High-pressure behavior of red mercuric iodide: in situ X-ray diffraction and optical absorption studies

We have investigated the behavior of red mercuric iodide (α-HgI2) under high pressures using in situ X-ray diffraction and optical absorption techniques. Our experimental results indicate that the tetragonal→orthorhombic phase transformation, observed at 1.4 GPa, is accompanied by an abrupt increase in the band gap while the nature of the gap does not change. However, across the orthorhombic→hexagonal phase transformation, observed at ∼7.2 GPa, the gap decreases discontinuously and changes from direct to indirect type. These studies suggest that HgI2 may metallize at ∼40 GPa, if not prevented by any other structural change.