Widespread Solid Solutions of Hexahydrated Cobalt and Nickel

Article pubs.acs.org/jced

Widespread Solid Solutions of Hexahydrated Cobalt and Nickel Nitrates: Solid−Liquid Equilibria at 273.15 and 303.15 K and Characterization of Mixed Crystals of Homogeneous Composition Angélique Teyssier,*,† Bahija El Goundali,‡ Mohammed Kaddami,‡ Jean-Jacques Counioux,† and Christelle Goutaudier† †

Laboratoire des Multimatériaux et Interfaces, UMR CNRS 5615, Université de Lyon, Université Claude Bernard Lyon 1, F-69622 Villeurbanne, France ‡ Laboratoire de Physico-chimie des Procédés et des Matériaux, Université Hassan 1 Faculté des Sciences et Techniques, B.P: 577. Settat, Morocco S Supporting Information *

ABSTRACT: The isothermal solubility curves of the H2O−Co(NO3)2− Ni(NO3)2 ternary system were studied at 273.15 and 303.15 K, using a synthetic method based on conductivity measurements. The analysis of the equilibrium phases’ composition was conducted by UV−visible spectroscopy. The solid phases were first characterized by powder X-ray diffraction, which showed the formation of two substitutional solid solutions: Co(1−σ)Niσ(NO3)2·6H2O, which has a monoclinic structure, and Ni(1−ω)Coω(NO3)2·6H2O, which has a triclinic structure. Then the evolution of the lattice parameters of the crystals at 303.15 K, depending on the solid solution composition, was studied by single-crystal X-ray diffraction. The shrinkage of the monoclinic lattice increases with the increase of the nickel ions in the structure. The stability area of this solid solution is very wide as more than 7 cobalt ions in 10 could be replaced by nickel ions. However, the variation of the lattice parameters of the triclinic solid solutions remains limited, although a small shift of the diffraction peaks is observed. This is consistent with a limited composition area of the triclinic solid solution, where less than 1 nickel ion in 10 could be replaced by cobalt ions.

1. INTRODUCTION

239.85 to 326.75 K and crystallizes into a monoclinic structure.8 The study of the solid−liquid equilibria of the H2O− Co(NO3)2−Ni(NO3)2 ternary system at 293.159and 248.15 and 258.15 K10 highlighted the formation of two widespread solid solutions, derived from the stable hydrates of the H2O− Co(NO3)2 and H2O−Ni(NO3)2 binaries. At 293.15 K,9 the solid solution derived from the triclinic nickel nitrate hexahydrate is stable from 0 to 18 x% Co(NO3)2·6H2O; the second comes from the monoclinic cobalt nitrate hexahydrate and is stable from 35 to 100 x% Co(NO3)2·6H2O. At 258.15 K,10 the first one is stable up to about 35.5 x% Co(NO3)2· 6H2O, and the second one is stable from 67.3 to 100 x% Co(NO3)2·6H2O. These studies show that Co2+ and Ni2+ cations, which present similar physicochemical properties (very close ionic radii: 65 and 69 pm for cobalt and nickel ions, respectively, and the same electrovalence), can then easily be exchanged on the crystallographic sites of the structures leading to substitutional solid solutions. However, as cobalt and

The solid−liquid equilibria of the H2O−Co(NO3)2 and H2O− Ni(NO3)2 binary systems have been the subjects of a number of experimental studies, completed by a critical assessment of the bibliographical data.1 The quasi-ideal model has already been applied successfully to several binary systems of concentrated electrolytes in aqueous solutions.2−4 By taking into account the ionic solvation, this model proved that it was possible to establish the equations of the liquidus curves and to refine and extrapolate the experimental curves. The solid− liquid equilibrium description in the water−cobalt nitrate and water−nickel nitrate binary systems also highlighted the formation of several hydrates:5,6 the di-, tetra-, and hexahydrate for both nitrates and the enneahydrate for the nickel nitrate. At temperatures close to the ambient temperature, the stable species are the hexahydrates. Nickel nitrate hexahydrate Ni(NO3)2·6H2O, which is a component presenting a noncongruent fusion, i.e., which decomposes in a liquid phase and in the tetrahydrate species, is stable in the temperature range 266.35−327.45 K and crystallizes in a triclinic structure.7 Cobalt nitrate hexahydrate Co(NO3)2·6H2O, which also decomposes into the tetrahydrate species, is stable from © 2017 American Chemical Society

