Phase Equilibria in the PrO_x–CoO_x–NiO System, Structure, and Oxygen Content in the Formed Oxides

Abstract

Using the homogenizing annealing method and X-ray powder diffraction, it was shown that in the quasi-ternary PrO_x–CoO–NiO system at 1373 K in air, two series of solid solutions are formed: PrCo$_{{1 - x}}$Ni_xO₃ (0.0 ≤ x ≤ 0.4) with the orthorhombically distorted perovskite structure and Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ (0.7 ≤ y ≤ 1.5) with the Ruddlesden–Popper type structure with n = 3. The oxygen content in both series of solid solutions is close to stoichiometric. It is assumed that Ni²⁺ ions are predominantly located in octahedra located in the middle of the perovskite block, while Ni³⁺ and Co³⁺ ions are in octahedra adjacent to the rock salt layers. Phase diagrams of the PrO_x–CoO_x and PrO_x–NiO systems were constructed in “T–composition” coordinates in air using literature data.

INTRODUCTION

Rare earth nickelates with a perovskite-like structure are promising materials for use as cathodes in fuel cells [1–13]. Unlike oxide systems based on other 3d-transition metals (for example, Co, Fe, Mn), rare earth nickelates formed in Ni-containing systems and belonging to the Ruddlesden–Popper homologous series Ln$_{{n + 1}}$Ni_nO$_{{3n + 1}}$ can be obtained in air without introducing acceptor-type substituents into the A-sublattice (for example, alkaline earth metal cations). Among other rare earth elements, praseodymium attracts particular attention, as it exhibits high catalytic activity in redox reactions involving oxygen. Partial replacement of nickel with cobalt in the B-sublattice leads to increased stability of oxides with the ABO₃ perovskite structure, as well as an increase in electrical conductivity, electrochemical activity of oxides, and acceleration of oxygen exchange processes between the solid and gas phases [14–25]. However, the available information on selected compositions with the simultaneous presence of nickel and cobalt in the resulting oxides with a perovskite-like structure does not provide a complete understanding of the phase equilibria in the PrO_x–NiO–CoO_x system. Another important parameter responsible for many functional properties is the oxygen content in the oxides. The available information concerning oxygen content in oxides in the aforementioned system is insufficient. Thus, the purpose of this work is to establish phase equilibria in the PrO_x–NiO–CoO system at 1373 K in air and determine the oxygen content in the resulting phases.

Phase equilibria in the constituent binary oxide systems have been studied quite well earlier.

PrO_x–CoO_х system. The results of studying phase equilibria in the PrO_x–CoO_x system [26–29] in a wide range of temperatures and oxygen pressures allow to conclude that only PrCoO₃ with the orthorhombically distorted perovskite structure (a = 5.3390 Å, b = 5.3513 Å, c = 7.5762 Å, sp. gr. Pbnm [30]) is stable in air conditions, while other cobaltites Pr₄Co₃O₁₀ and Pr₂CoO₄, belonging to the Ruddlesden-Popper homologous series, are stable at lower oxygen pressures.

PrO_x–NiO system. The thermodynamic stability of nickelates is shifted towards higher oxygen pressures compared to cobalt-containing analogues. A single-phase oxide with a perovskite structure PrNiO₃, is obtained only when using oxygen pressures increased relative to air [2, 31–34], while phases with a Ruddlesden–Popper type structure, Pr₂NiO₄ and Pr₄Ni₃O₁₀, can be obtained in an air conditions [3, 5, 7, 9, 11–13, 27–29, 35]. Synthesis of PrNiO₃ at 973 K in air does not allow obtaining a single-phase product; the sample contained large amounts of unreacted initial oxides PrO_x and NiO [36]. The difficulty of preparing single-phase PrNiO₃ in air is associated with kinetic reasons, since its thermal stability at ${{P}_{{{{{\text{O}}}_{2}}}}}$ = 0.21 atm is limited by relatively low temperatures for solid-phase processes. The single-phase sample obtained at higher ${{P}_{{{{{\text{O}}}_{2}}}}}$ found to be stable in an air atmosphere up to 1223 K [2]. At approximately 873 K, PrNiO₃ undergoes a reversible phase transition from a high-temperature rhombohedral cell (sp. gr. R‑3c) to a low-temperature orthorhombic cell (sp. gr. Pnma) [2]. When the temperature increased above 1223 K, at the first stage the decomposition products of PrNiO₃ were NiO and Pr₄Ni₃O₁₀, and above 1328 K, the latter, in turn, decomposed into Pr₂NiO₄ and NiO [2]. The stability of Pr₂NiO₄ at relatively low temperatures has also been the subject of researches [13, 37, 38]. The temperature of oxidative decomposition of Pr₂NiO₄ into Pr₄Ni₃O₁₀ and PrO_x in air lies in the range of 1123–1173 K.

