Subsolidus Phase Equilibria in the La₂O₃–(Ni/Со)O–Sb₂O₅ Systems

Abstract

Subsolidus phase equilibria in the La₂O₃–(Ni/Со)O–Sb₂O₅ systems have been studied. A previously unknown compound La₄Sb₂O₁₁ was found to exist in the La₂O₃–Sb₂O₅ system. La₄Sb₂O₁₁ has been shown to decompose at 1060°C to form La₃SbO₇ and LaSbO₄. Two ternary oxides (LaNi₂SbO₆ and La₂NiSb₂O₉) have been discovered in the La₂O₃–NiO–Sb₂O₅ system. These new compounds are stable and do not undergo polymorphic transformations throughout the range of temperatures studied (25–1350°C). The existence of previously known ternary oxides, namely perovskite La₃Ni₂SbO₉ and rosiaite LaNi_1/3Sb_5/3O₆, has been verified. In the La₂O₃–CoO–Sb₂O₅ system, two new compounds (LaCo₂SbO₆ and La₂CoSb₂O₉) have been found along with previously known perovskite La₃Co₂SbO₉, rosiaite LaCo_1/3Sb_5/3O₆, and rhombohedral pyrochlore La₃Co₂Sb₃O₁₄. These new compounds are isostructural to those found in the nickel oxide system. La₂CoSb₂O₉, unlike its nickel analogue, decomposes at 990°C. For LaCo₂SbO₆, no thermal events associated with polymorphic transitions or melting have been detected on DSC curves up to 1350°C. An inspection of diffuse reflectance spectra of the newly synthesized LaNi₂SbO₆, La₂NiSb₂O₉, LaCo₂SbO₆, and La₂CoSb₂O₉ phases showed that the oxidation state of nickel and cobalt in them is 2+. The 1050°C isothermal sections of La₂O₃–(Ni/Co)O–Sb₂O₅ systems have been constructed.

INTRODUCTION

Phase equilibria in multicomponent systems are directly related to the search for new functional materials based on complex oxides and the development of optimal methods for their synthesis. Despite an enormous amount of research work carried out to date into phase equilibria in binary and multicomponent oxide systems, a great many of the systems remain poorly studied. Such are systems comprising antinomy oxide, whose high volatility makes investigations difficult. Meanwhile, many promising functional materials, in particular catalysts, have been created based on antimonates. Various M–Sb–O double oxides (where M = Mg, Ca, Sr, Al, Cr, Fe, Co, Ni, Cu, Zn, Y, Ag, In, La, Pb, or Bi) have shown their efficacy in photocatalytic water splitting [1–8]. Transition-metal antimonates MSb₂O₆, where M = Mn, Fe, Co, and Ni, have been used as low-cost (free of precious metals) catalysts, which are distinguished by their good oxygen binding energy, conductivity, thermal phase stability, and stability in aqueous media. These catalysts can be used in the electrocatalytic reduction of oxygen [3–7] and for the electrochemical release of chlorine [2, 6]. The reason for the catalytic activity of antimonates in redox reactions is the low redox potential of the Sb³⁺ → Sb⁵⁺ transformations [9]. It is due to this feature that iron antimonate FeSbO₄ is an efficient oxidation catalyst in various organic syntheses [10–13]. We showed recently [14–20] that pyrochlore ternary oxides (Ln_1.8Fe_0.2)FeSbO₇ (Ln = Pr–Tb, or Bi) and rosiaites LnFe_0.5Sb_1.5O₆ (Ln = La–Sm) and LaNi_1/3Sb_5/3O₆ exhibit high activity and stability in catalytic CO oxidation and complete methane oxidation.

The study of phase equilibria in antimony-containing multicomponent oxide systems will elucidate previously unknown phases and, possibly, expand the range of promising antimonate-based functional materials. In particular, of interest are La₂O₃–(Ni/Co)O–Sb₂O₅ systems, which have not been studied but the boundary binary systems of which have been in sufficient detail.

