Th₄(PO₄)₄P₂O₇, a New Thorium Phosphate: Synthesis, Characterization, and Structure Determination

The chemistry of thorium phosphate reported in the literature has been found to be erroneous. It was reconsidered in terms of careful chemical preparations and specific analytical methods. Special attention has been paid to the atom ratio value referred to r = thorium/phosphorus, which was experimentally fixed in order to obtain the correct composition of the final compound. A new compound with the chemical formula Th 4 (PO 4 ) 4 P 2 O 7, derived from the crystal structure determination, has been obtained. The unit cell parameters were obtained from powder and single-crystal X-ray diffraction data. It is orthorhombic (space group Pcam, Z = 2) with the cell dimensions a = 12.8646(9) Å, b = 10.4374(8) Å, c = 7.0676(5) Å, and V = 949.00(9) Å 3 . The atomic positions were derived from Patterson and Fourier methods and the structure was refined to an R value of 0.039. The structure consists of layers parallel to (010) containing both PO 4 and P 2 O 7 groups. These layers alternate with planes of Th atoms. The coordination sphere of the two independent heavy atoms is formed by eight O atoms from five PO 4 and one P 2 O 7 groups. The formula of thorium phosphate Th 4 (PO 4 ) 4 P 2 O 7 is in good agreement with the elementary composition derived from electron microprobe analysis, which gave a ratio r = 2 / 3 . Any other value of r ( 1 / 2 < r < 3 / 4 ) induces the formation of polyphase systems: Th 4 P 6 O 23 and ThO 2 for r > 2 / 3; Th 4 P 6 O 23 and ThP 2 O 7 for r < 2 / 3 . The characterization of thorium phosphate diphosphate by means of infrared spectroscopy confirmed the presence of diphosphate groups in the compound.