Infrared and X-ray diffraction spectra of ammonium uranyl carbonate

241

Materials Chemistry and Physics, 36 (1994) 241-245

Infrared and X-ray di~raction carbonate

spectra of ammonium

uranyi

N.H. Rofail Nuclear Chemistly ~e~a~rnent, Nuclear Research Centre, Atomic Energv Post Code 13759, Cairo (Egypt)

(Received December 4, 1992; accepted May 21, 1993)

Abstract Ammonium uranyl carbonate was precipitated at various temperatures (50-65 “C), flow rates of COz (25-40 1 h-i) and NH, (35-65 I h-‘) and precipitation times (160-320 min). In the X-ray diffraction patterns, all products exhibited the diffraction lines characteristic of (NH,)zU02(C03)3, with small differences in intensity. The IR spectra (400&200 cm-‘) of the fresh samples as well as of the stored solid showed exactly the same patterns. They exhibited the characteristic vibrations of (NI&)+ at 3230 cm-’ (r+) and at 1445 cm-’ (ZQ) and of (CO,)‘at 1520, 1050, 720 and 690 cm-‘. The urany1 ion antis~metric stretching (Q) vibrations appeared at 890 cm-‘. The bending and stretching modes of (UO,)** are greatly hindered, owing to hexagonal coordination with the three bidentate carbonate groups.

Ammonium uranyl carbonate (AUC) has become an important product in the conversion of UF, and uranyl nitrate to UO, powder, used in the fabrication of fuel elements for nuclear reactors. The two industrial wet chemical processes involve the precipitated intermediate products, ammonium uranate and AUC. The AUC process was developed to obtain a precipitate with a lower F- content that is more amenable to further processing than a~onium uranate [l, Z].’ In comparison with other processes, the AUC process has the following advantages: (a) purification during the process of crystallization is good, and (b) UO, obtained from AUC has better reactivity and better powder characteristics for producing reactor elements P]Baran [4] reported the precipitation of AUC through precipitative reextraction of uranyl nitrate solvate in a 30% kerosene solution of tri-n-butyl phosphate by means of an aqueous ammonium carbonate solution. Products with a N/U ratio lower than 2.65 and a C/N ratio lower than 2.34 showed a structure other than that of AUC, which corresponds to the hydroxocarbonate 141. The crystal structure of the coordination AUC compound has been determined by X-ray diffraction by Malcic [.5], Graziani er al. [6] and Bachmann ef al. [7]. They reported a monoclinic lattice, and listed data for cell constants, space groups and other structural information. Graziani et al. [6] assigned a structure in

0254-0584/94/$07.00 0 1994 Elsevier Sequoia. All rights reserved

which the uranium~V1) atom is in an eight-coordinate distorted hexagonal environment. The linear uranyl group is perpendicular to the equatorial plane, in which three carbonate groups are chelated. The NW ions that fill holes in the structure link the anions through hydrogen bonding. In spite of such detailed structural studies concerning AUC, there seem to have been few infrared studies investigating the effect of such a coordination on the well-established vibrations for the three main groups, namely uranyl, carbonate and ammonium, and their interdependent. This promoted the present study, which reports the X-ray diffraction and IR spectra of laboratory-prepared AUC samples.

Reagents AUC samples were precipitated using the following three solutions: (1) 1.6 M uranyl nitrate (feed solution), prepared by dissolving 235 g U,O, in 170 ml of 65% HNO,; (2) 0.7 M a~onium carbonate solution (starting solution), prepared by dissolving 54 g I-’ (NH&CO,; (3) 1.5 M ammonium carbonate, used as a washing solution.

