The enthalpy of formation of hexagonal Am/sub 2/O/sub 3/ at 298.15 K has been determined to be -1690.4 +- 7.9 kj mol/sup -1/ by solution microcalorimetry. Systematic comparison of the measured enthalpies of formation of hexagonal Pu/sub 2/O/sub 3/ and Am/sub 2/O/sub 3/, monoclinic Cm/sub 2/O/sub 3/, and all lanthanide sesquioxides has been utilized to develop a predictive understanding of stability relationships among all actinide sesquioxides and other actinide species

Morss, L. R.

Sonnenberger, D. C.

UNT Digital Library

Presented at IUPAC Conference on Chemioal Thermodynamics a Hamilton, Ontario* Canada* August 1Z~1?3 1984,To be published in Joi&nal of Nuelear_ Materials,Tha iwbmJmd manuKript ha* tmtn authoredbv • eontncior of in* U. S- Govtrnmtntundtr contract No. W-3M09-ENC-38-Accordingly, iha V. S. Government troinj anoniKlUlivt. tovtftyU* lic*nM to puWi»hor raproduct th« publiahad (onn oi thiicontribution, or allow OthKt to do ao, forU. S. GoMmmcm purtwm.ENTHALPY OF FORMATION OF AMERICIUM SESQUIOXIDE; SYSTEHATICS OF ACTINIDE SESQU10XIDETHERMOCHEMISTRYLester R. HORSS and David C. SONNENBERGERChemistry Div is ion, Argonne National Laboratory, Argonne, IL 60439 U.S.A.*CONF-840 807—5DE85 000081The enthalpy of formation of hexagonal A1112O3 at 298.15 K has been determined to be -1690.4+ 7.9 kJ moi' l by solution microcalorimetry. Systematic comparison of the measuredenthalpies of formation of hexagonal PU2O3 and AJT12O3, monocHnic. Cm2Oa, and al l lanthanidesesquioxides has been uti l ized to develop a predictive understanding of stabil i tyrelationships among a l l actinide sesquioxides and other actinide species.1. INTRODUCTIONAs part of a systematic study of actinideoxide thermochemistry, and of structure-stability relationships in actinidecompounds, we decided to determine theenthalpy of formation of M12O3. Thiscompound is one of only three actinidesesquioxides (the other two are Cm20a andCf^a) that can be prepared stoichiometri-cally at moderate temperature in order forsolution calorimetry to be carried outefficiently. These data can then be used topredict similar properties for other .acti-nide sesquioxides.2. EXPERIMENTALBeginning with a stock solution of 50 fl>gof isotopically pure (>99.9 atom percent)a^Am (half l i f e 7370 y) as i ts trivalatitaqueous ion, we purified I t In two batchesfrom a l l common cationic impurities bycation exchange using ultrapure reagents.For each betch, americium(III) oxslate wasprecipitated, calcined to AmO* at 625"C inoxygen, and reduced to A112O3 in a H2-N2 gasmixture at 630 to 665°C. Complete reductionwas assured by monitoring the weight lossand by handling the tan-colored sesquioxideonly in a dry box with oxygen content ~ 5ppm.Both reduced oxide samples were found tobe hexagonal fim^03 by X-ray powder dif -' fraction films taken 3 hr after completionof reduction; least-squares refinementyielded lat t ice parameters an =0.001 and cn 5.972 t 0.002 A.3.818 ±The bestliterature values, from 2<tiAn2o3 films taken1 iir after reduction, are a. 3.8155 +0.0006 and c0 * 5.975 + 0.002 A.4 We notethat previous studies of the phase tran-sitions of AT12O3 (al l of which used theshorter-lived isotope ^ A m , for which thehalf l i f e is 433 y) suggest that at 675°Cthe body-centered cubic isomorph should bestable and that the bcc-hexagonal transitionoccurs between 700°C and SOO'C.1'4Calorimeter samples were weighed in thedry box on a Cahn 27 microbalance and sealedin th1n»walled borosiiicate-glass ampouleskindly furnished by Professor J . Fuger(University of Liege, Belgium).