Investigation of Structural and Dielectric Properties of Sr doped LaCrO3Synthesized by Auto-Combustion Method

We synthesized the undoped lanthanum chromium oxide, LaCrO3 (LCO), and Strontium (Sr) doped La1-xSrxCrO3 (x=0.1) perovskite compounds using auto-combution method. X-ray Diffraction technique reveals that both the samples possess Cubic crystal structure with Pm-3m space group. Furthermore, room temperature dielectrical properties of both the samples were emphasized. Dielectric constant and loss value of both the samples decreases as frequency increases attributed to Maxwell-Wagner type of relaxation.


Introduction
During the last few decades, Pervoskite type compounds have been study due to its numerous intriguing characteristics, such as high superconducting transition temperature (Tc), colossal magnetoresistance (CMR), multiferroicity, etc. [1,2].Perovskite rare earth chromites (RCrO3; R=La, etc.) have received a lot of attention as a significant subclass of functional materials due to their intriguing electrical, catalytic, and magnetic properties [3,4].Since, the RCrO3 often exhibits three different types of magnetic interactions, including Cr 3+ -Cr 3+ , R 3+ -R 3+ , and Cr 3+ -R 3+ due to which the magnetic characteristics are particularly complex and interesting.The antiferromagnetic Neel temperature (TN), which results from Cr 3+ -Cr 3+ superexchange interaction via intervening oxygen, ranges from 110 to 300K depending on the ionic size of the rare-earth ion, which is substantially greater than that (below 10K) resulting from R 3+ -R 3+ superexchange interaction [5].The antisymmetric Dzyaloshinsky-Moriya (DM) interaction of canted Cr 3+ moments, Cr 3+ sublattice exhibits mild ferromagnetism as well as strong antiferromagnetism in the temperature range between the two Neel temperatures, whereas R 3+ sublattice exhibits strong paramagnetism.Moreover, the weak ferromagnetic moment would produce an internal field on the R 3+ ions, causing an antiparallel coupling between the magnetic R 3+ and Cr 3+ ions and leading in spin reorientation, negative magnetization, and exchange bias (EB) effects [6][7][8].
The objective of the present study is to investigate how Strontium, Sr, dopant affects the structural and dielectric properties of LCO.It is well known that the parent materials structural and dielectric properties are significantly influenced by the ionic radius and oxidation states of the doped/foreign cations.The lattice mismatch between Sr and La causes the replacement of Sr to the La site to cause distortion of the LCO unit cell.The structural and dielectric properties of the LCO compound will be affected by this deformation.

Synthesis
LaCrO3 and Sr-doped LaCrO3 can be synthesized using the auto-combustion method.First, prepare a solution containing the appropriate amounts of La(NO3)3, Cr(NO3)3, Sr(NO3)2 and a suitable fuel, such as glycine or citric acid.Mix the solution well until a homogeneous mixture is obtained.Heat the mixture on an oven with constant stirring until the solution starts to boil and a gel-like substance is formed.Continue heating the mixture until it ignites and a self-sustaining combustion reaction occurs.After the reaction is complete, allow the material to cool down to room temperature.The resulting powder can be calcined at a high temperature, typically around 1000-1200°C, to obtain the desired LaCrO3 and Sr-doped LaCrO3 materials.

Characterization
The room temperature XRD spectrum of LaCrO3 and Sr-doped LaCrO3 were recorded by using A PROTO AXRD Benchtop Diffractometer with Cu-Ka1 radiation (k = 1.5406Å) to identify the crystal structure and space group.For the purpose of determining the dielectric properties of the sample, a silver paste was put to both sides of the pellets.The dielectric properties were measured using a Hioki LCR Meter (IM 3536).

X-ray Diffraction Analysis
X-ray Diffraction spectrum of Sr doped LaCrO3 was illustrated in the Figure 1.The results reveal single phase crystal structure with minute impurity peaks.Also, the majority of the peaks are high and sharp suggests that the sample has a high degree of order and regularity in its crystal structure, as can be seen from Figure 1.The XRD data exhibited Cubic crystal structure with Pm-3m space group [9].The hkl parameters of the corresponding peaks are also display in the Figure 1.The primary distinctive reflexes in the Sr doped LaCrO3 sample are indexed to the (020), ( 200), ( 220), (040), ( 240), ( 242), ( 402) and (044) diffraction planes and are positioned at 2θ of 22.93˚, 32.67˚, 40.18˚, 46.81˚, 52.65˚, 58.36˚, 68.47˚ and 77.93˚, respectively.The average crystal size of Sr doped LaCrO3 was 47.84 nm which was calculated from Debye Scherrer's formula [10].

