High-temperature stabilities of Rb3CrO4, Rb4CrO4 and Rb2Cr2O7 by solid electrolyte EMF

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journal of nudear makials

Journal of Nuclear Materials 217 (1994) 104-109

High-temperature stabilities of Rb,CrO,, Rb,CrO, and Rb,Cr20, by solid electrolyte EMF R. Pankajavalli, O.M. Sreedharan,

J.B. ~nanamoorthy

Metallurgy Diuision, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 10.2, India

Received 31 January 1994; accepted 30 June 1994

Abstract

The oxygen potentials in the co-existing mixtures Rb~CrO~/Rb~CrO~/Cr*O~ (I) and Rb,Cr,O,(l)/ Rb,CrO,/ Cr,O, (II) were measured by using solid electrolyte galvanic cells with a 15 mol% calcia-stabilized zirconia tube as the electrolyte and air (PO* = 0.21 atm)/Pt as the reference electrode. The EMF values of these electrodes yielded the least-squares expressions (E, f 2.57) (mV) = 1216.81- 0.500627’ (K) (798-967 K), (E, * 3.45) (mV) = 1167.13 0.44982T (K) (994-1189 K) and (E,, f 1.7) (mV) = 335.66 - 0.25697T(K) (973-1153 K), respectively. From the expressions for E,, the transition temperature Ttrans and the standard enthalpy of transformation, AG>,trans for Rb,CrO, were determined to be 978 K and 8.0 f 1.0 kJ/mol, respectively. By making use of the standard Gibbs energy data for tw-Rb&rO, and Cr,O, from the literature, the standard Gibbs energies of formation of Rb,CrO, and Rb,Cr,O, (1) were determined to be (AGF,, (Rb&rO,,

s) j, 2.22) &J/mol)

= - 1537.62 + 0.38342T(K)

and W&-

(Rb,Cr,07,

I) & 2.89) W/mol)

= - 2049.89 + 0.55197T(K).

By combining with Gibbs energy and phase equilibrium data on Rb,CrO, diagram for the Rb-Cr-0 system at 773 K has also been proposed.

1. Introduction

Rubidium is a fission product in nuclear reactors formed by the @-decay of “Kr [1,2]. Though its yield is reiatively minor, compared to cesium, still its interaction with oxide fuel and containment materials continues to be of interest and importance in nuclear technology. Further, identification of systematic trends if any, in the phase diagrams and thermodynamic stabilities of ternary phases in the A-Cr-0 system, where A is an alkali metal (and Cr is presumably the most electropositive component of structural alloys) was found to be hampered by the conspicuous gaps in the 0022-3115/94/$07.00

the phase

information on the Rb-Cr-0 system as could be seen from the following. The the~odynamic stability of NaCrO, had been reported in the literature by employing solid electrolyte EMF [3-61 as well as Knudsen effusion [7,8] techniques. The standard Gibbs energy data on KCrO, were reported by Sreedharan et al. [Q,lO] using EMF measurements and by Ganesan and Borgstedt [ll] employing an oxygen probe dipped in liquid potassium. These measurements were facilitated by the availability of reliable phase diagrams on Na-Cr-0 after Knights and Phitlips [7] and on K-Cr-0 after PankajavalIi [12] together with calorimetric data reported by O’Hare

0 1994 Elsevier Science B.V. All rights reserved

SSDI 0022-3115(94)00342-4

assessed from the literature,

R. Pankajavalliet al./Journal of Nuclear Materials217 (1994) 104-109

and Boerio [13] on AzCrO, (A = Li, Na or K). Phase diagrams for Li-Cr-0 and Cs-Cr-0 systems have also been reliably reported in the literature [7,14,151. But no such equilibrium could yet be found for the Rb-Cr-0 system. However, the existence of the ternary phases namely Rb,CrO, and Rb,CrO, in this systems have been confirmed by Lindemer et al. 1141 and Gadd and Borgstedt [1,2]. The latter investigators reported the results of their oxygen potential measurements carried out in liquid rubidium at 773 K using an electrochemical oxygen probe. They suggested that Rb,CrO, formed in the presence of insufficient quantities of liquid rubidium could be metastable in the temperature range of their investigation namely 673773 K. Precise thermodynamic data on cu-RbzCrO, were reported by O’Hare and Johnson [16] from calorimetry, but the transition temperature from (Y-to p-form was considered to be unreliable [17]. In view of the above survey, this investigation was undertaken in order to determine the thermodynamic stabilities of Rb,CrO, and Rb,Cr,O, with reference to a- RbzCrO, and to characterize its (Yto l3 transformation with the help of a sensitive EMF technique.