Received: September 9, 2016 Accepted: January 25, 2017 Published: February 13, 2017 980

DOI: 10.1021/acs.jced.6b00795 J. Chem. Eng. Data 2017, 62, 980−987

Journal of Chemical & Engineering Data

Article

Table 1. Specification of the Chemical Products chemical name

formula

CAS number

Source

mass fraction purity

analysis method

cobalt nitrate hexahydrate nickel nitrate hexahydrate

Co(NO3)2·6H2O Ni(NO3)2·6H2O

10026-22-9 13478-00-7

Acros Organics Acros Organics

0.994 0.999

complexometric titration complexometric titration

were stirred in thermostated reactors. The liquid addition was automatically done using an automatic buret. Besides, a three hole cap (for the thermometer, the conductivity cell and the buret tip) ensured the closure of the tubes. The conductivity of the solutions was measured using a commercial conductivity meter (Amel Instruments, model 160). The simultaneous study of the solid and liquid phases in equilibrium at 303.15 K was conducted using the Schreinemaker’s method:20 a mixture with a defined composition was stirred at a fixed temperature until the equilibrium state was reached (around 30 days). After settling and centrifugation, the liquid and the wet solid phases were analyzed by UV−visible spectroscopy (SAFAS Spectrophotometer UVmc 2). The absorption spectra presented maxima at the wavelengths λ = 721, 657, and 511 nm. At these wavelengths, the Beer−Lambert law could be applied in the linearity range of absorbance values up to 1.2. In order to avoid the cobalt contribution at the chosen nickel wavelength and vice versa, the molar extinction coefficients (ε) of the two species were obtained by linear regression of the monoelement calibration curves at the considered wavelengths:

nickel nitrate hexahydrates do not have the same crystalline structure, one of the Hume−Rothery rules11−13 (close ionic or atomic radii, same crystal structure, same valence, similar electronegativity) is not followed; thus, the two solids are not totally miscible in all the composition domain, leading to the formation of two series of solid solutions. In general, the hydrated double salts are frequently employed as mixed oxide precursors for several applications.14−17 The cobalt and nickel nitrates, which easily form substitutional solid solutions in large domains of temperatures and compositions, could lead, after decomposition, to mixed oxides of a controlled and homogeneous composition. They could benefit from the advantages of the soft chemistry processes, particularly their formation at low temperatures (99% purity) and deionized water. The characteristics of these products are available in Table 1. The isothermal study at 273.15 K was first carried out using a synthetic method based on conductivity measurements.18,19 To a known saline mixture, a liquid (pure water or water containing dissolved salts with a defined composition) was progressively added, and the conductivity of the solution in thermodynamic equilibrium was measured. The curves, which represented the conductivity depending on the added liquid volume, showed a break in the slope when there was a change in phase domain, for example, from a diphasic to a triphasic or a monophasic region. A stable conductivity meant that an invariant domain (liquid in equilibrium with two solid phases in that case) was crossed, as the composition of the liquid phase remained constant. The analysis of a mixture series enabled not only the liquidus curves to be delineated but also the invariant solution composition and the solid phases in equilibrium to be determined. The laboratory tubes, containing initial mixtures,

657nm 657nm A657nm = εCo(NO l[Co(NO3)2 ] + εNi(NO l[Ni(NO3)2 ] 3)2 3)2 511nm 511nm A511nm = εCo(NO l[Co(NO3)2 ] + εNi(NO l[Ni(NO3)2 ] 3)2 3)2

where A is the measured absorbance at the different wavelengths (λ = 511, 657, or 721 nm), [X] is the molar concentrations, expressed in mol L−1, and l is the path length of the beam of light through the material sample, in cm. After a rapid drying on a filter paper, the solid phases were characterized by powder X-ray diffraction, collected on a Panalytical X’PertPro MRD diffractometer with a copper anticathode tube (λ (Cu Kα1) = 1.54060 Å, λ (Cu Kα2) = 1.54443 Å), in air at room temperature. This diffractometer had a Bragg−Brentano configuration in θ/θ (fixed sample). For the solid phases of the isotherm at 303.15 K, an internal standard, silicon (00-027-1402 ICDD record), was added to the sample during the characterization to adjust the diffractograms and to be able to compare them. A statistical study of structural parameter determination by single-crystal X-ray diffraction was also carried out on all the solid phases of the ternary system at 303.15 K. Between 4 and 981



