Local environment of iodine dissolved as iodate in high-pressure aluminoborosilicate glasses: A I K-edge x-ray absorption spectroscopic study

Abstract

The use of high-pressure synthesis conditions to produce I-bearing aluminoborosilicate represents a promising issue for the immobilization of ¹²⁹I radioisotope. Furthermore, iodine appears to be more solubilized in glasses under its iodate (I⁵⁺) form rather than its iodide (I^-) form. Currently, the local atomic environment for iodine is poorly constrained for I^- and virtually unknown for I⁵⁺ or I⁷⁺. We used I K-edge x-ray absorption spectroscopy conducted at 20 K for determining the local atomic environment of iodine dissolved as I^-, I⁵⁺, and I⁷⁺ in a series of aluminoborosilicate glasses. We determined that I^- is surrounded by either Na⁺ or Ca²⁺ in agreement with previous studies. The signal collected from EXAFS reveals that I⁵⁺ is surrounded invariably by three oxygen atoms forming an IO₃ ^- cluster charge compensated by Na⁺ and/or Ca²⁺. The I-O distance in iodate dissolved in glass is comparable to the I-O distance in crystalline compounds at ∼1.8 Å. The distance to the second nearest neighbor (Na⁺ or Ca²⁺) is also constant at ∼3.2 Å. This derived distance is identical to the distance between I^- and Na⁺ or Ca²⁺ in the case of iodide local environment. For one sample containing iodate and periodate, the distinction between the local environment of I⁵⁺ and I⁷⁺ could not be made, suggesting that both environments have comparable EXAFS signals.