2. Experimental 2.1. Materials Reagent grade RbzCO, (Merck, Germany), RbCl (Fluka, Germany), Na,Cr,O, and Cr,O, (Johnson and Mathey, UK) all of which were of 99.9% purity or better were used as the starting materials. Mixtures of Rb,CO, and Cr,O, in the mole ratio 3 : 1 and 1: 1 were used to synthesize Rb,CrO, and RbzCrO,. These mixtures were compacted into cylindrical pellets of 12 mm diameter and 3 mm thickness in an hydraulic press at a pressure of 100 Mpa followed by heating in a stream of purified argon at 973 and 1050 K for a total duration of 20 h. The above procedure was repeated 2-3 times to ensure completion of the reaction as checked by the absence of starting materials within the 5 mass% limit of detection by the powder XRD method. The compound Rb,Cr,O, was made by a two-step fractional crystallization from an aqueous mixture of Na,Cr,O, and RbCl exploiting the difference in the solubilities of sodium and rubidium dichromates [18] especially near the ice point. 2.2. Phase equilibrium studies Mixtures of Rb,CrO,/Rb,CrO,/Cr,O, and Rb, Cr,O,/ Rb,CrO,/ Cr,O, were prepared by grinding and compacting as mentioned above followed by equilibration in flowing argon atmosphere at 773 K for 80-120 h. The products of equilibration were ascertained by XRD.

105

2.3. EMF method The test electrodes were made by intimate mixing of the co-existing phases in equimass ratio in the first run followed by about 20% variation in the mass ratio in the two subsequent runs in order to ascertain the attainment of equilibrium. The compaction of the electrode discs were carried out as described above. The EMF measurements were made on the following galvanic cells:

Pt, Rb,CrO,(s),

Rb,CrO,(s),

Cr,O,(s)/lSCSZ/air

(PO, = 0.21 atm) , Pt

(I)

and Pt, Rb,Cr,O,(l),

Rb,CrO,(s),

Cr,O,(s)/lSCSZ/air

( PO, = 0.21 atm) , Pt

(II)

where 15CSZ represents a 15 mol% calcia-stabilized zirconia electrolyte. The electrolyte is in the form of a tube having the dimensions 12.7 mm outer diameter, 9.5 mm inside diameter and 305 mm length with closed end flat (Corning USA). The solid electrolyte being gas-impervious also served the purpose of isolating the test electrode environment from that of the reference electrode. The temperature of the galvanic cell was measured by using a standard Pt-10% Rh versus pure Table 1 Temperature dependence of EMF of cell (I): Pt,Rb,CrO,(s), RbaCrO,(cY or p), Cr,O,(s)/lSCSZ/air Serial No. a

T (K)

kVj

Serial No. a

Temperature range A-l 798.75 A-2 820.35 A-3 867.15 A-4 891.25 A-5 914.55

of a-Rb,CrO, 817.76 A-6 807.72 A-7 780.90 B-l 767.45 B-2 755.67 B-3

Temperature range A-l 1087.15 A-2 1110.75 A-3 1134.85 A-4 1158.15 A-5 1185.15 A-6 1001.85 A-7 1036.05 A-8 1055.45 A-9 1079.45 A-10 1104.65 B-l 1036.05 B-2 1056.15 B-3 1078.55

of B-RbaCrO, 680.66 B-4 667.74 B-5 655.40 B-6 644.08 B-7 631.62 B-8 711.93 B-9 697.95 C-l 686.75 c-2 676.88 c-3 665.59 c-4 705.96 c-5 696.72 C-6 685.85

(PO, = 0.21 atm), Pt T (Kl

c”mv,

938.75 963.25 843.95 931.65 966.85

744.35 734.07 795.54 754.62 736.31

1100.75 1093.45 1128.85 1147.15 1166.65 1188.55 994.55 1015.15 1030.05 1048.15 1068.25 1093.45

675.44 680.39 663.93 653.63 643.17 632.50 722.44 713.42 702.94 692.95 685.01 670.69

’ A to C are different series of measurements based on different pellets with marginal (20%) variation in the equimass composition of three phases in order to ascertain the equilibrium nature of the EMF values.