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Table 2. Liquid and Wet Solid Phase Compositions of Several Mixtures in the Ternary System H2O−Co(NO3)2−Ni(NO3)2 at 303.15 K and 101 kPaa initial mixture composition (xd) mixture b

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13b

liquid phase composition (xd)

wet solid composition (xd)

Co(NO3)2

Ni(NO3)2

Co(NO3)2

Ni(NO3)2

Co(NO3)2

Ni(NO3)2

identified solid phasec

/ 0.0023 0.0130 0.0196 0.0243 0.0271 0.0344 0.0397 0.0486 0.0605 0.0811 0.1042 /

/ 0.1035 0.1005 0.0982 0.0913 0.0863 0.0810 0.0778 0.0670 0.0545 0.0345 0.0089 /

0.0000 0.0021 0.0149 0.0211 0.0214 0.0209 0.0265 0.0304 0.0378 0.0482 0.0625 0.0848 0.0980

0.0952 0.0894 0.0793 0.0762 0.0751 0.0748 0.0690 0.0662 0.0582 0.0493 0.0315 0.0084 0.0000

0.0000 0.0018 0.0104 0.0195 0.0261 0.0350 0.0434 0.0494 0.0563 0.0753 0.0990 0.1235 0.1429

0.1429 0.1263 0.1194 0.1194 0.1036 0.1001 0.0950 0.0910 0.0718 0.0652 0.0400 0.0115 0.0000

Ni(NO3)2·6H2O SSCo.Ni SSCo.Ni SSCo.Ni + SSNi.Co SSCo.Ni + SSNi.Co SSCo.Ni + SSNi.Co SSNi.Co SSNi.Co SSNi.Co SSNi.Co SSNi.Co SSNi.Co Co(NO3)2·6H2O

The standard uncertainties u are u(x) = 0.0006, u(P) = 4 kPa, and u(T) = 0.1 K. bCalculated solubility of Ni(NO3)2·6H2O and Co(NO3)2·6H2O at 303.15 K.1 cSSNi.Co: Solid solution Co(1−σ)Niσ(NO3)2·6H2O with σ ∈ [0; Σ]. SSCo.Ni: Solid solution Ni(1−ω)Coω(NO3)2·6H2O with ω ∈ [0; Ω]. d x: mole fractions of Co(NO3)2 and Ni(NO3)2. a

10 representative crystals of the concerned sample were analyzed after their removal from the solid phase of the mixture. Single-crystal X-ray diffraction data were collected at room temperature on an Oxford Gemini diffractometer with a molybdenum anticathode tube (λ (Mo Kα) = 0.71073 Å) and equipped with a CCD camera. An analytical absorption correction based on the crystal shapes was applied. The structures were solved by direct methods using the SIR97 program combined with Fourier difference analyses and refined against F (I/σ(I) > 3) by using the CRYSTALS program. All non-hydrogen atoms were anisotropically refined.

