Phase equilibria, crystal structure and oxygen nonstoichiometry of the complex oxides in Sm – (Sr, Ba) – (Co, Fe) – O systems

Present paper contains available information on the phase equilibria in the Sm – (Sr, Ba) – (Co, Fe) – O systems, including the synthesis routes used, crystal structure, which is often depended on oxygen nonstoichiometry, the data on thermodynamic stability of complex oxides, the obtained results on the homogeneity ranges of solid solutions, formed in the systems, and graphical presentation of phase relations in a form of phase diagrams.

Since the overall information concerning the crystal structure, phase equilibria, phase stability, oxygen nonstoichiometry and defect structure constitutes the physicochemical basis of the preparation and usage of these materials, it is vitally important.Thus, the present work was aimed to overview the available data concerning the phase equilibria within the Sm -(Sr, Ba) -(Co, Fe) -O system, as well as the crystal structure and oxygen nonstoichiometry of the complex oxides formed in these systems.

Phase equilibria in Sm -Co -O system
A systematic study of phase equilibria in the Sm -Co -O system was performed for the first time by Kropanev et al [20,21], and later by Kitayama [22].Samarium cobaltate SmCoO 3 was the only phase found to exist in this system.This complex oxide was first described by Wold and Ward [23] as perovskite type with the cubic structure (a = 3.75±0.01Å), although later it was suggested that the ideal perovskite structure is orthorhombically distorted (Table 1) [22,[24][25][26][27][28].
Different techniques have been used for preparation of SmCoO 3 : a conventional ceramic technique from oxides [20,21], or from mixture of nitrates dried from their solution [24,27], or from the mixture of cobalt carbonate and samarium nitrate [23]; via co-precipitation from the nitrates solution by Na 2 CO 3 with following annealing in air [22].The mechanism and kinetics of solid state SmCoO 3 synthesis from oxides has been studied in [29][30][31][32].It was shown that the diffusion stage of synthesis occur by transport of Co n+ (n = 2, 3) and O 2-ions through the layer of product to the reaction zone that is located on the SmCoO 3 -Sm 2 O 3 interphase boundary [29,30].The kinetics of synthesis depends on the grain size, oxygen partial pressure and compacting pressure of oxides mixture for both Sm 2 O 3 -CoO and Sm 2 O 3 -Co 3 O 4 systems [30,31].
Samarium cobaltate SmCoO 3 is stable in air up to the incongruent melting point equal to 1344±4 °C [21].The subsolidus part of the "T -composition" phase diagram for the Sm -Co -O system in air is shown in Fig. 1.
Thermodynamic properties and stability ranges measured by means of EMF technique in the galvanic cells with solid electrolyte are presented in [20,21,33,34] and those measured by thermogravimetric (TGA) method -in [22].The equilibrium oxygen partial pressure for the reaction: examined in the galvanic cell with the solid electrolyte (ZrO 2 doped by Y 2 O 3 ) can be written as follows [20]: where is a standard pressure, and standard Gibbs energy corresponding to the reaction (1) is expressed by the equations: Ref. [33]: Ref. [34]: The standard Gibbs energy of formation from elements ΔG °f (SmCoO 3 ) was presented in [33] by following equation: The cross sections of phase diagram for the Sm -Co -O system corresponding to the different fixed parameters are shown in Figs.2-4.
It was shown that despite of Sm 2 O 3 / Fe 2 O 3 ratio SmFeO 3-δ always appears as the first product in the initial stages of synthesis within the temperature range 700-1300 °C; the reaction rate was greater in the mixtures with the iron oxide excess [46].This is consistent with the fact that most of the samples with the nominal Fig. 2. The cross sections of phase diagram for the Sm -Co -O system at fixed metal ratio (ε Sm ): a -0.75; b -0.5 and c -0.33.Filled circles -single-phase, half-filled circles -double-phase samples [20].Dashed lines are SmCoO 3 decomposition oxygen partial pressure calculated from [34] composition of Sm 3 Fe 5 O 12 fired at 700 °C contained SmFeO 3-δ as the impurity phase, even if the citrate technique had been used as preparation method [43].Taking into account these findings, the temperature of final synthesis' anneals in order to get a single phase samarium ferrite with garnet structure has to be high enough (≥ 1200 °C).The unit cell parameters for the samarium ferrites SmFeO 3-δ and Sm 3 Fe 5 O 12 are listed in Table 2.
A detailed study of crystal structure performed on the single crystal of Sm  3.
The heat capacity anomaly that was detected for SmFeO 3 at 673 K and for Sm 3 Fe 5 O 12 at 560 K was attributed to the second-order magnetic order  disorder transformation [36].

