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Synthesis and characterization of crystalline phosphates of plutonium(III) and plutonium(IV)
Journal
of the Less-Common
Metals,
97 (1984) 349-356
349
SYNTHESIS AND CHARACTERIZATION OF CRYSTALLINE PHOSPHATES OF PLUTONIUM(II1) AND PLUTONIUM(IV)
C. E. BAMBERGER,
R. G. HAIRE, H. E. HELLWEGE’
and G. M. BEGUN
Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN37830(U.S.A.) (Received August 25,1983)
Summary The formation of both PuPO, and PuP,O, from reactions of PuO, with (NH,),HPO, or BPO, was studied by X-ray diffraction and Raman spectroscopy. The oxidation of plutonium(III), in PuPO,, to plutonium(IV), in PuP,O,, by air in the presence of BPO, and the thermal reduction of PuP,O, to PuPO, were demonstrated. In addition plutonium(II1) trimetaphosphate Pu(PO,), was synthesized and characterized. Attempts to prepare plutonium(IV) orthophosphate, Pu,(PO,),, by high temperature reactions were unsuccessful; instead, mixtures of PuP,O, and a new phase identified tentatively as “(PuO),P,O,” were obtained.
1. Introduction The concept of immobilizing actinides in synthetic monazites for waste disposal is attractive because natural homologues appear to have been stable for millions of years to radiation, heat and sea water [l, 21. The preparation of metal phosphates by a novel solid-solid reaction which uses BPO, as the phosphate source has been reported [3]. During the course of the work it was found that the reaction of BPO, with lanthanide sesquioxides and with CeO, and Tb,O,, yielded the corresponding orthophosphates LnPO, (Ln = lanthanide) [S]. When the higher oxides of cerium and terbium were used, the formation of their orthophosphates involved the evolution of oxygen. The results of reactions of PuO, with BPO,, expected to be similar to those with CeO, and to yield PuPO, and oxygen, were different; the products consisted of mixtures of PuPO, and PuP,O,, even when the initial phosphorus-to-plutonium ratio was varied between 1 and 3 [S]. For comparison PuO, was reacted at 1000 “C under nitrogen with (NH&HPO, (phosphorus-to-plutonium ratio, 1.33), and the products were *On leave from the Department of Chemistry, Rollins College, Winter Park, FL 32789, U.S.A. Elsevier Sequoia/Printed
in The Netherlands
350
found to be similar to that obtained with the BPO, preparations [S]. These results, which indicated that only a part of the plutonium(IV) in PuO, was reduced, could not be interpreted on the basis of a simple equilibrium. A similar behavior has been reported by Bjorklund [4] who on igniting plutonium(IV) oxalatophosphate precipitates in air obtained PuPO,, PuP,O, or mixutres of these compounds, depending on the ratio of phosphorus to plutonium in the precipitates. Bjorklund reported that both PuPO, and PuP,O, were quite stable in air at 950-1000 “C, but the duration of the heating was not reported [4]. If PuP,O, is thermodynamically more stable in air than PuPO,, we would expect that reactions involving phosphorus-to-plutonium ratios between 1 and 2 would yield mixtures of PuP,O, and PuO,. However, if PuPO, is the more stable compound, the same phosphorus-to-plutonium ratios should give complete conversion to PuPO,. In order to establish which of these species is the more stable, we studied the formation and interconversion of these plutonium phosphates from solid-solid reactions by means of Raman spectra, visible absorption spectra and X-ray diffraction. Attempts to synthesize Pu,(PO,), were monitored with Raman spectroscopy and X-ray diffraction. In addition, Pu(PO,), was synthesized, its lattice parameters were determined and its Raman and visible absorption spectra were recorded.
2. Experimental
details
The preparative experiments consisted of heating intimately ground mixtures of PuO, or PuF, and BPO, (200-400 mg of plutonium) for between 6 h and 2 weeks at elevated temperatures. The mixtures were contained in platinum boats (except when the phosphate source was (NH,),HPO, in which case silica boats were used) placed in a silica tube provided with gas inlet and exit ports and a thermocouple well, located above the boats. Dry or moist air, nitrogen or Ar-4%H, was passed through the system at a rate of 50-100 ml mini. When oxygen evolution was expected from the reaction, nitrogen was used and the exiting gases were monitored continuously by means of a model 741 Beckman oxygen analyzer. 242Pu02 was obtained from the Isotopes Sales Department, Oak Ridge National Laboratory, and the PuF, was obtained by repeated heating to 900 “C of mixtures of PuO, with excess NH,HF, in an Ar-4%H, atmosphere. The BPO, was either prepared [3] or procured from Alfa-Ventron and calcined to 1000 “C in air to eliminate traces of organic material present. After each heat treatment, the solid products were ground and sampled and the samples were loaded into glass capillaries 0.3 mm in diameter. The samples were then used consecutively for X-ray diffraction, Raman spectroscopy and absorption spectrophotometry. X-ray diffraction powder patterns were obtained with DebyeScherrer cameras using Cu Kcc radiation. The Raman spectroscopy [S] and absorption spectrophotometry [6] instruments and equipment have been described earlier. Raman spectra were excited with the 514.5 nm line of an argon ion laser.
