Incorporation of iodine into apatite structure: a crystal chemistry approach using Artificial Neural Network
Materials with apatite crystal structure provide a great potential for incorporating the long-lived radioactive iodine isotope (129I) in the form of iodide (I-) from nuclear waste streams. Because of its durability and potentially high iodine content, the apatite waste form can reduce iodine release rate and minimize the waste volume. Crystal structure and composition of apatite was investigated for iodide incorporation into the channel of the structure using Artificial Neural Network. A total of 86 experimentally determined apatite crystal structures of different compositions were compiled from literature, and 46 of them were used to train the networks and 42 were used to test the performance of the trained networks. The results show that the performances of the networks are satisfactory for predictions of unit cell parameters a and c and channel size of the structure. The trained and tested networks were then used to predict unknown compositions of apatite that incorporates iodide. With a crystal chemistry consideration, chemical compositions that lead to matching the size of the structural channel to the size of iodide were then predicted to be able to incorporate iodide in the structural channel. The calculations suggest that combinations of A site cations of Ag+, K+, Sr2+, Pb2+, Ba2+, and Cs+, and X site cations, mostly formed tetrahedron, of Mn5+, As5+, Cr5+, V5+, Mo5+, Si4+, Ge4+, and Re7+ are possible apatite compositions that are able to incorporate iodide. The charge balance of different apatite compositions can be achieved by multiple substitutions at a single site or coupled substitutions at both A and X sites. The results give important clues for designing experiments to synthesize new apatite compositions and also provide a fundamental understanding how iodide is incorporated in the apatite structure. This understanding can provide important insights for apatite waste forms design by optimizing the chemical composition and synthesis procedure.
Thermal and mechanical properties of AgPb9(VO4)6I and AgBa9(VO4)6I
Abstract AgPb 9 (VO 4 ) 6 I and AgBa 9 (VO 4 ) 6 I were synthesized as a potential waste form to immobilize radioactive iodine. The thermal and mechanical properties such as the Young’s modulus and thermal conductivity were measured. The crystal systems of AgBa 9 (VO 4 ) 6 I and AgPb 9 (VO 4 ) 6 I are rhombohedral and monoclinic, respectively. They are stable up to 950 K under Ar atmosphere. The thermal conductivity of the samples is less than 1.0 W m −1  K −1 , which is lower than that of borosilicate glasses used for high-level nuclear waste forms.
Heavy-ion irradiation effects in the ABO{sub 4} orthosilicates: Decomposition, amorphization, and recrystallization
The effects of displacive, heavy-ion irradiation on the properties of synthetic single-crystals of ZrSiO{sub 4} (zircon), HfSiO{sub 4} (hafnon), tetragonal ThSiO{sub 4} (thorite), and monoclinic ThSiO{sub 4} (huttonite) have been investigated. All four ABO{sub 4}-type orthosilicate materials became amorphous in a process that exhibited a two-stage dependence on temperature when irradiated by 800 keV Kr{sup +} or Xe{sup +} ions in the temperature range of 20 to 1100 K. The critical amorphization temperature above which amorphization did not occur increased in the following order: huttonite, zircon, hafnon, thorite. At temperatures below 500 K, the tetragonal and monoclinic polymorphs of ThSiO{sub 4} required approximately the same ion fluence for amorphization. Monoclinic ThSiO{sub 4} is, therefore, not {open_quotes}more resistant{close_quotes} to radiation damage than the tetragonal-symmetry form, but instead, its amorphous phase recrystallizes at a lower temperature. A model that accounts for the observed two-stage behavior was developed and used to calculate the recrystallization activation energies for both stages. When irradiated with heavy ions above a certain characteristic temperature, an unexpected decomposition was observed in which all four ABO{sub 4} orthosilicates phase separated into the crystalline oxides: ZrO{sub 2}, HfO{sub 2}, or ThO{sub 2} plus amorphous SiO{sub 2}. {copyright} {ital 1999} {ital Themore » American Physical Society}« less