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Thermochemical studies on the plutonium fluorides and protactinium ovides
Journal of the Less-Common
Metals,
121 (1986)
61
61 - 66
THERMOCHEMICAL STUDIES ON THE PLUTONIUM FLUORIDES AND PROTACTINIUM OXIDES* P. D. KLEINSCHMIDT and J. W. WARD Materials Science and Technology NM 87545 (U.S.A.)
Division,
Los Alamos National Laboratory,
Los Akamos,
summary The ionization and appearance potentials of parent and fragment ions from the neutral molecules PuF6 and PuF, were measured using electron impact ionization. From this data the bond dissociation energies of the molecules PuF,-F (n = 0 to 5) were calculated. Vapor pressures of PaO*(g), PaO(g) and Pa(g) were measured above the two-phase system Pa(l) + PaOz _ Y(s), and the congruently vaporizing composition PaOZ_ x(s). From these measurements, the enthalpies of formation of Pa(g), PaO(g) and PaO,(g) were calculated to be respectively 131.7 + 6.1 (551 + 26), 2.4 f 4.1 (10 + 17), and -122.7 f 4.0 (-513 f 17) kcal mol-’ (kJ mol-l). Bond dissociation energies of PaO-0 and Pa-O were found to be respectively 184.6 f 4.1 (772 * 17) and 188.4 f 4.1 (788 + 17) kcal mol-’ (kJ mol-l).
1. Plutonium fluorides Plutonium hexafluoride was leaked into the ion source of a quadrupole mass spectrometer. The ion PuFl was not observed, but the appearance potentials of the fragment ions were measured using the extrapolated voltage difference method [ 11. These ionization efficiency curves are shown in Fig. 1 and the appearance potentials are listed in Table 1. Plutonium pentafluoride was generated by placing a heated piece of aluminium oxide over the nozzle of the leak inlet. The ionization efficiency curves of the parent and fragment ions were measured and are shown in Fig. 2. The values of the appearance potentials are given in Table 1. Approximate bond energies are calculated from the appearance potentials by taking the difference for the two reactions: PuF,+e-+PuF:+F+2e PuF,+e-+PuF:+2e This can be expressed algebraically by the equation *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. 0022-5088/86/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
62
20
30
40
50
E(eV) Fig. 1. Ionization efficiency curves for the fragment ions from PuF6. TABLE 1 Ionization and appearance potentials for the ions PuF,,+ Neutral ion
Pu’ PuF+ PuF: PLlF: PuF; PuF; PuF;
PuF6 (eV)
PuF5 (ev)
PuF4 (eW
PuF3 (eV)
PuFz (eW
PuF (eV)
39.2 32.8 27.7 24.1 20.5 17.4
26.5 22.7 18.5 14.8
_
25.0 [3] 17.8 [3] 13.0 [3] a.a=
6.4 [3]
5.9 [3]
_ 128
6.1 [2]
b
aEstimated. bnot observed in mass spectrum. 100
az w
0
0
10
20
30
40
Pu (eV)
50
E(eV) Fig. 2. Ionization efficiency curves for the parent and fragment ions from PuFs.
63
D 6.5
= A%,5
-
Ips
AP,,, is the where D6,s is the bond dissociation energy of PuFs-F, appearance potential of the first reaction and IPs is the ionization potential of PuF+ Similar equations can be used to calculate the bond dissociation energies of the other molecules from appearance potentials. These results are shown in Table 2. Comparisons are given with the Knudsen effusion measurements of Kent [ 31 on the plutonium fluorides and Knudsen effusion measurements on ID,-F [4]. The bond dissociation energies for PuF, -F follow a pattern similar to that measured for UF,-F. TABLE 2 Bond energies of the plutonium Bond
PiiF (eV)
Pll-F PuF-F PuF,-F PuF3-F PuF4-F PuFsF
6.24 5.58 6.12 6.77 5.90 2.55
and uranium fluorides PuFS (eW
6.23 7.42 6.44
Kent [ 33 (eV)
UF,--F (eV)
5.68 5.85 6.37 -
6.81 5.90 6.46 6.42 4.42 2.82
141
One error inherent in this type of measurement is that the state of the product ion is not known (see, for example, ref. 1). If the product ion is produced in an excited electronic state, the measured appearance potential will be too high. Errors in the calculated bond energy are probably ranged from 0.2 to 1.5 eV, with the error being greatest for the appearance potential with the highest energv and least for the appearance potential with the lowest energy.
