title: Phononic Structure Engineering: the Realization of Einstein Rattling in Calcium Cobaltate for the Suppression of Thermal Conductivity.
abstract: Phonons in condensed matter materials transmit energy through atomic lattices as coherent vibrational waves. Like electronic and photonic properties, an improved understanding of phononic properties is essential for the development of functional materials, including thermoelectric materials. Recently, an Einstein rattling mode was found in thermoelectric material Na0.8CoO2, due to the large displacement of Na between the [CoO2] layers. In this work, we have realized a different type of rattler in another thermoelectric material Ca3Co4O9 by chemical doping, which possesses the same [CoO2] layer as Na0.8CoO2. It remarkably suppressed the thermal conductivity while enhancing its electrical conductivity. This new type of rattler was investigated by inelastic neutron scattering experiments in conjunction with ab-initio molecular dynamics simulations. We found that the large mass of dopant rather than the large displacement is responsible for such rattling in present study, which is fundamentally different from skutterudites, clathrates as well as Na analogue. We have also tentatively studied the phonon band structure of this material by DFT lattice dynamics simulation, showing the relative contribution to phonons in the distinct layers of Ca3Co4O9.
authors.affiliation: School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
authors.forename: Ruoming
authors.initials: R
authors.lastname: Tian
authors.affiliation: School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
authors.forename: Gordon J
authors.initials: GJ
authors.lastname: Kearley
authors.affiliation: Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2232, Australia.
authors.forename: Dehong
authors.initials: D
authors.lastname: Yu
authors.affiliation: School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia.
authors.forename: Chris D
authors.initials: CD
authors.lastname: Ling
authors.affiliation: School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
authors.forename: Anh
authors.initials: A
authors.lastname: Pham
authors.affiliation: Paul Scherrer Institute, 5232 Villigen PSI, Switzerland.
authors.forename: Jan P
authors.initials: JP
authors.lastname: Embs
authors.affiliation: School of Physics, University of the Witwatersrand, Braamfontein 2000, Johannesburg, South Africa.
authors.forename: Elvis
authors.initials: E
authors.lastname: Shoko
authors.affiliation: School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
authors.forename: Sean
authors.initials: S
authors.lastname: Li