Phase transitions of silver and silver-palladium alloys immersed in mercury

A series of Ag-Hg and Ag-Pd-Hg phases have been prepared by immersion in mercury and subsequent heat treatment of pure Ag and a series of Ag-Pd alloys. Phase transitions, along with the changes involved in morphology and chemistry, are investigated. Experimental results indicate that when mercury-immersed Ag discs are heat-treatd at 90 °C for 12 h, large (>50 μm) crystals with Ag/Hg atomic ratios close to those of stoichiometric AgHg₂ form on the Ag surface. Heat treatment for 36 h causes these crystals to disintegrate into smaller (majority <10 μm) Ag₂Hg₃ (γ₁) crystals. After 60 h, the stoichiometric γ₁ crystals further breakdown into even smaller stoichiometric AgHg(β₁) crystals. The oft-referenced β₁ phase Ag_1.1Hg_0.9 is not supported in the present study. Addition of Pd to Ag has a dramatic effect on the morphology of the 71 phase. With more addition of Pd, large-faceted and elongated γ₁ crystals tend to grow in the midst of smaller matrix γ₁ crystals. All three different Pd-containing crystals have (Ag + Pd)/Hg ratios very close to those of the stoichiometric γ₁ phase, indicating that added Pd tends to replace Ag, rather than Hg, in the γ₁ phase.