351
3. Results and discussion 3.1. Preparation of PuP,O, and PuPO, When mixtures of PuO, and BPO, (phosphorus-to-plutonium ratios, 1.31.9) were heated to 1200“C for 6-24 h under flowing nitrogen, a blue solid was produced and approximately 3 of the oxygen expected for total reduction of the initial plutonium(IV) was evolved. The X-ray diffraction patterns of the products indicated that they were mixtures of PuPO, and PuP,O,. This was based on a comparison of the diffraction patterns with those calculated for PuPO, and PuP,O, from data in ref. 4 and for PuO, from data in ref. 7. The same results were obtained when the starting mixture was PuO, and (NH,),HPO, (phosphorus-to-plutonium ratio, 1.33), except that oxygen evolution was not measured. The BPO, reactions can be represented by the general equation PuO, +xBPO,
+ (2%x)PuPO,+(x-l)PuP,O,
+;B,O,
2-x +40,
(1)
When the reaction mixture was made deficient in phosphate (phosphorus-toplutonium ratio, 0.75), the resulting solid consisted of PuPO, and PuO,, and the amount of oxygen evolved was greater than from mixtures with excess BPO,. It is evident that under the conditions of temperature and length of heating used in the above experiments the formation of PuP,O, is favored by phosphorus-toplutonium ratios greater than or equal to unity. The presence of PuP,O, (eqn. (1))may be due to its formation as an intermediate which has a slow rate of decomposition, e.g. slower than the 20 h required at 900 “C to complete the following reaction, as reported in ref. 8: CeP,O,
+ CePO,++P,O,+$O,
(2)
In order to prepare PuP,O, free of PuPO, (which Bjorklund [4] had reported to be stable in air to 1000 “C) we decided to favor the air oxidation of plutonium(II1) to plutonium(IV) by providing the additional phosphate according to the reaction PuPO, + BPO, +aOz -+ PuP,O, ++B,O,
(3)
Mixtures of PuPO, and PuP,O, with BPO, were heated in nitrogen for 4 h at 1000 “C and showed no change in color. The atmosphere was then changed to air and the samples kept for 6 h at 960 “C. The color of the solids changed dramatically from dark blue to light green. The Raman spectra of these green solids differed from that of PuPO,; X-ray diffraction analysis revealed that they were PuP,O, and that no PuPO, was present. Solid state absorption spectra and Raman spectra of pure PuPO, and PuP,O, are shown in Figs. 1 and 2 respectively. The significant overlap of absorption bands in the visible absorption spectra of both plutonium(II1) and plutonium(IV) compounds makes the identification of PuP,O, in the presence of PuPO, by absorption spectrophotometry difficult. The Raman spectra of PuPO, and of PuP,O, (Fig. 2) are significantly different and allow us to identify the presence of each compound in mixtures of these materials. The Raman spectrum of monoclinic CePO, is included in Fig. 2 to show its similarity to that of PuPO,. We have observed that the absorption spectrum of cubic PuP,O, shown in Fig. 1 is different from that
352
reported for plutonium(IV) doped in monoclinic LaPO, [9], and we conclude that the variation arises from the difference in the crystal environments. In additional experiments PuP,O, was prepared by heating a mixture of PuO, with BPOd in air PuO, + 2BPO,=
PuP,O,
+ B20,
(4)
In order to confirm that the apparent thermal stability of PuP20, was due to a slow rate of decomposition, samples of PuP,O, were held at 1060 “C! under WTWELENGTH , nm)
14
350
400 I
500 I
600 I
I
800 lllll
1200
d 42 -
I
I
40 -
I
I
I
08 0.6 04 02 O-
L 30
26
Fig. 1. Absorption
22 18 Wn,“EN”MBERl”IO5lK’l
cl
14
zoo &
400 600 WAVENUMBER
IO00 800 (CM’-‘,)
spectra of PuPO, (spectrum A), PuPz07 (spectrum B) and Pu(PO,),
Fig. 2. Raman spectra of PuPO, “(PuO),P,O,” (spectrum D).
(spectrum
A), CePO,
(spectrum
B), PuP,O,
I200
(spectrum C).
