Gibbs Energy of Formation of Eu3O4 and EuO - Journal of Chemical

Article pubs.acs.org/jced

Gibbs Energy of Formation of Eu3O4 and EuO Kallarackel T. Jacob* and Arneet Rajput Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India ABSTRACT: Thermodynamic data for Eu3O4 are not available in standard compilations. However, data for EuO and Eu2O3 are available. Data for Eu2O3 in the compilations are in agreement while those for EuO differ significantly. Two solid−state electrochemical cells incorporating yttria−doped thoria as the electrolyte are used to measure the standard Gibbs energy of formation of Eu3O4 and EuO relative to that for Eu2O3 in the temperature range from 1000 to 1300 K. A mixture of Nb and NbO is used as the reference electrode since the oxygen chemical potential associated with the mixture is close to that of the working electrodes EuO + Eu3O4 and Eu3O4 + Eu2O3. The standard Gibbs energy of formation of Eu3O4 can be represented by the following equation: ΔfG°(±860)/J·mol−1 = −2267816 + 403.18(T/K) {1090−1300 K}. Below the melting point of Eu, the Gibbs energy of formation of Eu3O4 is given by ΔfG°(±860)/J·mol−1 = −2240177 + 377.824(T/K) {1000−1090 K}. Also, the standard Gibbs energy of formation of EuO can be represented by the equations: ΔfG°(±350)/J·mol−1 = −601046 + 96.878(T/K) {1000−1090 K} and ΔfG°(±350)/J·mol−1 = −610259 + 105.33(T/ K) {1090−1300 K}. Based on these results and the Neumann−Kopp rule, a complete set of thermodynamic data for Eu3O4 is generated. The results also permit refinement of data for EuO. occupying the Ca site and two Eu3+ ions residing at the Fe site.2 Hence, the oxide does not display valence fluctuation.3 A large magnetic moment is associated with Eu2+ ion and a weak moment with Eu3+ ion. Below ∼5 K, Eu3O4 is antiferromagnetically ordered4,5 and displays metamagnetic behavior in low magnetic field below the Néel temperature with Hc = 2.4 kOe at 2 K. EuO is one of the very rare ferromagnetic oxides with saturation moment close to 7 μB and Curie temperature (TC) around 69 K, which is also associated with a spectacular metal− insulator transition. Resistivity drops by approximately 8 orders of magnitude below TC. EuO also exhibits colossal magnetoresistance (CMR).6 While EuO and Eu3O4 are near line compounds, Eu2O3 exhibits a range of stoichiometry caused by oxygen vacancies. Bedford and Catalano7 have suggested Eu2O2.9 as the limit of hypostochiometry at 1773 K. Unpublished measurements by the authors using a thermogravimetric determination of mass loss accompanying reduction by dry hydrogen at 1300 K suggest smaller oxygen deficiency, Eu2O2.98. Thermodynamic data for Eu3O4 are not available in standard compilations.8,9 However, data for EuO and Eu2O3 are available. Data for Eu2O3 in the two compilations are in good agreement, while those for EuO differ significantly. Sukhushina and Vasil’eva10 attempted to measure the Gibbs energy of formation of europium oxides using solid state electrochemical cells under vacuum. By application of an external potential, they partially reduced Eu2O3 and followed the evolution of emf with

1. INTRODUCTION The most common applications of europium containing oxides are based on phosphorescence in either the +2 or +3 oxidation state. In these applications, the host crystal is doped with europium. Trivalent europium gives red phosphors. Although the luminescence of divalent europium depends to some degree on the host lattice, generally it is on the blue side. Some types of glasses used in lasers also contain europium. Europium functions as an activator in yttrium-based phosphors. Thermodynamic data on europium oxides are useful to compute their solubility limits in host lattices and determine the valence state under different high-temperature processing conditions. Since europium is one of the least abundant elements on the earth’s crust (∼3 ppm), designing the most economic and efficient process is a matter of importance. Mixed phase polycrystalline EuxOy thin films containing both Eu2O3 and Eu3O4 show unipolar resistive switching, with the resistance ratio between high-resistance and low-resistance states as high as 108.1 The relative concentration of the two oxide phases depends on the oxygen partial pressure during deposition using magnetron sputtering. Higher oxygen concentration leads to broadening of the distribution of switching voltages. The polycrystalline thin films can be used as memory elements in RRAM devices. Three oxides of europium are known to exist: EuO (space group Fm3m), containing divalent europium ion, Eu2O3 (space group Ia3), containing trivalent ion, and Eu3O4 (space group Pnam), containing both types of ions. The heterogeneous mixed-valancy compound Eu3O4 has crystallographically distinct cites for the two ions. It crystallizes in the orthorhombic CaFe2O4-type structure; with one Eu2+ ion © XXXX American Chemical Society

