Background

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 [16], 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 [18] 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 [16]. 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 [22].

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.

Methods

Micro- and nanocrystalline samples of the mixed lanthanum-samarium ferrites were prepared by two different experimental routes. Samples with nominal compositions La1−x Sm x FeO3 (x = 0.2, 0.4, 0.6 and 0.8) were obtained by solid state reactions technique. Precursor oxides La2O3, Sm2O3 and Fe2O3 were ball-milled in ethanol for 5 h, dried, pressed into pellets and annealed in air at 1473 K for 40 h with one intermediate regrinding. The synthesis of La1−x Sm x FeO3 can be presented by following reaction scheme:

$$ \left(1- x\right){\mathrm{La}}_2{\mathrm{O}}_3+ x{\mathrm{Sm}}_2{\mathrm{O}}_3+{\mathrm{Fe}}_2{\mathrm{O}}_3\to 2{\mathrm{La}}_{1- x}{\mathrm{Sm}}_x{\mathrm{Fe}\mathrm{O}}_3 $$

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 [23]. 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

X-ray phase and structural analysis revealed that all La1−x Sm x FeO3 samples synthesized by solid state method at 1473 K for 40 h adopt orthorhombic perovskite structure isotypic with GdFeO3. No additional crystalline phases were found. Full profile Rietveld refinement, performed in space group Pbnm, shows excellent agreement between experimental and calculated diffraction patterns (Fig. 1) thus proving phase purity and crystal structure of the samples.

Fig. 1
figure 1

Graphical results of Rietveld refinement of the La0.6Sm0.4FeO3 structure. Experimental X-ray powder diffraction pattern (red dots) is shown in comparison with the calculated pattern (blue line). The difference between measured and calculated profiles is shown as a curve below the diagrams. Short vertical bars indicate the positions of diffraction maxima in space group Pbnm. Inset shows the view of the structure as corner-shared FeO6 octahedra with La/Sm species located between them

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X-ray powder diffraction examination of sol-gel derived La0.5Sm0.5FeO3 sample shows that even short-term heat treatment of the dried xerogel at 1073 K for 2 h led to formation of pure perovskite structure, without any traces of precursor components or other parasitic phases (Fig. 2). Substantial broadening of the diffraction maxima observed at the XRD pattern of La0.5Sm0.5FeO3@1073 sample clearly indicates the nanoscale particle size of the as-obtained product.

Fig. 2
figure 2

XRD patterns of La0.5Sm0.5FeO3 samples obtained at different conditions. Inset shows evolution of microstructural parameters vs the temperature and duration of heat treatment of the specimens

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Indeed, evaluation of microstructural parameters of the La0.5Sm0.5FeO3@1073 sample from the analysis of the XRD profile broadening by full profile Rietveld technique lead to the average grain size D ave = 78 nm and microstrains <ε> = < Δd>/d = 0.17%. Additional heat treatment of the sample at 1173 and 1473 K does not affects on the phase composition and crystal structure parameters of the sample; the main changes occurs in the microstructural parameters, as it is evidenced from the significant narrowing of the Bragg’s maxima, especially pronounced in La0.5Sm0.5FeO3@1473 sample (Fig. 2). Evolution of the average grain sizes and microstrains in La0.5Sm0.5FeO3 specimens vs synthesis temperature (Fig. 2, inset) clearly shows systematic increase of the average grain sizes, D ave, accompanied with simultaneous reducing of the lattice strains. The D ave values increases weakly from 78 to 103 nm after additional annealing at 1173 K for 2 h, whereas further heat treatment of the sample at 1473 K for 4 h lead to the drastic increase of the crystallite size up to >2000 nm. Similar evolution of the microstructural parameters of La0.5Sm0.5FeO3 was obtained from the Williamson-Hall analysis (Fig 3). No obvious selective hkl-dependent peak broadening was observed for the samples heat treated at different temperatures.

Fig. 3
figure 3

Graphical results of the Williamson-Hall analysis of La0.5Sm0.5FeO3 microstructure parameters

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Scanning electronic microscopy of the pristine La0.5Sm0.5FeO3, obtained at 1073 K, revealed sheets-like morphology of the powder consisting of the particles of irregular form with linear dimensions 200–500 nm (Fig. 4a). Taking into account that average grain size of La0.5Sm0.5FeO3@1073 sample derived from XRD data is 51–73 nm, it is evident that the particles observed at SEM picture of the sample consist of several smaller crystallites. SEM examination also confirms the temperature evolution of microstructural parameters of La0.5Sm0.5FeO3, derived from the X-ray powder diffraction data. As it is evidenced from Fig. 4b, additional heat treatment of the La0.5Sm0.5FeO3 sample at 1173 K for 2 h leads to the particle agglomeration and formation of 1–5 μm agglomerates, consisting of several submicron particles. Finally, further heat treatment of the sample at 1473 K for 4 h lead to coalescence of small grains and particles and formation of 10–50 μm crystallites with clear signs of the facet growth (Fig. 4c, d).

Fig. 4
figure 4

SEM pictures of La0.5Sm0.5FeO3 synthesized at 1073 K (a), 1173 K (b) and 1473 K (c, d)

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Refined values of unit cell dimensions and positional and displacement parameters of atoms for sol-gel-derived La0.5Sm0.5FeO3 samples, heat treated at 1073 and 1473 K, as well as the La1−x Sm x FeO3 specimens with x = 0.2, 0.4, 0.6 and 0.8, obtained by traditional ceramic technology are presented in Table 1.

Table 1 Lattice parameters, coordinates and displacement parameters of atoms in La1−x Sm x FeO3 structures
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Structural parameters of the mixed ferrites La1−x Sm x FeO3 synthesized by different experimental techniques agree well with the parent LaFeO3 and SmFeO3 compounds [14, 15], as well as with the lattice parameters for Sm-doped LaFeO3 recently reported [24]. An analysis of the concentration dependence of unit cell dimensions in La1−x Sm x FeO3 series clearly proves the formation of continuous solid solution in the LaFeO3–SmFeO3 pseudo-binary system. The lattice parameters of La1−x Sm x FeO3 change systematically between LaFeO3 and SmFeO3 showing divergence behaviour with increasing x: gradual decrease of the a- and c-parameters is accompanied with detectable increasing b parameter (Fig. 5).

Fig. 5
figure 5

Concentration dependencies of unit cell dimensions of La1−x Sm x FeO3. Orthorhombic lattice parameters and unit cell volume are normalized to the perovskite ones as follows: a p   = a o /√2, b p   = b o /√2, c p   = c o /2, V p   = V o /4. The dashed lines are polynomial fits: a p   = 3.9295(9) − 0.115(4) × x + 0.004(4) × x 2 ; b p  = 3.935(1) + 0.0119(5) × x + 0.0119(4) × x 2 ; c p  = 3.9278(8) −0.067(3) × x − 0.006(3) × x 2 . Arrow indicates the lattice crossover region

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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.

Conclusions

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.