High-resolution electron-microscopic studies of the polymorphs in Ag_2±δSe films

The polymorphs that appear in the low-temperature phase of silver selenide have been studied by high-resolution electron microscopy. The specimen films are intentionally prepared with excess silver or selenium over stoichiometric composition by flash evaporation, as depositing carbon films on both sides of the specimen films to protect them from selenium sublimation and to maintain the composition throughout the heat treatment. It is shown that four different types of low-temperature phase exist: tetragonal (a = 6.98, c = 4.96 Å) for a metastable phase only formed with a small grain size of less than 50 nm; face-centred cubic (a = 10.9 Å) for a non-stoichiometric phase with excess silver; monoclinic (a = 7.05, b = 8.17, c = 4.34 Å, α = 101.0°) for a non-stoichiometric phase with excess selenium; and orthorhombic (a = 7.05, b = 7.82, c = 4.34 Å) for the stoichiometric stable phase. The topotactic relations between the orthorhombic and monoclinic types are found to be fully coherent, having the same a and c lattice parameters.