Electron microscopic studies of modulated structures in (Au, Ag) Te 2: Part II. Sylvanite AgAuTe 4

Abstract

Sylvanite (AuAgTe 4) occurs under two forms at room temperature. Ideal sylvanite has a commensurate modulated superstructure of calaverite. The second form is an incommensurate modulated structure. The modulation is due to periodic displacements from the ideal atom positions, occupied in the commensurate form, and to the presence of periodic antiphase boundaries. The two structural modulations are closely coupled however and occur along almost parallel sets of planes. It is believed that the incommensurability is related to deviations from the ideal gold-silver ratio. The antiphase boundaries provide a means to incorporate deviations from the ideal AuAg ratio without changing the deformation modulation period. These conclusions were reached on the basis of the combined use of electron diffraction, electron microscopy, and computer simulation of the high resolution images.