pressed into a shape of circular pellet of the size 13.11 mm in diameter and 1.43 mm in thickness with the help of hydraulic press. The pellet was heated up to a temperature of 250 ∘ C at a heating rate of 2 ∘ C/min. before performing any measurement in order to overcome the effect of binder. The conducting silver paste was applied on both sides of the circular pallet which acted as capacitor with the material as dielectric. The material was then mounted on specially designed two terminal sample holders. The instrument directly provides the value of capacitance ( ) and dielectric loss (tan ). The dielectric constant was calculated using the relation: where " " is the capacitance (F), " " is the thickness (m), " " is the area (m 2 ), and " 0 " is the absolute permittivity in the free space having a value of 8.854 ×10 −12 Fm −1 .

Dependence of the Dielectric Constant and Loss Tangent
on Temperature. Figure 12 shows the variation of dielectric constant ( / ) with temperature (40-500 ∘ C) as a function of frequencies (5 kHz-1 MHz). The dielectric curve shows two anomalies, one at 60 ∘ C and another at 260 ∘ C. This behaviour was observed for other materials which exhibit two-phase transition [26]. In first phase of transition, the value of / increases with increase in temperature attaining a maximum value of 31 followed by a decreasing pattern. The second phase transition occurs at 260 ∘ C and the maximum value of / is 34. The first transition is attributed to the space charge polarization effect and the second phase transition may be due to ferroelectric to paraelectric transition. Similar behaviour was also observed in the thermal analysis results. From the thermal analysis graph, the peak observed at 285 ∘ C is nearly equal to the peak observed at 260 ∘ C in the dielectric results suggesting that the transition takes place in the material. During this phase transition, the peak observed is broadened and as frequency increases from 5 kHz to 50 kHz the peak around the dielectric constant becomes more broadened and the dielectric shifts towards the higher temperature. At higher frequencies (1 MHz), the value of dielectric constant shows less variation with temperature because space charge polarization decreases as the frequency increases. So, one can observe that the value of transition temperature shifts towards higher temperature with increase in frequency thereby showing the relaxational behaviour of the material. For most of the relaxor ferroelectrics, the peak of the dielectric constant ( / ) shifts to higher temperature with increasing frequency, which has been attributed to decreasing relaxation time with increasing temperature [27]. However, after the second phase transition and above 300 ∘ C the value of dielectric constant ( / ) shows further increase with increase in temperature. The maximum value of dielectric constant ( / ) obtained for 5 kHz at 500 ∘ C is 77, but due to instrument constraint we cannot perform our experiment beyond 500 ∘ C. The dielectric loss (tan ) was observed in the temperature range of 315-500 ∘ C ( Figure 13) and behaviour is almost similar to that of variation of dielectric constant ( / ) within this temperature range.

Diffusivity Study.
The region around the dielectric peak is broadened, which is a diffuse phase transition. The diffuse phase transition (DPT) takes place in materials where the composition fluctuations lead to large fluctuations in the Curie temperature. The structural disorder arises due to the presence of a number of voids and impurities of different size. The degree of diffusivity ( ) is calculated using the expression:  Here / max is the peak value of the dielectric constant and is the critical exponent which lies in the range 1 < ≤ 2. = 1 represents the ideal Curie-Weiss behaviour while being between 1 and 2 indicates diffuse behaviour [28]. The diffuse phase transition in ferroelectric is characterized by extending the phase transition in a wide temperature interval around the transition temperature where the dielectric constant assumes its maximum value. Figure 14 shows the variation of ln (1/ / − 1/ / max ) with ln( − ) at 5 and 10 kHz. The value of calculated from the slope of the curves comes out to be 1.8 at 5 kHz and 2.0 at 10 kHz. Thus, the value of " " suggests the diffuse phase transition, which may be due to the compositional fluctuations and structural disordering in the arrangement of cations in one or more crystallographic sites in the structure that finally results in a microscopic heterogeneity in the grown materials with local Curie points [29].

Dependence of Dielectric Constant and Loss Tangent on
Frequency. The dependence of dielectric constant ( / ) and loss tangent (tan ) on frequency of the applied ac field was studied in the temperature range of 40-500 ∘ C and a frequency range of 5 kHz to 1 MHz. Figures 15 and 16 show that the dielectric constant ( / ) and dielectric loss go on decreasing continuously with frequency in the lower frequency range and to attain saturation at 1 MHz. The continuous decrease in the value of the dielectric constant ( / ) and loss tangent with increasing frequency may be attributed to different types of polarisation, namely, ionic, electronic, dipolar, and interfacial or space charge polarisation at low frequencies [30]. This continuous fall occurs because dipoles are not able to follow faithfully the impressed oscillating field and the low value of dielectric constant ( / ) was observed at high frequencies.
Similarly, the loss tangent also decreases in the same manner like dielectric constant. The observed dielectric constant at Journal of Nanoparticles   low frequencies can be explained on the basis of the Maxwell-Wagner model. In this model, the dielectric structure consists of two layers: the first layer shows a large number of grains that act as a conducting medium at higher frequencies and the second layer consists of grain boundaries that act as a highly resistive medium at low frequencies [31]. In cerium orthophosphate nanoparticles, the polarisation mechanism at low frequency is explained as a local displacement of electrons by the hopping mechanism between Ce 3+ /Ce 4+ and an orientation of dipoles in the direction of the applied ac field. At low frequencies, the dielectric constant value is very high because the charge carriers are collected at the boundaries between the conductive grain region and the highly resistive grain boundary region. When the frequency increases, the electric dipoles and the Ce 3+ /Ce 4+ electrons transfer decreases the net displacement of charge in one direction thereby decreasing the dielectric constant value

