High-Pressure Synthesis and Characterization of New Actinide Borates, AnB(4)O(8) (An=Th, U)

New actinide borates ThB(4)O(8) and UB(4)O(8) were synthesized under high-pressure, high-temperature conditions (5.5 GPa/1100 °C for thorium borate, 10.5 GPa/1100 °C for the isotypic uranium borate) in a Walker-type multianvil apparatus from their corresponding actinide oxide and boron oxide. The crystal structure was determined on basis of single-crystal X-ray diffraction data that were collected at room temperature. Both compounds crystallized in the monoclinic space group C2/c (Z=4). Lattice parameters for ThB(4)O(8): a=1611.3(3), b=419.86(8), c=730.6(2) pm; β=114.70(3)°; V=449.0(2) Å(3); R(1)=0.0255, wR(2)=0.0653 (all data). Lattice parameters for UB(4)O(8): a=1589.7(3), b=422.14(8), c=723.4(2) pm; β=114.13(3)°; V=443.1(2) Å(3); R(1)=0.0227, wR(2)=0.0372 (all data). The new AnB(4)O(8) (An=Th, U) structure type is constructed from corner-sharing BO(4) tetrahedra, which form layers in the bc plane. One of the four independent oxygen atoms is threefold-coordinated. The actinide cations are located between the boron–oxygen layers. In addition to Raman spectroscopic investigations, DFT calculations were performed to support the assignment of the vibrational bands.