(spectrum
C) and
353
flowing nitrogen for up to 2 weeks. The samples lost weight and examination of the resulting solids by X-ray diffraction, Raman spectroscopy and absorption spectrophotometry showed that the PuP,O, decomposed slowly according to the reaction PuP,O,
4 PUPO, ++p,o,
+$O,
(5)
A consideration of all the above results suggests that the reaction of PuO, with BPO, can proceed to produce PuPO, or PuP,O,. Because oxygen evolution (eqn. (1)) was observed to begin at 700-750 “C and nearly stops after several hours of heating at 1000 “C while PuP,O, still remains in the solid, we conclude that both PuPO, and PuP,O, were initially formed. We speculate further that because their formation takes place in mixtures of powders it may be controlled by the local concentration of BPO, surrounding the PuO, particles. In order to convert PuP,O, to PuPO, more effectively, pure PuP,O, and several mixtures of it with PuPO, were calcined at 1030 “C for 3-19 h under flowing Ar-4%H, instead of nitrogen. X-ray diffraction and Raman spectra confirmed the conversion to PuPO, and all the samples registered the proper weight losses according to the reaction PuP,O,
+fH2 + PuPO,+$P,O,
++H,O
Pure PuPO, was also prepared by heating nitrogen atmosphere. The expected reaction PuF, + BPO, -+ PuPO, + BF,
(6) PuF, with BPO, to 1000 “C in a
(7)
was confirmed by the measured weight loss (BF,) and by the results of X-ray diffraction, Ramari spectroscopy and absorption spectrophotometry. The X-ray powder diffraction analysis showed that the PuPO, samples existed in the monoclinic (monazite) structure and that the PuP,O, samples were cubic. The average lattice parameters for PuPO, were a, = 6.772(7) A, b0 = 6.968(6) A, c0 = 6.427(7) A and B = 103.7(l)“. These values are in excellent agreement with the parameters determined for the transplutonium orthophosphates [lo], when allowance is made for the regular change in metal ionic radii, and with the values for PrPO, (a0 = 6.768(l) A, b, = 6.987(l) A, c,, = 6.442(l) A and p = 10356(l)“) [ll]. The latter are in accord with the similar trivalent ionic radii r of praseodymium and plutonium (rp,,+ = 1.32 A and rpU3+= 1.33 A, corrected for a coordination of nine) [12]. The lattice parameters determined for our PuPO, also agree with those measured by Bjorklund [4] (a = 6.73 (0.02) A, b = 7.00 (0.02) A, c = 6.42 (0.02) A and /I = 103.8” (0.4”)) within the larger uncertainties assigned to the latter. The average lattice parameter a, for the PuP,O, samples was 8.562(4)A which is in good agreement with the value of 8.560(6) A reported by Bjorklund [4]. 3.2. Preparation of Pu(PO,), Plutonium(II1) trimetaphosphate Pu(PO,), was prepared by means of the following reaction, analogous to those used to prepare lanthanide trimetaphosphates [13] :
354
PuPO, + B(NH,),HPO,
+ Pu(PO,),
+ 4NH, + 3H,O
(8)
This reaction was carried out under an Ar-4%H, atmosphere at a maximum temperature of 1040 “C. The product was amorphous, being very probably a glass. Additional heating of this material for 3 days at 820 “C yielded polycrystalline Pu(PO,),. The identity of this compound was confirmed by comparing an X-ray diffraction pattern of it with that of orthorhombic Nd(PO,), [14] and Pr(P03)3 [El. The analogy of Pu(PO,), with trimetaphosphates of the light lanthanides is further evident when their corresponding Raman spectra (Fig. 3) which are significantly different from the spectra of the monoclinic trimetaphosphates of the heavier rare earths (from gadolinium to lutetium), are compared. The absorption spectrum of crystalline Pu(PO,), is very similar to that obtained from the glassy form, except that the former has sharper and more defined peaks. The absorption spectra of PuPO, and crystalline Pu(PO,), (Fig. 1) are quite similar, thus indicating a lack of effect due to their crystal environment. In addition Raman spectra (Figs. 2 and 3) can be used to resolve mixtures of PuPO, and Pu(POJ3. Orthorhombic lattice parameters were derived for Pu(PO,), from the X-ray powder data and are as follows: a, = 11.23(2) A; b, = 8.57(l) A; cg = 7.30(l) A.
I
I, 200
A
I1 400
I
I
600
so0
WAVENUMBER
I
I
1000
I
I 1200
(CM-‘)
Fig. 3. Raman spectra ofPu(PO,),
(spectrum A), La(PO,),
(spectrum B) and Ho(PO,),
(spectrum C).
