''Hidden'' high-to-low cristobalite type transition in HgSe and HgTe at high pressure

A transition from the zinc blende (ZB) phase to a distorted-ZB structure has been found in both HgSe and HgTe. This is a structure type not previously reported in any of the II-VI or III-V systems. The transition occurs similar to 1 GPa above the pressure at which the ZB phase becomes unstable against the cinnabar phase, and is thus observable only because of the sluggishness of the ZB to cinnabar transformation. The atomic coordinates of the distorted-ZB structure are remarkably similar to those of the cations in the low-cristobalite phases of AlPO4 and GaPO4.