Received: August 28, 2015 Accepted: April 12, 2016

A

DOI: 10.1021/acs.jced.5b00728 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Source, Treatment, and Purity of Materials Used in This Study Chemical Name

Initial Mass Fraction Purityb

Source

Europium (Eu)

Aldrich

Europium(III) Oxide (Eu2O3) Europium(II) Oxide (EuO) Europium(II,III) Oxide (Eu3O4) Niobium (Nb)

Aldrich

Alfa Aesar

0.9999

Niobium(V) Oxide (Nb2O5) Niobium(II) Oxide (NbO)

Alfa Aesar

0.9995

a

Synthesized

0.999 0.9999 a

Synthesizeda

Synthesizeda

Purification/Synthesis/Treatment

Final Mass Fraction Purity

Analysis Method

Washed in dilute acid, cleaned with acetone and dried in vacuum. Heated in vacuum at 1173 K.

0.998

ICP-AES for metallic impurities.

0.9994


Reduction of Eu2O3 by Eu at 1373 K in closed Mo container. Mixture of EuO and Eu2O3 heated at 1573 K under Ar gas. Washed in dilute acid, cleaned with acetone and dried in vacuum. Heated in dry air at 1273 K.

0.997

0.9995

ICP-AES for trace metals. Mass gain on oxidation for oxygen. ICP-AES for trace metals. Mass gain on oxidation for oxygen. ICP-AES for metallic impurities.

0.9993


Mixture of Nb and Nb2O5 heated at 1273 K in vacuum.

0.9994

ICP-AES for trace metals. Mass gain on oxidation for oxygen.

0.998

Synthesized as described in the manuscript. bInformation from supplier.

Nb2O5 in the required ratio in vacuum, NbO was produced. The XRD pattern of NbO was almost identical with that in PDF #431290. The reference electrode was prepared by mixing Nb and NbO in equimolar ratio, compacting the mixture into pellets using a steel die and sintering at 1373 K in a stream of pre−purified Ar gas for ∼30 ks. Measuring electrodes of EuO + Eu3O4 and Eu3O4 + Eu2O3 were also prepared by mixing equimolar mixtures of oxides, compacting the mixture and then sintering the pellet. The flat surfaces of electrode and electrolyte pellets were polished to obtain good contact. The purity of the oxides used in the study on metal basis was confirmed by ICPAES. The oxygen content of NbO, EuO and Eu3O4 were confirmed by oxidative gravimetry using a thermobalance. Ar gas of mole fraction purity greater than 0.99999 was used to flush the electrodes. The gas was first dried by passing through glass columns containing silica gel, magnesium perchlorate, and diphosphorus pentoxide and then deoxidized by copper turnings at 723 K and titanium granules at 1173 K. 2.2. Electrochemical Measurements. The reversible emf values of two solid−state cells were measured as a function of temperature:

time. They did not observe time-invariant emf that would identify a two-phase equilibrium at any temperature. By linear extrapolation of different parts of the depolarization curve, they identified values corresponding to two-phase equilibria. There is some ambiguity in the data derived from such nonstationary emf measurements. Their results indicate that Eu3O4 is thermodynamically unstable below 1045 (±90) K. Hence, two solid-state electrochemical cells incorporating yttria-doped thoria (YDT) as the electrolyte and Nb + NbO as the reference electrode were designed to measure the standard Gibbs energy of formation of Eu3O4 relative to EuO and Eu2O3 in the temperature range from 1000 to 1300 K.

