Pressure-Induced Phase Transitions and Amorphization in HgCN(NO₃)

Abstract

In situ high-pressure Raman spectroscopic, X-ray diffraction (XRD), and optical absorption studies on negative thermal expansion material, HgCN­(NO3), were carried out under a nonhydrostatic pressure environment. High-pressure X-ray diffraction study illustrates a structural phase transition from hexagonal (P6/mmm) to monoclinic-I (P21/n) at 0.4 GPa and monoclinic-I (P21/n) to a second high-pressure phase above 2.6 GPa. Vibrational modes corresponding to the Hg–C–N bending mode and N–OII stretching in bidentate nitrate mode show softening up to ∼8.1 GPa. Broadening of the diffraction peaks beyond ∼8 GPa indicates the onset of a disordered phase, which eventually transforms into an amorphous phase beyond ∼15.8 GPa. The first structural transition at 0.4 GPa is accompanied by an indirect-to-indirect band-gap transition, resulting in an increase in the optical band gap. However, at the onset of the disorder, there is an indirect-to-direct band-gap transition, leading to an abrupt decrease (∼20.5%) in the band gap.