In the Ni–Sb–O system, only two mixed oxides exist: NiSb₂O₄ and NiSb₂O₆ [21]. NiSb₂O₆ has the trirutile structure (space group P4₂/mnm) [22, 23]; it is stable in air (at least to 1523°C) and can be used in C₃H₈ and CO sensors [24] and liquefied petroleum gas sensors [25]. NiSb₂O₆ has a metastable rosiaite-type polymorph (space group P$\bar {3}$1m) [26] with the unit cell parameters a = 4.691(7) Å, c = 9.299(3) Å. When heated for more than 1 h at 500°C, however, this compound transforms to a stable phase. NiSb₂O₄ belongs to the Pb₃O₄ structure type (space group P4₂/mbc) [27] and can be prepared only via heating NiO and Sb₂O₃ mixtures in vacuo or in flowing nitrogen [21].

In the Co–Sb–O system, as in the nickel system, there is a trirutile phase [28]. Also known are spinel Co_7/3Sb_2/3O₄ (space group Fd$\bar {3}$m) [29] and pyrochlore Co₂Sb₂O₇ (space group Fd$\bar {3}$m) [30, 31]. A Co_7/3Sb_2/3O₄ phase was prepared from cobalt sulfate and antimony oxide (Sb₂O₃) by annealing at 900°С; Co₂Sb₂O₇ was prepared from cobalt acetate and antimony oxide (Sb₂O₅) at 450°С, but it is yet unknown whether this phase is stable at high temperatures.

The La–Ni–O system was studied in detail at 1200°C [32]. Under air, the system can form LaNiO₃ and Ruddlesden–Popper homologous phases of general formula La_n+1Ni_nO_3n+1 (n is the number of octahedral layers) [33]: La₂NiO₄, La₃Ni₂O₇, and La₄Ni₃O₁₀. Their crystal lattices are formed by the alternation of perovskite layers and NaCl-type layers. LaNiO₃ has the perovskite structure [34]; it is stable below 850°C in air or under an elevated oxygen pressure. Above 850°C, this phase gradually decomposes to La_n+1Ni_nO_3n+1 oxides [32, 33, 35]. According to Kitayama [32], an La₆Ni₅O₁₅ phase exists; it was prepared at 1200°C under air. According to Demina at el. [36], however, a sample of nominal composition La₆Ni₅O₁₅ has an X-ray diffraction pattern that features reflections from two phases: La₄Ni₃O₁₀ and NiO. LaNiO₂ exists between 300 and 400°C and only under a reductive atmosphere [37].

In the La–Co–O boundary system, double oxides are thermodynamically stable phases: perovskite LaCoO₃ and Ruddlesden–Popper phases La₂CoO₄ and La₄Co₃O₁₀ [38]. All phases contain cobalt ions in various degrees of oxidation [39]. The stability of phases depends on the partial oxygen pressure and temperature. Cobalt(+3) phases have higher stability at low temperatures and at higher partial oxygen pressures. When synthesis is performed under an air atmosphere (p(O₂) = 0.2 atm), LaCoO₃ is the only stable compound up to 1600 K.

No systematic inventions have been performed into phase equilibria in the La–Sb–O system in air. There is evidence for the existence of three compounds: La₃SbO₇, LaSbO₄, and LaSb₃O₉ [40–42]. The orthorhombic LaSb₃O₉ phase (space group Cmcm) is stable only below 1100°C and melts incongruently above this temperature to yield a LaSbO₄ phase [40]. LaSbO₄ has a monoclinic structure (space group P2₁/m [43]) and exists up to 1450°С, at which temperature it decomposes to form orthorhombic La₃SbO₇ (space group Cmcm) [41]. The antimony in all of the above-listed compounds is antimony(+5).

