Structural and thermal studies on PuTe2O6

Journal of Alloys and Compounds 307 (2000) 114–118

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Structural and thermal studies on PuTe 2 O 6 K. Krishnan, K.D.S. Mudher*, V. Venugopal Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received 7 January 2000; accepted 22 February 2000

Abstract PuTe 2 O 6 was synthesised by the solid state reaction route and characterized by X-ray diffraction and thermal methods. The structure of PuTe 2 O 6 was derived by the Rietveld analysis of X-ray powder diffraction data in the monoclinic system with cell parameters a50.69937(1), b51.10014(2), c50.73404(2) nm, b 5107.98(2)8, Z54 in the space group P21 /n. In the structure, each plutonium atom is coordinated to eight oxygen atoms. The structure is made up of zigzag strings of PuO 8 distorted edge-sharing polyhedron parallel to the a axis. Tellurium atoms are coordinated to three oxygen atoms. PuTe 2 O 6 melts at 1125 K incongruently and decomposes according to the reaction: PuTe 2 O 6 (s)→PuO 2 (s)12TeO 2 (g). The kinetics of decomposition under isothermal heating conditions in flowing air were studied to determine the rate constants, activation energy and reaction mechanism.  2000 Elsevier Science S.A. All rights reserved. Keywords: PuTe 2 O 6 ; Crystal structure; Thermochemistry

1. Introduction Crystal structure and thermochemistry of the binary and ternary oxides of thorium, uranium, plutonium and fission products formed during the irradiation of oxide fuels is important in evaluating their performance in a reactor. Tellurium, one of the highly corrosive fission products that embrittles the steel claddings in fast breeder reactors [1], also exists in different chemical states in an irradiated fuel [2]. Various compounds of tellurium with uranium [3–5] and other actinides [6–8] have been synthesised and characterized by X-ray diffraction. In the systematic studies, we have earlier published the thermochemical, vaporisation behaviour and the Gibbs’ energy of formation of tellurium bearing compounds in the M–Te–O system, where M5Ce, Th, U and Ni [9–12]. In the Pu–Te–O system, Pu 2 O 2 Te is a well characterized phase and is reported to be isostructural with the corresponding rare earth oxide tellurides [13]. Though there is some data regarding the preparation of PuTe 2 O 6 [14], no detailed investigations have been carried out on its structure and thermochemistry. In the present work, we report the preparation, crystal structure and thermal studies on PuTe 2 O 6 . The crystal structure of PuTe 2 O 6 has been *Corresponding author. E-mail address: [email protected] (K.D.S. Mudher)

derived by Rietveld profile analysis of X-ray powder diffraction data, while the data on the rate of decomposition during isothermal heating experiments were used to evaluate kinetic parameters, such as activation energy and mechanism of decomposition.

2. Experimental PuTe 2 O 6 was prepared by the solid state reaction route, by heating well-ground mixtures of PuO 2 and TeO 2 in 1:2 molar ratio in an alumina boat in static air atmosphere at 975 K for 24 h. The product was removed intermittently, ground and reheated to obtain pure phase of the compound. The X-ray diffraction data was collected on a Diano diffractometer, enclosed in a glove box to handle radioactive material, using graphite monochromatised CuKa 1 radiation ( l50.15406 nm). The step scanned intensity data were recorded in the 2u range of 10–1008 with a step of 0.028, with a counting time of 5 s for each step. The X-ray data were analyzed using the Rietveld analysis program DBWS-9411 [15] to derive the structure. The thermal stability of PuTe 2 O 6 was studied by recording the TG and DTA patterns simultaneously using the Mettler thermoanalyser enclosed in a glove box. The experiments were carried out in platinum cups in flowing dry air, at the heating rate of 10 K / min up to 1673 K.

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00841-0

K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114 – 118

Pre-heated alumina was used as the reference standard for DTA measurements. For evaluating the kinetic parameters, isothermal heating measurements were carried out at 1193, 1208, 1223 and 1233 K in the same instrument.