CoO–NiO system. A continuous series of (Co,Ni)O solid solutions is formed in the CoO–NiO system at T > 1173 K in air [39].

PrO_x–CoO_x–NiO system. Several solid solutions in which cobalt and nickel are partially substituted for each other have been previously described. There is information about oxides with the perovskite structure PrCo$_{{1 - x}}$Ni_xO$_{{3 - \delta }}$ with nickel content x = 0.4 [24], x = 0.5 [16, 20], x = 0.6 [18], 0 ≤ x ≤ 0.7 [40], although not all works indicate the preparation conditions. On the side of nickelates with a Ruddlesden–Popper type structure, the oxides Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ with у = 0.3 [25, 41] and Pr₂Ni$_{{1 - y}}$Co_yO$_{{4 + \delta }}$, with y = 0.1–0.2 [14, 15, 21, 22] have been described. The boundaries of the homogeneity ranges of existing solid solutions, which undoubtedly depend on temperature, have not been determined to date.

EXPERIMENTAL

The samples were synthesized using the glycerol-nitrate method. Praseodymium oxide Pr₆O₁₁, metallic cobalt Co, and crystalline nickel acetate hydrate Ni(CH₃COO)₂⋅4H₂O were used as starting materials. To remove adsorbed moisture and gases, Pr₆O₁₁ was pre-calcined for 12 h at a temperature of 698 K; then it was removed from the heated furnace, cooled in a desiccator and weighed in closed beakers of known mass. The required masses of other components were calculated from the known mass of Pr₆O₁₁. Metallic cobalt was obtained by reduction of cobalt oxide Co₃O₄ in a hydrogen flow at 898 K. Weighed amounts of the starting components were dissolved in nitric acid when heated. An equimolar amount of glycerol was added to the resulting solution and heated until complete evaporation. The dry residue was slowly heated to 1373 K and kept at this temperature for 12 h. All subsequent anneals with intermediate grinding in ethyl alcohol after every 12 h were carried out at 1373 K. The total annealing time was 72 h. At the end of the annealing, the samples were quenched by abruptly removing them from the heated furnace onto a massive metal copper plate in air. The approximate cooling rate was 500 K/min.

The phase composition of the samples was controlled by XRD using an Inel Equinox 3000 and a Shimadzu XRD 7000 diffractometers in CuK_α radiation. The structural parameters were refined using the Rietveld full-profile analysis method in the Fullprof program.

The melting points of samples in the PrO_x–CoO system were determined by visual polythermal analysis using a platinum/platinum-rhodium thermocouple (PP-1).

The change in oxygen content in the samples with varying temperature was determined by thermogravimetric analysis using a STA 409 PC thermal analyzer. The absolute value of oxygen content was determined by the direct reduction of samples using a mixture of gases (90% H₂ + 10% N₂) at 1373 K in a TG cell to Pr₂O₃ and metallic Co and Ni. The completeness of reduction at the end of the experiment was monitored by XRD.

RESULTS AND DISCUSSION

Phase Diagrams of the PrO_x–CoO_х and PrO_x–NiO Systems

Although phase equilibria in the binary systems have been the subject of numerous studies, phase diagrams of these systems in air have not been presented previously. Based on our experimental data on the melting temperatures in the PrO_x–CoO system together with the results of homogenizing annealing at various temperatures and the available scattered literature data on the thermal stability of intermediate phases in air: PrCoO₃ [26–30, 40], PrNiO₃ [2, 10, 32, 36], Pr₄Ni₃O₁₀ [3, 9, 35, 41, 42], and Pr₂NiO₄ [5–7, 10, 13, 37, 38], we constructed phase diagrams of the PrO_x–CoO_х and PrO_x–NiO systems in the “T–composition” coordinates in air (Figs. 1 and 2). The relative mole fraction of the metallic component is a traditional way of expressing concentration in systems where cations can reveal different oxidation states in coexisting phases at fixed T and/or ${{P}_{{{{{\text{O}}}_{2}}}}}$. The oxygen content in the condensed phases cannot be calculated from the diagram, but is assigned in accordance with its real value.

Fig. 1.

Phase diagram of the PrO_x–CoO_x system in air.