Phase equilibrium in Sm -Sr -Co -O system
Two types of solid solutions were found to exist in the Sm -Sr -Co -O system: with the perovskite structure and with the K 2 NiF 4 type structure.
The perovskite-type solid solutions Sm 1-х Sr x CoO 3-δ can be prepared by the conventional ceramic technique [1,[50][51][52] or through the solution precursors me thods [53][54][55][56][57][58][59] within the temperature range 900-1200 °C in air or in the oxygen flow.It should be noted that using of conventional ceramic technique often yields the samples contaminated by small amounts of impurities, for example, they were detected in the Sm 0.5 Sr 0.5 CoO 3-δ sample after annealing at 1100 °C in air for 240 h.On the contrary, using the solution precursors routes allow to obtain single-phase samples much faster.Fig. 6.Thermodynamic stability of the SmFeO 3 and Sm 3 Fe 5 O 12 , constructed from the data in [36] (straight line); circle points are taken from [47], square points are calculated from [48,49] The synthesis conditions, structure type and unit cell parameters for the various Sm 1-х Sr х CoO 3-δ compositions are listed in Table 4.
Sm-enriched Sm 1-х Sr х CoO 3-δ (0 < х < 0.5) obtained at 1200 °C in air possesses the perovskite structure with orthorhombic [1,52] or tetragonal [53] distortions.The increase of strontium content leads to the decrease of orthorhombic distortions [1].It should be noted that annealing temperature not less than 1200 °C is important since the samples Sm 1-х Sr х CoO 3-δ with x ≤ 0.40 annealed at 1100 °C in air for 300 h were double-phase.
Depending on the preparation conditions, Sr-enriched samples could be obtained either with tetragonal (2a×2a×4a) Table 4 The synthesis conditions, structure type and unit cell parameters for the various Sm  Сontinuation of table 4 [54,59] or cubic [1,50] structure.One can see that the samples with the tetragonal structure appear in the relatively more oxidizing conditions and the samples with the cubic structures formed in the relatively more reducing conditions (Table 4).Fig. 8 illustrates the XRD pattern for Sr 0.8 Sm 0.2 CoO 3-δ with the 2a p ×2a p ×4a p superstructure.
Electron diffraction measurements uncovered the formation of 2a p ×2a p ×4a p superstructure (sp.gr.I4/mmm) within the tetragonal cell, but the intensity of reflections corresponding to the superstructure decreases with the increase of strontium content [54,55].The superstructure forms because of the ordering of Sm and Sr cations in the A-site sublattice accompanied by the ordering of oxygen vacancies.For Sr 1-x Sm x CoO 3-δ with x < 0.25, Sm atoms are first incorporated into the A1 position until substitution is complete, while the A2 and A3 sites remain fully occupied by Sr 2+ .Further increase of samarium content leads to the incorporation of Sm cations into the A3 position, while A1 is fully occupied by Sm 3+ and A2 is completely filled with Sr 2+ .The value of Sm content, x = 0.5, corresponding to the limiting composition of solid solution, represents the situation when half of A3 positions are occupied by Sm 3+ and the other half -by Sr 2+ [54,59].
Thermodynamic stability of the Sm 1-х Sr х CoO 3-δ solid solutions has not been studied yet.Usually partial substitution of alkaline-earth elements for rare-earth in the cobaltites with the perovskite structure decreases their thermodynamic stability [60].The only information concerning the behavior of Sm 0.5 Sr 0.5 CoO 3-δ under extremely reducing conditions at low temperature is available [61] [56,62], or at 1450 K in oxygen flow [63], or by the EDTA-citrate sol-gel method at 1000 °C in oxygen flow [64], or by the glycerinnitrate technique at 1100 °C in air [59].The homogeneity range of Sm 2-х Sr x CoO 4 solid solutions, estimated by EDX analysis, was reported as 0.79 ≤ х ≤ 1.68 [63].The samples quenched in air from 1100 °C were single-phase within the range 0.7 ≤ x ≤ 1.1 [59].The unit cell parameters for Sm 2-х Sr x CoO 4 are listed in Table 5.
Another representative of the Ruddlesden-Popper series Sm 2 SrCo 2 O 7 was reported earlier [64].It was prepared from Sm 2 O 3 , SrCO 3 and Co 2 O 3 by solid state synthesis at 1450 K in the flow of oxygen for 3 days [64].According to the powder X-ray diffraction measurements, it possesses the tetragonal structure with the unit cell parameters: a = 0.3801 nm, c = 1.9562 nm, V = 0.2826 nm 3 .It was shown that thermal stability of this phase is limited.The X-ray diffraction of the sample after heating at 1550 K for 6 h indicated that the compound decomposes to SmSrCoO 4 and SmCoO 3 [64].It worth to mention that Sm 2 SrCo 2 O 7 formation was not confirmed during the systematic study of phase equilibria in the ½ Sm 2 O 3 -SrO -CoO system at 1100 °C in air.
The projection of isothermal-isobaric phase diagram for the Sm -Sr -Co -O system to the compositional triangle ½ Sm 2 O 3 -SrO -CoO is shown in Fig. 9 [59].
The unit cell parameters for Sm 1-x Sr x FeO 3-δ are listed in Table 6.
Single-phase Sr 2-y Sm y FeO 4±δ samples were synthesized by the glycine nitrate route [56] or by the solid-state technique [62] with the final annealing temperature within the range 1000-1250 °C.The homogeneity range of Sr 2-y Sm y FeO 4±δ solid solution was reported to be equal to 0.5 ≤ y ≤ 1.2 [62].

