Formation of Fe4+ based on the Fe-O bond lengths in perovskite Eu1-xSrxFeO3

The physical properties of high oxidation state of transition metal oxide compounds have attracted many research interest for its potential applications. A systematic investigation of Fe4+ in Eu1-xSrxFeO3 with x = 0, 0.1, 0.2, 0.3, and 0.4 has been carried out in polycrystalline form obtained using sol-gel method. The single phase of Eu1-xSrxFeO3 is observed up to x = 0.3. The X-Ray powder pattern for low Sr content of x = 0.1, is best refined by using EuFeO3 crystal structure with Sr occupied the Eu-site. On the other hand, for higher Sr contents, two phases of EuFeO3 and Eu1/3Sr2/3FeO3 are required to obtained the best refinement. The result of the refinements shows that the volume and the average bond lengths of Fe-O decreases as increasing the Sr content. These results are consistent to support the existence of Fe4+ upon Sr doping in Eu1-xSrxFeO3. For the Sr content of x = 0.1, an anomalous change of the volume is observed.


Introduction
The physical properties RFeO3 (rare earth orthoferrite) have attracted many research interest for its interesting properties such as high Néel temperature, low dielectric loss, and high Seebeck coefficient [1][2][3][4].RFeO3 is a distorted perovskite material and belongs to the orthorhombic crystal system.The unit cell contains a FeO6 octahedron which construct a 3D perovskite structure [1].
RFeO3 is an antiferromagnetic material with antiparallel R and Fe spins [2,3].The Néel temperatures of RFeO3 were observed from 623 K to 738 K as increasing the ionic radii of the rare earth which corresponds to the increment of the bond angle of Fe-O-Fe between the octahedra [2][3][4][5].On the other hand, mixed series R1-xSrxFeO3 have been studied with pure SrFeO3 have cubic symmetry and low resistivity [6,7].Based on Mössbauer spectroscopy, substitution of the rare earth for strontium changes from Fe 3+ to Fe 4+ [7].Change of the valency of Fe change the length of Fe-O which affect the weak ferromagnetism in iron perovskite [8].
We obtained Eu1-xSrxFeO3 by using sol-gel method.Increase of Sr concentration will change the structure from orthorhombic to cubic-like crystal which decrease the number of peaks at X-ray diffraction patterns.Two-phase refinement EuFeO3 and Eu1/3Sr2/3FeO3 are required for transition phase with x ≈ 0.2-0.4.Based on each phase's fraction structure parameters of the perovskite were calculated.Unit cell volume and average bond lengths of Fe-O in Eu1-xSrxFeO3 decrease as Sr content increase.

Experiments
EuxSr1-xFeO3 was synthesized using sol-gel method [9].Eu2O3 and SrCO3 with the stoichiometric ratio of Eu 3+ :Sr 2+ :Fe 3+ = (1-x):x:1 (x = 0, 0.1, 0.2, 0.3, 0.4) are respectively put in beaker glasses and dissolved in 40 mL DI water.The solution then placed on a hot plate and heated to a temperature of 50 °C while stirring with a magnetic stirrer.The resulting solution will be white.HNO3 is dripped slowly until the solution is colourless.Fe(NO3)3 hydrate is dissolved in 20 mL DI water in the second beaker.Fe(NO3)3 solution and tartaric acid were mixed into the first beaker.The temperature of the hot plate was increased to 250 °C and waited until all the liquid evaporated.The beaker is then taken and the substance is collected in the beaker.The substance is carried out combustion at 500 °C to remove the remaining HNO3.Then, the sample was grinded and pressed into pellet.The pellet was sintered at 800 °C for 20 hours.The sample was then grinded, pressed into pellet, and sintered three times more until the single phase was obtained.
The XRD patterns of Eu1-xSrxFeO3 final samples were recorded with XRD Bruker D8.The radiation source used is Cu-Kα and the 2θ range is used from 10° to 90°.XRD data was carried out by Rietveld refinement using the GSAS-II program [10].Rietveld refinement results are stored in the form of CIF files.VESTA program was used to read the CIF files and determine the Eu1-xSrxFeO3 structure and the Fe-O bond lengths [11].

Results and discussions
XRD patterns of the Eu1-xSrxFeO3 are shown in figure 1. Single phase of Eu1-xSrxFeO3 is obtained up to x = 0.3.For x = 0.4, impurities phase of Eu3Fe5O12 appears at 2ߠ = 28.59° and 35.20°.The transition from orthorhombic to cubic are shown from the decreasing of peak numbers in XRD patterns.The peaks of EuFeO3 at 2ߠ ≈ 39.0, 39.9, 40.9, and 42.2 which correspond to Miller index of (2 1 1), (0 2 2), (2 0 2), and (1 1 3) reduce and merge into single peak with 2ߠ ≈ 40.4° at higher Sr-contents.For x = 0.1, the XRD pattern was refined using EuFeO3 with 0.1 fraction of Sr at Eu-site give a good result [5].At higher Sr content (x = 0.2 -0.4), single phase refinement using EuFeO3-like phase give the bad result due to new peaks from transition phase.A two-phase refinement using EuFeO3 and Eu1/3Sr2/3FeO3 mixed phase give better refinement results which can be seen at figure 2. The results obtained using GSAS refinement for each phase are shown in table 1. Lattice constants and volume of unit cell are calculated based on the fraction of each phase shown in figure 3.There is a trend for the unit cell volume to decrease when Sr content increase.This is due to the shortening of Fe-O bond lengths.However, there is an anomaly of unit cell volume when x = 0.1.Eu1-xSrxFeO3 has orthorhombic crystal structure that have two different positions of oxygen (O1 and O2).In addition, there are also two different bond lengths for Fe-O2 we labelled as Fe-O2 (1) and Fe-O2 (2) .So, we will calculate the weighted average distance for Fe-O as following [12]: Figure 4 shows the phase of Eu1/3Sr2/3FeO3 appeared when x = 0.2.The fraction of the phase showed the Sr content in the perovskite.The average bond lengths of Fe-O decreases as the increases of Sr content due to formation of Fe 4+ .Some of the Fe 3+ changed into Fe 4+ with higher valency which have shorter ionic radius.This result confirmed the trend of the unit cell volume also decreases as the increase of Sr content.This change of structures might change the physical properties of the perovskite, such as electrical conductivity, magnetic properties, etc.

Conclusions
We obtained single phase of Eu1-xSrxFeO3 by using sol-gel method up to x = 0.3.For ‫ݔ‬ ≈ 0.2 − 0.4, two phase refinement EuFeO3 and Eu1/3Sr2/3FeO3 are needed due to change of the perovskite structure from orthorhombic to cubic symmetry.Fraction of each phase is used to calculate the structure parameter of the perovskite.Based on the refinement, the unit cell volume and the average bond lengths of Fe-O decreases as the increases of Sr content.These trends appeared due to formation of Fe 4+ in the Eu1-xSrxFeO3.However, there is an anomaly in unit cell volume when x = 0.1.

Figure 3 .
Figure 3.The changes in (a) lattice parameters: (square) a, (circle) b, and (star) c and (b) unit cell volume of Eu1-xSrxFeO3 calculated based on fraction from each refinement phase.

Figure 4 .
Figure 4. Fraction of Eu1/3Sr2/3FeO3 represented by black circle with dashed line as regression line and average bond lengths of Fe-O represented as red square in Eu1-xSrxFeO3 of Fe-O (Å) 10th Asian Physics Symposium (APS 2023) Journal of Physics: Conference Series 2734 (2024) 012044