Effect of pressure on the superionic argyrodite Ag₇GeSe₅I

The effect of pressure on both the structure and the electrical conductivity of the superionic Ag₇GeSe₅I phase has been studied at room temperature (i) by powder X-ray diffraction from 0.1 MPa to 10 GPa, (ii) by single-crystal X-ray diffraction from 0.1 MPa to 2 GPa and (iii) by impedance spectroscopy from 0.1 MPa to 1.5 GPa. The decrease in conductivity of about one order of magnitude when the pressure increases up to 1.5 GPa can be explained by compression of the cell. For a pressure increase up to 10 GPa, compression of the cell is about 6% in volume. The compressibility of the cell affects mainly two sites, i.e. I and one of the two Ag sites. Both sites have a very large B_iso that increases even further with pressure, to reach a value close to 11.5 Å² at 1.5 GPa. The occupancy of both Ag sites changes with pressure and becomes similar above 0.8 GPa. On the whole, it appears that the conduction mechanism is affected by pressure. While the conduction pathway is not changed, the limiting jump for ionic diffusion changes from an intercluster jump with a pseudo-potential barrier of about 257 meV at ambient pressure to an indirect jump with a pseudo-potential value of 240 meV at 1.8 GPa.