**4.8 Iodoapatites**

Since, the apatite structure is capable of accommodating monovalent anions, strontium iodoapatites were investigated as a potential waste form to immobilize radioactive iodine [135],[192].

### **4.8.1 Calcium iodoapatite**

Calcium iodoapatite (Ca5(PO4)3I) does not exist as a separate phase but as oxoapatite. Iodooxyapatite (pentadecacalcium iodide oxide nanophosphate, Ca15(PO4)9(I,O)) was synthesized by the flux method (**Section 4.2**). The crystal structure was refined in the space group P63/M with lattice parameters *a* = 9.567 Å, *b* = 20.754 Å and *Z* = 2. Iodo-oxyapatite has typical hexagonal structure but the unit cell is tripled along the hexad (refer to **Footnote 16** in **Chapter 1**) and oxide ions along this direction [135].

#### **4.8.2 Strontium Iodoapatite**

Strontium iodoapatite (strontium iodoapatite, strontium iodine-apatite) is of academic interest due to large size of I <sup>−</sup> ions compared to other halide ions. However, the thermodynamic functions determined for the alkaline earth apatite series preclude the formation of stable iodoapatite because the cationic size of Sr2+ or Ba2+ is too small relating to that of iodide ion which must fit upon the *c*-axis of the structure next to the triads of Sr2+ or Ba2+ at (000), (010), and (001) positions in the lattice. Such crystals relevant to radioactive waste management include fluorapatite, and, in the end, iodoapatite which should be able to immobilize the radioactive species [135].