355
3.3. Formation of ‘((PuO)~P~O,” Attempts to prepare pure Pu,(PO,)~ by means of the reaction 3Pu0, + 4BP0,
+ Pu,(PO,),
tetravalent
+ 2B,O,
plutonium
orthophosphate (9)
were not successful. Mixtures of PuO, and BPO, were heated in air at 720 “C for 16 h and at 800, 900 and 960 “C for 3 days. Analysis of the solids by X-ray diffraction and Raman spectroscopy revealed that at 800 “C a reaction had produced some PuP,O,. At 900 “C unreacted PuO, was still present but not at 960 “C at which a new phase tentatively identified as ‘f(PuO),P,O,” was present together with PuP,O,. This tentative identification is based on the Raman spectrum obtained (Fig. 2) which shows new peaks in addition to those already assigned to other plutonium phosphates. In addition, the X-ray diffraction pattern of the mixture showed the expected lines for PuP,O, and an additional pattern which showed a strong resemblance to that of (UO),P,O, [163. Additional work on the high temperature synthesis of Pu,(PO,), was not done, as it was recognized that PuP,O, and/or PuPO, would probably be products of such treatment. Further no attempt was made to prepare Pu,(PO,), by precipitation from aqueous solutions because of the expected di~culties in controlling the oxidation states, stoichiometry and crystallinity of the precipitates [17,18]. 4. Summarizing
remarks
and conclusions
Our results suggest that PuPO, is the more stable species of plutonium phosphate near 1000 “C, that it is the product formed in inert or reducing atmospheres &rd that the presence of PuP,O, among the products of reaction may be due to its concurrent fast formation and slow decomposition rate in inert atmospheres. This is consistent with the results of Bjorklund [4] who observed rapid decomposition of PuP,O, only under high vacuum (5 x lo- ’ mmHg) and at temperatures of 1200-1400 “C. This thermal behavior of PuP,O, is intermediate between that of the pyrophosphates of neighboring actinides NpP,O, and AmP,O,. Neptunium(IV) pyrophosphate is very stable toward reduction and neptunium(II1) phosphate has not been synthesized [19], while AmP,O, is rapidly reduced thermally in the 600-1~0 ‘C temperature range to AmPO,? which is reported to be stable at temperatures above 1000 “C [19]. In addition, Pu(PO,), was synthesized and characterized but attempts to synthesize Pu,(PO,), were unsuccessful. During the course of the work, a new phase was formed that was tentatively identified as “(PuO),P,O,“. Finally, the Raman and absorption spectra of PuPO,, PuP,O, and Pu(PO,), are presented for the first time. Acknowledgments This research was sponsored by the Division of Chemical Sciences, Office of
356
Basic Energy Sciences, U.S. Department of Energy, under Contracts W-7405eng-26 with the Union Carbide Corporation and DE-AS0576ER04447 with the University of Tennessee, Knoxville. H.E.H. is a Visiting Research Associate, Department of Chemistry, University of Tennessee.
References 1 2
10
11 12 13 14 15 16 17
18 19
G. J. McCarthy, W. B. White and D. E. Pfoertsch, Synthesis of nuclear waste monazites, ideal actinide hosts for geologic disposal, Mater. Res. Bull., 13(1978) 1239. L. A. Boatner, G. W. Beall, M. M. Abraham, C. B. Finch, P. Huray and M. Rappaz, in C. J. Northrup (ed.), The Scientific Basis for Nuclear Waste Management, Vol. 2, Plenum, New York, 1980, p. 289. C. E. Bamberger, Preparation of metal phosphates by metathesis reaction with BPO,, J. Am. Ceram. Sot., 65 (7) (1982) C-107. C. W. Bjorklund, The preparation of PuP,O, and PuPO,, J. Am. Chem. Sot., 79 (24) (1958) 6347. G. M. Begun, G. W. Beall, L. A. Boatner and W. J. Gregor, Raman spectra of the rare earth orthophosphates, J. Raman Spectrosc., 11(4) (1981) 273. J, P. Young, R. G. Haire, R. L. Fellows and J. R. Peterson, Spectrophotometric studies of transcurium element halides and oxyhalides in the solid state, J. Radioanal. Chem., 43 (2) (1978) 479. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, International Center for Diffraction Data, Swarthmore, PA, 1955, Card 6-360. I. L. Botto and E. J. Baran, Crystallographic data, IR spectra and thermal behaviour of Ce(IV)diphosphate, 2. Anorg. Allg. Chem., 430(1977) 283. K. L. Kelly, G. W. Beall, J. P. Young and L. A. Boatner, Valence states of actinides in synthesis monazites. In J. G. Moore (ed.), The Scientific Basis for Nuclear Waste Management, Vol. 3, Plenum, New York, 1981, p. 189. R. G. Haire, H. E. Hellwege, D. E. Hobart and J. P. Young, Syntheses, lattice parameters and solid state absorption spectra of the first five transplutonium orthophosphates, J. Less-Common Met., 93 (1983) 358. J. G. Pepin and E. R. Vance, Crystal data for rare earth orthophosphates of the monazite structure-type, J. Inorg. Nucl. Chem., 43 (1981) 2807-2809. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A, 32(1976) 761-765. G. M. Begun and C. E. Bamberger, Raman spectra of rare earth trimetaphosphates, J. Raman Spectrosc., 13 (3) (1982) 284. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, International Center for Diffraction Data, Swarthmore, PA, 1977, Card 27-322. I. L. Botto and E. J. Baran, Crystal data for the trimetaphosphates of lanthanum, cerium and praseodymium, J. Appl. Crystallogr., 12(1979) 257. H. W. Dunn, X-ray diffraction data for some uranium compounds, Rep. ORNL-2092, 1956 (Oak Ridge National Laboratory). C. E. Bamberger, G. M. Begun, J. Brynestad and J. F. Land, Simultaneous precipitation of phosphates of Bi(II1) and Ce(IV) or Ce(II1): characterization of precipitates and their ignition products, Radiochim. Acta, 31(1982) 57. G. T. Seaborg and J. J. Katz (eds.), The Actinide Elements, in National Nuclear Energy Ser., Vol. 14A, Division IV, Plutonium Project Record, Manhattan Project Technical Section, McGraw-Hill, New York, 1st edn., 1954, pp. 414,416,417. C. Keller and K. H. Walter, Preparation, lattice parameters and chemical properties of some ternary oxides of neptunium, americium and curium of the type Mes*XVO,, J. Inorg. Nucl. Chem., 27(1965) 1253.
