Effect of Zr addition on structure and electrical properties of SmFeO3 prepared by solid state reaction method

Series of Sm1-xFe1-3xZr3xO3 (SFZO, where x = 0.0, 0.01, 0.02, 0.033, 0.05, 0.1 and 0.15), was synthesised by the solid state reaction method and sintered at 1500 °C in air. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the structure of the materials. The XRD results indicated that the pure SmFeO3 and Sm0.99Fe0.97Zr0.03O3 samples are single phase materials with an orthorhombic symmetry and Pbnm space group. The increase of Zr content (x ≥ 0.02) causes a phase change from single to twophase system with orthorhombic (Pbnm) and fluorite symmetry (Fm3̄m). The microstructural analyses showed that the Zr doping resulted in an increase of porosity in the sintered SFZO compared to SmFeO3 sample. The direct current (DC) electrical conductivity at 850 °C in air was found to be 0.045 S/cm and 0.007 S/cm for Sm0.99Fe0.97Zr0.03O3 and SmFeO3, respectively, showing a significant enhancement compared to the pure SmFeO3.


I. Introduction
The material properties might be improved by substitution or doping.This is basically related to the atomic arrangement in the material structure [1,2].This development is important for the design of new materials and provides essential features for various applications, specifically in gas sensors and solid oxide fuel cell (SOFC) devices [3][4][5].Furthermore, ceramic materials play an important role in development of some modern energy technology.Ceramic materials can also offer not only an enhancement in both conductivity and durability, but also show a redox stability in harsh conditions [6][7][8][9].Perovskite materials have displayed interesting physical, chemical, mechanical, and electrical properties related to their crystal structure and ordering [10][11][12].Meanwhile, substitution of lanthanides Ln (La, Sm, Nd, Pr, etc.) in perovskite structures has shown a superior performance in SOFCs utilizations [13,14].Apparently, these are giving the possibility to use newly synthesised materials as a part of the SOFC component.Furthermore, thermal and electrical properties are greatly dependent on the microstructure and phase stability of the material.One of the interesting materials for these applications is doped SmFeO 3 .Therefore, investigation of the structure of Zr-doped SmFeO 3 [15][16][17] can measure the possibility of their application in energy devices with high stability based on the presence of Zr 4+ in the structure.
It was already reported that samarium oxide (Sm 2 O 3 ) is a good catalyst with high activation energy [18][19][20].It was also confirmed that doping with Ni 3+ and Ce 3+ to SmFeO 3 resulted in a noticeable stability [21] with structure having orthorhombic symmetry with Pnma or Pbnm space groups [15,16,22,23].The analyses of Sm 3+ doped materials [10] also showed that the conver-sion of the Sm 3+ to Sm 2+ species can happen at the same time as the oxidation.So these modifications may justify more research on the use of samarium based materials as possible electrode in SOFC devices.LnFeO 3 perovskites showed mixed ionic conductivity when applied as electrode materials (especially as cathodes).They exhibit high amount of oxygen vacancies and a good surface kinetics as well [24][25][26][27][28].Moreover, Sm doped Fe 2 O 3 has proven to have a stable, good catalytic activity which can enhance performance as a gas sensor material in reduced atmosphere [21].This easily leads to high electrical conductivity of the perovskite because of high electrochemical performance.Further on, tolerancy to the carbon coking and H 2 S poisoning depends on the B-site cations [29].Zn and Zr doped Fe 2 O 3 have also been investigated showing a superior performance in structural, magnetic and photocatalytic properties [30][31][32].High electrochemical performance of Zn doped materials was achieved in nanosphere Fe 2 O 3 [31].Meanwhile, Zr has shown a high stability and better performance when doped to Fe 2 O 3 nanostructure array in photo-electrochemical H 2 production [32].
In this study we have investigated structure of a series of new compounds Sm 1-x Fe 1-3x Zr 3x O 3 (x = 0.0, 0.01, 0.02, 0.033, 0.05, 0.1 and 0.15) synthesised by solid state reaction method in order to improve their electrical properties.The doping of the well-known ABO 3 perovskite structure with Zr 4+ was on the B site (Fe 3+ ).Hence, high stability of these materials made it very suitable for the catalytic activity of the perovskites [33].

