High pressure¹²⁹I Mössbauer studies of GeI₄ molecular crystals

The Mössbauer effect in¹²⁹I in conjunction with Diamond-Anvil-Cell high pressure techniques was applied to investigate the high pressure phase(s) of the molecular crystal GeI₄. The¹²⁹I Quadrupole Interaction was the main probe for characterizing the intermolecular structural transformation with pressure. With increasing pressure, at about 15 GPa, the onset of a partial molecular-association phase (HP1) is first observed. In HP1 two out of the four iodines strongly overlap to form linear chains of GeI₄. The HP1 phase coexists with the low pressure (LP) molecular phase, but its population increases with increasing pressure. At P=20 GPa a second high pressure phase (HP2) is identified where all four iodines strongly overlap to form a three dimensional, fully molecular-associated structure. With increasing pressure and at P>20 GPa, HP2 is the only phase up to P=34 GPa, the highest pressure used. A significant hysteresis of the relative abundances with pressure is observed. The isomer shift of the HP2 and HP1 structures is considerably larger than that of the LP one.