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Structure, thermal, and transport properties of the clathratesSr8Zn8Ge38,Sr8Ga16Ge30, andBa8Ga16Si30

Physical Review B, 2004

The structural parameters, thermal properties, and transport properties of three type I clathrates, namely Sr 8 Zn 8 Ge 38 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Si 30 , have been determined at or below room temperature. The structural parameters of these clathrates were determined by powder neutron diffraction. Their lattice thermal expansion is two to four times greater than that of the diamond phases of silicon and germanium, consistent with more anharmonic lattice vibrations. From the temperature dependence of the isotropic atomic displacement parameters, the estimated rattling frequencies of guests in the large cages of these clathrates are in the range 50-60 cm −1. The heat capacities of these three clathrate materials increase smoothly with increasing temperatures and approach the Dulong-Petit value around room temperature. The Grüneisen parameter of these materials is constant between 100 and 300 K but increases below 100 K, due to the dominance of the low-frequency guest-rattling modes. The room-temperature electrical resistivity and the Seebeck coefficient show that these materials are metallic. The temperature profile of the thermal conductivities and calculated phonon mean free paths of these materials show glasslike behavior, although they are crystalline materials, indicating strong resonant scattering of heat-carrying acoustic phonons via the rattling of the guests in the clathrate cages.

Off-center rattling modes and glasslike thermal conductivity in the type-I clathrateBa8Ga16Sn30

Physical Review B, 2010

Type-I clathrate Ba 8 Ga 16 Sn 30 is unique for showing glasslike behavior in the lattice thermal conductivity L irrespective of the charge carrier type. For better understanding the relation between this behavior and guest rattling, polarized Raman-scattering measurements have been performed on carrier-tuned single crystals. The appearance of a symmetry-forbidden mode in the E g symmetry spectrum indicates that the Ba atoms are rotating among the off-center positions in the tetrakaidecahedron. On cooling from 300 to 4 K, the energies of E g and T 2g guest modes decrease by 15% and 25%, respectively. This drastic decrease originates from the strongly anharmonic potential with a quartic term of the atomic displacement. The energy of 14 cm −1 for the T 2g mode at 4 K is the lowest among intermetallic clathrates. These results for Ba 8 Ga 16 Sn 30 are compared with those for type-I Eu 8 Ga 16 Ge 30 and Sr 8 Ga 16 Ge 30 whose L ͑T͒ show similar glasslike behavior with a plateau. The comparison between three systems indicates that the guest rattling energy is proportional to the temperature range of the plateau in L ͑T͒.

Impact of Rattlers on Thermal Conductivity of a Thermoelectric Clathrate: A First-Principles Study

Physical Review Letters, 2015

We investigate the role of rattling guest atoms on the lattice thermal-conductivity of a type-I clathrate Ba8Ga16Ge30 by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause 10-fold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining unusually low thermal-conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

Strong Phonon Anharmonicity of Clathrate Compound at High Temperature

arXiv (Cornell University), 2021

Effects of strong phonon anharmonicity of a type-I clathrate Ba8Ga16Sn30 induced by quadruple-well potential of guest atoms were investigated. Phonon transport including coherent interbranch component was analyzed using a first-principles-based self-consistent phonon (SCP) theory that gives temperaturedependent harmonic interatomic force constants and by solving off-diagonal components of group velocity operator. Experimentally observed thermal conductivities have been reasonably reproduced by considering both lattice and electron contributions. Through the analysis with the SCP theory, we found that hardening of guest modes leads to an increase in lattice thermal conductivity at frequencies below those of framework-dominant flat modes (< 40 cm-1), which finally results in the slow decay and slight increase in the total lattice thermal conductivity with increasing temperature. Detailed analyses revealed that the increase in lattice thermal conductivity at low frequency is attributed to (a) the increase in group velocities of phonon modes located at frequencies below that of the flat guest modes and (b) abnormal increase in lifetimes of phonon modes located between frequencies of the flat guest and framework modes with increasing temperature. From an engineering point of view, this effect may lead to an intriguing phenomenon, a larger decrease in thermal conductivity due to nanostructuring at higher temperatures.

