1. Introduction
Perovskite oxides exhibit a wide range of interesting and useful properties, such as ferroelectricity, [1] piezoelectricity, [2] superconductivity, [3] and catalytic activity. [4] [5] Due to these properties, perovskite oxides find applications in various fields, including electronics, catalysis, and energy storage. Perovskite oxides are recently the focus of research because of their potential applications in technology such as solid oxide fuel cells, [6] metal-air batteries, [7] Lithium battery, [8] electrocatalysis, [9] thermal insulation, [10] sensors, [11] and photovoltaics. [12] Oxygen plays an important role for the material to demonstrate a functional property, leading to exhibit excellent catalytic behavior in many transition-metal oxides. Perovskite-type systems, with the general formula ABO3, are especially interesting, where A is usually an alkaline-earth metal or lanthanide, and B is usually a transition metal. The large A cations are located in spaces between corner-sharing BO6 octahedra.
It is possible to form oxide perovskite materials with some degree of oxygen deficiency. [13] In some cases, the vacant sites created due to oxygen deficiency can be distributed in the structure arbitrarily, forming a disordered system. One such material with a vacancy-disordered system is CaSrFe0.75Co0.75Mn0.5O6−δ. [9] Among a series of compounds reported with different Mn concentrations in the composition CaSrFe1−xCo1−xMn0.2xO6−δ, the composition with x = 0.25 demonstrated high efficiency of catalytic performance in oxygen generation and green hydrogen generation by water splitting.
Structural properties are the backbone for the functional properties and efficiency of material toward any application performance. So, scientists generally study in depth the structural properties of a material that illustrates outstanding performance with a better functional property. Since CaSrFe0.75Co0.75 Mn0.5O6−δ outperformed the electrocatalytic behavior of water splitting for oxygen and hydrogen production, we are interested in studying its crystal structure by neutron diffraction. CaSrFe0.75Co0.75Mn0.5O6−δ has been reported for its structural analysis by powder XRD, SEM and XPS. However, its structural analysis has not been reported by powder neutron diffraction which can support the previously reported structural data of XRD for this material.
2. Experimental
CaSrFe0.75Co0.75Mn0.5O6−δ was synthesized by solid state reaction method at high temperatures by mixing Stoichiometric amount of CaCO3, SrCO3, Fe2O3, Co3O4 and Mn2O3. The precursor chemicals were mixed uniformly in agate mortar and pestle. The mixture is pelletized using pellet die and a hydraulic pressure at a pressure of 3 tons. The dimensions of the cylindrical pellets were 2 - 3 mm thick with a diameter of 10 mm. They were fired at 1000˚C in the air in a muffle furnace for 12 hours. The heating and cooling rate was 5 degrees per minute. Once the pellet was cooled down, it was powdered and repelletized which was followed by a second firing at 1200˚C. It was heated at 1200˚C for 24 hours. This time the heating and cooling ramp was maintained at a rate of 100 degrees per hour. The cold pellet was powdered which was subjected to phase purity and structure of the polycrystalline samples were determined by powder X-ray diffraction (XRD) [14] at room temperature using Cu Kα1 radiation (λ = 1.54056 Å) using Bruker phaser D2 diffractometer and neutron diffraction at room temperature. The GSAS software [15] and EXPEGUI [16] interface were used for Rietveld refinements.
3. Results and Discussion
CaSrFe0.75Co0.75Mn0.5O6−δ is an oxygen deficient cubic perovskite oxide. It has Pm-3m space group. Its XRD data and Rietveld refined cell parameters are shown in Figure 1 and Table 1, respectively. The results are in agreement with the previous report. [9] The crystal structure and its neutron diffraction data are shown in Figure 2 and Figure 3, respectively. Neutron diffraction data also shows the cubic structure with Pm-3m space group in agreement with the XRD data. Its refined cell parameters are shown in Table 2. As mentioned in introduction, oxygen deficient perovskites are represented by a general formula ABO3−δ or A2B2O6−δ where A is alkaline earth metal and B is 3d or 4d transition metal. In our material CaSrFe0.75Co0.75 Mn0.5O6−δ, A site is occupied by Ca and Sr and B site is occupied by Fe, Co and Mn.
