Rare-earth Metal Borosilicides R9Si15–xB3 (R = Tb,Yb): New Ordered Structures Derived from the AlB2 Structure Type

Two novel ternary borosilicides R₉Si_15–xB₃ (R = Tb, x = 1.80, R = Yb, x = 1.17) were synthesized from the initial elements using tin flux method. Their crystal structures were determined by means of X-ray single crystal diffraction. Both refer to space group R32, Z = 1: a = 6.668(2) Å, c = 12.405(4) Å [R₁ = 0.027, wR₂ = 0.031 for 1832 reflections with I_o > 2σ (I_o)] for Tb₉Si_15–xB₃, and a = 6.5796(3) Å, c = 12.2599(5) Å [R_F = 0.052, wR = 0.090 for 1369 reflections with I_o > 2σ (I_o)] for Yb₉Si_15–xB₃. The structures represent a new structure type, derived from that of AlB₂, with ordering in the metalloid sublattice resulting in distorted [Si₅B] hexagons. The presence or absence of boron in this ordered structure is discussed on the basis of difference Fourier syntheses, interatomic distances, structural analysis, and theoretical calculations in relation with the parent structures of the binaries AlB₂ and Yb₃Si₅ (Th₃Pd₅ type of structure). Theoretical calculations show substantial covalent interactions between the metal and nonmetal elements. The small percentage of silicon atoms, which are missing in these nonstoichiometric compounds, probably allows strengthening boron-metal and boron-silicon bonding.     

A relationship between macroradical decay kinetics and α-segmental dynamics in glassy amorphous polymers

For a series of five amorphous polymers with a broad range of T_g values the kinetics of macroradical decay was measured by ESR technique and evaluated by the second-order kinetic model. It was found that the temperature T_tr of the transition between two regions of different reactivity in free radical decay reaction agrees quite well with the temperature parameter T₀ of the Vogel-Fulcher-Tamman-Hesse (VFTH) equation for α-segmental dynamics. This parameter represents the onset of α-segmental mobility in glassy state below T_g. A nontraditional way of the estimation of T₀ values for α-segmental dynamics through study of the macroradical decay in glassy state of amorphous polymers has been suggested. © 1996 John Wiley & Sons, Inc.     

Brown colour of ruby

The defects characteristic of brown pure ruby crystals coloured by specific growth conditions, annealing and irradiation are described. The comparsion of UV- and X-rayed crystals is given. Brown or yellow colour is attributed to the O⁻ centre which is formed by the electron transfer from O²⁻.