Polymorphism in Thermoelectric As₂Te₃

Metastable β-As₂Te₃ (<i>R</i>3̅<i>m</i>, <i>a</i> = 4.047 Å and <i>c</i> = 29.492 Å at 300 K) is isostructural to layered Bi₂Te₃ and is known for similarly displaying good thermoelectric properties around 400 K. Crystallizing glassy-As₂Te₃ leads to multiphase samples, while β-As₂Te₃ could indeed be synthesized with good phase purity (97%) by melt quenching. As expected, β-As₂Te₃ reconstructively transforms into stable α-As₂Te₃ (<i>C</i>2/<i>m</i>, <i>a</i> = 14.337 Å, <i>b</i> = 4.015 Å, <i>c</i> = 9.887 Å, and β = 95.06°) at 480 K. This β → α transformation can be seen as the displacement of part of the As atoms from their As₂Te₃ layers into the van der Waals bonding interspace. Upon cooling, β-As₂Te₃ displacively transforms in two steps below <i>T</i>_S1 = 205–210 K and <i>T</i>_S2 = 193–197 K into a new β′-As₂Te₃ allotrope. These reversible and first-order phase transitions give rise to anomalies in the resistance and in the calorimetry measurements. The new monoclinic β′-As₂Te₃ crystal structure (<i>P</i>2₁/<i>m</i>, <i>a</i> = 6.982 Å, <i>b</i> = 16.187 Å, <i>c</i> = 10.232 Å, β = 103.46° at 20 K) was solved from Rietveld refinements of X-ray and neutron powder patterns collected at low temperatures. These analyses showed that the distortion undergone by β-As₂Te₃ is accompanied by a 4-fold modulation along its <i>b</i> axis. In agreement with our experimental results, electronic structure calculations indicate that all three structures are semiconducting with the α-phase being the most stable one and the β′-phase being more stable than the β-phase. These calculations also confirm the occurrence of a van der Waals interspace between covalently bonded As₂Te₃ layers in all three structures