3. RESULTS AND DISCUSSION Under atmospheric pressure (101 kPa), the equilibrium states of the ternary system isotherms are described graphically in a classical representation using an isosceles right-angled triangle. The right angle corresponds to pure water, and the anhydrous cobalt and nickel nitrate compositions are expressed in molar fractions. 3.1. Isotherm at 303.15 K. 3.1.1. Delineation of the Solubility Domains. A series of mixtures of suitable compositions were stirred at 303.15 K for 15 to 30 days, which was long enough to reach the thermodynamic equilibrium. To avoid the formation of metastable equilibrium and to reach the equilibrium state, a seeding with a very small quantity of nickel and cobalt nitrate hexahydrate was done. After separation, the phases (liquid and wet solids) were titrated by UV−visible spectroscopy; then, the solid phases were characterized by powder X-ray diffraction. The results of the chemical titrations are available in Table 2 and illustrated in Figure 1. The M1 and M13 points represent the solubilities in pure water of nickel nitrate hexahydrate Ni(NO3)2·6H2O and cobalt nitrate hexahydrate Co(NO3)2· 6H2O, respectively, at 303.15 K.1 As it was observed for the isotherms at 248.15 and 258.15 K10 and 293.15 K,9 the solubility curves of the solid phases, shown Figure 1, have an almost linear behavior. Similarly, the experimental points and the position of the tie-lines indicate that the precipitated solid phases are two series of solid solutions derived from the cobalt and nickel nitrate hexahydrate.

Figure 1. Experimental ternary system H2O−Co(NO3)2−Ni(NO3)2, at 303.15 K and 101 kPa (SS Co.Ni, Ni(1−ω)Coω(NO3)2·6H2O; SS Ni.Co, Co(1−σ)Niσ(NO3)2·6H2O; × , initial mixtures; ●, liquid phases; □, wet solid phases; ■, calculated dry solid phases; ○, calculated invariant solid phases).

The structure of the solid phases at 303.15 K was characterized by powder X-ray diffraction after a rapid drying on a filter paper. Figure 2 presents the diffractograms of the M8, M5, and M2 mixtures as examples. The diffractogram of type (a) was observed for the solid phases of the mixtures from M7 to M12. All the diffraction peaks are similar to those of the monoclinic cobalt nitrate hexahydrate (00-025-1219 ICDD record). Nevertheless, some peaks present a shift to higher angles (Figure 3a) compared to the reference record, which increases with the increase of the nickel ion concentration and is due to the formation of a solid solution. The cobalt and nickel cations have similar properties: same electrovalence and close ionic radii (65 pm for Co2+ and 69 pm for Ni2+). The Hume−Rothery rules11−13 thus provide the formation of 982



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Co(1−σ)Niσ(NO3)2·6H2O. The AB and BC curves represent the solubility curves of the solid solutions Ni(1−ω)Coω(NO3)2·6H2O and Co(1−σ)Niσ(NO3)2·6H2O, respectively. • An invariant triphasic region, where a liquid with a defined composition (B, 2.11 x% Co(NO3)2 and 7.54 x% Ni(NO3)2, average of the experimental values for the samples from M4 to M6) is in equilibrium with the two solid solutions Co ( 1 − σ ) Ni σ (NO 3 ) 2 ·6H 2 O and Ni(1−ω)Coω(NO3)2·6H2O, where σ and ω are fixed stoichiometric coefficients. 3.1.2. Structural Characterization of the Substitutional Solid Solutions. The shift in the diffraction peaks (compared to the reference records), observed in the powder X-ray patterns of the solid solutions derived from the hexahydrated cobalt and nickel nitrates (Figure 3), involves some modifications of the structural parameters of the crystal lattices. For this purpose, a statistical study on single-crystal X-ray diffraction was carried out on all the solid phases of the ternary system at 303.15 K. The evolution of the a, b, and c lattice lengths and of the β angle (α = γ = 90°) depending on the substitution of Co2+ ions by Ni2+ ions was quantified for the monoclinic solid solution. The Table 3 presents the variation of the lattice parameters and the number of characterized and relevant crystals by sample in that series. Indeed, a crystal was taken into account only if it satisfied these two conditions: • More than 80−100 peaks were observed • More than 75−80% of the peaks matched with the determined cell The lattice lengths (a, b, and c) and the β angle decrease with the increase of the substitution of cobalt ions by nickel ions, which corresponds to a lattice shrinkage, as its volume reduces from 1026.4(20) Å3 to 1009.5(39) Å3. In the mixtures from M4 to M6, which contain the monoclinic and the triclinic solid solutions, it was complex to isolate crystals of the monoclinic structure. Nevertheless, between the sample M4 and the sample M6, the lattice parameters seem to remain constant as, in the invariant area, the composition of monoclinic solid solution does not change. The modification of the b parameter and of the volume of the cell depending on the composition is represented Figure 4. The evolution is almost linear in the entire two-phase area

