1 research outputs found
Polymorphism in Thermoelectric As<sub>2</sub>Te<sub>3</sub>
Metastable β-As<sub>2</sub>Te<sub>3</sub> (<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 Bi<sub>2</sub>Te<sub>3</sub> and is known for similarly displaying good thermoelectric properties
around 400 K. Crystallizing glassy-As<sub>2</sub>Te<sub>3</sub> leads
to multiphase samples, while β-As<sub>2</sub>Te<sub>3</sub> could
indeed be synthesized with good phase purity (97%) by melt quenching.
As expected, β-As<sub>2</sub>Te<sub>3</sub> reconstructively
transforms into stable α-As<sub>2</sub>Te<sub>3</sub> (<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
As<sub>2</sub>Te<sub>3</sub> layers into the van der Waals bonding
interspace. Upon cooling, β-As<sub>2</sub>Te<sub>3</sub> displacively
transforms in two steps below <i>T</i><sub>S1</sub> = 205–210
K and <i>T</i><sub>S2</sub> = 193–197 K into a new
β′-As<sub>2</sub>Te<sub>3</sub> allotrope. These reversible
and first-order phase transitions give rise to anomalies in the resistance
and in the calorimetry measurements. The new monoclinic β′-As<sub>2</sub>Te<sub>3</sub> crystal structure (<i>P</i>2<sub>1</sub>/<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-As<sub>2</sub>Te<sub>3</sub> 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 As<sub>2</sub>Te<sub>3</sub> layers in all three structures