Identification of superlattice structure cI16 in the P-VI phase of phosphorus at 340 GPa and room temperature via x-ray diffraction

The structural phase transitions of phosphorus (P) were investigated at pressures up to 340 GPa by a synchrotron radiation x-ray powder diffraction technique at room temperature. The transition from the simple hexagonal P-V to the P-VI phases was confirmed at 262 GPa. The results of full-profile Rietveld refinements indicate that the structure of the P-VI phase is a 2x2x2 superlattice of the bcc structure [cI16 (space group: I-43d)], which is isotypic to high-pressure phases of Li and Na. This phase was found to be stable up to 340 GPa. The results have demonstrated the extension of structure refinement to a complex high-pressure phase above 300 GPa by the x-ray diffraction method.