2. EXPERIMENTAL METHODS 2.1. Materials. The source and purity of materials used in this study are listed in Table 1. The XRD pattern of Eu2O3 used in this study was almost identical to that in PDF # 431008, corresponding to a cubic (C-rare earth oxide, Mn2O3-type) structure. The lattice parameter a = 1.086 nm was obtained in this study. EuO was prepared by reduction of Eu2O3 by metal Eu at 1373 K. A mixture of oxide and metal powders, with metal 16% in excess over the stoichiometric requirement, was contained in a closed Mo container. Residual excess Eu in the product was distilled off at 1423 K in vacuum (P = 10−2 Pa). Because of the reactivity of Eu, mixing and containment operations were carried out in a glovebox with a purified Ar atmosphere. Eu3O4 was prepared by heating a pelletized equimolar mixture of EuO and Eu2O3 contained in a closed thoria crucible at 1573 K under flowing high-purity Ar gas. Formation of EuO and Eu3O4 was confirmed by XRD. EuO with characteristic red color had an XRD pattern almost identical with PDF #150886 and #180507, corresponding to rock-salt structure. The lattice parameter obtained was a = 0.5144 nm. The XRD pattern of Eu3O4 was almost identical to PDF #431041, corresponding to an orthorhombic structure. The lattice parameters obtained were a = 1.009, b = 1.206, and c = 0.3503 nm. Dense yttria-doped thoria (YDT) pellets, containing 10 mol % Y2O3 and having ∼98% theoretical density, were prepared by the method reported elsewhere.11 The lattice parameter of YDT with cubic fluorite structure was a = 0.5560 nm. The starting materials for the preparation of the reference electrode were powders of Nb and Nb2O5. Nb2O5 was heated at 1273 K for 3.6 ks in dry air to remove moisture and other volatiles. By compacting and heat−treating a mixture of Nb and

( −)Pt, Eu3O4 + Eu 2O3 //(Y2O3)ThO2 //Nb + NbO, Pt( +)

(1)

( −)Pt, EuO + Eu3O4 //(Y2O3)ThO2 //Nb + NbO, Pt(+ ) (2)

The (Y2O3)ThO2 solid-electrolyte was chosen because of its lower oxygen chemical potential boundary for the onset of electronic conduction. The average oxygen ion transport number of the electrolyte at the low oxygen potentials encountered in cell 2 is 0.99. The apparatus used and the experimental details are similar to those described previously for measurements on MgCr2O412 and niobium oxides.13 Pellets of reference and measuring electrodes were pressed on either side of the YDT electrolyte pellet by springs located at the cold end using a system of alumina rods, slab, and ring. The measuring electrode was placed at the top of the stack. Thin Pt gauze, placed between the electrolyte and the measuring electrode, was spot-welded to a Pt lead wire. On the reference side, a Pt gauze with an attached Pt wire was pressed against the Nb + NbO electrode. The emf was measured between the two Pt leads. The electrodes were examined by XRD before and after the experiments. No significant change in phase B



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Table 2. Emf of Cells 1 and 2 as a Function of Temperaturea

composition of the electrodes was observed. A standard Pt− Pt−13%Rh thermocouple, checked against the melting point of Au and placed in close proximity (∼5 mm) to the cell, measured the temperature. The expanded uncertainty (0.95 level of confidence) for each temperature measurement was 0.8 K. Because of the weak temperature dependence of emf, the uncertainty in temperature does not cause serious error in emf. The environment around the measuring electrode was isolated from that around the reference electrode by a separate alumina tube spring-loaded from the top against the electrolyte pellet. By thus separating the gas phase around the two electrodes, material transport between them through the gas phase was prevented. The space around each electrode was flushed by separate streams of pre-purified Ar gas. The entire cell assembly was suspended in a vertical alumina tube, closed at both ends by brass caps which had provision for gas inlet/ outlet and electrical leads. The outer alumina tube enclosing the cell was placed inside a vertical resistance furnace with the cell located in the constant-temperature zone of the furnace. An earthed metallic shield around the outer alumina tube prevented induced emf on cell leads from furnace winding. After putting the cell together, the outer alumina tube was evacuated and refilled with pre-purified Ar gas three times to remove gas species adsorbed on alumina tubes and rods. After heating the cell under flowing Ar gas to 850 K, the procedure was again repeated to remove residual oxygen bearing gas species from the apparatus. Titanium internal getters placed inside the outer alumina tube captured traces of oxygen desorbing from ceramic tubes at higher temperatures. The reversible emf of each cell was measured at regular intervals of temperature in the range from 1000 to 1300 K using a high impedance (>1012 Ω) digital voltmeter during both heating and cooling cycles. The electrochemical reversibility of the cell was verified by microcoulometric titration in both directions. By passing a small direct current (∼15 μA) for ∼0.3 ks, the cell was polarized in each direction. Full emf recovery (±0.5 mV) was obtained after each titration. Thermal reversibility was demonstrated by the reproducibility of the emf during heating and cooling cycles. The emf was also independent of the flow rate of Ar gas around the electrodes in the range from 2 to 5 mL/s, confirming noninterference of equilibrium at the electrodes by the gas phase. For cell 1 the expanded uncertainty (0.95 level of confidence) for each emf measurement was U = 0.4 mV; for cell 2 U = 0.3 mV. The electrochemical measurements were conducted at ambient pressure in the laboratory (90.35(u = 0.5) kPa).