3. Results and discussion

3.1. Crystal structure The X-ray diffraction data of PuTe 2 O 6 was indexed on a monoclinic cell. The least squares refined values of the lattice parameters are given in Table 1. The extinction conditions corresponded to the space group P21 /n. The similarity in the crystal data of PuTe 2 O 6 and CeTe 2 O 6 [16] suggests that both the compounds are isostructural. Wroblewska et al. [14] have reported an orthorhombic cell, ˚ for PuTe 2 O 6 . However, a55.60, b510.46 and c511.76 A the present X-ray data could not be indexed with the reported cell parameters. The crystal structure of PuTe 2 O 6 was derived using the atomic parameters reported for CeTe 2 O 6 as the trial model. Rietveld’s method was used for the refinement of the X-ray data, using the DBWS 11 program [15]. The variables include a scale factor, six background parameters, three half width parameters defining pseudo Voigt profile peak shape, the unit cell dimensions, atomic positions, thermal parameters for plutonium and tellurium atoms. Details of refinement are given in Table 1. The refinement of the parameters gave final profile agreement factors of R p 5 10.4%, R wp 512.6% and R exp 57.9% and S51.6. The fractional atomic coordinates and isotropic thermal parameters of the atoms in PuTe 2 O 6 are given in Table 2, and selected atomic distances and interatomic angles are given in Table 3. The observed, calculated and difference X-ray powder diffraction patterns are shown in Fig. 1. The structure of PuTe 2 O 6 was drawn using software Table 1 Details of Rietveld refinement for PuTe 2 O 6 Chemical formula Formula weight Space group a (nm) b (nm) c (nm) b (8) Z rcalc. (g cm 23 ) 2u range (8) Step size (2u, 8) Wavelength (nm) No. of data points Peak profile R p (%) R wp (%) R exp (%) S

PuTe 2 O 6 590.2 P21 /n 0.69937(1) 1.10014(2) 0.73404(2) 107.98(2) 4 7.29 10–100 0.02 0.15406 4500 Pseudo Voigt 10.4 12.6 7.9 1.6

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Table 2 Refined atomic coordinates and temperature factor for PuTe 2 O 6 a 2

Atom

x

y

z

˚ ) Biso (A

Pu Te1 Te2 O1 O2 O3 O4 O5 O6

0.2571(6) 0.2322(9) 0.2623(8) 0.145(3) 0.405(4) 0.063(3) 0.155(3) 0.524(4) 0.182(4)

0.0895(3) 0.2736(5) 20.0742(6) 0.249(2) 0.405(2) 0.388(2) 0.020(2) 20.101(2) 20.229(3)

0.0065(4) 0.4326(7) 20.4607(8) 0.160(2) 0.423(3) 0.483(4) 20.297(3) 20.323(2) 20.389(3)

0.3(1) 1.5(3) 1.1(4) 2.6(6) 3.5(8) 2.8(7) 2.1(7) 2.8(8) 2.9(7)

a

E.s.d. values are given in parentheses.

Powder cell [17] and is shown in Fig. 2, depicting the atoms in the unit cell projected along the b-axis. The structure of PuTe 2 O 6 is similar to that of CeTe 2 O 6 . In the structure, each plutonium atom is surrounded by eight oxygen atoms at distances ranging from 0.222 to 0.254 nm (average distance 0.235 nm). The corresponding Pu–O distances in Pu(SO 4 ) 2 ?4H 2 O range from 0.231 to 0.241 nm at an average distance of 0.23360.003 nm in an Archemedian square antiprism geometry [18]. The eight 41 coordination of Ce ion in CeTe 2 O 6 has been described as trigonal prism bicapped over two rectangular faces [16]. The structure of PuTe 2 O 6 consists of zigzag chains of PuO 8 distorted polyhedra running parallel to a axis. These polyhedra are linked to each other by the common O 2 –O 2 and O 3 –O 3 edges. Each tellurium atom is surrounded by three oxygen atoms at distances given in Table 3. Of the three oxygen atoms around Te 1 atom, O 2 and O 3 share common edge with PuO 8 polyhedra and O 1 atom is linked through the corner of other plutonium polyhedra. All the three oxygen atoms around Te 2 atom i.e. O 4 , O 5 and O 6 share corners with three plutonium polyhedra. The oxygen atoms coordinated to tellurium atoms belong to PuO 6 chains. Thus, in the structure there are infinite chains of (PuO 6 ) 8n2 2n , which are linked by tellurium atoms. Table 3 Selected bond lengths (nm) and interatomic angles in PuTe 2 O 6 ; estimated standard deviations are given in parentheses Pu–O 1 Pu–O 2 a Pu–O 2 b Pu–O 3 c Pu–O 3 Pu–O 4 Pu–O 5 d Pu–O 6 e

