Impact of temperature and mode polarization on the acoustic phonon range in complex crystalline phases: A case study on intermetallic clathrates

From phonons to the thermal properties of complex thermoelectric crystals: The case of type-I clathrates

“…1(d). Still, the positions of the Einstein modes extracted from the heat capacity model are consistent with the median energies of the optical phonon distributions at low energy reported by spectroscopic measurements and located at energies 5, 7.5, 10 and 12 meV [14,15]. This, on the other hand, is not true for the Debye temperature obtained with this model, which clearly has no meaning at the microscopic level.…”

“…Crystals with a high structural complexity and chemical bonding inhomogeneity [10], such as tetrahedrites [11], skutterudites [12] or type-I clathrates [13], often have a conductivity of a type-I Ge-based clathrate is typically two orders of magnitude lower than that of pure Ge at room temperature and this difference increases upon cooling. On the other hand, phonon measurements by means of inelastic neutron/X-ray scattering (INS/IXS) have revealed the existence of long-living acoustic phonons in clathrates, thus questioning the origin of their low lattice thermal conductivity [14][15][16][17]. However, this is not specific to clathrates and is found in many crystalline materials with a complex crystallographic structure such as in the thermoelectric skutterudites or tetrahedrites [11,18], in photovoltaic hybrid perovskites [2], or in quasicrystals [19,20].…”

“…The onset of this continuum defines the upper limit in energy of the acoustic regime such that it has been associated with a low-pass filter for acoustic phonons [17] or a modified Debye energy [21,24]. Its position in energy controls the room temperature value of the lattice thermal conductivity [24] while its temperature dependence was found to be responsible for the smooth variation of the lattice thermal conductivity with temperature [14,25]. On the other hand, the onset of the optical continuum can be changed by varying the chemical composition [15] or the structural topology [26], i.e.…”

“…In this work, starting from a microscopic understanding of the lattice dynamics in type-I clathrates [14][15][16][17]22,27,28], simple phenomenological models -based on a description in terms of Debye and Einstein modes -for thermal properties such as isochoric heat capacity (C 𝑣 ) and lattice thermal conductivity (𝜅 𝐿 ) are derived. While first principle approaches are in principle capable to give access to these quantities, the proposed models can easily be applied to fit experimental data and, despite their simplicity, allow to extract the relevant microscopic parameters of the underlying phonon spectrum.…”

Phonon behavior in a random solid solution: a lattice dynamics study on the high-entropy alloy FeCoCrMnNi

“…Interestingly, similar thermal transport properties have also been reported in some crystalline materials 12,[24][25][26][27] belonging to the family of the Complex Metallic Alloys (CMAs), described in average with a simple cubic cell containing a large number of atoms organized in local (sub)nanostructures and which are frequently associated with intrinsic disorder 25,28 . In this case, the microscopic origin of the glass-like thermal properties has been identified in a very different mechanism with respect to glasses: not a strong phonon attenuation, but a significant reduction of the acoustic phonon spectrum due to the presence of a large number of optic modes [29][30][31][32][33][34] .…”