Complex monoclinic superstructure in Sr-IV

The complex structural behavior of strontium under high pressure has been studied using angle-dispersive x-ray powder-diffraction techniques. The previously unknown structure of Sr-IV is found to be monoclinic, with space group Ia and 12 atoms per unit cell, and is a helical distortion of the beta-tin structure of Sr-III. This structure has not been observed before in an element Sr-IV is stable from 37.7(2) GPa to 46.3(3) GPa where it transforms to the composite incommensurate Sr-V phase. Diffraction patterns over the small range from 36.0 to 37.7 GPa cannot be fitted completely by either Sr-III or Sr-IV. The previously identified minority phase of unknown structure that appears at the Sr-II to Sr-III transition persists through Sr-IV and Sr-V.