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Figure 1. Rietveld refinement profile for powder XRD data refined in the space group Pm-3m. Crosses represent experimental data, the solid red line is the model, vertical green tick marks show Bragg peak positions, and the lower pink line represents the difference plot.
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Table 1. The unit cell parameters and Powder X-ray data refinement profile for CaSrFe0.75Co0.75Mn0.5O6-δ.
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Figure 2. Crystal structure (a) Crystallographic unit cell and corner-sharing (Fe/Co/Mn)O6 octahedra (black) are highlighted. The large white-gray spheres are the Ca and Sr atoms. (b) View along the unit cell axis. Because of the cubic symmetry, the three axes are identical. (c) Coordination geometry around the Ca/Sr atom, which is 12-coordinated.
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Figure 3. Neutron diffraction Rietveld refinement profile. Crosses represent experimental data, the solid red line is the Pm-3m model, vertical green tick marks show Bragg peak positions, and the lower pink line represents the difference plot.
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Table 2. The unit cell parameters and powder neutron diffraction data refinement profile for CaSrFe0.75Co0.75Mn0.5O6-δ.
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Table 3. Comparison of the bond lengths (Å) between neutron diffraction and powder X-ray diffraction data.
If we closely look at Figure 2, it can be seen that the Fe/Co/Mn atoms (seen as green spheres) are surrounded by 6 oxygen atoms (small red spheres) in octahedral positions. Here, imaginary planes (black planes) are drawn connecting oxygen atoms to make the octahedral structures clear. It can be represented as BO6 octahedra. So, Fe/Co/Mn atoms are 6 coordinated throughout the crystal lattice as shown in Figure 2(a) except at the oxygen deficient positions (which are not shown due to uncertainty). Since the composition is CaSrFe0.75Co0.75 Mn0.5O6−δ, Fe and Co occupy 75% of the B site positions, each occupying 37.5% of the total B site positions and Mn occupies 25% of the B-site positions. It can be seen in Figure 2(c) that the whitish grey largest sphere, which is Ca/Sr atom, is surrounded by 8 octahedra. Ca and Sr share equally the A-site positions, each occupying 50% of the A-sites. Ca/Sr is 12 coordinated except at the oxygen deficient positions (which are not shown due to uncertainty). The octahedra are connected to one another by corner sharing through oxygen. Thus, the bonding pattern is B-O-B where B is Fe/Co/Mn. The B-O-B bond angle is 180˚. The B-O-B bonds lengths are shown in Table 2 and Table 3 for XRD and Neutron diffraction.
4. Conclusion
A perovskite material with a composition of CaSrFe0.75Co0.75Mn0.5O6−δ was synthesized by solid-state reaction at 1200 C. Its crystal structure was investigated by neutron diffraction and compared with that of XRD data. Neutron data revealed its structure cubic with the Pm-3m phase which supported the XRD data. The study showed that the B cations are surrounded by 6 oxygens forming BO6 octahedra which are interconnected by corner sharing through O-atoms and A cations are surrounded by 8 such octahedra.
Acknowledgements
This work is supported in part by the National Science Foundation Tribal College and University Program Instructional Capacity Excellence in TCUP Institutions (ICE-TI) award # 1561004. A part of this work is also supported by NSF grant No. HRD 1839895. Additional support for the work came from ND EPSCOR STEM grants for the purchase of potentiostat and X-ray diffractometer. Permission was granted by United Tribes Technical Colleges (UTTC) Environ-mental Science Department to publish this information. The views expressed are those of the authors and do not necessarily represent those of United Tribes Technical College.
Funding
Instructional Capacity Excellence in TCUP Institutions (ICE-TI) award #1561004 and NSF Tribal Enterprise Advancement Center award grant No. HRD 1839895.
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