Powder neutron diffraction of α-UB 2C (α-UB 2C-type)

Abstract

The crystal structure of α-UB 2C (low temperature modification below T = 1675(25)°C) was determined from powder X-ray data (RT) and powder neutron diffraction data (at 29 K) employing the Rietveld-Young-Wiles profile analysis method. α-UB 2C crystallizes in the orthorhombic space group Pmma with a = 0.60338(3), b = 0.35177(2), c = 0.41067(2) nm, V = 0.0872 nm 3, Z = 2. The residuals of the neutron refinement were R 1 = 0.032 and R F = 0.043. The crystal structure of α-UB 2C is a new structure type where planar nonregular 6 3-U-metal layers alternate with planar nonmetal layers of the type (B 6C 2) 3. Boron atoms are in a typical triangular prismatic metal surrounding with a tetrakaidekahedral coordination B[U 6B 2C 1], whereas carbon atoms occupy the center points of rectangular bipyramids C[U 4B 2]. The crystal structure of α-UB 2C derives from the high temperature modification β-UB 2C (ThB 2C-type, R 3m ), which reveals a similar stacking of slightly puckered metal layers 6 3, alternating with planar layers B 6 · (B 6C 3) 2. The phase transition from β-UB 2C to α-UB 2C is thus essentially generated by carbon diffusion within the ∞ 2 B 6 · (B 6C 3) 2 layers to form ∞ 2 (B 6C 2) 3 layers.