of the Less-Common
Metals,
97 (1984) 349-356
349
SYNTHESIS AND CHARACTERIZATION OF CRYSTALLINE PHOSPHATES OF PLUTONIUM(II1) AND PLUTONIUM(IV)
C. E. BAMBERGER,
R. G. HAIRE, H. E. HELLWEGE’
and G. M. BEGUN
Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, TN37830(U.S.A.) (Received August 25,1983)
Summary The formation of both PuPO, and PuP,O, from reactions of PuO, with (NH,),HPO, or BPO, was studied by X-ray diffraction and Raman spectroscopy. The oxidation of plutonium(III), in PuPO,, to plutonium(IV), in PuP,O,, by air in the presence of BPO, and the thermal reduction of PuP,O, to PuPO, were demonstrated. In addition plutonium(II1) trimetaphosphate Pu(PO,), was synthesized and characterized. Attempts to prepare plutonium(IV) orthophosphate, Pu,(PO,),, by high temperature reactions were unsuccessful; instead, mixtures of PuP,O, and a new phase identified tentatively as “(PuO),P,O,” were obtained.
1. Introduction The concept of immobilizing actinides in synthetic monazites for waste disposal is attractive because natural homologues appear to have been stable for millions of years to radiation, heat and sea water [l, 21. The preparation of metal phosphates by a novel solid-solid reaction which uses BPO, as the phosphate source has been reported [3]. During the course of the work it was found that the reaction of BPO, with lanthanide sesquioxides and with CeO, and Tb,O,, yielded the corresponding orthophosphates LnPO, (Ln = lanthanide) [S]. When the higher oxides of cerium and terbium were used, the formation of their orthophosphates involved the evolution of oxygen. The results of reactions of PuO, with BPO,, expected to be similar to those with CeO, and to yield PuPO, and oxygen, were different; the products consisted of mixtures of PuPO, and PuP,O,, even when the initial phosphorus-to-plutonium ratio was varied between 1 and 3 [S]. For comparison PuO, was reacted at 1000 “C under nitrogen with (NH&HPO, (phosphorus-to-plutonium ratio, 1.33), and the products were *On leave from the Department of Chemistry, Rollins College, Winter Park, FL 32789, U.S.A. Elsevier Sequoia/Printed
in The Netherlands
350
found to be similar to that obtained with the BPO, preparations [S]. These results, which indicated that only a part of the plutonium(IV) in PuO, was reduced, could not be interpreted on the basis of a simple equilibrium. A similar behavior has been reported by Bjorklund [4] who on igniting plutonium(IV) oxalatophosphate precipitates in air obtained PuPO,, PuP,O, or mixutres of these compounds, depending on the ratio of phosphorus to plutonium in the precipitates. Bjorklund reported that both PuPO, and PuP,O, were quite stable in air at 950-1000 “C, but the duration of the heating was not reported [4]. If PuP,O, is thermodynamically more stable in air than PuPO,, we would expect that reactions involving phosphorus-to-plutonium ratios between 1 and 2 would yield mixtures of PuP,O, and PuO,. However, if PuPO, is the more stable compound, the same phosphorus-to-plutonium ratios should give complete conversion to PuPO,. In order to establish which of these species is the more stable, we studied the formation and interconversion of these plutonium phosphates from solid-solid reactions by means of Raman spectra, visible absorption spectra and X-ray diffraction. Attempts to synthesize Pu,(PO,), were monitored with Raman spectroscopy and X-ray diffraction. In addition, Pu(PO,), was synthesized, its lattice parameters were determined and its Raman and visible absorption spectra were recorded.
2. Experimental
details
The preparative experiments consisted of heating intimately ground mixtures of PuO, or PuF, and BPO, (200-400 mg of plutonium) for between 6 h and 2 weeks at elevated temperatures. The mixtures were contained in platinum boats (except when the phosphate source was (NH,),HPO, in which case silica boats were used) placed in a silica tube provided with gas inlet and exit ports and a thermocouple well, located above the boats. Dry or moist air, nitrogen or Ar-4%H, was passed through the system at a rate of 50-100 ml mini. When oxygen evolution was expected from the reaction, nitrogen was used and the exiting gases were monitored continuously by means of a model 741 Beckman oxygen analyzer. 242Pu02 was obtained from the Isotopes Sales Department, Oak Ridge National Laboratory, and the PuF, was obtained by repeated heating to 900 “C of mixtures of PuO, with excess NH,HF, in an Ar-4%H, atmosphere. The BPO, was either prepared [3] or procured from Alfa-Ventron and calcined to 1000 “C in air to eliminate traces of organic material present. After each heat treatment, the solid products were ground and sampled and the samples were loaded into glass capillaries 0.3 mm in diameter. The samples were then used consecutively for X-ray diffraction, Raman spectroscopy and absorption spectrophotometry. X-ray diffraction powder patterns were obtained with DebyeScherrer cameras using Cu Kcc radiation. The Raman spectroscopy [S] and absorption spectrophotometry [6] instruments and equipment have been described earlier. Raman spectra were excited with the 514.5 nm line of an argon ion laser.