Figure 2. Powder XRD patterns of solid phases: (a) Co(1−σ)Niσ(NO3)2·6H2O solid solution (M8), (b) Co(1−σ)Niσ(NO3)2· 6H2O and Ni(1−ω)Coω(NO3)2·6H2O solid solutions in the triphasic region (M5), and (c) Ni(1−ω)Coω(NO3)2·6H2O solid solution (M2), at 303.15 K (★, silicon peak).

substitutional solid solutions, in which part of the cobalt ions is replaced by nickel ions. The chemical formula of this first series can be written as Co(1−σ)Niσ(NO3)2·6H2O. The diffractogram of type (c) was observed for the solid phases of the M2 and M3 mixtures. The diffraction peaks match with the triclinic nickel nitrate hexahydrate (00-025-0577 ICDD record) and also present a shift, toward lower angles (Figure 3b), in comparison with the reference record. In this case, part of the nickel ions is progressively replaced by cobalt ions. The second solid solution series can be expressed as Ni(1−ω)Coω(NO3)2·6H2O. Between these two regions, the two solid solutions coexist. The diffractogram (b) in Figure 2 shows the diffraction peaks of the triclinic and monoclinic structures and was observed for the mixtures from M4 to M6. Then, the several domains of the H2O−Co(NO3)2− Ni(NO3)2 ternary system can be described as shown in Figure 1: • One area where variance is 2: only one liquid phase is observed. • Two monovariant diphasic domains, where a liquid phase is in equilibrium with a solid phase: liquid + Ni(1−ω)Coω(NO3)2·6H2O and liquid +

Figure 3. Partial representation of the powder X-ray pattern of the commercial Co(NO3)2·6H2O and the solid phases of the mixtures from M12 to M5 for 2θ ∈ [28.0; 28.5] (a) and of the commercial Ni(NO3)2·6H2O and the solid phases of the mixtures from M2 to M6 for 2θ ∈ [25.9; 26.2] (b). The star (a) corresponds to a silicon peak. 983



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Table 3. Lattice Parameters of the Monoclinic Co(1−σ)Niσ(NO3)2·6H2O Solid Solution Series a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

nb samplea

14.305(23) 14.301(19)c 14.279(9) 14.254(52) 14.291(24) 14.255(4) 14.258(24) 14.261(30) 14.264(25) 14.262(27)

6.141(6) 6.130(8) 6.112(5) 6.114(12) 6.086(11) 6.089(5) 6.076(26) 6.079(5) 6.076(21) 6.074(6)

12.666(32) 12.666(12) 12.659(16) 12.650(25) 12.646(46) 12.640(7) 12.631(18) 12.624(34) 12.633(21) 12.613(26)

112.74(26) 112.75(7) 112.75(11) 112.67(21) 112.62(32) 112.56(5) 112.45(19) 112.50(20) 112.56(38) 112.43(8)

1026.4(20) 1023.7(15)c 1019.0(10) 1016.7(21) 1014.4(13) 1013.0(16) 1011.1(22) 1012.0(41) 1012.5(17) 1009.5(39)

5 4 7 7 6 4 8 4 4 4

sample Co(NO3)2·6H2O M12 M11 M10 M9 M8 M7 M6 M5 M4

b

Number of characterized crystals. bCommercial product. c(number): standard deviation → ex: 14.301(19) means 14.301 ± 0.019 and 1023.7(15) means 1023.7 ± 1.5.

a

Figure 4. Representations of the b lattice length (a) and of the cell volume (b) depending on the molar fraction of nickel nitrate in the dry solid phases for the monoclinic solid solutions.