Phase equilibrium in Sm -Ba -Co -O system
Relatively large difference in ionic radii between samarium and barium, in comparison with that between samarium and strontium, results in the formation of socalled "112 type" phase with the formula SmBaCo 2 O 6-δ [72][73][74] instead of solid so-lution that is typical for the Sr-containing system.The structure of SmBaCo 2 O 6-δ is also called as double perovskite since Sm and Ba atoms are separated to the alternating layers along the c axis.Therefore, the value of the c parameter is doubled  relatively to the ordinary perovskite structure, and the unit cell can be represented as a p ×a p ×2a p .Another specific feature of this structure that is caused by the cation separation is the location of oxygen vacancies.It is generally acknowledged that oxygen vacancies are not distributed randomly in the lattice while the oxygen content changes within the range 5 < (6-δ) < 6, but are concentrated in the particular planes.According to the most widespread point of view, oxygen vacancies are located in the SmO δ planes while BaO planes remain completed [74][75][76], however, alternatively the opposite model was suggested in [77].Such accumulation of oxygen vacancies in the specific planes (doesn't matter what they are -either SmO δ or BaO δ ) results in the ordering of oxygen vacancies when the value of (6-δ) is equal approximately to 5.5, leading to the doubling of b-parameter and formation of the a p ×2a p ×2a p supercell.
SmBaCo 2 O 6-δ can be prepared by a conventional ceramic technique [3,74,[78][79][80][81] and via solution methods using different precursors [82][83][84].It possesses the orthorhombic structure (space group Pmmm) with the a p ×2a p ×2a p supercell.The value of oxygen content at room temperature in the sample slowly cooled in air was found to be 5.61 [81].This value corresponds to the orthorhombic structure.The X-ray diffraction pattern for SmBa-Co 2 O 5.61 refined by the Rietveld analysis is shown in Fig. 10 and the structural parameters are listed in Table 7.
The samples within the compositional range Sm 1-x Ba x CoO 3-δ with x < 0.5 annealed at 1100 °C in air were doublephase and consisted of SmBaCo 2 O 5.61 and SmCoO 3-δ , while the samples with x > 0.5 were the mixtures of SmBaCo 2 O 5.61 and BaCoO 3-δ [72,73].
High temperature in situ XRD measurements reveals the structural transfor-Table 7 The unit cell parameters and atomic coordinates for SmBaCo 2 O 5.61 [81] Space group Pmmm mation from orthorhombic to tetragonal cell between 450 and 550 °C (Fig. 11) that is in good agreement with the value of oxygen content in SmBaCo 2 O 6-δ within this temperature range.Temperature dependence of unit cell parameters for SmBaCo 2 O 6-δ is shown in Fig. 12.
Although the radius of samarium is significantly larger than radius of cobalt ions, it was found that the solid solutions represented by the formula BaCo 1-z Sm z O 3-δ can be prepared by citrate-nitrate method at 1100 °C in air within the range 0.1≤ z ≤0.2.Partial substitution of Sm for Co stabilized the cubic structure similarly to BaCo 1-z Y z O 3-δ [85].Fig. 13 illustrates XRD pattern for the single-phase cubic solid solution BaCo 0.85 Sm 0.15 O 3-δ as an example.The unit cell parameters refined by the Rietveld method are listed in Table 8.The sample with nominal composition z = 0.05 con-sisted of cubic BaCo 0.9 Sm 0.1 O 3-δ and hexagonal BaCoO 3-δ .
One more complex oxide with the formula Sm 2 BaCo 2 O 7 representing the Ruddlesden-Popper (RP) (n=2) phase was reported to exist in the Sm -Ba -Co -O system [63,86].It was obtained by solidstate reaction from Sm 2 O 3 , BaCO 3 and Co 2 O 3 at 1300 K in the flow of oxygen for 2 weeks.The crystal structure was described by the orthorhombic cell with the parameters a = 3.821 Å, b = 3.776 Å and c = 19.426Å [85].However, Gillie et al. [87] using same preparation method with prolonged annealing in flowing oxygen at 1100 °C did not obtain the single phase but the mixture composed of two distinct phases: an oxy- Rietveld method [72] genated 112-type phase SmBaCo 2 O 5+x (x ≈ 0.5), and double-layered RP target compound.From a set of obtained results it was concluded that the composition of RP phase is probably close to Sm 2.1 Ba 0.8 Co 2.1 O 7-δ (where δ ≈ 1).The unit cell parameters refined within the Pnnm space group were equal to a = 5.4371(4) Å, b = 5.4405(4) Å, and c = 19.8629(6)Å [87].
The only one complex oxide Sm 2 BaO 4±δ was described in the Sm -Ba -O system [73,88,89].It can be prepared as a single phase by a conventional ceramic technique at 1500 °C in air for about 24 h [88].Sm 2 BaO 4±δ demonstrates low stability at room temperature due to high hygroscopicity and reactivity with CO 2 [88,89].However, DTA curves in the temperature range 950-1400 °C in air indicated no phase transitions occurred.The presumed space group is Pbna with the lattice parameters a = 12.313 Å, b = 10.535Å, c = 3.564 Å [88].The standard Gibbs energy of Sm 2 BaO 4 formation from the binary oxides Sm 2 O 3 and BaO, determined by the hightemperature CaF 2 -based EMF method, was evaluated as -110 kJ/mol at 1100 K [89].
According to the XR results, partial dissolution of BaO in Sm 2 O 3 at 1100 °C in air was about 15 mol% [72].The unit cell parameters for the Sm 2-x Ba x O 3 solid solutions are listed in Table 9.
The phase diagram for the Sm -Ba -Co -O system at 1100 °C in air [72] is shown in Fig. 14.According to the obtained results, it could be assumed that RP phase is thermodynamically unstable at 1100 °C in air but could be synthesized in more oxidizing conditions.
The crystal structure of SmBaFe 2 O 6-δ is well described within the tetragonal or the orthorhombic unit cell (a p ×a p ×2a p ), depending on the oxygen content [90][91][92][93].Similarly to the Co-containing double perovskite, the appearance of the (a p ×2a p ×2a p ) supercell takes place in the vicinity of oxygen content equal to 5.5.The values of unit cell parameters and synthesis conditions for SmBaFe 2 O 6-δ are listed in Table 10.
The complex oxide SmBa 2 Fe 3 O 8+δ can be obtained at 500 °C in oxygen flow [93] or at 1100 °C for 200 h [94].The structural refinements were performed by the Rietveld method within the ideal perovskite cubic structure (space group Pm3m).

