High-pressure, high-temperature single-crystal study of Bi-IV

Bi-IV is the stable high-pressure, high-temperature phase of bismuth at similar to 4 GPa and similar to 500 K. It was first identified in 1958, but its structure has remained uncertain. An X-ray powder-diffraction study of Bi-IV reported the structure as monoclinic, but a subsequent reinterpretation of the same data concluded that the structure was C-centred orthorhombic (oC16), with the same atomic arrangement as in Cs-V and Si-VI. To resolve the uncertainty over the structure of Bi-IV, we investigated this phase at 3.2 GPa and 465 K by single-crystal synchrotron X-ray diffraction. All of the observed reflections could be indexed on the orthorhombic oC16 structure proposed by Degtyareva, with a = 11.191(5)angstrom, b = 6.622(1)angstrom and c = 6.608(1)angstrom. The spacegroup was confirmed as Cmce. Refinement of the data resulted in an excellent fit (R = 2.8%), and gave atomic coordinates very similar to those of the oC16 structures in Cs-V and Si-VI.