Table 4. Lattice Parameters of the Triclinic Ni(1−ω)Coω(NO3)2·6H2O Solid Solution Series a (Å)

sample Ni(NO3)2·6H2O M2 M3 M4 M5 M6

b

c

5.813(4) 5.808(5) 5.804(5) 5.803(8) 5.819(20) 5.815(7)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

V (Å3)

nb samplea

7.705(2) 7.708(5) 7.690(10) 7.695(13) 7.724(18) 7.702(24)

11.695(9) 11.691(7) 11.696(12) 11.707(28) 11.734(15) 11.709(18)

105.70(7) 105.78(4) 105.78(8) 105.70(15) 105.67(27) 105.70(9)

98.56(4) 98.47(7) 98.67(10) 98.47(15) 98.60(20) 98.49(6)

102.28(3) 102.25(9) 102.25(6) 102.26(7) 102.25(14) 102.28(5)

480.4(4) 480.2(7) 480.0(8) 479.6(16)c 483.9(33) 481.1(10)

4 6 7 7 7 7

Number of characterized crystals. bCommercial product. c(number): standard deviation → ex: 5.183(4) means 5.183 ± 0.004 and 479.6(16) means 479.6 ± 1.6.

a

Co(1−σ)Niσ(NO3)2·6H2O + liquid and spreads until compositions of 10 x% Ni(NO3)2; then it remains constant in the triphasic region. The similar statistical study by single-crystal X-ray diffraction of the triclinic solid solution close to the nickel nitrate hexahydrate was conducted to quantify this variation of the lattice parameters (Table 4). However, this substitution area is very restricted in composition, and so the evolution of the lattice parameters in this triclinic series remains limited and is not significant. 3.1.3. Composition of the Invariant Solid Solutions. In the previous publication,10 it was observed and demonstrated, by using the quasi-ideal model and disregarding the ionic solvation, that the solubility curves of the solid solutions at 248.15 and 258.15 K had a linear behavior. The same observation can be done for the isotherm at 303.15 K.

According to this analysis, the hypothetical species in liquid or solid solutions, which are taking into account in the quasiideal model application, are Co2+, Ni2+, NO3−, and H2O. The calculation of their molar fractions in the liquid (x((species))) and solid (x⟨species⟩) phases is presented in the Supporting Information. The cation distribution curves x((Co2+)) = f(x⟨Co2+⟩) and x((Ni2+)) = f(x⟨Ni2+⟩) between the liquid and the solid phases at 258.15 K enabled the composition of the invariant solid solutions to be defined precisely, by analogy with the Roozeboom classification.21−24 The same representation was used for the isotherm at 303.15 K. The distribution curves in Figure 5 each show two almost linear parts, which can be described by eqs 1−4), with very good correlation coefficients, close to unity. The coefficients of the first-order polynomial Amono, Bmono, and Cmono for the monoclinic solid solution and Atri, Btri, and Ctri for 984



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Figure 5. Distribution of cobalt or nickel in the liquid and solid phases at 303.15 K: x⟨M2+⟩ is the mole fraction of M2+ in the solid phase and x((M2+)) is the mole fraction of M2+ in the liquid phase. (■ (a) and ● (b), Co2+ and Ni2+, respectively, in the monoclinic region; □ (a) and ○ (b), Co2+ and Ni2+, respectively, in the triclinic region; dashed lines represent the Co2+ and Ni2+ invariant liquid composition).

