- Nano Express
- Open Access
Sol-Gel-Prepared Nanoparticles of Mixed Praseodymium Cobaltites-Ferrites
© Pekinchak et al. 2016
- Received: 7 December 2015
- Accepted: 2 February 2016
- Published: 9 February 2016
Two series of nanocrystalline powders of PrCo1 − x Fe x O3 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) of high purity were obtained by sol-gel citrate method at 700 and 800 °C. The formation of continuous solid solution with an orthorhombic perovskite structure (sp. group Pbnm) was observed. A peculiarity of the PrCo1 − x Fe x O3 solid solution is the lattice parameter crossovers, which occurred at certain compositions and revealed in the pseudo-tetragonal or pseudo-cubic metric. An average crystallite size of the PrCo1 − x Fe x O3 samples estimated from the analysis of the angular dependence of the X-ray diffraction (XRD) line broadening varies between 30 and 155 nm, depending on the composition and synthesis temperature.
- Mixed cobaltites-ferrites
- Crystal structure
Complex oxides with perovskite structure RMO3, where R and M are rare earth and transition metals, respectively, represent an important class of the functional materials. In particular, the “pure” and mixed rare earth cobaltites and ferrites are used in thermoelectric devices; solid oxide fuel cells, as membranes for partial oxidation of methane; and cleaning oxygen, as catalysts and sensory materials [1–5]. The interest in the rare earth cobaltites RCoO3 is also stimulated by their unique fundamental physical properties, such as different types of magnetic ordering and temperature-induced metal-insulator (MI) transitions conjugated with the spin-state transitions of Co3+ ions [6, 7]. These transitions are strongly affected by the chemical pressure caused by the exchange of cations either in A- or B-sites of perovskite structure [8–10].
Among the mixed rare earth cobaltites-ferrites RCo1 − x Fe x O3, the most extensively studied is a system with La [10–12], whereas information about phase and structural behaviour in the systems with other rare earths is rather limited. Our recent investigations of structural and thermal behaviour of the mixed cobaltites-ferrites with R=Pr, Nd, Sm and Eu obtained by a standard ceramic technique at 1200–1300 °C [13–16] proved a formation of the continuous solid solution with the orthorhombic perovskite structure. In situ high-temperature X-ray synchrotron powder diffraction revealed strong anomalies in the lattice expansion, which are especially pronounced in cobalt-rich specimens. They are reflected in a sigmoidal dependence of the unit cell dimensions, in extra increment of the unit cell volume and in clear maxima of the thermal expansion coefficients [16–19]. These anomalies are related to the changes in spin state of Co3+ ions and conjugated MI transitions. They become less pronounced with the decreasing of the cobalt content in the RCo1 − x Fe x O3 series.
Here, we report the results of structural characterization of nanocrystalline cobaltites-ferrites PrCo1 − x Fe x O3 prepared by sol-gel citrate route.
Nanocrystalline powders of PrCo1 − x Fe x O3 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) were prepared by sol-gel citrate method. Crystalline Pr(NO3)3·6H2O (99.99 %, Alfa Aesar), Co(NO3)2·6H2O (ACS, Alfa Aesar), Fe(NO3)3·9H2O (ACS, Alfa Aesar) and a citric acid (CC) were dissolved in water and mixed in the molar ratio of n(Pr3+):n(Co2+):n(Fe3+):n(CC) = 1:(1 − х):х:4 according to the PrCo1 − x Fe x O3 nominal compositions. Prepared solutions were gelled at ~90 °C and subsequently treated at the temperatures of 700 and 800 °C for 2 h. Thus, two series of the samples were obtained. Spot-check examination of the cationic composition of the samples was performed by energy dispersive X-ray fluorescence (EDXRF) analysis by using XRF Analyzer Expert 3L.
Laboratory X-ray powder diffraction investigation was performed on the Huber imaging plate Guinier camera G670 (Cu K α1 radiation, λ = 1.54056 Å). The high-resolution X-ray synchrotron powder diffraction examination was performed for the PrCo0.5Fe0.5O3@700 °C and PrCo0.5Fe0.5O3@800°C samples with equiatomic amount of Co and Fe. Corresponding experiments were carried out at the beamline ID22 of ESRF (Grenoble, France) during the beamtime allocated to the Experiment N° MA-2320. All crystallographic calculations were performed by means of the programme package WinCSD , which was also used for the evaluation of microstructural parameters of the samples. The average grain size of the powders (D) and lattice strains <ε> = <Δd>/d were estimated from the analysis of angular dependence of the X-ray diffraction (XRD) profile broadening by using the external Si standard for the correction of instrumental broadening. The morphology of the nanoaggregates was investigated by scanning electron microscopy (SEM) by means of an ESEM FEI Quanta 200 FEGi system operated in a low-vacuum mode (70 Pa) and at an acceleration voltage of 15 kV (FEI Company, Eindhoven, NL). The samples were mounted onto conductive carbon tapes adhered on aluminium holders. High-resolution images were obtained using an Everhart-Thornley detector (ETD) for secondary electrons or a solid-state backscattered electron (SSD-BSE) detector.
Full profile Rietveld refinement, performed in space group Pbnm, led to an excellent agreement between calculated and experimental profiles for all PrCo1 − x Fe x O3 samples. In the refinement procedure, the unit cell dimensions and positional and displacement parameters of atoms were refined together with background and peak profile parameters and correction of absorption and instrumental zero shift. No significant difference in the structural parameters was found between two series of the samples.
Precise high-resolution X-ray synchrotron powder diffraction examination confirms phase purity of PrCo0.5Fe0.5O3 samples obtained at 700 and 800 °C. No traces of foreign phases were detected in both samples even applying this very sensitive diffraction technique.
In spite of superb resolution (typical HWFM of the reflections of Si standard are in the limits of 0.003–0.006 2θ o), no reflection splitting was observed at the PrCo0.5Fe0.5O3@700°C and PrCo0.5Fe0.5O3@800°C diffraction patterns due to the rather pseudo-cubic metric of the orthorhombic lattice and essential nanocrystalline size effect on the XRD line broadening.
Lattice parameters, coordinates and displacement parameters of atoms in PrCo1 − x Fe x O3 (space group Pbnm)
x in PrCo1 − x Fe x O3
B iso, Å2
B iso, Å2
B iso, Å2
B iso, Å2
Two series of the nanocrystalline mixed cobaltites-ferrites PrCo1 − x Fe x O3 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) of high phase purity were prepared by sol-gel citrate method at 700 and 800 °C. The average grain size of the powders estimated from the analysis of angular dependence of the XRD line broadening varies between 30 and 155 nm, depending on the composition and synthesis temperature. Refined structural parameters of the PrCo1 − x Fe x O3@700 °C and PrCo1 − x Fe x O3@800 °C series prove the formation of continuous solid solution as it was shown earlier for the similar series obtained by the standard ceramic technique at 1300 °C. In comparison with a traditional energy- and time-consuming high-temperature solid-state synthesis technique, the low-temperature sol-gel citrate method is a very promising tool for the obtaining of fine powders of the mixed perovskite oxide materials, free of contamination of constituent metal oxides or other parasitic phases.
The work was supported in parts by the Ukrainian Ministry of Education and Sciences (Project “KMON”) and ICDD Grant-in-Aid programme. The authors thank A. Fitch for the kindly assistance with high-resolution synchrotron powder diffraction measurements at ID22 of ESRF during the beamtime allocated to the Experiment MA-2320. The authors thank N. Koval for the EDXRF analysis.
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