Direct TEM observation of the "acanthite α-Ag₂S-argentite β-Ag₂S" phase transition in a silver sulfide nanoparticle

For the first time, the α-Ag₂S (acanthite)-β-Ag₂S (argentite) phase transition in a single silver sulfide nanoparticle has been observed in situ using a high-resolution transmission electron microscopy method in real time. Colloid solutions of Ag₂S nanoparticles and nanostructured powders of silver sulfide have been synthesized by one-stage chemical bath deposition from an aqueous solution of silver nitrate, sodium sulfide and sodium citrate. Ag₂S nanoparticles were heated to different temperatures directly in an electronic microscope by regulating the energy of the electron beam. This allowed observation of the transition of acanthite into argentite and the reversible transition of argentite into acanthite in real time, and this phase transition to be filmed. Temperature dependence of the lattice constant a _arg of argentite β-Ag₂S in the temperature range 448-723 K is established by in situ high-temperature X-ray diffraction. The orientation relationships between the monoclinic acanthite α-Ag₂S and the body-centered cubic argentite β-Ag₂S are determined. It is shown that the possible distances between silver atoms in cubic argentite, in contrast to those in acanthite, are too small for the positions of the metal sublattice to be occupied by Ag atoms with a probability equal to 1.