In the La₂O₃–NiO–Sb₂O₃ quasi-ternary system, two ternary oxides are currently known to exist: LaNi_1/3Sb_5/3O₆, and La₃Ni₂SbO₉. LaNi_1/3Sb_5/3O₆ has a rosiaite-type layered structure (space group P$\bar {3}$1m) [44, 45]. Layers are formed of six-membered rings of edge-sharing (Ni/Sb)O₆ octahedra, in which Ni²⁺ and Sb⁵⁺ ions are arranged randomly. The La³⁺ ions reside in interlayer spaces, occupying sites on the triple axis in hexagonal-prismatic channels. The unit cell parameters of LaNi_1/3Sb_5/3O₆ are a = 5.26142(4) and с = 5.21945(6) Å. The compound exists up to 1050°С. At higher temperatures, it has not been studied. La₃Ni₂SbO₉ has a monoclinic perovskite-like structure (space group P2₁/n) with the unit cell parameters a = 5.0675(1), b = 5.6380(1), c = 7.9379(2) Å, β = 89.999(6)° at room temperature [46]. Two crystallographically different octahedral sites are occupied randomly by Ni²⁺ and Sb⁵⁺ ions. There is also evidence for the existence of LaNi_1–xSb_xO₃ (0 ≤ x ≤ 1/3) limited solid solutions in the range 850–950°С. Unlike LaNiO₃, which has a rhombohedrally distorted perovskite structure (space group R3c) [47], LaNi_1‒xSb_xO₃ (0.05 ≤ x ≤ 1/3) solid solutions have an orthorhombically distorted perovskite structure (space group Pbmn), in which Ni and Sb ions are distributed randomly over the B lattice sites [48].

Only mixed oxides LaCo_1/3Sb_5/3O₆ [44], La₃Co₂Sb₃O₁₄ [49], and La₃Co₂SbO₉ [50], which occur in the La₂O₃–CoO–Sb₂O₅ system, are described in the literature. LaCo_1/3Sb_5/3O₆ is isostructural to the above-mentioned rosiaite LaNi_1/3Sb_5/3O₆. La₃Co₂Sb₃O₁₄ has a rhombohedrally distorted pyrochlore structure (space group R$\bar {3}$m, а = 7.52954(2) Å, с = 17.59983(6) Å [49]). In its crystal lattice, Co²⁺ ions occupy both the La and the Sb sites in accordance with the formula [La₃Co′][Sb₃Co′′]О₁₄. The La, Sb, and Co ions are distributed orderly. Perovskite La₃Co₂²⁺Sb⁵⁺O₉ (space group P2₁/n) has a monoclinic structure. In La₃Co₂²⁺Sb⁵⁺O₉, cobalt and antimony ions occupy the same octahedral sites [50].

Therefore, the available body of data implies that the La₂O₃–(Ni/Co)О–Sb₂O₃ systems have been studied to an insufficient extent. Here, we will report the results of our study into subsolidus phase equilibria of the La₂O₃–(Ni/Co)O–Sb₂O₅ systems.

EXPERIMENTAL

The synthesis of antimonates is complicated by the high volatility of antimony. Long-term anneals at high temperatures are thereby undesirable. On the other hand, refractory precursors containing lanthanum, nickel, and cobalt react only at high temperatures. Therefore, the citrate method followed by stepwise annealing was chosen to reduce the synthesis temperature. In this synthetic method, the reaction occurs at lower temperatures (compared to those of solid-phase synthesis from oxides), and the differences between the individual behaviors of cations in solution (which lead to the loss of reagents and deviation from the required composition of samples during coprecipitation synthesis) is also leveled out. Other advantages of the citrate method are a high degree of homogenization of the batch and a shorter equilibration time.

The reagents used were La(NO₃)₃∙6H₂O (specialty grade), Ni(NO₃)₂∙6H₂O (pure for analysis grade), Co(NO₃)₂∙6H₂O (pure for analysis grade), and Sb₂O₃ (99%). Ethylene glycol C₂H₆O₂ (pure grade) and citric acid monohydrate (chemically pure grade) were taken in a twofold excess relative to metal ions. The components were mixed and kept in a water bath at 80°C. Dissolution of all components, thickening of the solution, and foaming were observed. After this, the reaction mixture was dried at 110°C until brittle foam formed and then pounded. The powder was first annealed at 350°C (4 h) and 450°C (4 h) to decompose the nitrates and burn out the organic components. The resulting powder was then annealed at 650 (24 h), 900 (24 h), and 1050°C (48 h) in Pt crucibles under air. The stepped annealing ensured the formation of nonvolatile intermediate products, which prevented the loss of antimony.