Sample preparation
New Sm 1-x Fe 1-3x Zr 3x O 3 (x = 0.0, 0.01, 0.02, 0.033, 0.05, 0.1 and 0.15) compounds were synthesized by the solid state reaction method in air.Initially, the stoichiometric amounts of high purity (>99.9%)precursors, Sm 2 O 3 , Fe 2 O 3 and ZrO 2 , were mixed together.To elim-inate the effect of hydration and remove residues, chemicals were heated in air at 300 °C for 2 h inside a muffle furnace.After that, 5 g of each composition was well mixed with ethanol in an agate mortar with pestle.The ethanol was evaporated at RT in a fume cupboard.Then the obtained powders were dried in the oven at 80 °C for 24 hours.The obtained powders were heated in air atmosphere at 900 °C for 10 hours.The collected powders were sonicated for 30 min to get well and homogeneous mixture and the materials were ball milled for 2 h.Afterwords, the obtained dry powder and binder (Decoflux ™ ) were mixed together and pressed into pellets using a hydraulic press (pressure was 2 tonnes) and die with diameter of 15 mm.The pressed pellets (15 mm in diameter and 2 mm thickness) were fired at 1300 °C for 12 hours in the first sintering step.The XRD analysis confirmed that the pure phase was not obtained.Because of that, the pellets were re-ground with ethanol in a mortar with pestle and re-pelletized for the final sintering at temperature of 1500 °C for 15 hours with a heating and cooling rate of 5 °C/min.

Characterization
The initial structural characterization of the SFZO samples was performed by XRD (Shimadzu-7000, Shimadzu, Japan) using CuKα radiation (λ = 1.5406Å) in the 2θ range from 10°to 90°with a scan step of 0.01°/s.The procedure utilized here was to identify the structure based on indexing and find the lattice parameters using the WINXPOW [34] and SPuDS [35], respectively.FullProf software [36] was used to perform the Rietveld refinement of the XRD data.The least-square refinement was converged with the minimum goodness of fit (χ 2 ) and agreed with the literature data [16,17,21].Scanning electron microscope (SEM-JSM-7610F) was used to investigate microstructure of the sintered samples.
DC electrical conductivity was measured using a four probes DC conductivity jig -SS scientific laboratory supplies at the temperatures up to 850 °C in air atmosphere.

Structure determination
The XRD diffraction patterns of Sm 1-x Fe 1-3x Zr 3x O 3 (x = 0.0, 0.01, 0.02, 0.033, 0.05, 0.10 and 0.15) pellets sintered at 1500 °C are presented in Fig. 1.It is obvious that the single phase materials were obtained for x = 0.0 and 0.01, whereas the secondary phase appeared for x ≥ 0.02.The selected 2θ regions, showing the appearance and growth of the secondary phase, are given in Fig. 2.
The Rietveld analyses of the XRD data (Figs. 3 and  4) give the correct atomic positions, space group and cell parameters.The analyses confirmed that the materials for x = 0.0 and 0.01 crystallize in the single phase orthorhombic structure with the space group Pbnm.A small amount of the second phase with cubic symmetry and space group Fm 3m was found to be coexisting in the SFZO ceramics with x = 0.02, 0.033, 0.05, 0.1 and 0.15.The structure with higher Zr-content (x ≥ 0.02) was found to be two-phase system with orthorhombic and cubic symmetry, characterized with the splitting of some major peaks.The atomic positions of the investigated samples are illustrated in Table 1.The cell parameters after the Rietveld refinement of the sintered SFZO ceramics are given in Table 2 and the atomic occupancies, bulk and theoretical densities of the SFO and SFZO are shown in Table 3.It was observed that the lattice parameters of the perovskite structure increase with Zr doping.This slightly increase is due to the differences in ionic radii of Zr 4+ and Fe 3+ (radius of Zr 4+ is higher than that of Fe 3+ ) and the Zr 4+ occupies the B-site.However, for the SFZO samples with x ≥ 0.02 the lattice parameter of the fluorite structure has decreasing trend, due to the oxidation state of iron ion.