2. Protactinium oxides Vapor pressures of the species above the compositions Pa(l) + PaOz_Y (s) and PaOz _-x (s) were measured using Knudsen effusion mass speetrometry. Pressure calibration was done using Knudsen effusion target collection [ 51. Second law fits to the data are shown in Figs. 3 and 4, and the equations for the pressure are given in Table 3. Pa(g) was not Oklerved over the congruently vaporizing composition PaOz_X (s) . The~od~ic functions were estimated for the species PaO,(g), Pa0 (g), Pa(g), PaOz(s) and Pa(s,l). With these functions, the enthalpies of reaction and formation are calculated at 298 K. The entbalpy of vaporization of PaOz(g) was calculated assuming that the enthalpy of PaO, _-x (s) is equal to the enthalpy of formation of PaOz(s). This difference can be
64
-7.2
-
0.42
0.44
0.46
0.46
0.50
1000/T(K)
Fig. 3. Pressures (in atmospheres) of PaOz(g), PaO(g), PaO.+ (S) system as a function of reciprocal temperature.
0.42
0.44
0.46
0.46
and Pa(g) above the Pa(l) +
0.50
1000/T(K)
Fig. 4. Pressures (in atmospheres ) of PaOz(g) and PaO(g) above the Pa02 _-x (s) system as a function of reciprocal temperature.
calculated if the oxygen potential is known as a function of temperature and composition, but such information is not available at this time. Using the value of -265.12 3.6 kcal mol-’ (-1109 f: 15 kJ mol-I) for the enthalpy of formation of PaO,(s) [5] and 142.2 f 1.6 kcal mol-’ (595 f 7 kJ mol-‘) for the enthalpy of sublimation of PaO,(s), a value of -122.9 f 4.0 kcal mol-’ (514 + 17 kJ mol-‘) is calculated for the enthalpy of formation of PaOz(s). The enthalpy of formation of Pa(g) can be calculated assuming that oxygen dissolved in the liquid protactinium does not change the Pa(g) activity. Combined second and third law calculations give a value of 131.0 f 6.1 kcal mol-’ (548 + 26 kJ mol-‘) for the enthalpy of formation of Pa(g). Pressure measurements in the two phase region Pa(l) + PaO%_,(s) give the enthalpy change for the reaction PaO,(g) + Pa(g) = 2PaO(g) (-18
The third law value of this enthalpy change is -4.3 + 3.0 kcal mol-’ f 13 kJ mol-‘). Using this value and the enthalpies of formation of
(e1.1)
-27.2
Pa0
1 Phase
= A/T + B.
(kO.3)
-28.8
Pa02
1 Phase
Lg {P(atm)}
132.7 (kO.8) (555) (*3) 142.5 (tO.3) (596) (H.3) _
129.3 (f 5.0) (541) (i21) 141.8 (k1.3) (593) (f5) -
6.4
(i0.5)
7.73 (tO.13)
_
_
(kO.7) (kO.8) (kO.5)
5.9 1.7 6.1
(tl.6) (t1.8) (tl.1)
-25.8 -16.1 -26.7
Pa02 Pa0 Pa
2 Phase 2 Phase 2 Phase
3rd law (kcal mol-‘) (kJ mol-I)
2nd law (kcal mol-' ) (kJ mol-‘)
B
A/1000
Species
system
Region
Summary of results on the protactinium-oxygen
TABLE 3
Pa(g) and Pa02(g) gives values of 184.6 * 4.1 kcal mol-’ (772 f 17 kJ mall’) for the bond energy of PaO-O(g), 188.4 f 4.1 kcal mol-l (788 + 17 kJ mol-‘) for the enthalpy of formation of PaO(g). Interpolating the bond dissociation energies of the actinide oxides found in the compilation of Ackermann [6] gives values of 180 kcal mol-’ (753 kJ mol-‘) and 190 kcal mol-’ (795 kJ mol-‘) for the bonds PaO-0 and Pa-O respectively. The values reported in this paper appear to follow the smooth trend established earlier for the other actinide oxides.