The phase composition of the samples was determined by X-ray powder diffraction on a Bruker D8 Advance diffractometer (CuK_α radiation, Ni filter, LYNXEYE detector). Phase identification was with reference to the PDF-2 (ICDD) database. The thermal behavior of La₄Sb₂O₁₁ was studied on a NETZSCH DSC 404 F1 differential scanning calorimeter. Experiments were carried out in platinum crucibles with a lid under dry argon (grade 5.5; 99.9995 vol % Ar) atmosphere at a flow rate of 10 K/min.

A diffuse reflectance spectrum in the range of 300–1100 nm was recorded on an Ocean Optics DH-2000 spectrometer equipped with an integrating sphere (50 mm ISP-50-8-R-GT). An HPX-2000 xenon lamp was used as the radiation source.

RESULTS AND DISCUSSION

As shown in the Introduction, phase equilibria in the La₂O₃–Sb₂O₅ system in air are studied insufficiently, unlike those in its La₂O₃–МO and МO–Sb₂O₅ (М = Ni or Co) boundary systems. Therefore, we considered the possibility of the existence of hitherto unknown phases in this system. A number of samples with different La₂O₃ : Sb₂O₅ ratios were synthesized, and a new compound La₄Sb₂O₁₁ was discovered (Fig. 1). The amount of oxygen indicated in the formula of this compound is based on the presence Sb⁵⁺ ions. Interplanar distances in La₄Sb₂O₁₁ are listed in the attachment (Table 1).

Fig. 1.

(a) X-ray diffraction pattern of the La₄Sb₂O₁₁ phase as annealed at 1050°С and (b) X-ray diffraction patterns of samples with various La₂O₃ : Sb₂O₅ ratios. PDF 36-950 (LaSbO₄) and PDF 23-1138 (La₃SbO₇) were used for phase identification.

Table 1.   Interplanar spacings for La₄Sb₂O₁₁

The La₄Sb₂O₁₁ exists up to 1060°С. The DSC heating curve in the range 1000–1500°С features three endotherm effects (Fig. 2a). The first one begins at 1060°C and corresponds to the decomposition of the La₄Sb₂O₁₁ phase to La₃SbO₇ and LaSbO₄ (Fig. 2b). The effect with the onset at 1320°С is due to incongruent melting of LaSbO₄ which results in the formation of a liquid phase. The presence of the liquid creates favorable conditions for the volatilization of antimony oxide; therefore, the composition of the sample upon cooling does not correspond to the original one. This explains the nonappearance of effects on the cooling curve. We have not discovered other phases that would have been previously unknown in the La₂O₃–Sb₂O₅ system.

Fig. 2.

(a) DSC heating and cooling curves for La₄Sb₂O₁₁ and (b) X-ray diffraction patterns of La₄Sb₂O₁₁ as-annealed at various temperatures. PDF 36-950 (LaSbO₄) and PDF 23-1138 (La₃SbO₇) were used for phase identification.

The triangulation of the La₂O₃–(Ni/Со)O–Sb₂O₅ system at 1050°С was carried out by the cross sections methods based on XRD data.

We were the first to discover new ternary oxides in the La₂O₃–NiO–Sb₂O₅ quasi-ternary system: LaNi₂SbO₆ and La₂NiSb₂O₉ (Fig. 3; Tables 2 and 3). These compounds were stable under heating throughout the range of temperatures studied (25–1350°С). The existence of two previously known phases, namely, perovskite La₃Ni₂SbO₉ and rosiaite LaNi_1/3Sb_5/3O₆ [44–46], was verified.

Fig. 3.

(a, b) X-ray diffraction patterns and (c, d) DSC heating curves for ternary oxides: (a, c) LaNi₂SbO₆ and (b, d) La₂NiSb₂O₉.

