Inelastic X-ray scattering study of incipient and realized structural transitions in mercurous halides

We present new x-ray scattering investigations of mercury halides Hg$_2$X$_2$, where X=I,Br in an effort to compare lattice dynamics and structural properties of an incipient (X=I) and realized (X=Br) structural phase transition. For the realized structural transition in Hg$_2$Br$_2$, we find mode softening centered on the X point in the high-temperature body-centered tetragonal Brillouin zone in a realized transition to a lower-symmetry orthorhombic phase below $T_c\simeq$140 K. For the incipient transition in Hg$_2$I$_2$, we also find dramatic mode softening along the $\Gamma$-X direction, implying a putative transition temperature of $T_c$=-22K. In addition, we report the first temperature dependent lattice parameter development in these two systems, and compare these structural properties of the mercurous halides (Hg$_2$X$_2$) with 3$d$ transition metal trifluorides (XF$_3$), which host novel phases including negative thermal expansion.