Acknowledgments The authors wish to thank the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences for support of this work. They also wish to thank J. C. Spirlet of the European Institute for the Transuranium Elements, Karlsruhe Establishment and David Brown of the Atomic Energy Research Establishment, Harwell for supplying samples of protactinium; and B. A. Dye and R. A. Breisemeister of Los Alamos National Laboratory for supplying samples of plutonium hexafluoride.
References 1 J. H. Beynon, R. G. Cooks, K. R. Jennings and A. J. Ferrer-Correia, Znt. J. Mass Spectrom. Zon Phys., 18 (1975) 87 - 89. 2 J. Sugar, J. Chem. Phys., 60 (1964) 4103. 3 R. A. Kent, J. Am. Chem. Sot., 90 (1968) 5657 - 5659. 4 J. W. Ward, P. D. Kleinschmidt and R. G. Haire, J. Chem. Phys., 71 (1979) 3920 3925. 5 J. Fuger, in N. Edelstein (ed.), Actinides in Perspective, Pergamon, New York, 1982, pp. 409 - 431. 6 R. J. Ackermann and E. G. Rauh, Rev. Znt. Hautes Temp. Refmctaires, 15 (1978) 259 - 280.
Metals,
121 (1986)
61
61 - 66
THERMOCHEMICAL STUDIES ON THE PLUTONIUM FLUORIDES AND PROTACTINIUM OXIDES* P. D. KLEINSCHMIDT and J. W. WARD Materials Science and Technology NM 87545 (U.S.A.)
Division,
Los Alamos National Laboratory,
Los Akamos,
summary The ionization and appearance potentials of parent and fragment ions from the neutral molecules PuF6 and PuF, were measured using electron impact ionization. From this data the bond dissociation energies of the molecules PuF,-F (n = 0 to 5) were calculated. Vapor pressures of PaO*(g), PaO(g) and Pa(g) were measured above the two-phase system Pa(l) + PaOz _ Y(s), and the congruently vaporizing composition PaOZ_ x(s). From these measurements, the enthalpies of formation of Pa(g), PaO(g) and PaO,(g) were calculated to be respectively 131.7 + 6.1 (551 + 26), 2.4 f 4.1 (10 + 17), and -122.7 f 4.0 (-513 f 17) kcal mol-’ (kJ mol-l). Bond dissociation energies of PaO-0 and Pa-O were found to be respectively 184.6 f 4.1 (772 * 17) and 188.4 f 4.1 (788 + 17) kcal mol-’ (kJ mol-l).
