Structural and valence properties of the amorphous-metallic high-pressure phase of SnI4

Mössbauer spectroscopy (MS) in Sn119 and I129 was employed to investigate the evolution and properties of the high-pressure amorphous-metallic phase of SnI4. Measurements were carried out at pressures up to 27 GPa using diamond-anvil cells. Both MS probes detect the onset of a new high-pressure phase at P10 GPa. The relative abundance of the high-pressure phase increases with pressure, reaching 100% at P20 GPa. Whereas the isomer shift IS(P) of Sn in the low-pressure phase has a positive slope, that of the high-pressure phase has a negative slope with an unusual plateau in the 15 20-GPa region. In the 0P10 GPa range the I129 MS reveals a single site. For P10 GPa a spectral component evolves with pressure that coexists with the molecular phase and reaches an abundance saturation of 50% at P18 GPa. In contrast to the low-pressure phase this component shows a positive quadrupole coupling constant, whose magnitude is about half that of the low-pressure phase, a large value, and nearly identical IS values. The high-pressure abundance determined by both Sn119 and I129 shows a dramatic pressure hysteresis. It is unequivocally determined that no SnI4SnI2+I2 dissociation takes place. A model proposed for the high-pressure phase of SnI4 consists of randomly oriented chains of SnI4 tetrahedral molecules where two bridging iodines per molecule provide for the intermolecular association. The Sn4+ ground state in this structure is formed by hybridizing the outer 5s5p with the inner 4p4d tin orbitals. This unique molecular association and the high-pressure Sn4+ ground state provide the proper path and mechanism for the one-dimensional charge delocalization.