Thermal synthesis and structural characterization of the orthorhombic Th2(PO4)(P3O10)

Polycrystalline thorium(IV) phosphate-triphosphate, Th₂(PO₄)(P₃O₁₀) (1), was obtained by (NH₄)₂Th(PO₄)₂·H₂O (2) heating from room temperature to 1,273 K. 1 crystallizes in the orthorhombic space group Pn2₁ a (a = 11.6846(2) Å, b = 7.1746(1) Å, c = 12.9320(3) Å, Z = 4). Combining powder synchrotron X-ray diffraction data and DFT geometry optimization, a structural model is proposed for 1. The structure is built on ThO₈ polyhedral chains along the b-axis. PO₄ ^3? and P₃O₁₀ ^5? groups coexist in the structure and the latter group forms non-linear chains. Cohesion of the structure is made by the linkage of ThO₈ chains by PO₄ and P₃O₁₀ groups. Thermal transformation from 2 to 1 was monitored by thermogravimetric analysis (activation energy as a function of the extent of conversion was obtained from Kissinger–Akahira–Sunose (KAS) isoconversional method) and powder X-ray thermo-diffraction. For 2, the dehydration process takes place in two steps, with the apparition of a layered intermediate phase, (NH₄)₂Th(PO₄)₂·nH₂O (0 < n < 1, d = 6.42 Å), previously to the formation of (NH₄)₂Th(PO₄)₂ (d = 6.31 Å). The condensation process produces an amorphous material that crystallizes to α-ThP₂O₇ (3) when the temperature increases. At 1,273 K, 3 slowly transforms to 1.