*Work performed under auspices of U.S. Department of Energy, Office of Basic Energy Sciences,Division of Chemical Sciences, under Contract W-31-109-ENG-38.DISTRIBUTION OF WS DOCUMENT IS UNLIMITEDSolution caiorimetry was carried out in anew vacuum-jacketed tantalum Isoperibolmicrocalorimeter modeled upon establisheddesigns.5"7 This instrument has a liquidcapacity of 5 on3, a thermal leakage modulusof 0.036 min'i , and a temperature noiselevel of 1 x 10"s K {resulting in a thermalsensitivity of 4 x 10"" 0 ) . Calibrations ofthe enthalpy of solution of Mg metal in 1moi dm" 3 HCl(aq) and of KC1 in water agreedwith literature values to better than + 0.5per cent. The enthalpies of solution ofseven A112O3 samples (each ~ 2 mg) in 6.0 moldm"3 HCl(aq) at 298.15 K are reporter inTable 1 . These measurements and auxiliarydata lead to AfH^Am^.cr.hexag,298.15 K) »-1690.4 + 7.9 kJ i » n (Table 2 ) .3. CONCLUSIONSAlthough the trivalent ions of all theactinides uranium through lawrencium havebeen observed in aqueous solution, only thesesquioxides of piutonium through einstein-ium have been prepared. Enthalpies offormation of hexagonal Pu^03, from high-temperature thermodynamic measurements,8>9and of monoclinic C1112O3, from solutioncalorimetry,10 have been reported. The re-sults of this research permit us to assessand to predict the thermochemital propertiesTable 1Enthalpies of solution of An203(cr,hexag) in 6.0 mol dm"J HCl(aq) at 298.15 K.,-3Prep.AAAA8BBm(solute)/mg2.1372.0292.9182.4722.6583.4263.8521.5261.5292.1931.8071.9402.5672.878Mean-381.4-402.2-402.2-390.4-389.8-400.2-399iO-395.0 + 7.4(95% confidence)DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesC o g e n t . N t t t a the United States Government nor any agency thereof 1 « « * * « £employees mates any warranty, express or implied, or assumes any legal liability or_resP°»»-S £ t e t e accurac^, completeness, or usefulne* of any irform.tion - H " " " " * ^process disclosed, or represents that its use vould not uifnnge privately owned nghts Reference herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its « " £ » « « ° T ;mendation, or favoring by the United States Government or any agency d m o t T t a c n mand opinions of authors expressed herein do tot necessarily state or reflect th<»e of theUnited States Government or any agency thereof.Table 2Reaction scheme tor enthalpy of formation of * n 2 0 3 at 298.15 KyMR203(crlhexag) + 6 HC1(6 mol dm'3) >2 ftnCl3dn 6 mol dm"3 HCl) + 3 t^Odn 6 mol dm"3 HC1) iHj • -395.4 + 7.4 k j aAm(cr) + 3 HC1(6 mol dm*3) • A»Cl3(1n 6 mol dm"3 HCl) W2 * -613.0 + 1.3 k jb+ 1/2 03(9) - HgOdn 6 mol dm"3 HCl) 4H3 * -286.6 + 0.4 Wc2 An(cr) + 3/2 0 2 (g) • AH203(cr,hexag) AfHro{Am203,cr,hexag) •-4Hj + 2 W2 + 3M3 " "1690.4 ± 7.9(95% confidence)aThis research.b J . Fuger and F. L. Oet t ing, The Chemical Thermodynamics of Astinide Elements and Compounds(IAEA, Vienna, 1976) Part 2 .CV. B. Parker, D. D. Wagman, and D. Garvin, U.S. Nat.. Bur. Stand. Tech. Rep. N8SIR-75-968(1976).of actiniae sesquioxides in comparison withthe lanthanide sesquioxides.Table 3 and Figure 1 show enthalpies ofsolution in dilute acid (calculated atInfinite dilution; of all lanthanide sesqui-oxides as well as of PuOj 5, AnOj 5, andOnOi^. For comparison, enthalpies ofsolution in water of trichlorides are alsoshown in Figure 1. It is seen that thevalues for the actinides are significantlyTable 3Thermochemical and Structural Data for Figure 1YLaCePrNdPmSmEuGdTbDyHoErTmYbLuUNpPuAnCmAfHJ(M3+,aq)-715+15a-709.4+1.6a-700.4+2.1*-706.2+1,6a-696.6+1.7a-691.1+1.7a-605.6+2.3a-687.0+2.1*-698±6a- -696+5°-707+88. -708.7+1.8C-705.2±3.0a-674.5+3.0*-702.6t2.6a-4B9.1+3.81-527.2+.2.11-592.0+.2.11-616.7+1.31-615.0+5.0*^ ( M O j g ,cr)-952.8+1.I(cub1c)d ' f-897.1+0.4(hexag)d 'e ' f-898.1+1.4(hexag)d«e-913.8+3.2(cubic)f-911.7+3.2(hexa9)d'f-904.0+0.5(hexag)d»e'f(hexag)-911.8+0.9(monocl)d'e-913.7+1,5(cubic)d»f-825.7+1.7(monocl)d*e-831.4+1.9(cub1c)d>e-907.8+1.8(monocl)d«f-913.4:i3.1(cub1c)d-932.6t3.8(cubic)d>e-931.6+1.8(cubic)d»e»f-94O.5±2.4(cubic)d>e«f-94B.9+0.9(cub1c)d»e«f-944.3i2.9(cub1c)d«e ' f-907.3tl . l (cub1c)d«e ' f-939.1+3.8(cub1c)d'e»f-728(hexag)J-761(hexag)J-842+10(hexag)1-828(hexag)"-845+4(hexag)P-841+6(monocl)(l-191+15 *-241+2-231+2-221+4-223+4-22112-208+2-206+2-209+3-203+3-Z08+3-202±4-194±7-193±5-195±8-189+2-190t4-196+3-192±5-190k-195k-179+10-193-200+4-203+69 Molar Vol/cm3 f » h44.949.647.952.246.745.944.945.049.244.048.443.347.646.545.644.944.143.442.842.245.6m45 .6m45.4P45.8''Table 3 (continued)aL. R. Morss, Chem. Revs. 76 (1976) 827."Reassessed from L. R. Morss, Chem. Revs. 76 (1976) 827 and L. R. Morss and J . A. Fa hey, Proc.12th Rare-Earth Res. Conf., Vol. 1 (1976) 443.c 0 . Fuger, L. R.Morss, and 0. Brown, J . Chem. Soc. Dalton (1980) 1076.dK. A. Gschneidner, Jr . , N. Kippenhan, and 0. D. McMasters, IS-RIC-6, Institute for AtomicResearch, Iowa State University, Ames, Iowa (1973).eR. H. Schumm, D. D. Wagman, S. Bailey, W. H. Evans, and v. B. Parker, U. S. Nat. Bur. Stand.Tech. Note 270-7, Washington, D. C , 1973.fGmel1n Handbuch der Anorg. Chemie, Seltenerdelemente, Teil Cl , pp. 118-122 and 131-133,Springer-Verlag, Berlin, 1974.9For the reaction M01#5(cr) + 3H+(aq) • H^aq) + 1.5 H2O(t), 4 s 0 i n ^ • AfHJJfM .̂aq) +1.5 A f h ^ O . t ) - AfH;(*M01#5,cr).h L. Eyring, The Rare Earth Oxides, In Handbook on the Physics and Chemistry of Rare Earths, ed.K. A. Gschneidner, Jr.'and L. Eyring, Vol. 3, Ch. 27, North-Holland, Amsterdam (1978).' ' j . Fuger and F. I.. Oetting, The Chemical Thermodynamics of Actinide Elements and Compounds(IAEA, Vienna, 1976) Part 2.^Calculated from ^ ( M ^ . a q ) + 1.5 ^ ( H g O . t ) - i s ^ , , ^ .Estimated from Figure 1 .1 Reference 9 .™H. E. Flotow and M. Tetenbaum, J . Chem. Phys. 74 (1981) 5269."Reference 11.PThis research.''Reference 10.less exothermic than the values for thecorresponding lanthanides. The result of anindependent assessment of iHffPuO^s.cr)11by Besmann and Lindewer 1s also plotted inFigure 1 and is seen to be much more con-sistent with the ft*0ia5 and CmOi.s data thanIs the PuOî s data point from the work ofChereau et a l . 9 We conclude that the acti-nide sesquioxides nnt slightly more stablethan the lanthanide sesquioxides in compar-ison with their aqueous ions. The rightside of Figure 1 shows that similar behavioris observed In the hexagonal trichlorides.These systematics allow us to predict4fHJl(U0i#s,cr,hexag) • -728 and(Np0i#5,cr,hexag) - -761 W mol'i (Table 3 ) .In Figure 2 the trivaVent correlationfunction P(M) is plotted for lanthanide andactinide aqueous Ions, sesquioxides, andtr ich lor ides .^*^ This figure has been up-dated by inclusion of a new measurement ofAfffttCf^.aq) • -577 + 5 W mol"!14 and bynewly assessed energy levels of Am(g)f6ds2,«H3/2, at 10684 cm"1, and Bk(g)f«ds2,*Gi3/2, at 9141 cm" i . 1 5 The P(M) points forthe actinIdes are shifted systematicallyfrom those of the lanthanides and appear tofal l nearly on a single straight line ratherthan a V-shaped l ine. Our1•120•1*0•1*0•1*0•200•MO W•240 -•3*0PuO,j (Charaau)AmO,., O^PuO,j (Baamann)LnCI,<h*xag)DTbCI,45 SO 56 WVohMM par M e l * M * (enf lF1SURE 1Enthalpies of solution of lanthanide andactfnfde sesquioxides and trichlorides inwater as a function of molar volume.and the new energy level assignments yieldmore consistent P(H) plots for actinidesthan did earlier studies.12 '13I t is possible to extend the correla-tions of Figures 1 and 2 by measuring theenthalpy of solution of Cf203, using mi l l i -gram quantities cf 2*9Cf Isolated from 2"»9Bkproduced at Oak Ridge National Laboratory.This