(3)

E2( ±0.4)/mV = 114.28 − 0.00171(T /K)

(4)

Emf of Cell 1 E1 (mV) (U = 0.4)

Emf of Cell 2 E2 (mV) (U = 0.3)

1000 1025 1050 1075 1100 1125 1150 1175 1200 1225 1250 1275 1300

61 59.5 59.6 58.4 58.2 57.8 56.8 56.9 55.8 55.1 54.9 54.4 53.9

112.9 112.2 112.6 112.3 112.6 112.2 112.3 112.2 112.1 112 112.2 112.3 112.1

a U represents expanded uncertainty (0.95 level of confidence) for each measurement.

Figure 1. Temperature dependence of the reversible emf of cells 1 and 2.

2Eu3O4 + 1/2O2 → 3Eu 2O3

(5)

At the reference electrode on the right-hand side of the cell, the reaction creating oxygen potential is

NbO → Nb + 1/2O2

3. RESULTS AND DISCUSSION 3.1. Standard Gibbs Energy of Formation for Eu3O4 and EuO. The reversible emf values of cells 1 and 2 in the range from 1000 to 1300 K are listed in Table 2 and plotted in Figure 1 as a function of temperature. The emf values of both cells are linear functions of temperature. The equations for emf obtained by least-squares regression analysis are E1( ±0.62)/mV = 83.01 − 0.02253(T /K)

T (K) (U = 0.8)

(6)

The virtual cell reaction obtained by combining the two halfcell reactions (5 and 6) is 2Eu3O4 + NbO → Nb + 3Eu 2O3

(7)

With the help of the Nernst equation, the standard Gibbs energy change for reaction 7 can be calculated as Δr(7)Go( ±120)/J·mol−1 = − 2FE1 = − 16018 + 4.342(T /K)

(8)

where n = 2 is the number of electrons involved in the electrode reactions, F = 96485.33 C·mol−1 is the Faraday constant, and E/V is the reversible emf of the electrochemical cell. Using eq 8 and auxiliary data for the standard Gibbs energy change associated with the formation of NbO13 and Eu2O3,8 the standard Gibbs energy of formation for Eu3O4 in the

The uncertainty limits in eq 3 and eq 4 are expanded uncertainties U (0.95 level of confidence) obtained from regression analysis. The chemical reaction establishing the oxygen potential at the measuring electrode on the left-hand side of cell 1 is C



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temperature range from 1000 to 1090 K, below the melting point of Eu, can be derived as Δf Go( ±860)/J·mol−1 = − 2240177 + 377.824(T /K) (9)

The uncertainty estimate includes the error in the emf measurement and the error in the auxiliary data for NbO and Eu2O3. Above the melting point of Eu, in the temperature range 1090 to 1300 K, the standard Gibbs energy of formation for Eu3O4 is obtained as Δf Go( ±860)/J·mol−1 = − 2267816 + 403.18(T /K) (10)