Thermal conductivity of transition metal containing type-I clathrates

MRS Proceedings, 2015

ABSTRACTConcerning a materials ability to convert heat to electrical energy, the electrical power factor S2/ρ as well as the thermal conductivity at elevated temperatures are of special interest. Since Flash experiments measure the thermal diffusivity and standard steady-state heat-flow experiments are inaccurate at elevated temperatures due to radiation errors inherent to this technique, direct and accurate thermal conductivity data on type-I clathrate single crystals at elevated temperatures are scarce in literature. Here we report 3ω thermal conductivity data on single crystalline Ba8Cu5.09Ge40.91 (BCG), La1.23Ba6.99Au5.91Si39.87, and Ce1.06Ba6.91Au5.56Si40.47 in the temperature range between 80 and 330 K, and specific heat data on BCG between 2 and 300 K. The comparison of our room temperature phonon thermal conductivity data (κph) to results on transition metal (TM) free type-I clathrates in terms of the guest free space (Rfree) suggests a stronger dependence of κph on Rfree fo...

Why are Clathrates Good Candidates for Thermoelectric Materials

Journal of Solid State Chemistry, 2000

Clathrates are periodic solids in which tetrahedrally coordinated atoms form cages that surround a metal atom. We examine Slack's suggestion that the metal atoms scatter phonons but not electrons, thus lowering the thermal but not the electric conductivity. If this is true, as transport measurements indicate, these compounds are promising thermoelectric materials.

Thermal Conductivity and Phonon Engineering in Low-Dimensional Structures

1998

The use of first principles methods based on density functional theory to investigate novel thermoelectric materials is illustrated for several empty and filled skutterudite compounds, including CoSb 3 , C0P3, La(Fe,Co) 4 Sbi 2 and La(Fe,Co)P 12 . Band structures and their relationship to transport properties especially as regards optimization of thermoelectric properties is discussed. Phonon models constructed from calculations and existing experimental data for CoSb 3 are presented. These have been extended to the filled skutterudites, particularly LaFe 4 Sbi2 using additional first principles calculations to fix the La related parameters in the model. This model allows an interpretation of neutron scattering data as well as an understanding of the low frequency phonon modes that transport heat in these compounds.

Thermal and mechanical properties of the clathrate-II <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:msubmml:miNamml:mn24mml:msubmml:miSimml:mn136

Physical review, 2022

Thermal expansion, lattice dynamics, heat capacity, compressibility, and pressure stability of the intermetallic clathrate Na 24 Si 136 have been investigated by a combination of first-principles calculations and experimentation. Direct comparison of the properties of Na 24 Si 136 with those of the low-density elemental modification Si 136 provide insight into the effects of filling the silicon clathrate framework cages with Na on these properties. Calculations of the phonon dispersion only yield sensible results if the Na atoms in the large cages of the structure are displaced from the cage centers, but the exact nature of off-centering is difficult to elucidate conclusively. Pronounced peaks in the calculated phonon density of states for Na 24 Si 136 , absent for Si 136 , reflect the presence of low-energy vibrational modes associated with the guest atoms, in agreement with prior inelastic neutronscattering experiments and reflected in marked temperature dependence of the guest atom atomic displacement parameters determined by single-crystal x-ray diffraction. The bulk modulus is only weakly influenced by filling the Si framework cages with Na, whereas the phase stability under pressure is significantly enhanced. The roomtemperature linear coefficient of thermal expansion (CTE) is nearly a factor of 3 greater for Na 24 Si 136 compared to Si 136. Negative thermal expansion (NTE), observed in Si 136 below 100 K, is noticeably absent in Na 24 Si 136. In contrast to Si 136 , the thermal expansion behavior in Na 24 Si 136 is relatively well described by the conventional Grüneisen-Debye model in the temperature range of 10-700 K. First-principles calculations in the quasiharmonic approximation correctly predict an increase in high-temperature CTE with Na loading, although the increase is less than observed in experiment. The calculations also fail to capture the absence of NTE in Na 24 Si 136 , perhaps due to anharmonic effects and/or inadequateness of the ordered structural model.