Structural and valence properties of the amorphous-metallic high-pressure phase ofSnI4

Mössbauer spectroscopy (MS) in ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ and ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ was employed to investigate the evolution and properties of the high-pressure amorphous-metallic phase of ${\mathrm{SnI}}_4$. 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 P≃10 GPa. The relative abundance of the high-pressure phase increases with pressure, reaching 100% at P≥20 GPa. Whereas the isomer shift ${\delta }_{\mathrm{IS}\left(\mathrm{P}\right)}$ 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 0≤P≤10 GPa range the ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ MS reveals a single site. For P≥10 GPa a spectral component evolves with pressure that coexists with the ‘‘molecular’’ phase and reaches an abundance saturation of ≃50% at P≥18 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 η value, and nearly identical ${\delta }_{\mathrm{IS}}$ values. The high-pressure abundance determined by both ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ and ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ shows a dramatic pressure hysteresis. It is unequivocally determined that no ${\mathrm{SnI}}_4$→${\mathrm{SnI}}_2$+${\mathrm{I}}_2$ dissociation takes place. A model proposed for the high-pressure phase of ${\mathrm{SnI}}_4$ consists of randomly oriented chains of ${\mathrm{SnI}}_4$ tetrahedral molecules where two bridging iodines per molecule provide for the intermolecular association. The ${\mathrm{Sn}}^{4+}$ 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 ${\mathrm{Sn}}^{4+}$ ground state provide the proper path and mechanism for the one-dimensional charge delocalization.