SOLID STATE REDOX REACTIONS OF SILVER HALIDES IN THE DIAMOND ANVIL CELL

Electron and ion charge transfer processes help govern electrical conductivity, and mass and heat transport properties in deep Earth minerals. To investigate the chemical effects of electron transfer in ionic materials at high pressures and to explore the feasibility of performing redox reactions at high pressures, the halides silver iodide (AgI) and silver bromide (AgBr) were studied under the influence of an internally-imposed electric field in a diamond anvil cell.

A controlled-voltage power supply connecting two electrodes in the sample chamber of a hydrothermal diamond anvil cell forced the dissociation of AgI and AgBr into silver metal at the cathode and halide at the anode. Experiments were performed under non-hydrostatic conditions as well as under quasi-hydrostatic conditions. The progression of the reactions was measured by monitoring color changes and the precipitation of silver using image processing of movie files generated during the reactions. For both AgI and AgBr the reaction kinetics increased as a function of voltage and decreased as a function of pressure. For AgBr, the electrical resistance across the cell was also measured, and observed to photosensitive. As the AgBr sample was exposed to visible light, the resistance dropped instantaneously, and after the light was turned off, the resistance increased on a timescale of ~10’s of seconds. Notably, at higher pressures, the AgBr showed less photosensitivity.

Potential applications include methods to investigate the effect of pressure on electrochemical processes, point defect creation and transport behavior, redox reactions under pressure, transport and reactions in the deep Earth, and innovative technological potential, such as producing miniature photovoltaic and electronic devices at high pressures.