The crystal structure of γ-P4, a low temperature modification of white phosphorus

The crystal structure of gamma-P-4, one of three modifications hitherto reported on white phosphorus, was determined from X-ray powder diffraction data collected at T = 123 K on a Guinier-Simon camera equipped with a cold gas blower and an image plate detector. Crystallographic data at T = 123 K are: space group C2/m, a = 9.1709(5) angstrom, b 8.3385(5) angstrom, c = 5.4336(2) angstrom, and beta = 90.311(3)degrees, V 415.51(6) angstrom(3), Z = 4. The crystal structure Of gamma-P4 was solved by the method of simulated annealing. The subsequent Rietveld refinement in the range 12 degrees < 20 < 92 degrees employing rigid-body constraints on the P-4 molecule converged at R-p = 3.8, wR(p) = 5.0, and R-F(2) = 14.0%. The asymmetric unit Of gamma-P-4 contains three P atoms; two P atoms in a molecule are related by a mirror plane which bisects the molecule. The centers of gravity of these P-4 molecules show a distorted body-centered cubic arrangement. The four apices of the P-4 tetrahedron point to the largest possible voids formed by neighbor molecules. The difference to the crystal structures of SiF4 and GeF4 with an exact bcc arrangement of tetrahedral molecules is discussed as well as, in terms of layer stackings, the similarity of the structures of gamma- and beta-P-4.