High-pressure synthesis of alpha-PbO_2 and its crystal structure at 293, 203, and 113 K from single crystal diffraction data

摘要

The synthesis and the first single crystal structure refinements of alpha-PbO_2, a high-pressure modification of lead dioxide, at room and low temperatures are presented (R_(Fsigma)/R_(Funiq) 0.025/0.048, 0.028/0.047, 0.027/0.044, omega R_(Fsigma)/omega R_(Funiq) 0.056/0.061, 0.059/0.063, 0.059/0.062 at 293, 203, and 113 K, correspondingly). Single crystals were grown by a hydrothermal synthesis procedure (rho = 4.0 GPa, temperature range 600-300 deg C, cooling rate 1.25 deg/min). alpha-PbO_2 is orthorhombic, space group Pbcn (No. 60), Z = 4, with a = 4.9858(2) A, b = 5.9596(3) A, c = 5.4626(2) A. V = 162.31(1) A~3 for T = 293 K. The description of the structure is given from a cation- and an anion-centered perspective. The oxygen atoms are coordinated by three lead atoms and oscillate mainly perpendicularly to the Pb_3-plane. The principal axes of the thermal displacement parameters decrease under cooling, and the anisotropy of the thermal oscillations increases from 293 to 113 K. Anisotropic thermal expansion was observed. Upon heating, the crystal structure extends practically only along the a-axis.