The starting solution (2) was heated to the prescribed temperature (S-65 “C), and then CO*, NH, and N,

242 TABLE

1. Conditions

Sample

for precipitation

of AUC samples

Precipitation Temperature

Feed dose (ml h-‘)

COZ (1 h-‘)

NH, flow rate (1 h-‘)

Fixed parameters

(“C)

Time (min)

HA

60

160

180

40

65

uranyl nitrate hydrate concentration =1.6 mol 1-l

HB

50

160

180

40

65

slurry density =154 g u 1-l

HC

60

320

90

25

35

PH 8

HD

65

160

180

40

65

Nz flow rate =lOO 1 h-’

gases were passed through the hot solution at flow rates of 25-40, 35-65 and 100 1 h-l, respectively. The feed solution was added to the hot gas-agitated starting solution at the rate of 150 ml h-l for up to 180 min, as shown in Table 1. After addition of the feed solution, the passage of the three gases was continued for another 20 min; the flow of NH, was stopped before both CO, and N,. The resultant slurry of AUC was filtered under reduced pressure, and the cake washed twice by the washing solution (3) and then by methanol to remove the adhering water. The washed AUC was then left in air at room temperature to dry.

HO

Apparatus

X-ray diffraction analysis was carried out with a Siemens D500 instrument, using Cu Ka, radiation (A= 1.5405 A) and a chart speed of 2” min-‘. Infrared absorption measurements were performed using the KBr pellet technique. The spectra were recorded using a Beckmann Munchen Bell apparatus.

Results and discussion X-ray diffraction analysis

Figure 1 and Table 2 show the X-ray diffraction patterns of four AUC samples prepared under different conditions of temperature, feed dose, total precipitation time, and flow rates of CO, and NH,. The four samples have the same diffraction lines as those reported earlier, differing only in intensity. This confirms that AUC is a stoichiometric compound that has a definite monoclinic structure, while ammonium uranate is nonstoichiometric and its composition depends on the U:NH,:H,O ratio. All diffraction patterns are identical with the data published by Bachmann et al. [7]. It is noticed from the X-ray patterns that some diffraction lines characteristic of ammonium uranate start to appear if the temperature of precipitation is

Fig. 1. X-ray diffraction

spectra of AUC samples.

increased beyond 65 “C. Also, if UO,(NO& is present in excess, it may dissolve the formed AUC to form other ammonium derivatives, as shown by the following equations [8]: 5(NH&JO2(CO3)3

+ UOz(NO& + 6HzO -

3(NH&(UO,),(CO,),(H,O),

+ 2NHNO3

or 2(NI-IJ4UOZ(C03)3 + UOP(NO& + 6H,O 3(NH,)JJO,(CO,),(H,O),

+ 2NH?NO,

The pH value during precipitation affects the precipitate composition. At pH values less than 7, the hydroxocarbonate obtained is reported to have the following formula [9]:

243 TABLE

2. X-ray

Literature

data”

patterns of AUC d (A) for HA-HD samples, obs.

I est.

h k 1

7.05

50

1 1 0

7.06

6.36

broad 90

002 1 1 T

6.36

d (A) talc.

;x) obs.

7.011 y3;; 5.959 5.294 4.899 4.678 4.392 4.316 3.865 3.759 3.519 3.446 3.318 3.281 3.186 3.172 3.145 3.118 2.985 2.933 2.892 2.875 2.808 2.742 2.676 2.647 2.633 2.611 2.547 2.483 2.450 2.353 2.337 2.301 2.237 2.216 2.147 2.138 2.126 2.113 2.095 2.076 2.049 2.004 1.955 1.943 1.932 1.923 1.911 1.899 1.878 1.843 1.821 1.795 1.779

diffraction

5.97 5.30 4.88 4.68 4.39 4.32 3.86 3.76 3.51 3.44 3.31 3.28 3.18 3.17 3.14 3.12 2.988 2.930 2.890 2.874 2.808 2.739 2.674 2.644 2.632 2.608 2.548 2.483 2.450 2.352 2.337 2.301 2.237 2.216 2.147 2.137 2.126 2.113 2.094 2.076 2.050 2.006 1.956 1.944 1.933 1.922 1.910 1.899 1.878 1.842 1.822 1.795 1.780

80 80 10 20 50 50 80 50 50 15 50 50 IS 20 15 20 20 15 15 20 15 50 15 10 15 15 15 20 50 20 20 50 20 15 15 1.5 20 15 20 10 15 15 20 20 20 15 15 15 20 20 20 20 20

