title: Localised Ag(+) vibrations at the origin of ultralow thermal conductivity in layered thermoelectric AgCrSe2.
abstract: In materials science, the substructure approach consists in imagining complex materials in which a particular property is associated with a distinct structural feature, so as to combine different chosen physical characteristics, which otherwise have little chance to coexist. Applied to thermoelectric materials, it has been used to achieve simultaneously phonon-glass and electron-crystal properties. Mostly studied for its superionic conductivity, AgCrSe2 is a naturally layered compound, which achieves very low thermal conductivity, ~0.4 W.K(-1).m(-1) at RT (room temperature), and is considered a promising thermoelectric. The Cr atoms of the [CrSe2]∞ layer bear a spin S = 3/2, which orders below TN = 55 K. Here we report low temperature inelastic neutron scattering experiments on AgCrSe2, alongside the magnetic field evolution of its thermal and electrical transport. We observe a very low frequency mode at 3 meV, ascribed to large anharmonic displacements of the Ag(+) ions in the [Ag]∞ layer, and 2D magnetic fluctuations up to 3 TN in the chromium layer. The low thermal conductivity of AgCrSe2 is attributed to acoustic phonon scattering by a regular lattice of Ag(+) oscillating in quasi-2D potential wells. These findings highlight a new way to achieve localised phonon modes in a perfectly crystalline solid.
authors.affiliation: Laboratoire Léon Brillouin, CEA-CNRS UMR 12, 91191 GIF-SUR-YVETTE CEDEX, France.
authors.forename: F
authors.initials: F
authors.lastname: Damay
authors.affiliation: Laboratoire Léon Brillouin, CEA-CNRS UMR 12, 91191 GIF-SUR-YVETTE CEDEX, France.
authors.forename: S
authors.initials: S
authors.lastname: Petit
authors.affiliation: Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 GRENOBLE CEDEX 9, France.
authors.forename: S
authors.initials: S
authors.lastname: Rols
authors.affiliation: Laboratoire Léon Brillouin, CEA-CNRS UMR 12, 91191 GIF-SUR-YVETTE CEDEX, France.
authors.forename: M
authors.initials: M
authors.lastname: Braendlein
authors.affiliation: Laboratoire CRISMAT, CNRS UMR 6508, 6 bvd Maréchal Juin, 14050 CAEN CEDEX, France.
authors.forename: R
authors.initials: R
authors.lastname: Daou
authors.affiliation: Synchrotron Soleil, Saint-Aubin BP 48, 91192 GIF-SUR-YVETTE CEDEX, France.
authors.forename: E
authors.initials: E
authors.lastname: Elkaïm
authors.affiliation: Synchrotron ALBA, Carretera BP 1413, 08290 Cerdanyola del Vallès, BARCELONA, Spain.
authors.forename: F
authors.initials: F
authors.lastname: Fauth
authors.affiliation: Laboratoire CRISMAT, CNRS UMR 6508, 6 bvd Maréchal Juin, 14050 CAEN CEDEX, France.
authors.forename: F
authors.initials: F
authors.lastname: Gascoin
authors.affiliation: Laboratoire CRISMAT, CNRS UMR 6508, 6 bvd Maréchal Juin, 14050 CAEN CEDEX, France.
authors.forename: C
authors.initials: C
authors.lastname: Martin
authors.affiliation: Laboratoire CRISMAT, CNRS UMR 6508, 6 bvd Maréchal Juin, 14050 CAEN CEDEX, France.
authors.forename: A
authors.initials: A
authors.lastname: Maignan