1. Plutonium fluorides Plutonium hexafluoride was leaked into the ion source of a quadrupole mass spectrometer. The ion PuFl was not observed, but the appearance potentials of the fragment ions were measured using the extrapolated voltage difference method [ 11. These ionization efficiency curves are shown in Fig. 1 and the appearance potentials are listed in Table 1. Plutonium pentafluoride was generated by placing a heated piece of aluminium oxide over the nozzle of the leak inlet. The ionization efficiency curves of the parent and fragment ions were measured and are shown in Fig. 2. The values of the appearance potentials are given in Table 1. Approximate bond energies are calculated from the appearance potentials by taking the difference for the two reactions: PuF,+e-+PuF:+F+2e PuF,+e-+PuF:+2e This can be expressed algebraically by the equation *Paper presented at Actinides 85, Aix en Provence, September 2 - 6, 1985. 0022-5088/86/$3.50
0 Elsevier Sequoia/Printed in The Netherlands
62
20
30
40
50
E(eV) Fig. 1. Ionization efficiency curves for the fragment ions from PuF6. TABLE 1 Ionization and appearance potentials for the ions PuF,,+ Neutral ion
Pu’ PuF+ PuF: PLlF: PuF; PuF; PuF;
PuF6 (eV)
PuF5 (ev)
PuF4 (eW
PuF3 (eV)
PuFz (eW
PuF (eV)
39.2 32.8 27.7 24.1 20.5 17.4
26.5 22.7 18.5 14.8
_
25.0 [3] 17.8 [3] 13.0 [3] a.a=
6.4 [3]
5.9 [3]
_ 128
6.1 [2]
b
aEstimated. bnot observed in mass spectrum. 100
az w
0
0
10
20
30
40
Pu (eV)
50
E(eV) Fig. 2. Ionization efficiency curves for the parent and fragment ions from PuFs.
63
D 6.5
= A%,5
-
Ips
AP,,, is the where D6,s is the bond dissociation energy of PuFs-F, appearance potential of the first reaction and IPs is the ionization potential of PuF+ Similar equations can be used to calculate the bond dissociation energies of the other molecules from appearance potentials. These results are shown in Table 2. Comparisons are given with the Knudsen effusion measurements of Kent [ 31 on the plutonium fluorides and Knudsen effusion measurements on ID,-F [4]. The bond dissociation energies for PuF, -F follow a pattern similar to that measured for UF,-F. TABLE 2 Bond energies of the plutonium Bond
PiiF (eV)
Pll-F PuF-F PuF,-F PuF3-F PuF4-F PuFsF
6.24 5.58 6.12 6.77 5.90 2.55
and uranium fluorides PuFS (eW
6.23 7.42 6.44
Kent [ 33 (eV)
UF,--F (eV)
5.68 5.85 6.37 -
6.81 5.90 6.46 6.42 4.42 2.82
141
One error inherent in this type of measurement is that the state of the product ion is not known (see, for example, ref. 1). If the product ion is produced in an excited electronic state, the measured appearance potential will be too high. Errors in the calculated bond energy are probably ranged from 0.2 to 1.5 eV, with the error being greatest for the appearance potential with the highest energv and least for the appearance potential with the lowest energy.
2. Protactinium oxides Vapor pressures of the species above the compositions Pa(l) + PaOz_Y (s) and PaOz _-x (s) were measured using Knudsen effusion mass speetrometry. Pressure calibration was done using Knudsen effusion target collection [ 51. Second law fits to the data are shown in Figs. 3 and 4, and the equations for the pressure are given in Table 3. Pa(g) was not Oklerved over the congruently vaporizing composition PaOz_X (s) . The~od~ic functions were estimated for the species PaO,(g), Pa0 (g), Pa(g), PaOz(s) and Pa(s,l). With these functions, the enthalpies of reaction and formation are calculated at 298 K. The entbalpy of vaporization of PaOz(g) was calculated assuming that the enthalpy of PaO, _-x (s) is equal to the enthalpy of formation of PaOz(s). This difference can be
64
-7.2
-
0.42
0.44
0.46
0.46
0.50
1000/T(K)
Fig. 3. Pressures (in atmospheres) of PaOz(g), PaO(g), PaO.+ (S) system as a function of reciprocal temperature.
