MOSSBAUER STUDIES OF PRESSURE-INDUCED AMORPHIZATION IN THE MOLECULAR-CRYSTAL SNBR4

Properties of the molecular structure and the Sn(IV) valence of the pr essure-amorphized SnBr4 molecular crystal were investigated by Sn-119 Mossbauer spectroscopy. Studies were conducted at 80 K with pressures to 25 GPa, far beyond the amorphization onset at P-c=7-9 GPa as determ ined by previous Raman studies. No valence change in the Sn(IV) centra l ion takes place as deduced from the isomer shift (IS) variation with pressure, in contrast to the analogous molecular crystal SnI4 which u ndergoes amorphization and metallization under pressure. The IS value of 1.15(5) mm/s, with respect to a CaSnO3 source at 300 K, remains pra ctically constant over the whole pressure range, suggesting little var iation in both the Sn-Br intramolecular distances and covalency within the amorphous and/or insulator pressure regime. A single line spectru m is characteristic of the Mossbauer data obtained in the 0-5 GPa rang e, indicating the preservation of fourfold symmetry of the SnBr4 molec ule in the crystal. Above 5 GPa, near P-c, a quadruple interaction tak es place concurrent with a dramatic increase in the recoil-free fracti on (f). The value of the quadrupole splitting reaches a maximum of 0.9 mm/s at P similar to 15 GPa and remains constant thereafter. By simil ar to 9 GPa the absorption area, which is proportional to f, increases by 30-40 % over the lowest pressure value and then remains constant a t higher pressure. These results are consistent with the formation of a molecular species, e.g., a (SnBr4)(2) dimer, lacking the T-d symmetr y at the original Sn4+ site and having optical phonons hard enough not to be excited by the nuclear recoil process. Molecular association in to (SnBr4)(2) dimers, the building block of the high-pressure disorder ed state, also explains many of the experimental features of the Raman data. Upon decompression, Mossbauer (and Raman) data suggest that the se dimers dissociate into monomers at similar to 5 GPa; however, a dis ordered structure of SnBr4 persists as pressure is decreased further. Crystallization is fully recovered below 1 GPa. The nature of the pres sure-induced amorphization of the insulator SnBr4 is discussed in term s of the structural and valence properties of the analogous metallic S nI4.