Dielectric Constant (εʹ)
The frequency dependent dielectric properties of LaCrO3 and Sr doped LaCrO3 was studied at room temperature.Fig. 2 displays the plot of ε' as a function of a log of frequency.It is evident from the Fig. 2 as the dipoles obey the electric field at a low frequency.As the frequency rises, the dipoles will start to fall further behind the applied field, the dielectric constant will somewhat decrease, and dipoles will no longer affected by the field [11].
When the applied electric field is changed at low frequencies, the dipoles in the dielectric material can swiftly adjust, aligning with the field and raising the overall polarisation of the material.Higher dielectric constant depicts the measure of the material's ability to store electrical energy.The dipoles in the material start to fall behind the field as the frequency of the applied electric field rises.As a result, they are unable to react to changes in the field as quickly, and the material's overall polarization declines.The outcome is a small drop in the dielectric constant.The material's dipoles entirely lose sensitivity to the applied field at much higher frequencies.The total polarization of the material becomes very minor as a result, and they are no longer able to respond to changes in the field at all.The Maxwell-Wagner interfacial type of polarization, which is frequently observed in heterogeneous systems, and Koop's phenomenological theory, which claims that at low and high frequencies, the grain border and grain, respectively, have large contributions, can both be used to explain these phenomena [12,13].

Dielectric Loss (tan δ)
Figure 3 illustrates the frequency dependence of dielectric loss for LaCrO3 and Sr doped LaCrO3 ceramics.It is evident from the Figure 3 that with the increase in the frequency, the dielectric loss decreases smoothly in LaCrO3 ceramics.On the other hand, a hump in dielectric loss with increase in

LaCrO
frequency is observed in Sr doped LaCrO3.Its presence in the material is linked to the presence of grain boundaries, defects, and relaxation processes [14].
In general, the hump that appears in the dielectric loss graph of Sr doped LaCrO3 with an increase in frequency can be explained by the presence of oxygen vacancies and grain boundaries in the material, as well as the relaxation processes that are related with each of these features [15].

Ac electrical Conductivity(σac)
In Figure 4, the ac electrical conductivity of LaCrO3 and Sr-doped LaCrO3 was investigated as a function of frequency.It can be clearly seen that the spectra are split up into two separate regions that are easily identifiable.The first of these is the frequency-free region, also known as the plateau region, in the applied small frequencies.In this plateau region, frequency does not play a role in conductivity.The frequency-dependent portion of the electrical conductivity is referred to as the ac conductivity region.The conductivity is higher in this part of the region.This peculiar behavior of ac conductivity might be explained by the activation of hopping of charge carriers between various localized regions as a result of an increase in the field that is applied from the outside.Because of this, there is an increase in the colonization of electrons, which in turn causes a rise in conductivity at higher frequencies [16].
In addition, it has been demonstrated that the doping of Sr to La in LaCrO3 structures results in an increase in conductivity due to the production of an increased number of Cr 6+ ions [17,18].The variation in conductivity that occurs as a function of frequency can be interpreted in a number of different ways, one of which is as the presence of space charges and cationic disorder among neighboring sites [19].

Conclusion
In Summary, LaCrO3 and Sr-doped LaCrO3 compound were synthesized by auto combustion method.These samples were successfully characterized and analyzed for structural and dielectric properties.Xray diffraction data reveals that both the samples possess Cubic crystal structure with Pm-3m space group.Furthermore, room temperature dielectric properties of both the samples were emphasized.Dielectric constant and loss value of both the samples decreases as frequency increases attributed to Maxwell-Wagner type of relaxation.An electrical conductivity study indicated that the conduction process in both samples is attributed to small polaron hopping.

Figure 2 :
Figure 2: Frequency dependent dielectric properties of LaCrO3 and Sr doped LaCrO3

Figure 3 :
Figure 3: Frequency dependence of Dielectric Loss for LaCrO3 and Sr doped LaCrO3 ceramics

Figure 4 :
Figure 4: ac electrical conductivity of LaCrO3 and Sr-doped LCO