Structure and SHG of the high pressure phase IV of HgBr2

Angle dispersive X-ray diffraction experiments on mercuric bromide (HgBr2) under high pressure up to 11.0 GPa were carried out at room temperature using synchrotron radiation. In addition to the already known four different phases of HgBr2 in the pressure–temperature range of p<4.5 GPa, 90<T<600 K our observations show the existence of a new phase (V) above 9.0 GPa and, together with published material, support the phase transition sequence: (I) orthorhombic–(II) orthorhombic–(III) monoclinic–(IV) trigonal–(V) trigonal/hexagonal. The structure of phase IV with space group symmetry P3 has been determined from powder diffraction data. The observation of second-harmonic-generation signals confirms the absence of an inversion center. The structure of phase IV is a commensurately modulated variant of the CdI2 type layer structure, where part of the Hg atoms are displaced from the centers of the HgBr6 octahedra by a much as 0.76 Å in the direction perpendicular to the layers.