Mössbauer studies of pressure-induced amorphization in the molecular crystal SnBr4

Properties of the molecular structure and the Sn(IV) valence of the pressure-amorphized SnBr4 molecular crystal were investigated by Sn119 Mössbauer spectroscopy. Studies were conducted at 80 K with pressures to 25 GPa, far beyond the amorphization onset at Pc=79 GPa as determined by previous Raman studies. No valence change in the Sn(IV) central ion takes place as deduced from the isomer shift (IS) variation with pressure, in contrast to the analogous molecular crystal SnI4 which undergoes amorphization and metallization under pressure. The IS value of 1.15(5) mm/s, with respect to a CaSnO3 source at 300 K, remains practically constant over the whole pressure range, suggesting little variation in both the Sn-Br intramolecular distances and covalency within the amorphous and/or insulator pressure regime. A single line spectrum is characteristic of the Mössbauer data obtained in the 05 GPa range, indicating the preservation of fourfold symmetry of the SnBr4 molecule in the crystal. Above 5 GPa, near Pc, a quadruple interaction takes place concurrent with a dramatic increase in the recoil-free fraction $(f). The value of the quadrupole splitting reaches a maximum of 0.9 mm/s at P15 GPa and remains constant thereafter. By 9 GPa the absorption area, which is proportional to f, increases by 3040 % over the lowest pressure value and then remains constant at higher pressure. These results are consistent with the formation of a molecular species, e.g., a (SnBr4)2 dimer, lacking the Td symmetry at the original Sn4+ site and having optical phonons hard enough not to be excited by the nuclear recoil process. Molecular association into (SnBr4)2 dimers, the building block of the high-pressure disordered state, also explains many of the experimental features of the Raman data. Upon decompression, Mössbauer (and Raman) data suggest that these dimers dissociate into monomers at 5 GPa; however, a disordered structure of SnBr4 persists as pressure is decreased further. Crystallization is fully recovered below 1 GPa. The nature of the pressure-induced amorphization of the insulator SnBr4 is discussed in terms of the structural and valence properties of the analogous metallic SnI4.