The preparation of thorium phosphate-diphosphate (Th(4)(PO(4))(4)P(2)O(7), TPD) was developed through the precipitation of thorium phosphate-hydrogenphosphate hydrate (Th(2)(PO(4))(2)(HPO(4))center dot H(2)O, TPHPH) at 150-160 degrees C in closed PTFE container or in autoclaves. From EPMA analyses and SEM observations, the initial precipitate was single phase and multilayered, The behaviour of TPHPH (orthorhombic system with a = 21.368(2) angstrom, b = 6.695(1) angstrom and c = 7.023(1) angstrom) was followed when heating up to 1250 degrees C. It was first dehydrated leading to the anhydrous thorium phosphate-hydrogenphosphate (TPHP, orthorhombic, system with a = 21.229(2) angstrom, b = 6.661 (1) angstrom and c = 7.031 (1) angstrom at 220 degrees C) after heating between 180 and 200 degrees C. This one turned progressively into the new low-temperature variety of TPD (called alpha-TPD, orthorhombic system with a = 21.206(2) angstrom, b = 6.657(1) angstrom and c = 7.057(1) angstrom at 300 degrees C) correlatively to the condensation of hydrogenphosphate groups into diphosphate entities. These three phases (TPHPH, TPHP and alpha-TPD) exhibit closely related 2D layered structures. therefore different from the 3D structure of the thorium, phosphate-diphosphate (high-temperature variety), This latter compound. now called beta-TPD, was obtained by heating alpha-TPD above 950 degrees C. All the techniques involved in this Study (XRD, Raman and IR spectroscopy, (1)H and (31)P NMR) confirmed the successive chemical reactions proposed. (C) 2005 Elsevier Ltd. All rights reserved.