Structure Peculiarities of Micro- and Nanocrystalline Perovskite Ferrites La1−x Sm x FeO3
© The Author(s). 2017
Received: 25 January 2017
Accepted: 22 February 2017
Published: 27 February 2017
Micro- and nanocrystalline lanthanum-samarium ferrites La1−x Sm x FeO3 with orthorhombic perovskite structure were obtained by using both solid state reactions (x = 0.2, 0.4, 0.6 and 0.8) and sol-gel synthesis (x = 0.5) techniques. Obtained structural parameters of both series of La1−x Sm x FeO3 are in excellent agreement with the “pure” LaFeO3 and SmFeO3 compounds, thus proving formation of continuous solid solution in the LaFeO3–SmFeO3 system. Peculiarity of La1−x Sm x FeO3 solid solution is divergence behaviour of unit cell dimensions with increasing x: systematic decrease of the a and c lattice parameters is accompanied with increasing b parameter. Such behaviour of the unit cell dimensions in La1−x Sm x FeO3 series led to crossover of the a and c perovskite lattice parameters and formation of dimensionally tetragonal structure near x = 0.04. Linear decrease of the unit cell volume of La1−x Sm x FeO3 with decreasing x according with the Vegard’s rule indicate absence of short-range ordering of R-cations in the LaFeO3–SmFeO3 system.
The interest in the rare earth ferrites RFeO3 (R = rare earths) is stimulated by their unique properties, such as high electrical conductivity, specific magnetic properties including spin reorientation phenomena, as well as significant electrochemical and catalytic activity. RFeO3-based materials are used as electrode materials in solid oxide fuel cells [1, 2], as membranes for gases separation, sensory materials and catalysts [3–6], and as magnetic and multiferroic materials [7–10].
Among RFeO3 compounds, lanthanum and samarium orthoferrites are two of the most studied materials because of combination of several intrigue properties [10–13]. At the ambient conditions, both LaFeO3 and SmFeO3 display the orthorhombic perovskite structure isotypic with GdFeO3 [14, 15]. In situ high-resolution X-ray synchrotron and neutron powder diffraction examination revealed no structural changes in SmFeO3 in the temperature range of 300–1173 K , whereas LaFeO3 undergoes the first-order orthorhombic-to-rhombohedral structural phase transition at 1253–1260 K [17–19]. Lattice expansion of LaFeO3 and SmFeO3 shows non-linear and strongly anisotropic thermal behaviour: in both compounds relative expansion in b-direction is much lower than in a- and c-directions [16, 18–20]. As a result, lattice parameter crossovers occur in LaFeO3 at 750–950 K [18–20]. Subtle anomalies in the lattice expansion detected in LaFeO3 and SmFeO3 are associated with antiferromagnetic—to paramagnetic phase transition occurred in these compounds at 735 and 670 K, respectively [11, 12, 16, 21]. In LaFeO3, such anomalies are reflected in non-linear lattice expansion across the magnetic phase transition at the Néel temperature 735 K  and in the step of dilatometric thermal expansion coefficient at 723 ± 50 K. In SmFeO3, the b parameter exhibits a small anomalous kink around 670 K that is indicative for magnetoelastic coupling at the magnetic ordering temperature T N . Similar sign of magnetoelastic coupling was recently detected in the mixed ferrite system SmFeO3–PrFeO3, in which subtle maxima at the thermal expansion curves were observed in Sm0.5Pr0.5FeO3 at around 670 K .
The aim of the present work is synthesis of phase pure micro- and nanocrystalline powders of lanthanum-samarium orthoferrites La1−x Sm x FeO3 and their detailed structural investigation in whole concentration range.
For a preparation of nanocrystalline powders of nominal composition, La0.5Sm0.5FeO3 sol-gel citrate method was used. Crystalline salts La(NO3)3 · 6H2O (99.99%, Alfa Aesar), Sm(NO3)3 · 6H2O (ACS, Alfa Aesar) and Fe(NO3)3 · 9H2O (ACS, Alfa Aesar) and citric acid (CC) were dissolved in water and mixed in the molar ratio of n(La3+):n(Sm2+):n(Fe3+):n(CC) = 1:0.5:0.5:4 according to the nominal composition of the sample. Prepared solution was gelled at ~90 °C and heat treated at 1073 K for 2 h. After X-ray diffraction (XRD) examination, the part of the powder was additionally annealed at 1173 K for 2 h and then at 1473 K for 4 h. Thus three La0.5Sm0.5FeO3 specimens, synthesized at different conditions, were obtained.