351
3. Results and discussion 3.1. Preparation of PuP,O, and PuPO, When mixtures of PuO, and BPO, (phosphorus-to-plutonium ratios, 1.31.9) were heated to 1200“C for 6-24 h under flowing nitrogen, a blue solid was produced and approximately 3 of the oxygen expected for total reduction of the initial plutonium(IV) was evolved. The X-ray diffraction patterns of the products indicated that they were mixtures of PuPO, and PuP,O,. This was based on a comparison of the diffraction patterns with those calculated for PuPO, and PuP,O, from data in ref. 4 and for PuO, from data in ref. 7. The same results were obtained when the starting mixture was PuO, and (NH,),HPO, (phosphorus-to-plutonium ratio, 1.33), except that oxygen evolution was not measured. The BPO, reactions can be represented by the general equation PuO, +xBPO,
+ (2%x)PuPO,+(x-l)PuP,O,
+;B,O,
2-x +40,
(1)
When the reaction mixture was made deficient in phosphate (phosphorus-toplutonium ratio, 0.75), the resulting solid consisted of PuPO, and PuO,, and the amount of oxygen evolved was greater than from mixtures with excess BPO,. It is evident that under the conditions of temperature and length of heating used in the above experiments the formation of PuP,O, is favored by phosphorus-toplutonium ratios greater than or equal to unity. The presence of PuP,O, (eqn. (1))may be due to its formation as an intermediate which has a slow rate of decomposition, e.g. slower than the 20 h required at 900 “C to complete the following reaction, as reported in ref. 8: CeP,O,
+ CePO,++P,O,+$O,
(2)
In order to prepare PuP,O, free of PuPO, (which Bjorklund [4] had reported to be stable in air to 1000 “C) we decided to favor the air oxidation of plutonium(II1) to plutonium(IV) by providing the additional phosphate according to the reaction PuPO, + BPO, +aOz -+ PuP,O, ++B,O,
(3)
Mixtures of PuPO, and PuP,O, with BPO, were heated in nitrogen for 4 h at 1000 “C and showed no change in color. The atmosphere was then changed to air and the samples kept for 6 h at 960 “C. The color of the solids changed dramatically from dark blue to light green. The Raman spectra of these green solids differed from that of PuPO,; X-ray diffraction analysis revealed that they were PuP,O, and that no PuPO, was present. Solid state absorption spectra and Raman spectra of pure PuPO, and PuP,O, are shown in Figs. 1 and 2 respectively. The significant overlap of absorption bands in the visible absorption spectra of both plutonium(II1) and plutonium(IV) compounds makes the identification of PuP,O, in the presence of PuPO, by absorption spectrophotometry difficult. The Raman spectra of PuPO, and of PuP,O, (Fig. 2) are significantly different and allow us to identify the presence of each compound in mixtures of these materials. The Raman spectrum of monoclinic CePO, is included in Fig. 2 to show its similarity to that of PuPO,. We have observed that the absorption spectrum of cubic PuP,O, shown in Fig. 1 is different from that
352
reported for plutonium(IV) doped in monoclinic LaPO, [9], and we conclude that the variation arises from the difference in the crystal environments. In additional experiments PuP,O, was prepared by heating a mixture of PuO, with BPOd in air PuO, + 2BPO,=
PuP,O,
+ B20,
(4)
In order to confirm that the apparent thermal stability of PuP20, was due to a slow rate of decomposition, samples of PuP,O, were held at 1060 “C! under WTWELENGTH , nm)
14
350
400 I
500 I
600 I
I
800 lllll
1200
d 42 -
I
I
40 -
I
I
I
08 0.6 04 02 O-
L 30
26
Fig. 1. Absorption
22 18 Wn,“EN”MBERl”IO5lK’l
cl
14
zoo &
400 600 WAVENUMBER
IO00 800 (CM’-‘,)
spectra of PuPO, (spectrum A), PuPz07 (spectrum B) and Pu(PO,),
Fig. 2. Raman spectra of PuPO, “(PuO),P,O,” (spectrum D).
(spectrum
A), CePO,
(spectrum
B), PuP,O,
I200
(spectrum C).