project has been initiated in our lab-oratory. I t 1s unlikely that solution cal-orimetry can be employed for determinationof AfH^PusOs) or flHfH£(Bk203) because ofthe extremely high temperatures required forreduction of their dioxides. Since classi-cal methods cannot be used for thermodynamicmeasurements on compounds of elements beyondCf, the measurements on AnzOa, Cm^s, andCf20$ will be useful In predicting thermo-chemical properties for trivalent compoundsof heavier elements as well as predictingor U, Np, and Bk sesquioxides.14H1440140013*0132012*012401200,112010*0104010001 1 1 t 1 1Vrv~*~r~r~a~~-P(M, LnO,,)• * • •Am*\V1 1 1 I 1 I1 1 1 1 1 1P(M, LnCI.)• aimn• CmOiSvA, .V, , ,-aaa---a-0 1 2 3 4 5 C 7 8 B 10 11 IS 13 14La Ca Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb LuAc Th Pa U Np Pu Am Cm Bk Cl E« Fm Md No LrFIGURE 2Correlation function P(M) for lanthanide andactinide trivalent species.The estimated values of AfH£ for U203and Np20a lead, as expected, to highlypositive free energies for any possiblesyntheses of these oxides and to highlynegative free energies for theirdisproportionate, e.g.162 Np203(s) Np{s) -169The instability of these sesquioxides isconfirmed by their impossibility of pre-paration. Since Figure 1 shows essentiallyconstant differences between enthalpies ofTable 4Differences In Enthalpies of Formation of Trivalent Actinide Species (kj mol*1).Diff . Diff . MCl3{cr,hexag)buNpPuAnCm-489-527-592-617-615-239-234-236-228-226-728-761-828C-845-841-138-137-132-133-133-866-898-960-978-974aSame references as Table 3 .b J . Fuger, V. B. Parker, H. N. Hubbard and F. L. Oett ing, The Chemical Thermodynamics ofActinide Elements and Compounds (IAEA, Vienna, 1983), Part 8 .(•Recommended by Reference 11 and consistent with our resu l ts .solution of act inide sesquioxides and t r i -chlor ides, we compare the differences be-tween enthalpies of formation in Table 4 .The constancy of these differences confirmspur confidence in our estimates of AfHjJ forUjtOa and NP2O3 as well as Besmann's11 e s t i -mate for PU2O3.REFERENCES1 . J . C. Wallmann, J . Inorg. Nucl . Chem.26 (1964) 2053.2 . T. D. CM kai l a and L. Eyring, J .Inorg. Nucl. Chem. 30 (1968) 133.3 . U. Berndt, R. Tanamas, D. Maier, andC. Ke l le r , Inorg. Nucl. Chem. Le t t . 10(1974) 315.4 . c . Hurtgen and G. Fuger, Inorg. Nucl.Chem. Le t t . 13 (1977) 179.5 . J . Fuger, 1965, 1966 Annual ReportsEURATOM-Univ. of Liege ResearchContract 011-64-6 TPUB.6 . L. R. Morss, Ph.D. Thesis, Univ. ofC a l i f . , Berkeley, Report UCRL-18951(1969) .7 . L. R. Morss and M.C. NcCue, Inorg.Chem. 14 (1975) 1624.8 . The Plutonium-Oxygen and Uranium-Pi utonium-Oxygen Systems: AThermochemical Assessment, Tech. Rep.Series 79 (IAEA, Vienna, 1967) .9 . P. Chereau, G. Dean, M. de Franco, andP. Gerdanian, J . Chem. Thermodyn. 9(1977) 2 1 1 .10. I . R. Horss, J . Fuger, J . Goffart , andR. G. Haire, Inorg. Chen. 22 (1963)1993.1 1 . T . N. Besmann and T. B. Lindemer, J .Am. Ceram. Soc. 66 (1983) 782.12. F. David, K. Samhoun, R. Guillaumont,and N. Edelstein, J . Inorg. Nuci.Chan. 40 (1978) 69 .13 . L. R. Morss, J . Less-Common Metals 93(1983) 301.*14. J. Fuger, R. G. Haire, and 0. R.Peterson, J. Less-Common Metals 98(1984) 315.15. L. Brewer, High Temp. Sc1., in press.16. L. R. Morss, Thermodynamic Properties,in: The Chemistry of the ActinideElements, eds. 0. 0. Katz, G. T.Seaborg, and L. R. Morss, to bepublished.

Enthalpy of formation of americium sesquioxide; systematics of actinide sesquioxide thermochemistry

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Enthalpy of formation of americium sesquioxide; systematics of actinide sesquioxide thermochemistry

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