Fig. 2.

Phase diagram of the PrO_x–NiO system in air.

The crystal structure of PrCoO₃ quenched from 1373 K to room temperature was indexed in an orthorhombic cell sp. gr. Pbnm with unit cell parameters: a = 5.3775(1) Å, b = 5.3439(1) Å, c = 7.5773(2) Å, which is in good agreement with the data available in the literature [40]. The only thermodynamically stable oxide at 1373 K in the PrO_x–NiO system is Pr₂NiO₄, which crystallizes in an orthorhombic structure (sp. gr. Fmmm) with the unit cell parameters: a = 5.3976(1) Å, b = 5.4527(1) Å, c = 12.4397(1) Å. After annealing at 1373 K in air the samples which overall composition corresponded to PrNiO₃ and Pr₄Ni₃O₁₀, were mixtures of two phases: Pr₂NiO₄ and NiO. Annealing of Pr₂NiO₄ in air at T ≥ 1123 K indicates its oxidative decomposition into Pr₄Ni₃O₁₀ and PrO_x [13, 37, 38].

PrCo$_{{1 - x}}$Ni_xO₃ Solid Solutions

XRD of samples with nominal composition PrCo$_{{1 - x}}$Ni_xO₃ in the range 0 ≤ x ≤ 0.5 (∆x = 0.05), quenched from 1373 K to room temperature, confirmed the formation of single-phase solid solutions in the composition range 0 ≤ x ≤ 0.4. The structural parameters of the solid solutions are listed in Table 1. Starting from x = 0.45, lines of impurity phases such as Ruddlesden–Popper type with n = 3 and NiO were detected in the XRD patterns.

Table 1.   Structural parameters of the PrCo$_{{1-x}}$Ni_xO₃ solid solution

The oxygen content in all the obtained solid solutions is close to stoichiometric and practically does not change when the temperature is varied from room temperature to 1373 K, which corresponds to the oxidation state of 3+ for cobalt and nickel ions and is in good agreement with the results obtained in other works [40, 43, 44]. Practically linear increase in the unit cell parameters with increasing nickel content (Fig. 3) suggests that cobalt is in a low-spin state r(${\text{Co}}_{{{\text{VI}}}}^{{3 + }}$, LS) = 0.545 Å, r(${\text{Co}}_{{{\text{VI}}}}^{{3 + }}$, HS) = 0.61 Å, r(${\text{Ni}}_{{{\text{VI}}}}^{{3 + }}$, LS) = 0.56 Å, r(${\text{Ni}}_{{{\text{VI}}}}^{{3 + }}$, HS) = 0.6 Å [45].

Fig. 3.

Unit cell parameters of PrCo$_{{1 - x}}$Ni_xO₃ at various nickel content (x).

Examples of XRD patterns for the samples whose composition lies outside the homogeneity range are shown in Fig. 4. It should be noted that at a high nickel content (x = 0.9), the perovskite phase is no longer formed, and the coexisting phases are a Ruddlesden–Popper type solid solution with n = 3, Pr₂NiO₄, and NiO.

Fig. 4.

XRD patterns of multi-phase samples with nominal composition PrCo$_{{1 - x}}$Ni_xO₃ at various x values.

Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ Solid Solutions

Although the decomposition temperature of Pr₄Ni₃O₁₀ in air is estimated to be approximately 1323 K, partial replacement of nickel by cobalt leads to stabilization of the Ruddlesden–Popper type structure with n = 3 at 1373 K. According to the XRD results of quenched samples, single-phase Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ solid solutions were obtained in the composition range 0.7 ≤ y ≤ 1.5. A lower cobalt content in the solid solution (y = 0.3) was achieved by the authors of [25, 41] by using lower annealing temperatures (T = 1223 K). All obtained solid solutions have a monoclinic structure (sp. gr. P121/c1), similar to that reported in the literature for unsubstituted praseodymium nickelate Pr₄Ni₃O₁₀ [42] and a solid solution with y = 0.3 [41]. Studying the effect of temperature on the crystal structure of a solid solution with у = 0.3, the authors of [25] found that at T > 873 K the monoclinic phase begins to gradually transform into a tetragonal phase (sp. gr. I4/mmm), and both phases coexist in a fairly wide temperature range (923–1173 K). Without sufficient grounds to talk about the reasons for this behavior, of which there may be several, Berger et al. [25] chose not to discuss them.

The oxygen content in Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ (Table 2), similar to solid solutions with a perovskite structure, is also not very different from the stoichiometric one, and its change with temperature is also not too large. Table 2 also shows the average oxidation states of 3d metals (z).

