High-temperature breakdown of the synthetic iodine analogue of vanadinite, Pb ₅(VO ₄) ₃I: An apatite-related compound for iodine radioisotope immobilization?

The thermal stability of the synthetic iodine analogue of vanadinite, Pb ₅(VO ₄) ₃I, in air has been investigated by in situ high temperature X-ray powder diffraction between 300 and 1070 K. Rietveld refinement of phase fractions shows that breakdown to lead orthovanadate, Pb ₃(VO ₄) ₂, begins at temperatures above 540 K, with complete loss of iodine above 680 K. More than 50 K below the onset of breakdown, the unit-cell parameters of Pb ₅(VO ₄) ₃I show anomalous contraction in the crystallographic x-y plane (reduction of a) which we associate with movement of iodine within the [0001] channels of the structure. The implications of these results for immobilization of ¹²⁹I in potential apatite-related crystalline radioactive waste forms are discussed.