R. Pankajavalli et al. /Journal

106

of Nuclear Materials 217 (1994) 104-109

Table 2 Temperature dependence of EMF of cell (II): Pt,Rb,Cr,O,(l), Rb,CrO,(s), CrrO&s)/lSCSZ/air (PO2 = 0.21 atm), Pt

760

?

w 700 -

630

773

873

973

1073

1173

1273

Serial

T

Serial

T

E

No. =

WI

;“,

No. a

UC)

(mV)

A-l A-2 A-3 A-4 A-5 A-6 A-7 B-l B-2

973.35 1005.15 1016.65 1038.95 1063.15 1085.75 1107.45 1080.95 1097.95

86.19 79.64 74.59 68.45 61.38 54.46 49.68 54.56 51.38

B-3 B-4 B-5 B-6 B-7 B-8 B-9 B-10

1118.55 1134.15 1152.55 1028.65 1044.85 1061.65 1080.55 1096.15

48.43 45.14 41.53 70.17 66.47 63.71 60.45 56.59

T/K

Fig. 1. The temperature

dependence

a A to B are different series of measurements based on different pellets with marginal (20%) variation in the equimass composition of three phases in order to ascertain the equilibrium nature of the EMF values.

of EMF of cell (I):

Pt, Rb,CrO.,(s), Rb,CrO,(a or g), Cr,O,(s)/l5CSz/o,(Po, = 0.21 atm, air), Pt

electrode to yield the following equations. Pt thermocouple calibrated at the freezing points of Zn, Sb and Ag. The absence of asymmetric potentials that could arise from the chemical as well as crystallographic inhomogeneities or from the temperature gradient across the cell head was confirmed by null-EMF measurements using air/h as the reference electrode material on both sides. The reproducibility of the EMF values was verified by thermal cycling and by the use of three-phase electrodes of different compositions from one run to another for each cell configuration. Other experimental details were given elsewhere [ 19,201.

f 2.57) (mV) = 1216.81 - 0.466927’(K), ( %X~.Xtl%i (4) ( Ecorrectedk 3.45) (mV) = 1167.13 - 0.41612T(K). (5) The overall galvanic cell reaction for the passage of 10 faraday of electricity could be represented by the equation 4Rb,CrO,(s) =

3. Results The results on the EMF of cell I given in Table 1 and plotted in Fig. 1 could be represented by leastsquares expressions corresponding to the temperature ranges 798-967 K and 994-1189 K, respectively: (E f 2.57) (mV) = 1216.81-

050062T(K),

(1)

(E & 3.45) (mV) = 1167.13 - 044982T(K).

(2)

+ Cr,O,(s)

6Rb,CrO,(

+ $0,(g)

(x or p).

(6)

Using the Nernst equation, the standard Gibbs energy change, AG& corresponding to the corrected EMF expressions (4) and (5) valid for the o- and p-forms of Rb,CrO, were calculated to be (AG;,,,

f 2.48)(kJ/mol)

= - 1174.07 + 0.450527’(K),

(7)

Likewise the results on cell II as given in Table 2 and Fig. 2 have been fitted into the following least-squares expression valid over the range 973-1153 K: (E f 1.7) (mV) = 335.66 - 0.25697T(K).

(3)

4. Discussion 4.1. (Y to p-phase transition in Rb,CrO, The EMF expressions (1) and (2) were corrected for the standard state of oxygen in the air/Pt reference

Fig. 2. The temperature dependence of EMF of cell (II): Pt,Rb,CrsO,U), Rb,CrO,W, Cr,03(s)/15CSZ/O;?(Poz = 0.21 atm, air), Pt

It. P~n~j~~~~ et al. /.kwmI

of~~lear Materials217 (1994) 104-109

( AG;,,, f 3.33) (kJ/mol) = - 1126.13 + 0.40150T(K),

(8) respectively. Solving Eqs. (7) and (8), the a to l3 transformation temperature could be found to be 978 K from the following equation: ~AG~~-~~(Rb~CrO~) f 0.97) (~/rnol~ = 7.99 - O.~817T(K). (9) This is in good agreement with 973 K reported in the compilation by Levin et al. 1211but in poor agreement with 998 K reported by O’Hare et al. [16]. In order to cross check the transition temperature, a differential scanning calorimetric run was taken (using PerkinElmer DSC-2) during this investigation and was found to yield a peak with an inception temperature of 978 1 1 K at a scanning rate of lO”/min. Further, drop calorimetry has not been considered to be a reliable method for the determination of such transition temperatures according to O’Hare [17]. The standard enthalpy of transition at 978 K could be found to be 7.99 kJ/mol in fair agreement with 5.48 kJ/mol at 998 K

Ml. For the standard Gibbs energy of fo~ation of ol-Rb&rO,, the following linear expression was computed for further use from the data tabulated by O’Hare et al. [16].