Table 2.   Interplanar spacings for La₂NiSb₂O₉

Table 3.   Interplanar spacings for LaNi₂SbO₆

The 1050°С isothermal section of the La₂O₃–NiO–Sb₂O₅ system (Fig. 4) features four ternary oxides: LaNi_1/3Sb_5/3O₆, La₃Ni₂SbO₉, LaNi₂SbO₆, and La₂NiSb₂O₉. Since the stable antimony oxide species at 1050°С in air is Sb₂O₄, the area between LaSb₃O₉, NiSb₂O₆, and antimony oxide is the projection of a more complex system: LaSb₃O₉–NiSb₂O₆–Sb₂O₃–Sb₂O₅. We have not studied the La₂O₃–NiO_x–La₃SbO₇ area in detail, as it should be regarded the projection of a quaternary system because of a mixed oxidation state of nickel in this area. We have not found new ternary oxides in the areas belonging to quaternary systems. The La₃SbO₇–NiO–NiSb₂O₆–LaSb₃O₉ section may be represented by 11 triangles of coexisting phases: La₃SbO₇–La₃Ni₂SbO₉–La₄Sb₂O₁₁, La₄Sb₂O₁₁–La₃Ni₂SbO₉–LaSbO₄, La₃Ni₂SbO₉–LaSbO₄–La₂NiSb₂O₉, La₃Ni₂SbO₉–LaNi₂SbO₆–La₂NiSb₂O₉, La₃Ni₂SbO₉–LaNi₂SbO₆–NiO, LaNi₂SbO₆–NiO–NiSb₂O₆, LaNi₂SbO₆–LaNi_1/3Sb_5/3O₆–NiSb₂O₆, LaNi₂SbO₆–LaNi_1/3Sb_5/3O₆–La₂NiSb₂O₉, LaSbO₄–LaNi_1/3Sb_5/3O₆–La₂NiSb₂O₉, LaSbO₄–LaNi_1/3Sb_5/3O₆–LaSb₃O₉, and LaNi_1/3Sb_5/3O₆–NiSb₂O₆–LaSb₃O₉.

Fig. 4.

1050°С isothermal section of the La₂O₃–NiO–Sb₂O₅ system.

Unlike in the nickel-containing system, three compounds have been hitherto known in the La₂O₃–СоO–Sb₂O₅ system: rosiaite LaСо_1/3Sb_5/3O₆, rhombohedrally distorted pyrochlore La₃Со₂Sb₃O₁₄, and perovskite La₃Со₂SbO₉ [44, 49, 50]. In addition to these compounds, we showed the existence of two more compounds, namely La₂СоSb₂O₉ and LaСо₂SbO₆, which are isostructural to those found in the nickel system (Fig. 5; Tables 4 and 5). LaСо₂SbO₆ is stable over the entire range of investigation up to 1350°С. La₂СоSb₂O₉, unlike its isostructural nickel analogue, decomposes at 1003°С. In the La₂O₃–СоO_x–La₃SbO₇ area, no ternary oxides have been found. Therefore, the 1050°С isothermal section of the La₂O₃–CoO–Sb₂O₅ system in the La₃SbO₇–CoO–LaSb₃O₉–CoSb₂O₆ area may be represented by 14 triangles of coexisting phases (Fig. 6): La₃SbO₇–La₃Co₂SbO₉–La₄Sb₂O₁₁, La₄Sb₂O₁₁–La₃Co₂SbO₉–La₃Co₂Sb₃O₁₄, La₃Co₂Sb₃O₁₄–La₄Sb₂O₁₁–La₂CoSb₂O₉, La₄Sb₂O₁₁–La₂CoSb₂O₉–LaSbO₄, La₃Co₂SbO₉–La₃Co₂Sb₃O₁₄–LaCo₂SbO₆, La₃Co₂SbO₉–LaCo₂SbO₆–CoO, LaCo₂SbO₆–CoO–Co₇Sb₂O₁₂, LaCo₂SbO₆–Co₇Sb₂O₁₂–La₃Co₂Sb₃O₁₄, Co₇Sb₂O₁₂–La₃Co₂Sb₃O₁₄–CoSb₂O₆, La₃Co₂Sb₃O₁₄–CoSb₂O₆–LaCo_1/3Sb_5/3O₆, La₃Co₂Sb₃O₁₄–La₂CoSb₂O₉–LaCo_1/3Sb_5/3O₆, La₂CoSb₂O₉–LaSbO₄–LaCo_1/3Sb_5/3O₆, LaSbO₄–LaCo_1/3Sb_5/3O₆–LaSb₃O₉, and LaCo_1/3Sb_5/3O₆–LaSb₃O₉–CoSb₂O₆.

Fig. 5.

(a, b) X-ray diffraction patterns and (c, d) DSC curves of new ternary oxides: (a, c) LaСо₂SbO₆ and (b, d) La₂СоSb₂O₉.