Phase equilibrium in Sm -Co -Fe -O system
The solid solutions between samarium ferrite and samarium cobaltite Sm-Fe 1-x Co x O 3-δ were extensively studied [52,[97][98][99][100][101][102] because of their possible application as gas sensors.Polycrystalline samples of SmFe 1-x Co x O 3-δ can be prepared by the pyrolysis of cyanide complexes [97,99], sol-gel method [98,100,102] or conventional solid-state technique [52] at 800-1100 °C.It was shown that the homogeneity range of SmFe 1-x Co x O 3-δ solid solutions extended to the entire range of compositions (0 ≤ x ≤ 1).Similarly to the undoped parent oxides SmFeO 3-δ and SmCoO 3-δ , the structure of all SmFe 1-x Co x O 3-δ solid solutions was identified as orthorhombic.The unit cell parameters and unit cell volume values are listed in Table 11 [98,102].
Another solid solution in the Sm -Fe -Co -O system was obtained by partial substitution of Sm for Fe in the cobalt ferrite CoFe    within the range 0 ≤ y ≤ 0.5 [106] by coprecipitation by sodium hydroxide with following annealing at 800 °C.However, further increase of temperature (> 800 °C) led to the decomposition of CoFe 2-y Sm y O 4 (y > 0.1 [105,107] or y ≥ 0.3 [106]) with a decrease in Sm content and formation of SmFeO 3 as a secondary phase.The increase of calcination temperature up to 1000 °C resulted in the complete decomposition of the CoFe 2-y Sm y O 4 solid solution even with y = 0.1 [107].The phase equilibria in the Sm-Fe-Co-O system at T = 1100 °C in air is presented in Fig. 16 [102] in the form of the isothermal-isobaric projections to the compositional triangle.