The temperature independent term in eq 9 gives the enthalpy of formation of Eu3O4 (−2240.18 kJ·mol−1) from solid Eu and oxygen gas at the mean temperature (1045 K). The temperature dependent term with sign reversed gives the entropy of formation of Eu3O4 (−377.82 J·mol−1·K−1) at 1045 K. Similarly, from eq 10 the enthalpy and entropy of formation of Eu3O4 from liquid Eu and oxygen gas at the mean temperature (1195 K) are −2267.816 kJ·mol−1 and −403.18 J·mol−1·K−1 respectively. The virtual cell reaction corresponding to cell 2 is NbO + 3EuO → Nb + Eu3O4

Figure 2. Comparison of the Gibbs energy of formation of EuO obtained in this study with data reported in the thermodynamic compilations of Pankratz8 and Knacke et al.9

ΔμO (Eu3O4 + Eu 2O3)(± 350)/J ·mol−1 2

= ΔμO (Nb + NbO) − 4FE1 2

= −829972 + 173.722(T /K) − 4FE1

(11)

= −862008 + 182.406(T /K)

and the corresponding standard Gibbs energy change for reaction 11 is

(15)

In Figure 3 the oxygen potential obtained in this study for the Eu3O4 + Eu2O3 equilibrium is compared as a function of

Δr(11)Go( ±275)/J·mol−1 = − 2FE2 = − 22052 + 0.329(T /K)

(12)

The uncertainty estimate in eq 12 includes random error in the emf obtained from regression analysis and a possible systematic error caused by 1% electronic conduction in the YDT electrolyte. With the help of eq 12 and the Gibbs energy of formation of Eu3O4 obtained in this study (eqs 9 and 10) and auxiliary data on the standard Gibbs energy of formation for NbO,13 the standard Gibbs energy of formation for EuO can be calculated. In the temperature range from 1000 to 1090 K, the Gibbs energy of formation of EuO is given by Δf Go( ±375)/J·mol−1 = − 601046 + 96.878(T /K)

(13)

Above the melting point of Eu, in the range from 1090 to 1300 K, the standard Gibbs energy of formation for EuO is Δf Go( ±375)/J·mol−1 = − 610259 + 105.33(T /K)

Figure 3. Comparison of the oxygen chemical potential for the Eu3O4 + Eu2O3 equilibrium measured in this study with data reported by Sukhushina and Vasil’eva10 using a non-steady-state emf method.

(14)

The standard Gibbs energy of formation of EuO obtained in this study is compared with data from the compilations of Pankratz8 and by Knacke et al.9 in Figure 2. The new results are in closer agreement with data from Pankratz8 than those from Knacke et al.9 3.2. Oxygen Chemical Potential for Biphasic Equilibria: Eu3O4 + Eu2O3 and Eu3O4 + EuO. The oxygen potential for the Eu3O4 + Eu2O3 equilibrium, defined by the equation 4Eu3O4 + O2 → 6Eu2O3 (C), computed from the emf of cell 1, can be expressed as

temperature with the data reported by Sukhushina and Vasil’eva.10 Although the results agree at 1201 K, the slopes of the lines differ significantly. The entropy change for the oxidation of Eu3O4 to Eu2O3 obtained from the data of Sukhushina and Vasil’eva10 (−251.1 J·mol−1·K−1) is significantly more negative than that derived from the present results (−182.406 J·mol−1·K−1). Although Sukhushina and Vasil’eva10 used Eu2O3 with B-rare earth oxide structure in their measurements, the relatively small entropy of structure D



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transformation does not explain the large difference. Thus, although the non-steady-state emf technique can give approximately correct data depending on the method of data analysis, very precise information cannot be derived therefrom. The oxygen potential corresponding to the Eu3O4 + EuO equilibrium, defined by the equation 6EuO + O2 → 2Eu3O4, obtained from the emf of cell 2 is

o ΔC podT represents the )=∫ where ∑ (HTo − H298.15K 298.15 enthalpy increment of the product minus the reactants, each multiplied by their respective stoichiometric coefficients. T

