Structure and stability of the modulated phase Sb-II

The crystal structure of Sb-II has been determined using angle-dispersive x-ray diffraction of synchrotron radiation. The crystal structure comprises an interpenetrating assembly of a tetragonal host sublattice with symmetry $I422$ and a tetragonal guest sublattice of symmetry $I422,$ which realize common a axes but different c axes. Weak extra peaks that are also reported in recent investigations performed independently are attributed to modulation waves of the atomic positions in both sublattices. A refinement using full profiles of powder-diffraction data was performed in the four-dimensional superspace group L_-111I422:L_-111I422. The structural investigation is accompanied by ab initio full-potential band-structure calculations confirming the experimentally determined modification sequence. Fitting equations of state to the results of these computations generates pressure-volume relations which are in excellent agreement with the experimental data. On the basis of total-energy calculations, an earlier proposed crystal structure of the tetragonal phase has to be reconsidered in the light of the composite arrangement in Sb-II.