Microstructural study
The microstructure of the materials was examined by using SEM (Fig. 5).It is obvious that addition of Zr increases level of porosity in the sintered samples.Hence, it is recommended that this material can be used for oxygen sensor and SOFCs electrode applications [37][38][39][40][41] depending on their electrochemical and electrical conductivity with a good chemical and thermal stability.Thus, based on the previous results, i.e.SmFeO 3 with another dopant like Ce [20,21], it is possible to obtain high performance material for electrode application in fuel cell systems, with fast oxygen transport [42,43] as well as high electrocatalitic performances.
From the density calculations, from both experimental and theoretical [44], it is clear that there is a very  3 in the samples with higher amount of Zr the lower density was obtained.Thus, the pure SmFeO 3 has relative density of 98.2 %TD and relative density of Sm 0.85 Fe 0.55 Zr 0.45 O 3 is 82.9 %TD.This significant decrease in density resulting from Zr addition was confirmed by SEM analyses.Moreover, the obtained microstructure gives an excellent stability and reversibility of oxygen storagerelated process [45,47].Accordingly, it is very easy to predict the occurrence of high transportation in SFZO oxides because of the porosity increases without adding any pore former [41,43].

Electrical conductivity
Three sintered samples: SmFeO 3 , Sm 0.99 Fe 0.97 Zr 0.03 O 3 (with single phase structures) were selected to investigate the electrical properties in terms of DC conductivity in air atmosphere.Four thin gold wires were attached on different positions on the sample surface (Fig. 6) with the gold paste and heated at 700 °C for 1 hour with 2 °C/min heating and cooling rate.Afterwards, the bar was fixed on the measuring jig and the probes of the jig were attached to the four terminals of the sample.Then the set up was inserted into the chamber and tested at the temperature up to 850 °Cin air with 4 °C/min heating and cooling rate.Figure 7 shows the conductivity results of the three tested samples.The conductivity was calculated by using the Uhlir equation [47]: where d is the spacing between two attached wires, t is the bar thickness and L av is the effective length during the measurements.The continuous increase of conductivity with temperature is obvious in all three samples (Fig. 7).The highest electrical conductivity at 850 °C of 0.045 S/cm has the sample Sm 0.99 Fe 0.97 Zr 0.03 O 3 , considerably higher in comparison to the value of 0.007 S/cm for the pure SmFeO 3 .The sample Sm 0.98 Fe 0.94 Zr 0.06 O 3 , which contains a small amount of secondary phase, still has relatively high electrical conductivity of 0.025 S/cm at 850 °C in air (shown in Fig. 7c).

IV. Conclusions
Standard solid state reaction method was used to synthesize a series of new materials Sm 1-x Fe 1-3x Zr 3x O 3 (x = 0.0, 0.01, 0.02, 0.033, 0.05, 0.10, and 0.15) successfully and investigate the structural, microstructural and electrical properties.The Rietveld refinement of XRD data indicated that these samples possess an orthorhombic single phase crystal structure for x = 0.0 and 0.01 in the Pbnm space group.However, slightly higher Zr-doped materials (≥ 0.02) have a small amount of cubic fluorite phase which was refined together with the main phase to get accurate structural information in Sm 1-x Fe 1-3x Zr 3x O 3 at x ≥ 0.02.Those materials have shown two-phase structures of orthorhombic (perovskite) and cubic (fluorite) type.Microstructures of the single phase materials demonstrated a slight increase in porosity with Zr doping with almost the same particle size of 20 µm (both in SFO and SFZ oxides).The increase of Zr dopant resulted in an increase in electrical conductivity compared to the un-doped SmFeO 3 material.Based on the obtained DC conductivity results, Zr doped SmFeO 3 showed a noticeable improvement which indicates their ability to be applied in various applications, such as gas sensors and SOFC electrodes.

Figure 2 .Figure 3 .
Figure 2. Highlighted XRD peak 011 of the cubic phase of the prepared samples for x ≥ 0.02

Figure 6 .
Figure 6.Enlarged view of the sintered bar attached with gold wires and fixed in the test jig position

Table 1 .
Atomic positions (x, y, z, O1, O2) of all atoms in the compounds

Table 3 .
Cell parameters and R factors after the Rietveld refinements of the compositions