Structural, Morphological, and Dielectric Properties of Gd3+ modified LaPO4 Ceramics

Gd3+ modified LaPO4 (Gd x La1-x PO4, where x = 0.02, 0.04 & 0.06) ceramics have been prepared using Auto Combustion. X-Ray diffractograms analysis shows that samples exhibits pure monoclinic structural phase upto x = 0.04 without presence of any other secondary phases whereas in x = 0.6, some secondary phases appears. The FESEM micrographs show grain growth, densification along with compactness of grains whereas EDS micrographs confirm the presence of elements according to stoichiometric proportion. Elemental mapping gives information about uniform distribution of elements. The dielectric relaxation behavior of prepared ceramics confirms from Cole-Cole model whereas dielectric loss gives information about conductivity of samples which has also be confirmed from conductivity profile.


Introduction
Rare earth modified phosphate materials commonly known as orthophosphates materials attracts focus of researchers for their wide usage in industrial applications such as light emitting diodes, laser technology, plasma display panels, compact fluorescent lamps etc. Apart from electronic applications, the orthophosphate materials also used for other industrial application such as in medical industry as a biological implant materials and host materials for nuclear waste management.In solid state illumination, light emitting diode is one of a milestone for its high resolution.Orthophosphate materials are important for above said industrial applications due to 4f orbitals electrons.Among such category of industrial applications, lanthanum phosphate is one of most promising materials due to its high physical and thermal stability and also for water insolubility.Rare earth modified phosphate with monoclinic , monazite or xenotime phase become are very much important as requirement of emitting diodes as a refractory application, high temperature photonic conductors, fuel cell etc. Lanthanum-Phosphate exhibits monoclinic crystal phase in two polymorphic phase such as monoclinic monazite and quardratic xenotime phase [1][2][3][4][5][6][7][8].
In this paper, we report the effect of partial substitution of Gd 3+ on structural, optical as well as dielectric properties LaPO4 (Gd 3+ at La 3+ site in LaPO4) ceramics prepared using Combustion method with citric as a fuel agent.The prepared modified LaPO4 with rare earth ion ceramics sintered at 1250˚C for 2 hours.

Synthesis and Characterization 2.1 Synthesis Procedure:
Rare earth ion modified LaPO4 (GdxLa1-xPO4) where x = 0.02, 0.04 & 0.06 ceramics have been synthesis using Auto Combustion method.The required metal nitrates such as GdNo3, LaNo3, and 1291 (2023) 012013 IOP Publishing doi:10.1088/1757-899X/1291/1/012013 2 phosphates have been first weighed in above mentioned stoichiometric proportion and made their clear solution with distilled water.Solution of Citric acid with water has also been prepared for using as fuel agent.The individually prepared solutions of above mentioned metal nitrates mixed one by one with continuous stirring and heating.The fuel agent mixed in last and heated solution along with continuous stirring till the liquid get converted into powder.The powder was then calcined at 1000 ˚C for 1 hour for crystal phase formation.The PVA (Poly Vinyl Alcohol) mixed calcined powder was pressed in circular disc for sintering and characterization.The PVA acts as binder.The circular disc were sintered at 1200 ˚C for 2 hours and termed as 'a', 'b' for analysis of various properties that have been characterized.

Characterization Procedure:
The powder of sintered disc was used for structural characterization using X-ray diffractometer from Shimadzu Maxima equipped with anode of Cu Kα (λ = 1.54 Å) whereas morphological as well as elemental analysis was carried out over pallets using Scanning Electron Microscope from Carl Zeiss Supra 55 equipped with X-ray detector from Oxford Instruments.For dielectric measurements such as dielectric permittivity, dielectric loss and ac conductivity measurements have been performed over sintered pallets using Impedance Analyzer (E4990A) from KEYSIGHT Technologies.

Structural Analysis
X-Ray diffraction data of GdxLa1-xPO4, where x = 0.02, 0.04 & 0.06 has been shown in figure 1 (a) & (b).Sharp, high intense crystalline peaks clearly validate the crystalline nature of prepared samples.Primarily, all the three graphs of prepared samples has merged to a single graph for confirmation of single phase formation.It has been clearly seen that diffraction peaks in prepared samples for x = 0.02, 0.04 overlapped each other reveals the single phase formation whereas in sample x = 0.06, beyond 2θ = 70, peaks does not overlapped meaning presence of some secondary phases.(14).All the peaks upto x = 0.04 indexed according to above mentioned JCPDS card whereas in x = 0.06, some secondary phases also appear which reveals that the solublesolubility limit of Gd 3+ substitution in LaPO4 is only upto x = 0.04.The graphs with indexed peaks have been shown in figure 1(b).