( AG;,,(a: - Rb,CrO,)

it 0.9) (kJ/mol)

= - 1405.76 + 0.37190T(K) Combining

(800-978 K),

(10)

Eqs. (9) and (lo),

( AG';,r(l3 - Rb,CrO,)

f 2.0) (~/mol)

= - 1397.76 + 0.363731”(K)

(11)

could be obtained.

107

For the passage of 4 faraday of electricity, the overall galvanic cell reaction for the cell II could be represented by :Rb,CrO,(s) c

f $&O,(s)

+ O,(g)

zRb,Cr,O,(f).

(14)

Using the Nernst equation, the oxygen potential RT In PO, of the test electrode was calculated from the expression (13) and is given by (RT In Po, f 0.66) (kJ/mol) = - 129.55 + O.O8617T(K).

(15)

Substituting the expressions (11) and (15) together with that for AG;,r(Cr,O,, s) into the reaction (141, the equation for AG~,=(Rb~~r*O,, I) was calculated to be ( AG;,r(Rb,Cr,O,, = -2049.89

E) f 2.89) (kJ/mol) + 0.55197T(K),

(16)

valid over the range 973-1153 K. 4.4. Computation ture data

of AG~,,(Rb4Cr04, s) from the litera-

Gadd and Borgstedt [1,2] reported AG$Rb,CrO,, s) values from EMF measurements carried out on a mixture of Rb(Z)/Rb,CrO,(s) taken in a stainless steel capsule. As these calculations did not take into account the thermodynamic activity of Cr (a,) of the container materials which was a Fe-Cr-Ni-Mo alloy stabilized with Ti, it was found necessary to recalculate the AGF,, of Rb,CrO,. As these are no direct measurements of a,, in this alloy (designated as X-lo), we used the following expression for a,-, directly determined for an almost similar alloy (designated as D-9) reported by Azad et al. [241

4.2. Standard Gibbs energy data on Rb,CrO,(s) (log a&D-9) Combining the expressions (7) and (10) with the stoichiometric galvanic cell reaction (6) together with the values for AGT,(Cr,O,, s) assessed in the literature 122,231,the A&- of Rb,CrO, could be derived to be ( AG;,r( Rb,CrO,,

f 0.01) = 1.113 - 1713.92/T

(976-1132 K).

(17)

The above expression was combined with the relations (RT In Po,)

(W) = -576.63

+ 0.2055T - 385.948E’, (18)

s) rfi 2.22) (kJ/mol)

= - 1537.62 + 0,38342T(K).

(K)

and (12) (E’) (V) = (4 x 10-5)T + 0.48108,

4.3. Gibbs energy data on Rb,Cr,O,Of The EMF expression (3) corresponding to the galvanic cell II could be corrected for the standard state of oxygen in the air/Pt reference electrode to be as follows ( J%*IWte*& 1.7) (mV) = 335.66 - 0.22327T(K). (13)

(19)

both of which were given by Gadd and Borgstedt to yield the corrected expression for AG;,, of Rb,CrO,

( AG;,r(Rb$r%

s)) W/mol)

= - 1557.4 + 0.40142T(K).

(20)

However the validity of such a correction has to be confirmed by independent experimental values. If one could back-calculate the EMF value, E’, making use of

R. Pankajuualli et al. /Journal

108

of Nuclear Materials 217 (1994) 104-109

the fact that AGy,,(Rb,CrO,, s) (from Eq. (20)) = 2 RT In Po, at 773 K, it would yield a value of 0.533 V. In an earlier experiment Gadd and Borgstedt used excess chromium powder along with Rb,O in liquid rubidium and followed the attainment of equilibrium at 773 K with the help of the oxygen probe. They found that the meter reached a constant value of 0.536 V after 300 h. The proximity of the value with 0.533 V computed by correcting for a,, in the stainless steel container in their later experiment with Rb(f)/Rb,CrO,(s) in the stainless steel vessel without excess Cr upholds the validity of the Gibbs energy data given in Eq. (20). 4.5. Rb-Cr-0

Table 3 Equilibrium the Rb-Cr-0

Rb pressures system

Serial No.