o Similarly, ∑ (STo − S298.15K )=∫ (ΔC po/T )dT . From the 298.15 standard Gibbs energy change for the reaction at different temperatures, the standard enthalpy of formation at 298.15 K (ΔfHo298.15 K) can be derived. Since the heat capacity data for Eu3O4 is not available in the literature, it is estimated using the Neumann−Kopp rule {Cop(Eu3O4) = Cop(EuO) + Cop(cubic Eu2O3)}.8 The auxiliary data for Eu is taken from Pankratz8 and for oxygen gas from NIST-JANAF.14 Since a reliable value for the standard entropy of Eu3O4 is also not available in the literature, values of both ΔfHo298.15 K and ΔfSo298.15 K are derived simultaneously from the experimental Gibbs energy of formation data as a function of temperature using multivariate analysis. In this iterative procedure, both ΔfHo298.15 K and ΔfSo298.15 K are treated as independent variables to get the best fit to the high-temperature Gibbs energy of formation at different temperatures using least-squares optimization. The results of the analysis are shown in Figure 5. The analysis yields

ΔμO (Eu3O4 + EuO)(± 1500)/J·mol−1 2

= ΔμO (Nb + NbO) − 4FE2 2

= −874076 + 174.38(T /K)

(16)

The data for oxygen potential corresponding to the Eu3O4 + EuO equilibrium, as reported by Sukhushina and Vasil’eva10 as a function of temperature, is compared with that obtained in this study in Figure 4. Although the results cross at 1262 K,

Figure 4. Comparison of the oxygen chemical potential for the Eu3O4 + EuO equilibrium measured in this study with data reported by Sukhushina and Vasil’eva10 using a non-steady-state emf method. Figure 5. Third law analysis of the high-temperature Gibbs energy of formation of Eu3O4 obtained in this study.

they diverge with temperature in both directions. The entropy change for the oxidation of EuO to Eu3O4 obtained from the data of Sukhushina and Vasil’eva10 (−126.4 J·mol−1·K−1) is significantly less negative than that derived from the present results (−174.05 J·mol−1·K−1). The data of Sukhushina and Vasil’eva10 indicate that Eu3O4 is unstable below 1045 K and will decompose to a mixture of EuO and Eu2O3. In contrast, the results of this study show that Eu3O4 is marginally stable at all temperatures below the melting points of the oxides. 3.3. Enthalpy of Formation of Eu3O4 and EuO at 298.15 K: Third−Law Analysis. Using the third-law method, the standard enthalpy of formation (Δf Ho298.15 K) for Eu3O4 can be computed from the standard Gibbs energy of formation at each experimental temperature. For Eu3O4 the formation reaction,

3Eu + 2O2 → Eu3O4

ΔfHo298.15 K = −2265.60 (±2.5) kJ·mol−1 and ΔfSo298.15 K = −423.01 (±2.2) J·mol−1·K−1. The corresponding second-law enthalpies and entropies of formation of Eu3O4 at 298.15 K are −2264.6 (±4.3) kJ·mol−1 and −421.3 (±4.1) J·mol−1·K−1, respectively. Thus, there is good agreement between the second and third law values. The standard entropy of Eu3O4 at 298.15 K obtained from the third-law assessment is 220.75 (±2.2) J· mol−1·K−1. It is interesting to note that the sum of the entropies of EuO and Eu2O3(c) at 298.15 K from Pankratz8 is 223.8 J· mol−1·K−1. The sum of the standard entropy of EuO assessed in this study and that for Eu2O3(c) from Pankratz8 is 218.0 J· mol−1·K−1. Both values are close to the value obtained in this study for Eu3O4. The enthalpy of formation Eu3O4 suggested by Sukhushina et al.15 is ΔfHo298.15 K = −2255.6 (±9.9) kJ·mol−1. From the vaporization study of Eu3O4 in the temperature range from 1604 to 2016 K using target-collection and masso spectrometry, Haschke and Eick16 derived ΔfH298.15 K = o −1 −2269.4 (±15.1) kJ·mol and S298.15 K = 203.34 (±10.9) J· mol−1·K−1, with a questionable estimate of heat capacity for Eu3O4 as Cop(Eu3O4) = 1.5 Cop(Eu2O3) − 0.5Cop(O2). Revising the calculations of Haschke and Eick,16 Rard17 has suggested

