Polymorphism in Thermoelectric As2Te3.

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• Metastable β-As2Te3 is isostructural to Bi2Te3 and shows promising thermoelectric properties around 400 K.
• Synthesis of β-As2Te3 via melt quenching yields high phase purity (97%).
• Glassy As2Te3 crystallization results in multiphase samples.

Purpose of the Study:

• To investigate the phase transitions and crystal structures of arsenic telluride (As2Te3) under varying temperatures.
• To characterize the allotropes formed during phase transitions and their electronic properties.
• To understand the structural changes associated with temperature-induced transformations in As2Te3.

Main Methods:

• Melt quenching technique for synthesizing β-As2Te3.
• X-ray and neutron powder diffraction for Rietveld refinements at low temperatures.
• Calorimetry and resistance measurements to detect phase transitions.
• Electronic structure calculations to determine stability and semiconducting nature.

Main Results:

• β-As2Te3 reconstructively transforms to stable α-As2Te3 at 480 K, involving As atom displacement.
• Upon cooling, β-As2Te3 undergoes two displacive phase transitions below 210 K, forming a new β'-As2Te3 allotrope.
• The new monoclinic β'-As2Te3 structure (P2(1)/m) was solved, showing a 4-fold modulation along the b axis.
• All three structures (α, β, β') are semiconducting, with α being most stable and β' more stable than β.

Conclusions:

• Arsenic telluride exhibits complex phase behavior with reversible, first-order transitions.
• The discovered β'-As2Te3 allotrope possesses a unique modulated structure and distinct stability compared to β-As2Te3.
• Understanding these phase transitions is crucial for optimizing thermoelectric applications of As2Te3-based materials.