Structural Phase Transitions in AuIn2 at High Pressure

The intermetallic compound AuIn2 provides an analog for the high-pressure phases of SiO2, as it is initially in the Fm3m fluorite (CaF2) structure. Synchrotron-based angular-dispersive x-ray diffraction (Advanced Light Source beamline 12.2.2) reveals subtle anomalies in the pressure variation of normalized stress (F) versus Eulerian strain (f) around 3 GPa, coinciding with anomalies observed in fusion, transport and optical data, and potentially associated with the onset of an electronic phase transition. Our diamond-cell experiments (gasketted sample with methanol-ethanol pressure medium) show continuous broadening of diffraction peaks beyond 12 GPa, leading to amorphization near 24 GPa. On further increase of pressure, a crystalline phase appears around 28 GPa and persists upon unloading from 30 GPa to about 5 GPa, then reverting back to the original CaF2 phase. We find the sequence of pressure-induced phase transition documented for CaF2 (fluorite structure Fm3m - PbCl2 Pnma - Ni2In-type P63/mmc and a combination of PbCl2 and Ni2In) to be inadequate in fitting the observed high-pressure diffraction patterns of AuIn2. However, the post-cotunnite structure of PbCl2, BaCl2, BaBr2 and SnCl2 (P1121/c, Z=8) is able to account for most of the prominent peaks in our high-pressure diffraction patterns (a=10.983, b=9.875, c=4.350, À=96.6). Many oxides of geophysical interest occur in the CaF2 structure, and study of intermetallic compounds such as AuIn2 may prove useful in suggesting high-pressure metallic phases for these oxides.