Table 2.   Oxygen content and average oxidation states of 3d metal ions (z) in Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ (y = 0.7, 0.9, 1.1, 1.2, 1.4) at 298 and 1373 K

It is obvious that with the simultaneous presence of Co and Ni in the oxide, cobalt ions will predominantly be in the Co³⁺ state, while nickel ions will tend to lower their oxidation state to Ni²⁺. Taking this fact and the oxygen content into account, the formulas of solid solutions were presented as follows:

298 K	1373 K
${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{0.98}}^{{2 + }}{\text{Ni}}_{{1.32}}^{{3 + }}{\text{Co}}_{{0.7}}^{{3 + }}{{{\text{O}}}_{{10.01}}}$,	${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.14}}^{{2 + }}{\text{Ni}}_{{1.16}}^{{3 + }}{\text{Co}}_{{0.7}}^{{3 + }}{{{\text{O}}}_{{9.93}}}$,
${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{0.94}}^{{2 + }}{\text{Ni}}_{{1.16}}^{{3 + }}{\text{Co}}_{{0.9}}^{{3 + }}{{{\text{O}}}_{{10.03}}},$	${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.04}}^{{2 + }}{\text{Ni}}_{{1.06}}^{{3 + }}{\text{Co}}_{{0.9}}^{{3 + }}{{{\text{O}}}_{{9.98}}},$
${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{0.98}}^{{2 + }}{\text{Ni}}_{{0.92}}^{{3 + }}{\text{Co}}_{{1.1}}^{{3 + }}{{{\text{O}}}_{{10.01}}}$,	${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.06}}^{{2 + }}{\text{Ni}}_{{0.84}}^{{3 + }}{\text{Co}}_{{1.1}}^{{3 + }}{{{\text{O}}}_{{9.97}}},$
${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{0.92}}^{{2 + }}{\text{Ni}}_{{0.88}}^{{3 + }}{\text{Co}}_{{1.2}}^{{3 + }}{{{\text{O}}}_{{10.04}}},$	${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.00}}^{{2 + }}{\text{Ni}}_{{08}}^{{3 + }}{\text{Co}}_{{1.2}}^{{3 + }}{{{\text{O}}}_{{10.00}}},$
${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.02}}^{{2 + }}{\text{Ni}}_{{0.58}}^{{3 + }}{\text{Co}}_{{1.4}}^{{3 + }}{{{\text{O}}}_{{9.99}}}$,	${\text{P}}{{{\text{r}}}_{4}}{\text{Ni}}_{{1.1}}^{{2 + }}{\text{Ni}}_{{0.5}}^{{3 + }}{\text{Co}}_{{1.4}}^{{3 + }}{{{\text{O}}}_{{9.95}}}$.

The crystal structure of oxides of the homologous Ruddlesden–Popper series with n = 3 (А₄В₃О₁₀) is an alternation of three perovskite blocks (ABO₃) and a layer of rock salt (AO) along the c axis. According to the crystallographic approach of the sum of valence bonds method, the distribution of doubly and triply charged cations with simultaneous presence in the oxide is as follows: the former will stay in the center of octahedra located between two layers of perovskite, and the latter—in octahedra located near the rock salt layer. Thus, it should be assumed that Ni²⁺ ions, which content is approximately 1/3 of the total number of cations in B-sites, are located in the “internal” octahedra of the perovskite block. The remaining part of the nickel ions, with a predominant oxidation state of 3+, and Co³⁺ ions are located in the perovskite layers adjacent to the rock salt (AO) layer. The amounts of Ni³⁺ and Co³⁺ ions in these layers are comparable. Since phases with a Ruddlesden–Popper type structure in the PrO_x–CoO system are not stable in air (in rare earth and cobalt oxides, the latter, under the studied conditions, tends to take Co³⁺ state and, therefore, form the perovskite structure LnCo³⁺O₃), an increase in the content of ions cobalt in layers near the rock salt layer becomes thermodynamically unfavorable. Further introduction of cobalt into the system leads to separation, and along with the “cobalt-saturated” boundary solution Pr₄Ni_1.5Co_1.5O$_{{10-\delta }}$, two more phases are formed: PrCo$_{{1 - x}}$Ni_xO₃ perovskite and PrO_x. Thus, the gradual replacement of nickel with cobalt at the first stage stabilizes the А₄В₃О₁₀-type structure due to the appearance of triply charged Co³⁺ ions, which, together with nickel ions, fill octahedrons near the rock salt layers. On the other hand, an increase in the proportion of cobalt ions makes it more advantageous to form a perovskite structure, which provides a higher degree of oxidation of 3d metals in the B sublattice. Similar observations about the determining role of the nature of 3d metals on the stability of certain perovskite-like phases and changes in phase equilibria in systems based on rare earth elements and 3d transition metals were reported earlier [46].

