title: Polymorphism in Thermoelectric As2Te3.
abstract: Metastable β-As2Te3 (R3̅m, a = 4.047 Å and c = 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 (C2/m, a = 14.337 Å, b = 4.015 Å, c = 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 T(S1) = 205-210 K and T(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 (P2(1)/m, a = 6.982 Å, b = 16.187 Å, c = 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 b 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.
authors.affiliation: Institut de Chimie et des Matériaux Paris Est (ICMPE), UMR 7182 CNRS- Université Paris-Est Créteil , Thiais, France.
authors.forename: Cedric
authors.initials: C
authors.lastname: Morin
authors.affiliation: Institut Charles Gerhardt (ICG), UMR 5253 CNRS- Université de Montpellier , Montpellier, France.
authors.forename: Serena
authors.initials: S
authors.lastname: Corallini
authors.affiliation: Science des Procédés Céramiques et de Traitements de Surface (SPCTS), UMR CNRS 7315- Université de Limoges , Limoges, France.
authors.forename: Julie
authors.initials: J
authors.lastname: Carreaud
authors.affiliation: Institut Jean Lamour, UMR 7198 CNRS - Université de Lorraine , Nancy, France.
authors.forename: Jean-Baptiste
authors.initials: JB
authors.lastname: Vaney
authors.affiliation: Science des Procédés Céramiques et de Traitements de Surface (SPCTS), UMR CNRS 7315- Université de Limoges , Limoges, France.
authors.forename: Gaelle
authors.initials: G
authors.lastname: Delaizir
authors.affiliation: Institut de Chimie et des Matériaux Paris Est (ICMPE), UMR 7182 CNRS- Université Paris-Est Créteil , Thiais, France.
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Crivello
authors.affiliation: C2TN, Instituto Superior Técnico, Universidade de Lisboa , Lisboa, Portugal.
authors.forename: Elsa Branco
authors.initials: EB
authors.lastname: Lopes
authors.affiliation: Institut Charles Gerhardt (ICG), UMR 5253 CNRS- Université de Montpellier , Montpellier, France.
authors.forename: Andrea
authors.initials: A
authors.lastname: Piarristeguy
authors.affiliation: Institut de Chimie et des Matériaux Paris Est (ICMPE), UMR 7182 CNRS- Université Paris-Est Créteil , Thiais, France.
authors.forename: Judith
authors.initials: J
authors.lastname: Monnier
authors.affiliation: Institut Jean Lamour, UMR 7198 CNRS - Université de Lorraine , Nancy, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Candolfi
authors.affiliation: Université Grenoble Alpes , Grenoble, France.|CNRS, Institut Néel , Grenoble, France.
authors.forename: Vivian
authors.initials: V
authors.lastname: Nassif
authors.affiliation: Institut Laue Langevin , Grenoble Cedex 9, France.
authors.forename: Gabriel Julio
authors.initials: GJ
authors.lastname: Cuello
authors.affiliation: Institut Charles Gerhardt (ICG), UMR 5253 CNRS- Université de Montpellier , Montpellier, France.
authors.forename: Annie
authors.initials: A
authors.lastname: Pradel
authors.affiliation: C2TN, Instituto Superior Técnico, Universidade de Lisboa , Lisboa, Portugal.
authors.forename: Antonio Pereira
authors.initials: AP
authors.lastname: Goncalves
authors.affiliation: Institut Jean Lamour, UMR 7198 CNRS - Université de Lorraine , Nancy, France.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Lenoir
authors.affiliation: Institut de Chimie et des Matériaux Paris Est (ICMPE), UMR 7182 CNRS- Université Paris-Est Créteil , Thiais, France.
authors.forename: Eric
authors.initials: E
authors.lastname: Alleno