Structure Peculiarities of Micro- and Nanocrystalline Perovskite Ferrites La1−xSmxFeO3

Micro- and nanocrystalline lanthanum-samarium ferrites La1−xSmxFeO3 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−xSmxFeO3 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−xSmxFeO3 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−xSmxFeO3 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−xSmxFeO3 with decreasing x according with the Vegard’s rule indicate absence of short-range ordering of R-cations in the LaFeO3–SmFeO3 system.


Background
The interest in the rare earth ferrites RFeO 3 (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. RFeO 3based materials are used as electrode materials in solid oxide fuel cells [1,2], as membranes for gases separation, sensory materials and catalysts [3][4][5][6], and as magnetic and multiferroic materials [7][8][9][10].
Among RFeO 3 compounds, lanthanum and samarium orthoferrites are two of the most studied materials because of combination of several intrigue properties [10][11][12][13]. At the ambient conditions, both LaFeO 3 and SmFeO 3 display the orthorhombic perovskite structure isotypic with GdFeO 3 [14,15]. In situ high-resolution X-ray synchrotron and neutron powder diffraction examination revealed no structural changes in SmFeO 3 in the temperature range of 300-1173 K [16], whereas LaFeO 3 undergoes the first-order orthorhombic-to-rhombohedral structural phase transition at 1253-1260 K [17][18][19]. Lattice expansion of LaFeO 3 and SmFeO 3 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][19][20]. As a result, lattice parameter crossovers occur in LaFeO 3 at 750-950 K [18][19][20]. Subtle anomalies in the lattice expansion detected in LaFeO 3 and SmFeO 3 are associated with antiferromagnetic-to paramagnetic phase transition occurred in these compounds at 735 and 670 K, respectively [11,12,16,21]. In LaFeO 3 , 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 SmFeO 3 , 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 SmFeO 3 -PrFeO 3 , in which subtle maxima at the thermal expansion curves were observed in Sm 0.5 Pr 0.5 FeO 3 at around 670 K [22].
The aim of the present work is synthesis of phase pure micro-and nanocrystalline powders of lanthanumsamarium orthoferrites La 1−x Sm x FeO 3 and their detailed structural investigation in whole concentration range.

Micro
For a preparation of nanocrystalline powders of nominal composition, La 0.5 Sm 0.5 FeO 3 sol-gel citrate method was used. Crystalline salts La(NO 3 ) 3 · 6H 2 O (99.99%, Alfa Aesar), Sm(NO 3 ) 3 · 6H 2 O (ACS, Alfa Aesar) and Fe(NO 3 ) 3 · 9H 2 O (ACS, Alfa Aesar) and citric acid (CC) were dissolved in water and mixed in the molar ratio of n(La 3+ ):n(Sm 2+ ):n(Fe 3+ ):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 La 0.5 Sm 0.5 FeO 3 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 La 1−x Sm x FeO 3 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 La 0.5 Sm 0.5 FeO 3 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, LaB 6 external standard was used for the correction of instrumental broadening. The morphology of sol-gel derived La 0.5 Sm 0.5 FeO 3 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 La 1−x Sm x FeO 3 samples synthesized by solid state method at 1473 K for 40 h adopt orthorhombic perovskite structure isotypic with GdFeO 3 . 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.
X-ray powder diffraction examination of sol-gel derived La 0.5 Sm 0.5 FeO 3 sample shows that even shortterm 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 La 0.5 Sm 0.5 FeO 3 @1073 sample clearly indicates the nanoscale particle size of the as-obtained product.
Indeed, evaluation of microstructural parameters of the La 0.5 Sm 0.5 FeO 3 @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 La 0.5 Sm 0.5 FeO 3 @1473 sample (Fig. 2). Evolution of the average grain sizes and microstrains in La 0.5 Sm 0.5 FeO 3 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 La 0.5 Sm 0.5 FeO 3 was obtained from the Williamson-Hall analysis (Fig 3). No obvious selective hkldependent peak broadening was observed for the samples heat treated at different temperatures.
Scanning electronic microscopy of the pristine La 0.5 Sm 0.5-FeO 3 , 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 La 0.5 Sm 0.5 FeO 3 @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 La 0.5 Sm 0.5 FeO 3 , derived from the X-ray powder diffraction data. As it is evidenced from Fig. 4b, additional heat treatment of the La 0.5 Sm 0.5 FeO 3 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).
Refined values of unit cell dimensions and positional and displacement parameters of atoms for sol-gel-derived La 0.5 Sm 0.5 FeO 3 samples, heat treated at 1073 and 1473 K, as well as the La 1−x Sm x FeO 3 specimens with x = 0.2, 0.4, 0.6 and 0.8, obtained by traditional ceramic technology are presented in Table 1.
Structural parameters of the mixed ferrites La 1−x Sm x FeO 3 synthesized by different experimental techniques agree well with the parent LaFeO 3 and SmFeO 3 compounds [14,15], as well as with the lattice parameters for Sm-doped LaFeO 3 recently reported [24]. An analysis of the concentration dependence of unit cell dimensions in La 1−x Sm x FeO 3 series clearly proves the formation of continuous solid solution in the LaFeO 3 -SmFeO 3 pseudo-binary system. The lattice parameters of La 1−x Sm x FeO 3 change systematically between LaFeO 3 and SmFeO 3 showing divergence behaviour with increasing x: gradual decrease of the aand c-parameters is accompanied with detectable increasing b parameter (Fig. 5).     Such strongly anisotropic behaviour of the unit cell dimensions in La 1−x Sm x FeO 3 series is explained by crystal structure peculiarities of the end members of the system--LaFeO 3 and SmFeO 3 . In spite of both compounds belong to the same GdFeO 3 -type of crystal structure (space group Pbnm), they show different order of the perovskite cell parameters: b p > a p > c p for LaFeO 3 and b p > c p > a p for SmFeO 3 . Consequently, a crossover of a p -and c p -parameters and formation of dimensionally tetragonal structure occurs in La 1−x Sm x FeO 3 series near x = 0.04 (Fig. 4). Similar phenomena of the lattice parameters crossover were earlier observed in the mixed cobaltite-ferrites PrCo 1-x Fe x O 3 and NdCo 1-x Fe x O 3 [25,26], as well as in the related rare earth aluminates and gallates R 1-x R' x AlO 3 and R 1-x R' x GaO 3 [27][28][29][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 La 1−x Sm x FeO 3 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 La 1−x Sm x FeO 3 perovskite lattice and absence of short-range ordering in LaFeO 3 -SmFeO 3 system.

Conclusions
Single-phase micro-and nanocrystalline ferrites La 1−x Sm x FeO 3 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 La 1−x Sm x FeO 3 structures, as well as microstructural parameters of La 0.5 Sm 0.5 FeO 3 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 La 1−x Sm x FeO 3 agree well and prove the formation of continuous solid solution in LaFeO 3 -SmFeO 3 pseudobinary system. Peculiarity of La 1−x Sm x FeO 3 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 hightemperature ceramic technique, the low-temperature solgel citrate method is very promising tool for a synthesis of fine powders of the mixed perovskite oxide materials, free of contamination of parasitic phases.