AC Conductivity and Activation Energy.
In order to understand and authenticate the conduction mechanism, the alternating current conductivity ac (Ω −1 m −1 ) was calculated in the high temperature region using the relation: where " ac " is the frequency of the applied field (kHz) " o " is the absolute permittivity of free sample having a value of 8.854 × 10 −12 Fm −1 , " / " is the dielectric constant and "tan " is the dielectric loss. The variation of ac conductivity was measured at different frequencies from 5 kHz to 1 MHz in the temperature range of 315-500 ∘ C. Figure 17 shows the variation of ac conductivity "ln( ac )" with temperature. It was observed from the graph that the ac conductivity goes on increasing with increasing temperature. The influence of conductivity has been explained by considering the mobility of charge carrier responsible for hopping. As temperature increases, the mobility of hopping ions also increases thereby increasing conductivity. The variation of ac conductivity "ln( ac )" with frequency at different temperatures ranging from 340 to ) 500 ∘ C is shown in Figure 18. From the graph, it can be observed that the dependence of ac conductivity increases with increasing frequency at all temperature which is characteristics feature of material where hopping mechanism dominates. In the low frequency region, the conductivity was identical to the dc conductivity and was independent of frequency [32]. In the low frequency range, the applied field is not enough to initiate the hopping conduction. Therefore, the range of the low frequency conductivity plateau ( ac ) was found to increase with temperature. As frequency increases, ac conductivity increases due to the strong mobility of charge carriers. One can conclude that the conductivity is a function of both temperature and frequency of the applied field as has been reported for other materials [33][34][35]. At high frequency, ac conductivity ( ac ) increases linearly with frequency obeying the power law: ac = , where " " is the temperature dependent constant and " " is the frequency exponent [33]. The variation of frequency exponent parameter " " with temperature gives information about the specific mechanism. Figure 19 shows the variation of the exponent " " with temperature and it is clear that with the increase of temperature, the value of " " goes on decreasing. The decrease of ( ) suggests that the correlated barrier hopping (CBH) [36] model is the predominant ac conduction mechanism, which considers the hopping of charge carriers between two sites over a barrier separating them.
The variation of ac conductivity ( ac ) with temperature favours the thermally activated transport properties obeying Arrhenius formula. The activation energy for cerium orthophosphate nanoparticles was calculated from Arrhenius plot of ln ac using the relation: where " ac " is the conductivity at temperature " , " " " is the activation energy for the electrical process, and " " is the Boltzmann constant. Activation energy was calculated from the slope of the graph (Figure 20) showing the variation of ln( ac ) with 1000 −1 at different frequencies. The activation energy at frequencies 5, 10, 20, 50, 100, and 1000 kHz was calculated and is given in Table 4. The values of activation This type of behaviour suggests that the conduction mechanism may be due to the hopping of the charge carriers. The polarisation process in CePO 4 nanoparticles explained as a local displacement of electrons (i.e., n or p type) by the hopping mechanism between Ce 3+ /Ce 4+ and an orientation of electric dipoles in the direction of the applied field from one site to another. Therefore, a very small amount of energy is required to activate the charge carriers/electrons for electrical conduction. It has been shown by Ang et al. [37] and Moretti and Michel-Calendini [38] that the value of activation energy depends on ionization level of the oxygen vacancy. Usually, activation energy less than 1.0 eV is connected to singly ionized vacancies [37] and/or electronic mobility in space charge region. Thus, the conduction process within this temperature range may be due to the hopping of charge carriers and/or singly oxygen vacancies.

Conclusions
In summary, cerium orthophosphate (CePO 4 ) nanoparticles having monoclinic monazite phase were prepared by coprecipitation method. From XRD results, the crystallite size comes out to be 49.3 nm and from particle size analyser one can confirm that the particles grown in the nanoscale range. Transmission and scanning electron microscopic analysis reveals that the particles prepared by this technique are nanosphere having grain size in the range of 30−50 nm which is comparable to the crystallite size. Thermogravimetric and differential thermal analysis (TGA/DTA) suggests that the structural transition from hexagonal to monoclinic phase is taking place above 800 ∘ C. Optical studies using UV-VIS-NIR absorption spectrophotometer shows the maximum absorption in the ultraviolet region and the optical band gap energy of the material was found to be 1.07 eV for direct transition. The photoluminescence spectrum gives strong ultraviolet band at 400 nm which is caused by the 5d-4f emission. The electrical properties of these nanospheres show dependence of dielectric constant on both temperature and frequency. The variation of dielectric constant with temperature shows the transition peak shift towards higher temperature as the frequency increases. Therefore, it is suggested to have relaxor ferroelectric behaviour. From the ac conductivity behaviour, the value of activation energy decreases with increase in frequency, suggesting that conduction mechanism is due to the hopping of the charge carriers from one site to another.