Phase Diagram of the PrO_x–CoO–NiO System at 1373 K in Air

An analysis of phase equilibria in the PrO_x–CoO–NiO system at 1373 K in air was carried out based on the XRD results of 47 quenched samples. The phase diagram in the form of a Gibbs triangle is shown in Fig. 5. Here, as in quasi-binary systems, the composition is presented in terms of the relative mole fraction of metal components. The phase composition in the fields of phase diagram is presented in Table 3.

Fig. 5.

Phase diagram of the PrO_x–CoO–NiO system in air at 1373 K and ${{P}_{{{{{\text{O}}}_{2}}}}}$ = 0.21 atm.

Table 3.   Phase composition of fields in the phase diagram of the PrO_x–CoO–NiO system at 1373 K in air

The composition of the PrCo$_{{1 - x}}$Ni_xO₃ solid solution, dividing it into a part coexisting with PrO_x and a part coexisting with Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$, was determined from the unit cell parameters of the perovskite phase in samples from field II and the concentration dependence of the solid solution unit cell parameters (Fig. 3).

Attempts to partially replace nickel with cobalt in Pr₂NiO$_{{4 + \delta }}$ at 1373 K in air were unsuccessful. The phase composition of all studied samples corresponded to field III of the phase diagram (Fig. 5). It is likely that the preparation of such solutions with a noticeable content of cobalt Pr₂Ni$_{{1 - y}}$Co_yO$_{{4 + \delta }}$ will be possible under more reducing conditions (high temperatures and/or lower ${{P}_{{{{{\text{O}}}_{2}}}}}$ values relative to air).

A comparison of the phase equilibria in the studied system and a similar one based on lanthanum 1/2La₂O₃–CoO–NiO [47] reveals a number of analogies, although there are some differences. The differences are associated with the stability of the Ruddlesden–Popper homologous series phases for different REEs. Three homologues are stable in the lanthanum-containing  system  at  1373  K  in  air: La₂NiO₄, La₃Ni₂O₇, and La₄Ni₃O₁₀ [48–50], therefore, solid solutions with partial replacement of nickel by cobalt start from the 1/2La₂O₃–NiO side. As a result, the homogeneity ranges of such solid solutions are somewhat wider compared to the Pr-containing system: for La₂Ni$_{{1 - y}}$Co_yO$_{{4 + \delta }}$ y_max = 0.1, for La₄Ni$_{{3 - y}}$Co_yO₁₀ y_max = 1.8. Phase equilibria in the 1/2Sm₂O₃–CoO–NiO system [51] with smaller REE confirms the observed trend. The only stable intermediate phase in it is SmCo$_{{1 - x}}$Ni_xO₃ with a homogeneity range of 0 ≤ x ≤ 0.15 [51], while for LaCo$_{{1 - x}}$Ni_xO₃ it is 0 ≤ x ≤ 0.6 [47], and for PrCo$_{{1 - x}}$Ni_xO₃ it has an intermediate value (0.0 ≤ x ≤ 0.4).

CONCLUSIONS

Analysis of phase equilibria in the PrO_x–CoO–NiO system at 1373 K in air showed the presence of two series of solid solutions: PrCo$_{{1 - x}}$Ni_xO₃ (0.0 ≤ x ≤ 0.4) with the structure of an orthorhombically distorted perovskite and Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ (0.7 ≤ y ≤ 1.5) with the Ruddlesden–Popper (n = 3) structure. There is no noticeable substitution of nickel for cobalt in praseodymium nickelate Pr₂NiO$_{{4 + \delta }}$ under these conditions. The obtained solid solutions are practically stoichiometric in oxygen. An increase in the unit cell parameters of PrCo$_{{1 - x}}$Ni_xO₃ with increasing nickel content may indirectly indicate that cobalt ions are in a low-spin state. In the Pr₄Ni$_{{3 - y}}$Co_yO$_{{10 - \delta }}$ structure, Ni²⁺ ions are settled in octahedra located in the middle of the perovskite block, while Ni³⁺ and Co³⁺ are placed in octahedra adjacent to the rock salt layer.