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

Abstract

M\ossbauer 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\ensuremath{\simeq}10 GPa. The relative abundance of the high-pressure phase increases with pressure, reaching 100% at P\ensuremath{\ge}20 GPa. Whereas the isomer shift ${\ensuremath{\delta}}_{\mathrm{IS}(\mathrm{P})}$ 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 0\ensuremath{\le}P\ensuremath{\le}10 GPa range the $^{129}\mathrm{I}$ MS reveals a single site. For P\ensuremath{\ge}10 GPa a spectral component evolves with pressure that coexists with the ``molecular'' phase and reaches an abundance saturation of \ensuremath{\simeq}50% at P\ensuremath{\ge}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 \ensuremath{\eta} value, and nearly identical ${\ensuremath{\delta}}_{\mathrm{IS}}$ 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 ${\mathrm{SnI}}_{4}$\ensuremath{\rightarrow}${\mathrm{SnI}}_{2}$+${\mathrm{I}}_{2}$ 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.