(spectrum
C) and
353
flowing nitrogen for up to 2 weeks. The samples lost weight and examination of the resulting solids by X-ray diffraction, Raman spectroscopy and absorption spectrophotometry showed that the PuP,O, decomposed slowly according to the reaction PuP,O,
4 PUPO, ++p,o,
+$O,
(5)
A consideration of all the above results suggests that the reaction of PuO, with BPO, can proceed to produce PuPO, or PuP,O,. Because oxygen evolution (eqn. (1)) was observed to begin at 700-750 “C and nearly stops after several hours of heating at 1000 “C while PuP,O, still remains in the solid, we conclude that both PuPO, and PuP,O, were initially formed. We speculate further that because their formation takes place in mixtures of powders it may be controlled by the local concentration of BPO, surrounding the PuO, particles. In order to convert PuP,O, to PuPO, more effectively, pure PuP,O, and several mixtures of it with PuPO, were calcined at 1030 “C for 3-19 h under flowing Ar-4%H, instead of nitrogen. X-ray diffraction and Raman spectra confirmed the conversion to PuPO, and all the samples registered the proper weight losses according to the reaction PuP,O,
+fH2 + PuPO,+$P,O,
++H,O
Pure PuPO, was also prepared by heating nitrogen atmosphere. The expected reaction PuF, + BPO, -+ PuPO, + BF,
(6) PuF, with BPO, to 1000 “C in a
(7)
was confirmed by the measured weight loss (BF,) and by the results of X-ray diffraction, Ramari spectroscopy and absorption spectrophotometry. The X-ray powder diffraction analysis showed that the PuPO, samples existed in the monoclinic (monazite) structure and that the PuP,O, samples were cubic. The average lattice parameters for PuPO, were a, = 6.772(7) A, b0 = 6.968(6) A, c0 = 6.427(7) A and B = 103.7(l)“. These values are in excellent agreement with the parameters determined for the transplutonium orthophosphates [lo], when allowance is made for the regular change in metal ionic radii, and with the values for PrPO, (a0 = 6.768(l) A, b, = 6.987(l) A, c,, = 6.442(l) A and p = 10356(l)“) [ll]. The latter are in accord with the similar trivalent ionic radii r of praseodymium and plutonium (rp,,+ = 1.32 A and rpU3+= 1.33 A, corrected for a coordination of nine) [12]. The lattice parameters determined for our PuPO, also agree with those measured by Bjorklund [4] (a = 6.73 (0.02) A, b = 7.00 (0.02) A, c = 6.42 (0.02) A and /I = 103.8” (0.4”)) within the larger uncertainties assigned to the latter. The average lattice parameter a, for the PuP,O, samples was 8.562(4)A which is in good agreement with the value of 8.560(6) A reported by Bjorklund [4]. 3.2. Preparation of Pu(PO,), Plutonium(II1) trimetaphosphate Pu(PO,), was prepared by means of the following reaction, analogous to those used to prepare lanthanide trimetaphosphates [13] :
354
PuPO, + B(NH,),HPO,
+ Pu(PO,),
+ 4NH, + 3H,O
(8)
This reaction was carried out under an Ar-4%H, atmosphere at a maximum temperature of 1040 “C. The product was amorphous, being very probably a glass. Additional heating of this material for 3 days at 820 “C yielded polycrystalline Pu(PO,),. The identity of this compound was confirmed by comparing an X-ray diffraction pattern of it with that of orthorhombic Nd(PO,), [14] and Pr(P03)3 [El. The analogy of Pu(PO,), with trimetaphosphates of the light lanthanides is further evident when their corresponding Raman spectra (Fig. 3) which are significantly different from the spectra of the monoclinic trimetaphosphates of the heavier rare earths (from gadolinium to lutetium), are compared. The absorption spectrum of crystalline Pu(PO,), is very similar to that obtained from the glassy form, except that the former has sharper and more defined peaks. The absorption spectra of PuPO, and crystalline Pu(PO,), (Fig. 1) are quite similar, thus indicating a lack of effect due to their crystal environment. In addition Raman spectra (Figs. 2 and 3) can be used to resolve mixtures of PuPO, and Pu(POJ3. Orthorhombic lattice parameters were derived for Pu(PO,), from the X-ray powder data and are as follows: a, = 11.23(2) A; b, = 8.57(l) A; cg = 7.30(l) A.
I
I, 200
A
I1 400
I
I
600
so0
WAVENUMBER
I
I
1000
I
I 1200
(CM-‘)
Fig. 3. Raman spectra ofPu(PO,),
(spectrum A), La(PO,),
(spectrum B) and Ho(PO,),
(spectrum C).
355
3.3. Formation of ‘((PuO)~P~O,” Attempts to prepare pure Pu,(PO,)~ by means of the reaction 3Pu0, + 4BP0,
+ Pu,(PO,),
tetravalent
+ 2B,O,
plutonium
orthophosphate (9)
were not successful. Mixtures of PuO, and BPO, were heated in air at 720 “C for 16 h and at 800, 900 and 960 “C for 3 days. Analysis of the solids by X-ray diffraction and Raman spectroscopy revealed that at 800 “C a reaction had produced some PuP,O,. At 900 “C unreacted PuO, was still present but not at 960 “C at which a new phase tentatively identified as ‘f(PuO),P,O,” was present together with PuP,O,. This tentative identification is based on the Raman spectrum obtained (Fig. 2) which shows new peaks in addition to those already assigned to other plutonium phosphates. In addition, the X-ray diffraction pattern of the mixture showed the expected lines for PuP,O, and an additional pattern which showed a strong resemblance to that of (UO),P,O, [163. Additional work on the high temperature synthesis of Pu,(PO,), was not done, as it was recognized that PuP,O, and/or PuPO, would probably be products of such treatment. Further no attempt was made to prepare Pu,(PO,), by precipitation from aqueous solutions because of the expected di~culties in controlling the oxidation states, stoichiometry and crystallinity of the precipitates [17,18]. 4. Summarizing
remarks
and conclusions
Our results suggest that PuPO, is the more stable species of plutonium phosphate near 1000 “C, that it is the product formed in inert or reducing atmospheres &rd that the presence of PuP,O, among the products of reaction may be due to its concurrent fast formation and slow decomposition rate in inert atmospheres. This is consistent with the results of Bjorklund [4] who observed rapid decomposition of PuP,O, only under high vacuum (5 x lo- ’ mmHg) and at temperatures of 1200-1400 “C. This thermal behavior of PuP,O, is intermediate between that of the pyrophosphates of neighboring actinides NpP,O, and AmP,O,. Neptunium(IV) pyrophosphate is very stable toward reduction and neptunium(II1) phosphate has not been synthesized [19], while AmP,O, is rapidly reduced thermally in the 600-1~0 ‘C temperature range to AmPO,? which is reported to be stable at temperatures above 1000 “C [19]. In addition, Pu(PO,), was synthesized and characterized but attempts to synthesize Pu,(PO,), were unsuccessful. During the course of the work, a new phase was formed that was tentatively identified as “(PuO),P,O,“. Finally, the Raman and absorption spectra of PuPO,, PuP,O, and Pu(PO,), are presented for the first time. Acknowledgments This research was sponsored by the Division of Chemical Sciences, Office of
356
Basic Energy Sciences, U.S. Department of Energy, under Contracts W-7405eng-26 with the Union Carbide Corporation and DE-AS0576ER04447 with the University of Tennessee, Knoxville. H.E.H. is a Visiting Research Associate, Department of Chemistry, University of Tennessee.
