TH-4(PO4)(4)P2O7, 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 chemic al 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 composi tion of the final compound. A new compound with the chemical formula T h-4(PO4)(4)P2O7, 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 grou p Pcam, Z = 2) with the cell dimensions a = 12.8646(9) Angstrom, b = 1 0.4374(8) Angstrom, c = 7.0676(5) Angstrom, and V = 949.00(9) Angstrom (3). The atomic positions were derived from Patterson and Fourier meth ods and the structure was refined to an R value of 0.039. The structur e consists of layers parallel to (010) containing both PO4 and P2O7 gr oups. 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 PO4 and one P2O7 groups. The formula of thorium phosphate Th -4(PO4)(4)P2O7 is in good agreement with the elementary composition de rived from electron microprobe analysis, which gave a ratio r = 2/3. A ny other value of r (1/2 < r < 3/4) induces the formation of polyphase systems: Th4P6O23 and ThO2 for r > 2/3; Th4P6O23 and ThP2O7 for r < 2 /3. The characterization of thorium phosphate diphosphate by means of infrared spectroscopy confirmed the presence of diphosphate groups in the compound.