(17)

o ∑ (HTo − H298.15K ) o o + T {Δf S298.15K + ∑ (STo − S298.15K )}

o Δf H298.15K = Δf GTo −

(18) E



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ΔfHo298.15 K = −2270.4 (±12) kJ·mol−1 and So298.15 K = 209.2 (±8.8) J·mol−1·K−1. The results obtained in this study are more accurate and comprehensive than sparse information available in the literature for Eu3O4. For EuO, both low-temperature (16 to 300 K)18 and hightemperature (300 to 1725 K),19 heat capacity data are available and in principle a proper third-law analysis is possible. However, when using the standard entropy of EuO (83.64 J· mol−1·K−1) for third-law analysis, derived values of ΔfHo298.15 K were found to increase linearly with temperature. This suggests possible error in So298.15 K. Hence, a multivariate analysis was conducted to obtain a consistent set of values for the enthalpy and entropy of formation of EuO. The results, shown graphically in Figure 6, are ΔfHo298.15 K = −603.4 (±1.5) kJ· mol−1 and ΔfSo298.15 K = −102.52 (±1.3) J·mol−1·K−1 for the formation reaction,

Eu + 1/2O2 → EuO

83.64 J·mol−1·K−1 are suggested in the compilations of Knacke et al.9 and Pankratz,8 respectively. For enthalpy of formation of EuO at 298.15 K, Knacke et al.9 suggest −589.944 kJ·mol−1, while Pankratz8 suggests −597.894 kJ·mol−1 based on the assessment of McMasters et al.19 Vaporization studies of Haschke and Eick20 yield a value of −607.52 (±17.2) kJ mol−1, solution calorimetric measurements of Burnett and Cunningham21 suggest a value of −607.52 (±9.2) kJ·mol−1 and combustion calorimetry by Huber and Holley22,23 generates a value of −589.94 (±5.4) kJ·mol−1. 3.4. Thermodynamic Data Tables for Eu3O4 and EuO. The thermodynamic data as evaluated in this study for Eu3O4 and EuO are used to create the thermodynamic data tables for both the compounds. All the thermodynamic properties are listed at regular intervals of temperature for Eu3O4 and EuO in Tables 3 and 4, respectively. The auxiliary data from NISTJANAF14 is used here for oxygen gas and that from Pankratz8 for Eu. A comprehensive data table does not exist in the literature for Eu3O4, and Table 3 fills the gap. For EuO Table 4 provides improved information. 3.5. Relative Stability of Europium Oxides. A useful way to display the relative stability of europium oxides is through enthalpy and Gibbs energy of mixing diagrams. The enthalpy of mixing at 298.15 K and Gibbs energy of mixing at 1000 K for the system Eu−O are presented in Figure 7. The minimum in enthalpy and Gibbs energy of mixing occurs at the composition corresponding to Eu2O3. It is seen from each plot that EuO and Eu2O3 are very stable oxides and Eu3O4 is only marginally stable relative to the other two. The value for Eu3O4 is only marginally more negative compared to the line joining the values for EuO and Eu2O3. The oxygen chemical potential corresponding to the reduction of europium oxides is compared with that for other rare−earth elements and some typical oxides in Figure 8. It is seen that the oxygen potential for the reduction EuO to metal Eu is comparable to that for the reduction of La2O3 to La metal. However, the oxygen potentials for the reduction of Eu2O3 to Eu3O4 and for the reduction of Eu3O4 to EuO are substantially higher, but below those for the reduction of NbO and MnO. It is seen that Ca metal can be used as a reductant for EuO at high temperatures.

(19)

Figure 6. Third law analysis of the high-temperature Gibbs energy of formation of EuO obtained in this study.

The corresponding second-law enthalpies and entropies of formation of EuO at 298.15 K are −603.5 (±2.3) kJ·mol−1 and −102.7 (±2.2) J·mol−1·K−1, respectively. Thus, there is excellent agreement between the second and third law values. The value of So298.15 K for EuO obtained in this study from thirdlaw analysis is 77.88 (±2.2) J·mol−1·K−1. Values of 81.588 and

4. CONCLUSIONS Accurate values of the standard Gibbs energy of formation of Eu3O4 and EuO are derived from new electrochemical

Table 3. Thermodynamic Properties of Eu3O4 at Regular Intervals of Temperature at P = 101.3 kPa (Tr = 298.15 K) T (K)

Cop (J·K−1·mol−1)

So (J·K−1·mol−1)

−[Go − Ho(Tr) ]/T (J·K−1 ·mol−1)