References 1 2
10
11 12 13 14 15 16 17
18 19
G. J. McCarthy, W. B. White and D. E. Pfoertsch, Synthesis of nuclear waste monazites, ideal actinide hosts for geologic disposal, Mater. Res. Bull., 13(1978) 1239. L. A. Boatner, G. W. Beall, M. M. Abraham, C. B. Finch, P. Huray and M. Rappaz, in C. J. Northrup (ed.), The Scientific Basis for Nuclear Waste Management, Vol. 2, Plenum, New York, 1980, p. 289. C. E. Bamberger, Preparation of metal phosphates by metathesis reaction with BPO,, J. Am. Ceram. Sot., 65 (7) (1982) C-107. C. W. Bjorklund, The preparation of PuP,O, and PuPO,, J. Am. Chem. Sot., 79 (24) (1958) 6347. G. M. Begun, G. W. Beall, L. A. Boatner and W. J. Gregor, Raman spectra of the rare earth orthophosphates, J. Raman Spectrosc., 11(4) (1981) 273. J, P. Young, R. G. Haire, R. L. Fellows and J. R. Peterson, Spectrophotometric studies of transcurium element halides and oxyhalides in the solid state, J. Radioanal. Chem., 43 (2) (1978) 479. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, International Center for Diffraction Data, Swarthmore, PA, 1955, Card 6-360. I. L. Botto and E. J. Baran, Crystallographic data, IR spectra and thermal behaviour of Ce(IV)diphosphate, 2. Anorg. Allg. Chem., 430(1977) 283. K. L. Kelly, G. W. Beall, J. P. Young and L. A. Boatner, Valence states of actinides in synthesis monazites. In J. G. Moore (ed.), The Scientific Basis for Nuclear Waste Management, Vol. 3, Plenum, New York, 1981, p. 189. R. G. Haire, H. E. Hellwege, D. E. Hobart and J. P. Young, Syntheses, lattice parameters and solid state absorption spectra of the first five transplutonium orthophosphates, J. Less-Common Met., 93 (1983) 358. J. G. Pepin and E. R. Vance, Crystal data for rare earth orthophosphates of the monazite structure-type, J. Inorg. Nucl. Chem., 43 (1981) 2807-2809. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., Sect. A, 32(1976) 761-765. G. M. Begun and C. E. Bamberger, Raman spectra of rare earth trimetaphosphates, J. Raman Spectrosc., 13 (3) (1982) 284. Powder Diffraction File, Joint Committee on Powder Diffraction Standards, International Center for Diffraction Data, Swarthmore, PA, 1977, Card 27-322. I. L. Botto and E. J. Baran, Crystal data for the trimetaphosphates of lanthanum, cerium and praseodymium, J. Appl. Crystallogr., 12(1979) 257. H. W. Dunn, X-ray diffraction data for some uranium compounds, Rep. ORNL-2092, 1956 (Oak Ridge National Laboratory). C. E. Bamberger, G. M. Begun, J. Brynestad and J. F. Land, Simultaneous precipitation of phosphates of Bi(II1) and Ce(IV) or Ce(II1): characterization of precipitates and their ignition products, Radiochim. Acta, 31(1982) 57. G. T. Seaborg and J. J. Katz (eds.), The Actinide Elements, in National Nuclear Energy Ser., Vol. 14A, Division IV, Plutonium Project Record, Manhattan Project Technical Section, McGraw-Hill, New York, 1st edn., 1954, pp. 414,416,417. C. Keller and K. H. Walter, Preparation, lattice parameters and chemical properties of some ternary oxides of neptunium, americium and curium of the type Mes*XVO,, J. Inorg. Nucl. Chem., 27(1965) 1253.