Co-existence mixture

1 2 3

Rb,CrO, /Rb,CrO, Rb,CrO, /Rb,CrO, Rb,CrO,/CrzO,/Cr

over some co-existent

logw,,l /mm Hg (A + B/T 7.7526-10932/ 8.067-5061/T 8.097-5061/T

(K)) T

mixtures

in

log1 P,,l /mm Hg at 773 K - 6.39 + 1.52 + 1.55

Rb,CrO, (in Rb(Z)). The formation of RbCrO, could be explained by the driving of the following reaction,

phase diagram

The phase equilibrium studies as well as the powder XRD patterns of electrode pellets used in the present investigation have confirmed the existence of the three-phase fields, Rb,CrO,/Cr,O,/Rb,CrO,, Rb, Cr,O,/Cr,O,/Rb,CrO, over the range 773-1200 K. From Section 4.4, the co-existence of Rb(l)/Rb,CrO, /Cr at 773 K is well substantiated by the experimental evidence from the literature. However, when Gadd and Borgstedt had distilled the reaction products of Cr/Rb,O in liquid Rb(l), they found the residue to be RbCrO, by chemical analysis and by the likeness of its XRD pattern to that of RbGaO,. On the contrary, the temperature dependence of such an interaction product (RbCrO,) in Rb(f) and excess Cr was interpreted by them as that corresponding to below - the - saturation level of oxygen. As this interpretation was deemed to be correct, the dissolved ternary oxide could well be

Rb,CrO,(s)

+ Cr(s) e

2RbCr0,

+ 2Rb( f)

(21)

towards the forward direction during distillation under high vacuum, perhaps at a temperature lower than 773 K. Thus, dispelling the existence of RbCrO,, the isothermal section of the Rb-Cr-0 phase diagram at 773 K could be drawn as shown in Fig. 3. The triangular fields Rb,Cr0,/Rb,Cr0,/Cr,03 and Rb,CrO,/ Cr,O,/Cr were drawn not only on the basis of the similarities with the system Cs-Cr-0 [14,15] but also on account of the trend in the values of log P,, given in Table 3. This table was constructed using the Gibbs energy data on rubidium chromates and the vapour pressure data of Rb from the literature [25].

5. Conclusion Determination of the Gibbs energy data for the formation of Rb,Cr,O,(l) and Rb,CrO,(s) besides the assessment of that for Rb,CrO, from the literature in conjunction with the tabulated Gibbs energy values of cu-Rb&rO, have enabled the construction of a RbCr-0 phase diagram. The transition temperature and the standard enthalpy at Ttrans were found be 978 K and 7.99 kJ/mol, respectively, from the precise EMF measurements.

Acknowledgement

This investigation forms part of the thesis to be submitted by one of the authors (R.P.) for the Ph.D, degree from the University of Madras.

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Fig. 3. Isothermal section Cr-0 system at 773 K.

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R. Pankajavalli et al. /Journal

of Nuclear Materials 217 (1994) 104-109

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[13] P.A.G. O’Hare and J. Boerio, J. Chem. Thermodyn. 7 (1975) 1195. [14] T.B. Lindemer, T.M. Besmann and C.E. Johnson, J. Nucl. Mater. 100 (1981) 178. [15] A.M. Azad, O.M. Sreedharan and P. Rodriguez, J. Alloy Phase Diagr. 5 (1989) 1. [16] P.A.G. O’Hare and G.K. Johnson, J. Chem. Thermodyn. 17 (1985) 391. [17] P.A.G. O’Hare, Private communication (September 1993). [18] H. Stephen and T. Stephen, Solubilities of Inorganic and Organic Compounds (Pergamon Press, Oxford, 1963). [19] O.M. Sreedharan, E. Athiappan, R. Pankajavalli and J.B. Gnanamoorthy, J. Less-Common Metals 68 (1979) 142. [20] A.M. Azad, R. Pankajavalli and O.M. Sreedharan, J. Chem. Thermodyn. 18 (1986) 255. [21] E.M. Levin, C.R. Robins and H.F. McMurdie, Phase Diagrams for Ceramists (American Ceramic Society, Columbus, OH, 19641. [22] O.M. Sreedharan and C. Mallika, Indira Gandhi Centre for Atomic Research, Kalpakkam, T.N., India, RRC-69, (19841. [23] O.M. Sreedharan and J.B. Gnanamoorthy, J. Nucl. Mater. 89 (1980) 113. [24] A.M. Azad, O.M. Sreedharan, C. Narayanan and J.B. Gnanamoorthy, Stand. J. Metals (19921, in press. [25] 0. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th ed. (Pergamon Press, Oxford, 1983).