X-ray phase and structural characterization of the samples were performed by using Huber imaging plate Guinier camera G670 (Cu K α1 radiation, λ = 1.54056 Å). Spot-check examination of the cationic composition was performed by energy dispersive X-ray fluorescence (EDXRF) analysis by using XRF Analyzer Expert 3L. Based on the experimental powder diffraction data, the unit cell dimensions and positional and displacement parameters of atoms in the La1−x Sm x FeO3 structures were derived by full profile Rietveld refinement technique using software package WinCSD . This programme package was also used for the evaluation of microstructural parameters of La0.5Sm0.5FeO3 powders from angular dependence of the Bragg’s maxima broadening. Average grain size, D ave and microstrains <ε> = <Δd>/d were derived both by full profile Rietveld refinement and by using Williamson-Hall analysis, which allows to separate the effect of size and strain broadening due to their different dependence on the scattering angle. In both cases, LaB6 external standard was used for the correction of instrumental broadening. The morphology of sol-gel derived La0.5Sm0.5FeO3 samples synthesized at different conditions was investigated by means of Hitachi SU-70 scanning electron microscope.
Results and Discussion
Lattice parameters, coordinates and displacement parameters of atoms in La1−x Sm x FeO3 structures
x = 0.2
x = 0.4
x = 0.5a, 1073 K
x = 0.5a, 1473 K
x = 0.6
x = 0.8
B iso, Å2
B iso, Å2
B iso, Å2
B iso, Å2
Such strongly anisotropic behaviour of the unit cell dimensions in La1−x Sm x FeO3 series is explained by crystal structure peculiarities of the end members of the system—LaFeO3 and SmFeO3. In spite of both compounds belong to the same GdFeO3-type of crystal structure (space group Pbnm), they show different order of the perovskite cell parameters: b p > a p > c p for LaFeO3 and b p > c p > a p for SmFeO3. Consequently, a crossover of a p- and c p-parameters and formation of dimensionally tetragonal structure occurs in La1−x Sm x FeO3 series near x = 0.04 (Fig. 4). Similar phenomena of the lattice parameters crossover were earlier observed in the mixed cobaltite-ferrites PrCo1-x Fe x O3 and NdCo1-x Fe x O3 [25, 26], as well as in the related rare earth aluminates and gallates R 1-x R’ x AlO3 and R 1-x R’ x GaO3 [27–30], in which the end members of the systems show different relations of the lattice parameters. In spite of the observed peculiarities lattice parameters behaviour, the unit cell volume in La1−x Sm x FeO3 series decreases almost linearly with decreasing R-cation radii according to the Vegard’s rule. This observation indicates statistical distribution of La and Sm species over positions of R-cations in La1−x Sm x FeO3 perovskite lattice and absence of short-range ordering in LaFeO3–SmFeO3 system.
Single-phase micro- and nanocrystalline ferrites La1−x Sm x FeO3 with orthorhombic perovskite structure were prepared by solid state reactions (x = 0.2, 0.4, 0.6 and 0.8) and sol-gel citrate route (x = 0.5). The lattice parameters and coordinates and displacement parameters of atoms in La1−x Sm x FeO3 structures, as well as microstructural parameters of La0.5Sm0.5FeO3 nanopowders were derived from X-ray powder diffraction data by full profile Rietveld refinement technique. Obtained structural parameters of both solid state and sol-gel synthesized ferrites La1−x Sm x FeO3 agree well and prove the formation of continuous solid solution in LaFeO3–SmFeO3 pseudo-binary system. Peculiarity of La1−x Sm x FeO3 solid solution is divergence behaviour of unit cell dimensions with increasing samarium content and crossover of the a and c perovskite lattice parameters near x = 0.04. In comparison with a traditional energy- and time-consuming high-temperature ceramic technique, the low-temperature sol-gel citrate method is very promising tool for a synthesis of fine powders of the mixed perovskite oxide materials, free of contamination of parasitic phases.
The work was supported in parts by the Ukrainian Ministry of Education and Sciences (Project “RZE”), ICDD Grant-in-Aid program and by the Polish National Science Center (Project 2015/17/B/ST5/01658).
OP synthesized the samples by solid state reactions technique, contributed to the data evaluation and wrote the manuscript. LV performed the laboratory X-ray powder diffraction measurements, made the structural characterization of the samples and contributed to the manuscript writing. IL performed the sol-gel synthesis of the samples. NK performed the examination of the cationic composition of the samples by energy dispersive X-ray fluorescence (EDXRF) analysis. YZ and AP performed the scanning electron microscopy measurements and contributed to the manuscript writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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