Localised Ag(+) vibrations at the origin of ultralow thermal conductivity in layered thermoelectric AgCrSe(2)

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, AgCrSe(2) 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 [CrSe(2)](∞) layer bear a spin S = 3/2, which orders below T(N) = 55 K. Here we report low temperature inelastic neutron scattering experiments on AgCrSe(2), 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 T(N) in the chromium layer. The low thermal conductivity of AgCrSe(2) 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.