1 1 1 2 0 0 1 1 2 0 2 0 0 2 1 i 0 2 2 0 2 1 1 3 1 1 3 2 2 i 2 2 1 3 1 1 004 2 2 i 0 2 3 3 1 1 1 1 4 1 3 i 1 3 1 i 0 4 2 2 9 1 3 i 1 3 2 4 0 0 0 2 4 2 2 3 4 0 2 1 3 3 2 2 4 4 0 2 3 3 0 0 4 1 4 2 i 1 3 4 1 3 4 2 2 5 2 4 i 4 2 5 2 4 1 1 1 6 0 4 3 5 1 1 3 3 rt 5 1 3 4 0 4 3 1 5 135 206 316 1 5 0 3 3 4 1 I i I 5 2.

5.97 5.30 4.89 4.68 4.39 4.31 3.86 3.76 3.51 3.44 3.32 3.28 3.17 3.14 3.12 2.99 2.931 2.889 2.874 2.808 2.74 2.678 2.646 2.632 2.61 2.55 2.487 2.45 2.35 2.32 2.303 2.24 2.22 2.15 2.136 2.129 2.111 2.09 2.072 2.03 2.006 1.956 1.946 1.931 1.922 1.877 1.844 1.822 1.80 1.78

I& HA

HB

30 35 5 10 15 90 20 25 10 10 10 20 -

-

5 5 5 5 10

WD

10

15

15

100

100

100

40 55

40 35 5 5 30 30 20 30 2s 15 10 25

50 50

5 15 20 60 25 30 5 1.5 20 -

50 10 10 10 5 20 10 15 60 5 5 5 5 10 10 20 5 5 5 20 10 10 5 25 40 5 15 10 5 20 15 5 5

HC

-

30 5 15 10

15 1.5 10 5 5 10 15 10 5 5

40 5 10 20 10 15 10 25 50 5 5 5 5 5 10 35 5 5 10 20 15 10 5 10 20 5 5 5 5 25 20 10 5

5 5 5 5 5

10 10 10 10 10

10 5 15 45 5 5 5 5 5 15 15 15 10 10 15 10 5 -

-

2. (conkwed)

Literature

of samples

10 loo

TABLE

samples

-

-

-

10 15 15 60 25 25 10 10 25 25 10 25 15 5 10 5 15 35 5 5 5 5 5 15 20 15 10 10 10 15 5 5 1.5 10 5 10 5 10 10 10 10 10

5 5 5 5 5

(curztinued)

data8

I

d

d (A) for H.&HD samples, obs.

it k I

(A) talc.

&) obs.

est.

1.758

1.758

20

;:;‘,;

1.751

20

1.731 1.707 1.703 1.675 1.662 1.653 1.640

1.731 1.706 1.704 1.675 1.662 1.652 1.639

20 15 15 1.5 15 20 15

531 530 1 602 _ 5 3 i 136 153 535 335 514 622

;I;;; 1.621

1.632 1.620

20 15

> 425 336 352

1.580

1.580

20

352 443

Z/I,, of samples HA

HB

HC

I-ID

1.756

1.5

10

20

10

1.733 _

5

5

10

5

-

-

-

-

1.679 1.665 1.655 1.640

5 5 5 5

10 5 5 5

10 10 10 10

5 5 5 5

1.632

5

5

10

5

1.620 1.600

5 5

5 5

10 10

5 5

1.580

5

5

10

5

“Ref. 7. Cell constants of (~~)~UU~(~O~)~: a = 10.654 f 0.001 A; b = 9.356 + 0.001 A; c = 12.824 f 0.002 A; j3= 96.42” j, 0.01”; 2=4.

I

4m

1

35cfJ

3000

2500

I

I

zoo3

Wave

.

,

1400

number

,

,

(100

,

I

200

cm”

Fig. 2. Infrared absorption spectra of AUC samples: (I) spectrum of the four prepared AUC samples; (II) spectrum of an AUC sample stored for 45 days in a closed bottle.