0.235(2) 0.235(3) 0.246(2) 0.221(2) 0.254(2) 0.225(2) 0.236(2) 0.227(3)

O 1 –Te 1 –O 2 O 1 –Te 1 –O 3 O 2 –Te 1 –O 3

93.9(9) 106.7(1.0) 86.7(9)

a

2 1 / 2 1 x,1 / 2 2 y, 2 1 / 2 1 z. 1 / 2 2 x, 2 1 / 2 1 y,1 / 2 2 z. c 1 / 2 1 x,1 / 2 2 y, 2 1 / 2 1 z. d 1 2 x, 2 y, 2 z. e 1 / 2 2 x,1 / 2 1 y, 2 1 / 2 2 z. b

Te 1 –O 1 Te 1 –O 2 Te 1 –O 3

0.192(2) 0.190(3) 0.184(2)

Te 2 –O 4 Te 2 –O 5 Te 2 –O 6

0.191(2) 0.182(2) 0.192(3)

O 4 –Te 2 –O 5 O 4 –Te 2 –O 6 O 5 –Te 2 –O 6

105.2(9) 96.0(1.0) 91.9(1.0)

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K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114 – 118

Fig. 1. Observed (dots) and calculated X-ray diffraction Rietveld plot for PuTe 2 O 6 . Difference curve is shown at the bottom of the plot.

Fig. 2. Structure plot of PuTe 2 O 6 projected along the b-axis showing the coordination around Pu and Te atoms. .

K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114 – 118

3.2. Thermal studies The thermogravimetric curve shows that PuTe 2 O 6 is stable in air up to 1173 K, beyond which the compound starts losing weight due to its decomposition. The weight loss is complete around 1573 K. The product at the end of the decomposition was identified by X-ray diffraction as PuO 2 . The observed weight loss of 55.1% was in agreement with the expected loss of 54.0% for two moles of TeO 2 , during the decomposition of PuTe 2 O 6 according to the Eq. (1) PuTe 2 O 6 (s) → PuO 2 (s) 1 2 TeO 2 (g)

(1)

DTA showed an sharp endothermic peak at 1125 K and did not show any reversible peak due to solidification, indicating that the compound PuTe 2 O 6 melts incongruently. The compound was heated in a furnace up to its melting temperature and cooled slowly to room temperature. The X-ray diffraction pattern of the product was identified as a mixture of PuO 2 and TeO 2 confirming further the incongruent melting of the compound.

3.2.1. Kinetic studies Kinetic parameters such as the rate constant k and activation energy E were evaluated from the isothermal data at temperatures 1193, 1208, 1223 and 1233 K. The weight loss at each isothermal temperature was followed with respect to time. The fraction decomposed a was calculated from the weight loss measurements at all the temperatures using the relation a 5 (Wt 2 W0 ) /(Wf 2 W0 )

(2)

where W0 , Wt and Wf are the initial weight, weight at time t and final weight for the completion of the reaction, respectively. The a values were fit in the range 0.15–0.80 into various kinetic models for all the diffusion controlled, nucleation growth and phase boundary controlled reaction

117

mechanisms as reported in the literature [19]. The regression coefficients (g ) calculated for all the kinetic models using iterative procedure are given in Table 4. The correlation coefficients for the phase boundary controlled mechanisms are more consistent with the value approaching unity at all the temperatures among the mechanisms assumed. Generally, the mechanism of the decomposition reactions resulting in solid / solid and gaseous products could be controlled either by diffusion or phase boundary reaction. Our earlier studies on ThTe 2 O 6 and CeTe 2 O 6 [9,10] have indicated similar mechanism as governed by the phase boundary controlled. Also, the observed linearity in the nature of vaporisation at all temperatures in the present study suggests that, the surface area of the reactant is constant throughout the course of the reaction and the rate governing equation can be represented by the phase boundary controlled mechanism as given in Eq. (3) g(a ) 5 1 2 (1 2 a )n 5 kt /r 0