determined in equilibrium with the invariant liquid B. Then, the cobalt nitrate and nickel nitrate compositions are calculated based on the equations in the table in the Supporting Information. At 303.15 K, the compositions of the invariant solid solutions are 1.3 x% Co(NO3)2 and 13.1 x% Ni(NO3)2 for the triclinic Ni(1−ω)Coω(NO3)2·6H2O, i.e., ω = 0.09, and 3.9 x% Co(NO3)2 and 10.4 x% Ni(NO3)2 for the monoclinic Co(1−σ)Niσ(NO3)2· 6H2O, i.e., σ = 0.72. The stoichiometry of the invariant monoclinic solid solution is thus Co0.28Ni0.72(NO3)2·6H2O, which corresponds to the substitution of more than 7 Co2+ cations in 10 by Ni2+ cations, whereas the invariant triclinic solid solution is Ni0.91Co0.09(NO3)2·6H2O, i.e., less than 1 Ni2+ ion in 10 is replaced by Co2+ ions. This is in good agreement with the evolution of the lattice parameters of the monoclinic structure, which is significant as the substitution of cobalt ions by nickel ions is high. On the contrary, in the triclinic series, a few nickel ions are replaced by cobalt ions, leading to a moderate variation of the lattice parameters. 3.2. Isotherm at 273.15 K. The solubility curves of the solid solutions of cobalt and nickel nitrate hexahydrate at 273.15 K were determined by conductivity measurements of the solutions according to the isoplethic method. The experimental values are presented in Table 5 and illustrated in Figure 6. By analogy with the isotherm at 303.15 K, the same regions are observed. A complementary analysis of the solid and liquid phases was conducted to define precisely the composition of the invariant liquid and to characterize the solid phases in equilibrium. Three mixtures, with suitable composition, were prepared. After a thermostated stirring, long enough (more than 30 days) to reach the equilibrium state, the liquid and wet solid phases were separated by settling and titrated by UV− visible spectroscopy. Besides, the structure of the solid phases was characterized by powder X-ray diffraction. To avoid modifications of their nature or composition, due to temperature change at the time of the analysis, the solids were collected at the last moment and dried rapidly on a filter paper, and the analyses lasted less than 10 min. The results of the analyses are available in Table 6 and presented in Figure 6.

the triclinic solid solution are constants at the considered temperature. • For the liquid in equilibrium with the monoclinic solid solution x((Co2+)) =

xl = A mono (1 + 2(x l + y l ))

x⟨Co2+⟩ + Bmono with A mono = 0.768, Bmono = − 0.006, and r 2 = 0.995 x((Ni 2+)) =

(1)

yl 1 + 2(x l + y l )

= Cmonox⟨Ni 2+⟩

with Cmono = 0.784 and r 2 = 0.997

(2)

• For the liquid in equilibrium with the triclinic solid solution x((Co2+)) =

xl = A tri x⟨Co2+⟩ 1 + 2(x l + y l )

with A tri = 1.816 and r 2 = 0.996

x((Ni 2+)) =

yl 1 + 2(x l + y l )

(3)

= Btri x⟨Ni 2+⟩ + C tri

with Btri = 1.829, C tri = −0.124, and r 2 = 0.98 (4)

The distribution curves also present a part where the cobalt and nickel ion compositions are constant in the liquid phase, which corresponds to the triphasic region of the ternary system (Figure 1): liquid B + Co (1−σ) Ni σ (NO 3 ) 2 ·6H 2 O + Ni(1−ω)Coω(NO3)2·6H2O, where σ and ω are fixed stoichiometric coefficients. The intersections of the straight lines (1) and (3) with the straight line x((Co2+)) = x((Co2+))invariant and the straight lines (2) and (4) with the straight line x((Ni2+)) = x((Ni2+))invariant enable the cobalt and nickel ion compositions of the solid solutions to be 985



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Table 5. Solubility Data in the H2O−Co(NO3)2−Ni(NO3)2 Ternary System at 273.15 K and 101 kPaa

The composition of the invariant liquid, in equilibrium with the two solid solutions Co (1−σ) Ni σ (NO 3 ) 2 ·6H 2 O and Ni(1−ω)Coω(NO3)2·6H2O with fixed σ and ω, is 3.46 x% Co(NO3)2 and 4.56 x% Ni(NO3)2. As only three mixtures were chemically titrated, each one being located in a different region, it appears bold to determine precisely the σ and ω values of the invariant solid solutions, as it was done at 303.15 K. Consequently, the three-phase area is delimitated in Figure 6 by dashed lines.

liquid phase composition (xb) Co(NO3)2

Ni(NO3)2

assumed solid phase in equilibrium

0.0000c 0.0000 0.0038 0.0106 0.0174 0.0223 0.0248 0.0329 0.0431 0.0490 0.0576 0.0688 0.0711 0.0751

0.0740c 0.0780 0.0695 0.0645 0.0594 0.0557 0.0535 0.0462 0.0360 0.0281 0.0216 0.0115 0.0071 0.0000

Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O Co(NO3)2·6H2O

4. CONCLUSION The solid−liquid equilibrium study of the H2O−Co(NO3)2− Ni(NO3)2 ternary system was performed at 273.15 and 303.15 K. Two liquidus curves, which delineate the diphasic liquid− solid domains, were experimentally defined by solubility measurements. The compositions of the equilibrium phases were established by chemical analyses of the pure liquid phase and of the wet solid phases. The two solid phases, in equilibrium with their solutions, were deeply studied at 303.15 K. The crystal structures come from the monoclinic cobalt nitrate hexahydrate and the triclinic nickel nitrate hexahydrate. The first solid solution, Co(1−σ)Niσ(NO3)2·6H2O, spreads over a wide composition region as σ ∈ [0; 0.72]. The second solid solution, whose formula is Ni(1−ω)Coω(NO3)2·6H2O, is more limited and corresponds to ω ∈ [0; 0.09]. The structural parameter study of each lattice, by single-crystal X-ray diffraction, highlighted a lattice shrinkage correlated with the increase of the substitution of the cobalt ions by nickel ions. However, the Ni(1−ω)Coω(NO3)2·6H2O substitutional solid solution domain is narrow, which involves a small evolution of the triclinic lattice, in which less than 1 Ni2+ ion in 10 is replaced. The two diphasic domains are separated by an invariant triphasic domain (one liquid + two solid solutions). The compositions of the isothermal invariant liquid solutions were accurately determined by chemical analysis. The increase in temperature involves a significant variation of the invariant liquid composition, as this phase enriches in nickel (3.46 x% Co(NO3)2 and 4.56 x% Ni(NO3)2 at 273.15 K; 2.11 x% Co(NO3)2 and 7.54 x% Ni(NO3)2 at 303.15 K). The solid solutions prepared are thus highly relevant as precursors for mixed oxides and offer a wide choice of cationic ratios. Indeed, as the solid−liquid equilibria are perfectly known, the choice of the liquid phase composition in equilibrium involves not only the control of the composition and constitution of the mixed nitrate crystals but also the control of their homogeneity at the nanometer scale.

a

The standard uncertainties u are u(x) = 0.0005, u(P) = 4 kPa, and u(T) = 0.1 K. bx: mole fractions of Co(NO3)2 and Ni(NO3)2. c Calculated solubility of Ni(NO3)2·6H2O at 273.15 K.1

Figure 6. Experimental ternary system H2O−Co(NO3)2−Ni(NO3)2, at 273.15 K and 101 kPa (SS Co.Ni, Ni(1−ω)Coω(NO3)2·6H2O; SS Ni.Co, Co(1−σ)Niσ(NO3)2·6H2O; ×, initial mixtures; ●, liquid phases; □, wet solid phases; ■, calculated dry solid phases; ◆, liquid phases by conductimetric measurements).

Table 6. Liquid and Wet Solid Phase Compositions of Several Mixtures in the Ternary System H2O−Co(NO3)2−Ni(NO3)2 at 273.15 K and 101 kPaa

a

initial mixture composition (xb)

liquid phase composition (xb)

mixture

Co(NO3)2

Ni(NO3)2

Co(NO3)2

Ni(NO3)2

Co(NO3)2

Ni(NO3)2

identified solid phase

M1 M2 M3

0.0256 0.0341 0.0431

0.0417 0.0575 0.0486

0.0266 0.0346 0.0388

0.0499 0.0456 0.0406

0.0250 0.0351 0.0576

0.1035 0.0982 0.0716

Ni(1−ω)Coω(NO3)2·6H2O Ni(1−ω)Coω(NO3)2·6H2O + Co(1−σ)Niσ(NO3)2·6H2O Co(1−σ)Niσ(NO3)2·6H2O

wet solid composition (xb)

The standard uncertainties u are u(x) = 0.0006, u(P) = 4 kPa, and u(T) = 0.1 K. bx: mole fractions of Co(NO3)2 and Ni(NO3)2. 986



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00795. Calculation details of the molar fractions of the cobalt, nickel, nitrate ions, and water (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(A.T.) E-mail: [email protected]. ORCID

Angélique Teyssier: 0000-0001-9968-6572 Notes

The authors declare no competing financial interest.

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REFERENCES

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Widespread Solid Solutions of Hexahydrated Cobalt and Nickel

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