Infrared spectroscopy

Figure 2(I) shows the IR absorption spectra exhibited by the four prepared AUC samples, the spectra being identical over the whole range 40~-2~ cm-‘. In the absence of free or combined water, AUC should indicate absorption bands characteristic of the three main groups, NH,“, UOz’and (CO,)‘-, combined in the crystalline coordination product. In order to adequately describe the absorption bands, the band absorbance (log To/T,), intensity and halfband with (AC,,,) indicative of its sharpness, in addition to the assignment, were established for each band, and are given in Table 3.

244 TABLE 3. Characteristic Peak position (cm-‘) 225 240 270 460 690 720 845 855

Absorbance log(TlJ/T,) ( x 104)

infrared

spectra

Relative intensity

peaks of AUC &tZ

366

0.144

5.0

1520 131 726

0.598

0.052 0.286

5.0 14.0 2.0

1827 322

0.719 0.128

2.0 2.0

890

2539

1.000

1050 1345 1445 1520 1678 2850 3230

863 2596 1867 2144 257

0.339 1.020 0.7353 0.8444 0.101

4.0 1.5 8.0 7.0 12.0 4.0

854

0.336

16.0

Assignment

doubly degenerate

bending vibration,

U-Ori,

stretching

U-O stretching vibration of v~(UO~)‘+~ U + 0 linkb q for COa*v3 for CO,*V, for CO?Raman frequency (150 cm-‘; a weak doublet symmetric many1 group”, b antisymmetric stretching of the uranyl group y(UOr)*+ %(CW*%(Co3)*v, useful diagnostic (NH,)+ v.&o,)*deformation vibration of v*(NH~)+ nonaxial v,(NH,J+ valence vibration asymmetric stretching vibration v&W,)+

vibration”

stretching

of the

“Ref. 16. bRef. 15.

The ammonium ion, as a representative of the tetrahedral X0, ions, should have four normal modes of vibration: the doubly degenerate vibration v,, the nondegenerate stretching vibration vl, and two triply degenerate vibrations v, and v,, the latter two being the only infrared-active vibrations [lo]. Oxton et al. [ll] stated that the valence vibrations v,(N-H) are differentiated into axial and nonaxial frequencies; in our compounds the axial frequency seems to be inactive or hidden in bands of the nonaxial frequency. As a result, the weak low-energy band at 2850 cm-’ confirms the presence of v1(NH4)+ with a strong hydrogen bond. According to Knop et al. [12] the deformation vibration u2(NH,) l may appear at 1678 cm-’ in the IR spectrum, activated by strong hydrogen bonds, which is clear in our case. The asymmetric stretching vibration r+(NH4) + occurs in the region 3350-3050 cm-’ and may be either obscured by or confused with the absorption arising from the presence of water; in our case no confusion is likely owing to the absence of water. However, v,(NH,)+ is found between 1450 and 1390 cm-’ and is a useful diagnostic absorption band. Vibrations are observed at both 1445 and 3230 cm-‘, characteristic of v, and v, absorption, and appear as clear prominent, bands in both materials. As for the (U0,)2’ ion, four vibrations were assigned: v,, as two bending modes that differ only in the direction of the bend and are degenerate, and the symmetric and asymmetric stretching modes y1 and v,. Jones and Penneman [13] quoted the following bands for the uranyl ions: r+ at 880-860 cm-‘, v, at 210-199 cm-’ and v, at 960-930 cm-‘. Since the symmetric stretching