0.42
0.44
0.46
0.46
and Pa(g) above the Pa(l) +
0.50
1000/T(K)
Fig. 4. Pressures (in atmospheres ) of PaOz(g) and PaO(g) above the Pa02 _-x (s) system as a function of reciprocal temperature.
calculated if the oxygen potential is known as a function of temperature and composition, but such information is not available at this time. Using the value of -265.12 3.6 kcal mol-’ (-1109 f: 15 kJ mol-I) for the enthalpy of formation of PaO,(s) [5] and 142.2 f 1.6 kcal mol-’ (595 f 7 kJ mol-‘) for the enthalpy of sublimation of PaO,(s), a value of -122.9 f 4.0 kcal mol-’ (514 + 17 kJ mol-‘) is calculated for the enthalpy of formation of PaOz(s). The enthalpy of formation of Pa(g) can be calculated assuming that oxygen dissolved in the liquid protactinium does not change the Pa(g) activity. Combined second and third law calculations give a value of 131.0 f 6.1 kcal mol-’ (548 + 26 kJ mol-‘) for the enthalpy of formation of Pa(g). Pressure measurements in the two phase region Pa(l) + PaO%_,(s) give the enthalpy change for the reaction PaO,(g) + Pa(g) = 2PaO(g) (-18
The third law value of this enthalpy change is -4.3 + 3.0 kcal mol-’ f 13 kJ mol-‘). Using this value and the enthalpies of formation of
(e1.1)
-27.2
Pa0
1 Phase
= A/T + B.
(kO.3)
-28.8
Pa02
1 Phase
Lg {P(atm)}
132.7 (kO.8) (555) (*3) 142.5 (tO.3) (596) (H.3) _
129.3 (f 5.0) (541) (i21) 141.8 (k1.3) (593) (f5) -
6.4
(i0.5)
7.73 (tO.13)
_
_
(kO.7) (kO.8) (kO.5)
5.9 1.7 6.1
(tl.6) (t1.8) (tl.1)
-25.8 -16.1 -26.7
Pa02 Pa0 Pa
2 Phase 2 Phase 2 Phase
3rd law (kcal mol-‘) (kJ mol-I)
2nd law (kcal mol-' ) (kJ mol-‘)
B
A/1000
Species
system
Region
Summary of results on the protactinium-oxygen
TABLE 3
Pa(g) and Pa02(g) gives values of 184.6 * 4.1 kcal mol-’ (772 f 17 kJ mall’) for the bond energy of PaO-O(g), 188.4 f 4.1 kcal mol-l (788 + 17 kJ mol-‘) for the enthalpy of formation of PaO(g). Interpolating the bond dissociation energies of the actinide oxides found in the compilation of Ackermann [6] gives values of 180 kcal mol-’ (753 kJ mol-‘) and 190 kcal mol-’ (795 kJ mol-‘) for the bonds PaO-0 and Pa-O respectively. The values reported in this paper appear to follow the smooth trend established earlier for the other actinide oxides.
Acknowledgments The authors wish to thank the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences for support of this work. They also wish to thank J. C. Spirlet of the European Institute for the Transuranium Elements, Karlsruhe Establishment and David Brown of the Atomic Energy Research Establishment, Harwell for supplying samples of protactinium; and B. A. Dye and R. A. Breisemeister of Los Alamos National Laboratory for supplying samples of plutonium hexafluoride.
References 1 J. H. Beynon, R. G. Cooks, K. R. Jennings and A. J. Ferrer-Correia, Znt. J. Mass Spectrom. Zon Phys., 18 (1975) 87 - 89. 2 J. Sugar, J. Chem. Phys., 60 (1964) 4103. 3 R. A. Kent, J. Am. Chem. Sot., 90 (1968) 5657 - 5659. 4 J. W. Ward, P. D. Kleinschmidt and R. G. Haire, J. Chem. Phys., 71 (1979) 3920 3925. 5 J. Fuger, in N. Edelstein (ed.), Actinides in Perspective, Pergamon, New York, 1982, pp. 409 - 431. 6 R. J. Ackermann and E. G. Rauh, Rev. Znt. Hautes Temp. Refmctaires, 15 (1978) 259 - 280.