3 12 a
Fe 5 O 12 within the range 20 ≤ T, K ≤ 297 reveals the second order phase transition at 68 and 40 K [44].The coefficients for the temperature dependency of unit cell parameter for Sm 3 Fe 5 O

Table
The values of unit cell parameter of orthorhombically distorted SmCoO 3 (Pbnm space group)

Table
The values of unit cell parameter of SmFeO 3-δ and Sm 3 Fe 5 O 12 Fig. 5. Phase equilibria in the Fe -Fe 2 O 3 -Sm 2 O 3 system at 1200 °C (mol%) [35].Numbers in the figure mean values of -log pO 2 at which three crystalline phases are in equilibrium state.Letters R, P, G, and M represent stoichiometric compositions of Sm 2 O 3 , SmFeO 3 , Sm 3 Fe 5 O 12 , and Fe 3 O 4 , respectively.M 1 is the end member of the magnetite solid solution with chemical composition Fe 2.957 O 4 .P ss , W ss , and M ss are the solid solutions of SmFeO 3 from P to P 1 , of FeO from W to W 2 , and Fe 3 O 4 from M to M 1 , respectively.W and W 2 are the end members of the wustite solid solution with chemical compositions FeO 1.049 and FeO 1.166 , respectively.P 1 is nonstoichiometric perovskite phase SmFeO 2.982 .

Table 6
The unit cell parameters for the Sr 1-x Sm x FeO 3-δ solid solution

Table 9
The unit cell parameters for the Sm 2-x Ba x O 3 solid solutions

Table 11
[98,102] cell parameters and unit cell volume of the SmFe 1-x Co x O 3-δ solid solutions[98,102]