Ho − Ho(Tr) (kJ·mol−1)

ΔfHo (kJ·mol−1)

ΔfGo (kJ·mol−1)

log Kf

298.15 300 400 500 600 700 800 900 1000 1090 1090 1100 1200 1300

173.427 173.837 182.920 188.426 192.435 195.866 198.882 201.824 204.669 207.062 207.062 207.330 209.869 212.300

220.75 221.824 273.662 315.369 350.248 380.275 406.696 430.338 451.784 469.543 469.543 471.435 489.603 506.513

220.75 220.753 228.264 241.917 257.299 272.869 288.043 302.608 316.502 328.433 328.433 329.724 342.319 354.321

0 0.321 18.159 36.726 55.769 75.184 94.922 114.957 135.282 153.810 153.810 155.882 176.742 197.850

−2265.602 −2265.539 −2261.991 −2258.077 −2254.240 −2250.599 −2247.198 −2244.218 −2241.813 −2240.257 −2267.897 −2267.669 −2265.342 −2262.834

−2139.481 −2138.695 −2097.136 −2056.507 −2016.652 −1977.411 −1938.669 −1900.326 −1862.277 −1828.249 −1828.252 −1824.191 −1783.995 −1743.989

374.8 372.4 273.9 214.8 175.6 147.6 126.6 110.3 97.27 87.61 87.61 86.62 77.65 70.07

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Table 4. Thermodynamic Properties of EuO at Regular Intervals of Temperature at P = 101.3 kPa (Tr = 298.15 K) T (K)

Cop (J·K−1·mol−1)

So (J·K−1·mol−1)

−[Go − Ho(Tr) ]/T (J·K−1·mol−1)

Ho − Ho(Tr) (kJ·mol−1)

ΔfHo (kJ·mol−1)

ΔfGo (kJ·mol−1)

log Kf

298.15 300 400 500 600 700 800 900 1000 1090 1090 1100 1200 1300

48.744 48.953 50.208 50.961 51.463 52.007 52.509 53.179 53.932 54.572 54.572 54.643 55.354 56.066

77.88 78.182 92.617 103.989 113.374 121.377 128.374 134.610 140.261 144.941 144.941 145.440 150.230 154.693

77.88 77.881 79.996 83.775 87.994 92.232 96.340 100.264 103.994 107.189 107.189 107.534 110.900 114.103

0 0.090 5.048 10.107 15.228 20.402 25.627 30.912 36.267 41.150 41.150 41.696 47.196 52.767

−603.4 −603.387 −602.697 −602.013 −601.434 −600.976 −600.640 −600.472 −600.513 −600.761 −609.975 −609.985 −610.071 −610.103

−572.835 −572.644 −562.562 −552.647 −542.857 −533.150 −523.5 −513.879 −504.264 −495.607 −495.608 −494.552 −484.058 −473.556

100.4 99.70 73.46 57.73 47.26 39.78 34.18 29.82 26.34 23.75 23.75 23.48 21.07 19.03

tables for Eu3O4 and EuO are compiled. Contrary to the suggestion of Sukhushina and Vasil’eva,10 the results of this study clearly indicate that Eu3O4 is marginally stable relative to EuO and Eu2O3 at all temperatures below their melting points. The oxygen chemical potential for the reduction of EuO to Eu metal is comparable to that for the reduction of La2O3 to La metal. Metal Ca may be used for the reduction of EuO to Eu metal.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 80 2293 2494; Fax: +91 80 2360 0472. Email: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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Figure 7. Enthalpy of mixing at 298.15 K and Gibbs energy of mixing at 1000 K for the system Eu−O.

REFERENCES

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Figure 8. Oxygen potential diagram a la Ellingham−Richardson−Jeffes comparing oxidation/reduction equilibria of the system Eu−O with that of other typical Ln−O systems.

measurements using solid state cells incorporating yittria-doped thoria as the electrolyte and a mixture of Nb and NbO as the reference electrode. Using multivariate analysis, values of both enthalpy and entropy of formation of Eu3O4 and EuO at 298.15 K are evaluated. Based on the results, thermodynamic data G



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Gibbs Energy of Formation of Eu3O4 and EuO - Journal of Chemical

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