Morphological l Analysis
FESEM Micrographs of GdxLa1-xPO4, where x = 0.02, 0.04 have been shown in figure 2. The irregular, cross linked, vary in size shows increase in grain size, grain growth, and decrease in porosity along with enhanced densification.The all micrographs were taken at same electric voltage of 15kV and magnification of 30KX using InLens detector.It has been clearly visualised that grain size increases with increase in content of substitution.To confirm the exact grain size, grain size has been calculated using length of scale bar given in SEM micrographs.For this approximately 8-10 grain have been selected randomly and calculate the size.From calculation, it appears that grain size increases from 3.21 m to 4.56 m as substitution of Gd 3+ has been increased from x = 0.02 to x = 0.04.

Dielectric Properties:
Frequency dependent dielectric data (Real Part of dielectric permittivity) i.e. ε' vs. Frequency at room temperature of GdxLa1-xPO4, x = 0.02, 0.04 in frequency range100Hz-100KHz have been shown in figure 4. Real part of dielectric permittivity (ɛ') starts decreases with increasing frequency reveals that sample show behaviour similar to general dielectrics.In the lower range of frequency, sample exhibits maximum value of dielectric constant value and decreases continuously upto certain value of frequency and beyond that, dielectric constant become almost constant.The maximum value of dielectric constant results due to contribution of all polarization such as electronic, ionic, dipolar, space charge polarizations but in higher frequency range, some of these polarization do not responds results in lower value of dielectric constant.The phenomenon of dipole relaxation can be used to explain these observations [9].shows that substitution Gd 3+ at La 3+ site in LaPO4 decreases the insulating behaviour of LaPO4 results in increase in conductivity as well as dielectric loss.The decreases in insulating behaviour has been confirmed from lower value of dielectric constant in comparison with pure LaPO4 is an evidence of this behaviour.

Dielectric Loss (Tanδ)
The figure 5 shows the variation of dielectric loss (Tanδ) vs. frequency of GdxLa1-xPO4, x = 0.02, 0.04.The graphs clearly shows that substitution of Gd 3+ increase in dielectric loss in pure LaPO4.This increase in dielectric loss is due to increase in oxygen vacancies which increases the conduction behaviour results in increase in dielectric loss.

Figure 1 (
Figure 1 (b): X-Ray Diffraction Pattern of GdxLa1-xPO4, (a) = 0.02, (b) = 0.04 & (c) = 0.06 Further, for studying the crystal phase of prepared samples, diffraction data has been matched with reported Crystallographic data published in JCPDS card.The diffraction peaks clearly shows matching with diffraction peaks reported in JCPDS card no.84-0600 for x = 0.02 & 0.04.Since it has been clearly visualized from figure 1(a) that peaks for x = 0.02 & 0.04 clearly overlapped reveals presence of similar crystal structure whereas in x = 0.06, some secondary phases appears.So all peaks matched with above mentioned JCPDS card for x = 0.02 & 0.04.This JCPDS card belongs to Monoclinic phase having space group P21/n (14).All the peaks upto x = 0.04 indexed according to above mentioned

Figure 2 :
Figure 2: SEM Micrographs of GdxLa1-xPO4, (a) = 0.02 & (b) = 0.04 To confirm the element as per stoichiometric proportion, Energy Dispersive X-ray spectroscopy has also been performed.The peaks accordingly binding energy (eV) are direct evidence of presence of elements as per stoichiometric proportion in prepared samples.The table, electro micrograph and graphs shown in figure 3. The table shows presence of elements in calculated wt% by software attached with EDS is direct relevance of presence of elements as per stoichiometric proportion.The show uniform distribution of elements according to stoichiometric formula has been confirmed from elemental mapping shown in figure 3.In elemental mapping, different-different colour assigned to different ion.The mixed micrograph shows uniform distribution.

Figure 3 :
Figure 3: EDS and Elemental Map Micrographs of GdxLa1-xPO4, (a) = 0.02 & (b) = 0.04 .1088/1757-899X/1291/1/012013 7 relaxation model, a modified version of the Debye relaxation model [10].According to this model, the ε‫׳‬ and ε‫״‬ vary with frequency as: where ε∞ = dielectric constant in higher frequency regime, εs = dielectric constant in lower frequency regime, ω is angular frequency and τ = characteristics relaxation time of the medium.The value of exponent parameter (α) usually varies from 0 and 1, and gives information about shape of spectral curves.The value of exponent parameter (α =0) means Cole-Cole model changes to the Debye model.The fitted dielectric data using Cole-Cole dispersion formulas has been shown in figure 3.4(a & b).The graphs clearly shows that in lower range of frequency, dielectric constant remains almost constant that