Study of Th 4 (PO 4 ) 4 P 2 O 7 and Solid Solutions With U(IV), Np(IV) and Pu(IV): Synthesis, Characterization, Sinterin
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thorium phosphate-diphosphate Th 4 (PO 4 )4 P2 0 7 (TPD) was obtained through several ways of synthesis (using wet and dry chemistry methods) then characterized. The ab initio structure determination on powder and single crystal led to an orthorhombic unit cell (space group Pcam, Z = 2) with a = 12.8646(9) A, b = 10.4374(8) A and c = 7.0676(5) A (V= 949.0(1) k3) [1]. This material is very resistant to aqueous corrosion [2]. Thus, it was specially tailored for the 239 pu immobilization and for the radiogenic 235 U. Nevertheless, we were also interested in the incorporation of the tetravalent neptunium (237 Np) or trivalent actinides like americium and 4 4 curium (24 1 Am, 244 Cm) for the long term storage. We studied the substitution of Th + by U +, 3 3 4 4 Np + and Pu + on one hand and by Am + and Cm + on the other hand even if the substitution of thorium by trivalent actinides could be difficult because of the great difference between ionic radii. On the contrary, for tetravalent actinides it should be possible to get solid solutions with U4 +, Np4 + and Pu 4 + because their ionic radii are close to the thorium one ([8 ]rTh = 1.05A, [8 ]rU =1.00A, [8 ]rNp = 0.98A and [8 ]rpu = 0.96A) in the eightfold coordination number.
85 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
SYNTHESIS AND CHARACTERIZATION Th_4_xM__P(Q4)P
2Q 7
solid solutions (M(V) = U, Np, Pu)
In our previous work concerning the ThO 2 - P20 5 system, we showed that for a mole ratio r Th/P0 4 in the range 0.5 < r < 1, only the thorium phosphate-diphosphate (r = 2/3) is well defined at high temperature. For all other ratios, the system obtained was polyphase [1]. For this reason, we performed all the syntheses considering a mole ratio r = (Th+M)/P0 4 equal to 2/3. Solid solutions of thorium-neptunium (IV) phosphate-diphosphate Th4_xNpx(PO 4 )4 P20 7 (TNPD) and thorium-plutonium (IV) phosphate-diphosphate Th 4 _xPux(PO 4 )4 P2 0 7 (TPPD) were synthesized for several x values from a mixture of thorium, tetravalent neptunium (or plutonium) nitrate and concentrated phosphoric acid solutions. For thorium-uranium (IV) phosphatediphosphate solid solutions (TUPD), solutions were prepared in hydrochloric acid in order to avoid the oxidization of U4 + into U0 2 +. The mixtures were slowly evaporated between 100°C =
and 200'C, ground then heated in alumina nacelles at 300'C for 2 hours and at 1000°C for 14 hours in air (for TNPD and TPPD solid solutions) or in inert atmosphere (for TUPD solid solutions). Some details concerning these syntheses were already reported [3, 4]. The solids were characterized by X-ray powder diffraction. Silicon metal (JCPDS file number 27-1402) was added as internal reference in order to calibrate the angular positions of the diffraction lines. As the structure of TPD is well known, we studied the deformation of the lattice when thorium is partly replaced by a tetravalent smaller actinide (M(IV) = U, Np, Pu). For each solid solution Th 4 _xMx(PO 4 )4 P2 0 7 , the variations of the unit cell parameters in term
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