(vl) is a Raman-active frequency (150 cm-‘), it was interpreted by Freymann et al. [ 141not as an independent frequency, but as the difference between the frequencies 860 and 720 cm-‘. So this confirms the presence of a Raman-active vibration, i.e., the symmetric vibration. From the fact that this band occurs only in the crystal, it seems most reasonable to assume that the appearance of this band results from crystal field effects that cause a symmetry lowering of the whole ion, which leads in part to the nonlinearity and/or nonequivalence of the uranyl bonds (O-U-O). A strong, sharp r+(U02)2f band at 890 cm-l with a strong, broad v~(UO~)~+ band at 275 cm- ’ are seen in Fig. 2. These observations may be understood by considering the crystal field effects as previously interpreted. As a result, the presence of the uranyl group is confirmed. A phenomenon similar to the doubly degenerate bending vibration also occurs in the far-infrared region between 250 to 210 cm-l, as shown in Fig. 2. The bands observed in this region can be assigned mainly to the U-O(C0,) and the U-X stretching vibrations, corresponding to the doubly degenerate bending vibration. The carbonate group (C03)2- shows generally strong infrared absorption in the region 1530-1320 cm-’ (rQ, usually restricted to 1450-1400 cm-‘, and medium strength bending vibrations between 890 and 800 cm- ’ (v2) and between 760 and 670 cm-’ (~~‘4). When present, as in aragonite-type carbonates, the symmetric stretch (Ye) occurs in the 1120-1040 cm-’ region. The v, and v, modes are doubly degenerate (and under certain symmetry conditions will split into doubles), and the

245

v1 stretching frequency becomes obvious. All of these vibrations are abo observed as prominent bands at 1520, 1050, 845, 720 and 690 cm-‘. The spectra for all the prepared samples have the same bands, represented by curve (I) in Fig. 2. Curve (II) in the same figure represents an AUC sample stored for 45 days in a closed bottle; a very weak, broad band at 3440 cm-’ is noticed, corresponding to the bending, stretching and deformation of the OH group [15], owing to the water absorbed during storage. This broad band disappeared from the IR spectra after the stored sample was dried for 3 h at 50 “C.

owing to steric hindrance by the three (CO,)*forming the hexagonal symmetry.

References

Conclusions

The results of this work show that the optimum precipitation temperature of ammonium uranyl carbonate is 60-65 “C. Higher temperatures result in the decomposition of the compound to ammonium uranate. The presence of the requisite amount of ammonia is necessary in order to obtain the optimum pH value for precipitation. In addition, CO, is passed while the pH is controlled to prevent the formation of ammonium uranate and ammonium carbonate. Regarding the structure of the ammonium uranyl carbonate obtained, IR spectral analysis showed that (C03)‘- and NH: ions are found in equatorial coordination with the uranium atom. The bending and stretching modes of the (UO$+ ion could not be observed in the spectrum, presumably

ligands

9 10 11 12 13 14 1.5 16

S.G. Brandberg, Nucl. Technol., 19 (1973) 177. H. Assman and W. Dorr, in P. Vincenzini (ed.), Ceramic Powders, Eisevier, Amsterdam, 1983, p. 707. G. Qingren and K. Shifang, Themzochim. Acta, I16 (1987) 71. V. Baran, Collect. Czech. Chem. Commu~., 47 (1982) 1269. S.S. Malcic, Bull. Boris Kidric Inst. Nucl. Sci., 8 (1958) 95. R. Graziani, G. Bornbieri and E. Forsellini, J. Chem. Sot., Dulton Trans., 19 (1972) 2059. H.G. Bachmann, K. S&bold, H.Z. Dokuzoquz and H.M. Muller, J. Inorg. Nucl. Chem., 37 (1975) 735. 1.1. Chernyaev, V.A. Golovnya, G.V. Ellert, R.N. Shcholokov and V.P. Markov, 2nd Int. Conj Peaceful Uses of Atomic Energy Geneva, 1958, Paper P/2138. 1.1. Chemyaev, V.A. Golovanya and G.V. Ellert, Zh. Neorg, Khim., 6 (1961) 386. J.A. Gadsen, Infrared Spectra of Minerals and Related Inorganic Compounds, Butterworths, London, 1975. J.A. Oxton, 0. Knop and M. Faik, Can. .I. Chem., 53 (1975) 2675. 0. I(nop, W.J. Westerhands and M. Fafk, Can. J. Chem., 58 (1980) 270. L.H. Jones and R.A. Penneman, J. Chem. Phys., 21 (1953) 542. M. Freymann, 7”. Guilmart and R. Freymann, CR. Acad. Sci., 223 (1946) 545; 223 (1946) 573. M. Rodor, Z. Poko and J. Mink, Mikrochim. Acta, 4/5 (1966) 865. Ken Ohwada, J. Coord. Chem., 6 (1976) 75.