A Revision of the Sb-Te Binary Phase Diagram and Crystal Structure of the Modulated γ-Phase Field

Abstract

A new experimental study of Sb-Te, based on long term annealed samples homogenized by intermediate powdering and re-annealing of the pressed powder pellets was performed. More than 25 samples were prepared, covering the entire composition range between 0 and 60 at.% Te. This part of the phase diagram was disputed for many years, as the number of phases and their crystal structures were uncertain. Samples were characterized by optical microscopy, SEM/EDX, powder XRD and DTA. Our results show the existence of a continuous single phase field (γ) ranging from 11.4 to 56.9 at.% Te at 520 °C. A comparison of all obtained diffraction patterns showed clear evidence for a continuously modulated crystal structure. It was possible to refine all powder patterns by applying a structural model in the (3 + 1)-dimensional super-space group R­3m(0,0,γ). Lattice parameters of the basic cell as well as the modulation vector are varying continuously throughout the phase field and the γq-component (q= γq· c∗) of the incommensurate q-vector shows a linear extrapolation to pure Sb (γq = 3/2) and Sb2Te3 (γq = 6/5). In agreement with previous authors, the single-phase field γ was found to have a melting point minimum (30 at.% Te, 535 °C). Furthermore, two peritectic reactions were observed and the course of liquidus lines was determined in the whole investigated composition range. The new phase diagram differs considerably from all previous versions, and, for the first time, gives a complete rationalization of the crystal structures observed in the debated composition area.