Table 4.   Interplanar spacings for La₂CoSb₂O₉

Table 5.   Interplanar spacings for LaCo₂SbO₆

Fig. 6.

(a) 900 and (b) 1050°С isothermal sections of the La₂O₃–СоO–Sb₂O₅ system.

The diffuse reflectance optical spectra of the new phases (Fig. 7) confirmed that nickel and cobalt are in the +2 oxidation state. The spectrum of octahedrally coordinated Ni²⁺ is known to feature three allowed d–d transitions from the ³A_2g ground level to ³T_2g, ³T_1g, and ³T_1g(P) [51]. In oxides, bands corresponding to these transitions appear in the ranges 7000–10 000, 12 000–16 000, and 22 000–27 000 cm^–1. Bands associated with ³A_2g → ¹E_g and ³A_2g → ¹T_2g spin-forbidden transitions can also be observable. The ¹E_g and ³T_1g states are close in energy to each other. As a result of spin–orbit coupling, the ³A_2g → ¹E_g forbidden transition grows in intensity due to the allowed transition, so a doublet appears in Ni²⁺ spectra in the range 12 000–16 000 cm^–1. The ³A_2g → ¹T_2g spin-forbidden transition usually appears as a weak band at the long-wavelength edge of the ³A_2g → ³T_1g transition. All the above-listed features appear in the LaNi₂SbO₆, La₂NiSb₂O₉, and LaNi_1/3Sb_5/3O₆ spectra (Fig. 7). LaNi_1/3Sb_5/3O₆ is shown as a reference. This indicates that nickel ions in these compounds are in the +2 oxidation state.

Fig. 7.

Diffuse reflectance spectra in the visible of (a) LaNi₂SbO₆, and La₂NiSb₂O₉ and (b) LaСо₂SbO₆ and La₂СоSb₂O₉.

The spectra of octahedrally coordinated Со²⁺ usually feature two strong bands in the ranges 8000–10 000 and 16 000–20 000 cm^–1 referring to ⁴T_1g(P) → ⁴T_2g and ⁴T_1g(P) → ⁴T_1g allowed transitions, respectively [51]. The ⁴T_1g(P) → ⁴T_1g band, as a rule, is a multiplet due to an admixture of spin-forbidden transitions. In the range 11 000–14 000 cm^–1, a less strong ⁴T_1g(P) → ⁴A_2g transition is observed; it frequently appears as a shoulder at the low-energy side of the ⁴T_1g(P) → ⁴T_1g transition band. The LaСо₂SbO₆ and La₃Со₂SbO₉ spectra, just as the La₃Со₂Sb₃O₁₄ spectrum given as a reference, have all the listed features, indicating the presence of Co²⁺ ions in an octahedral surrounding in these compounds. The different degrees of splitting of the observed transitions are explained by the structural specifics of the compounds.

Therefore, the fact that the nickel and cobalt are Ni²⁺ and Co²⁺ in all the prepared ternary oxides, implies that, indeed, the above-considered La₃SbO₇–NiO–NiSb₂O₆–LaSb₃O₉ and La₃SbO₇–CoO–LaSb₃O₉–CoSb₂O₆ areas belong to the La₂O₃–NiO–Sb₂O₅ and La₂O₃–СоO–Sb₂O₅ systems, respectively.

CONCLUSIONS

Altogether, the La₂O₃–(Ni/Со)O–Sb₂O₅ systems are not completely alike. They implement four pairs of isostructural compounds: La(Ni/Co)_1/3Sb_5/3O₆, La₃(Ni/Со)₂SbO₉, La(Ni/Со)₂SbO₆, and La₂(Ni/Со)Sb₂O₉; in the cobalt oxide system, however, one more compound exists: La₃Со₂SbO₉. The above-listed compounds differ from one another in (Ni/Со)²⁺/Sb⁵⁺ cation ratios and have different structures. The known ternary oxides were characterized previously. Some of them were shown to have functional properties that allowed them to be considered promising for use. For example, the rosiaite compound LaNi_1/3Sb_5/3O₆ exhibited high catalytic activity in СО oxidation [20]. Therefore, a further study of the new phases first synthesized in this work would be of undoubted interest.