(3)

where a is the fraction decomposed, n is equal to 1 / 3, 1 / 2 and 1 for 3-, 2- and 1-dimensional symmetries; r 0 is the radius of the reactant and g(a ) is the function of a. Since the change in radii of the reactant to the product is negligible for one-dimensional symmetry, the value of n becomes 1 and the Eq. (3) reduces to g(a ) 5 a 5 kt

(4)

The plot of a against time t for all the temperatures is shown in Fig. 3. The rate constant k for each temperature were calculated from the slopes of the curves and then fit into the Arrhenius equation, to derive the activation energy of decomposition of PuTe 2 O 6 . The rate constants k derived from the slopes of the linear fit are given in Table 5. The representation of 2ln k and 1 /T (K) in an Arrhenius plot as shown in Fig. 4, gave an activation energy value of 278610 kJ / mol for the decomposition of PuTe 2 O 6 .

Table 4 Regression coefficient values (g ) obtained for the most commonly used mechanisms in the a range of 0.15–0.80 at different temperatures Mechanism

g (a )

1193 K

1208 K

1223 K

1233 K

1. Diffusion controlled Parabolic law Valensi, two-dimensional Jander, three-dimensional Brounshteub–Ginstling Mampel-unimolecular law

a2 [(1 2 a ) log (1 2 a )] 1 a [1 2 (1 2 a )1 / 3 ] 2 [1 2 2a / 3 2 (1 2 a )2 / 3 ] [2log (1 2 a )]

0.9867 0.9767 0.9620 0.9721 0.9944

0.9892 0.9762 0.9547 0.9697 0.9882

0.9848 0.9711 0.9499 0.9465 0.9863

0.9877 0.9741 0.9511 0.9670 0.9873

2. Nucleation growth Two-dimensional Three-dimensional

2 log (1 2 a )1 / 2 2 log (1 2 a )1 / 3

0.9989 0.9952

0.9987 0.9988

0.9994 0.9991

0.9990 0.9985

3. Phase boundary controlled One-dimensional Two-dimensional Three-dimensional

a [1 2 (1 2 a )1 / 2 ] [1 2 (1 2 a )1 / 3 ]

0.9992 0.9991 0.9983

0.9993 0.9984 0.9960

0.9999 0.9970 0.9943

0.9994 0.9980 0.9955

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K. Krishnan et al. / Journal of Alloys and Compounds 307 (2000) 114 – 118

cross-linked with TeO 3 units. The structure of PuTe 2 O 6 is similar to that of CeTe 2 O 6 . PuTe 2 O 6 melts incongruently and decomposes above 1173 K with the loss of TeO 2 (g) as the vaporizing species. The kinetics of decomposition studied in flowing air under isothermal heating conditions showed that the decomposition of PuTe 2 O 6 is consistent with the phase boundary controlled mechanism. The activation energy evaluated for the decomposition of PuTe 2 O 6 was found to be 278610 kJ / mol.

Acknowledgements

Fig. 3. Plots of a vs. time (min) at various isothermal temperatures. Table 5 Rate constants for the decomposition of PuTe 2 O 6 at different temperatures (K)

2 ln k

1193 1208 1223 1233

4.9275 4.5401 4.2455 4.0172

4. Conclusion PuTe 2 O 6 belongs to the monoclinic crystal system. The structure of PuTe 2 O 6 consists of chains of distorted edgesharing polyhedra of PuO 8 , running parallel to a axis and

The authors are thankful to Shri D.S.C. Purushotham, Director Nuclear Fuels Group and Shri R. Prasad, Head, Fuel Development Chemistry Section for their keen interest in this work and Dr. G.A. Rama Rao for helpful discussion.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Fig. 4. Arrhenius plot of 2ln k vs. 1 /T (K) for the decomposition of PuTe 2 O 6 (s).

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