Electronic, elastic, vibrational, and thermodynamic properties of type-VIII clathrates Ba8Ga16Sn30 and Ba8Al16Sn30 by first principles (original) (raw)
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Journal of Alloys and Compounds, 2014
We present the results of first principles calculations of the structural, electronic, elastic, vibrational, and thermodynamic properties of the hypothetical silicon-based, guest containing type-VIII clathrates Ba 8 Si 46 and Ba 8 Al 16 Si 30. We obtained the lattice constant, formation energy, band structure, density of states, elastic constants, sound velocity, and Debye temperature using the density functional theory with generalized gradient approximation (GGA). We calculated phonon dispersion and vibrational density of states spectra using the density functional perturbation energy within GGA. We computed the temperature dependent specific heat, vibrational entropy, and vibrational Helmholtz free energy by utilizing quasi-harmonic approximation. We found that replacing some silicon atoms in the framework with aluminum atoms leads to the decrease of the fundamental band gap from 1.0 in Ba 8 Si 46 to 0.18 eV in Ba 8 Al 16 Si 30. Moreover, the guest Ba atoms produced localized phonon modes lying below 1.2, 1.8 THz for Ba 8 Si 46 , and Ba 8 Al 16 Si 30 , respectively, which lead to the reduction of the acoustic bandwidth of the host material. The effect of replacing Si atoms with Al on the properties of the interest is also discussed.
Physical Review B, 2005
We report the electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, specific heat C, and thermal conductivity κ on single crystals of the type-VIII clathrate Ba8Ga16Sn30 grown from Snflux. Negative S and RH over a wide temperature range indicate that electrons dominate electrical transport properties. Both ρ(T) and S(T) show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 µV/K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n=3.7×10 19 cm −3. Hall mobility µH shows a maximum of 62 cm 2 /Vs around 70 K. The analysis of temperature dependence of µH suggests a crossover of dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity (1.1 W/m K at 150 K). The potential of type-VIII clathrate compounds for thermoelectric application is discussed.
Materials (Basel, Switzerland), 2016
Type II clathrates are interesting due to their potential thermoelectric applications. Powdered X-ray diffraction (XRD) data and density functional calculations for NaxSi136 found a lattice contraction as x increases for 0 < x < 8 and an expansion as x increases for x > 8. This is explained by XRD data that shows that as x increases, the Si28 cages are filled first for x < 8 and the Si20 cages are then filled for x > 8. Motivated by this work, here we report the results of first-principles calculations of the structural and vibrational properties of the Type II clathrate compounds AxSi136, AxGe136, and AxSn136. We present results for the variation of the lattice constants, bulk moduli, and other structural parameters with x. These are contrasted for the Si, Ge, and Sn compounds and for guests A = Na, K, Rb, and Cs. We also present calculated results of phonon dispersion relations for Na₄Si136, Na₄Ge136, and Na₄Sn136 and we compare these for the three materials. Finall...
Applied sciences, 2019
Structural and vibrational properties of the aluminium-substituted ternary and quaternary clathrates R 8 Al 8 Si 128 (R = Cs, Rb), Cs 8 Na 16 Al 24 Si 112 are investigated. The equilibrium volume of R 8 Si 136 expands when all Si atoms at the 8a crystallographic sites are replaced by Al. Formation of the Al-Si bond is thus anticipated to correlate with decreased guest vibration modes. Underestimation of the predicted lattice phonon conductivity κ L (1.15 W m −1 K −1) compared to a previous experiment (1.9 W m −1 K −1) in Cs 8 Na 16 Si 136 is thought to arise from our evaluation on the phonon mean free path λ using the "scattering centers" model. Accordingly, we expect that the "three-phonon" processes dominate the determination of the phonon relaxation time, leading to a more reasonable λ in the R 8 Al 8 Si 128 system. Additionally, the "avoided-crossing" effect causes no appreciable difference in the sound speed for acoustic phonons in this framework. Starting with configuration optimization about aluminium arrangements in Cs 8 Na 16 Al 24 Si 112 , the calculated lattice parameter agrees well quantitatively with the experiment. The reduced U iso of Cs from this calculation is anticipated to be primarily related to temperature-dependent quartic anharmonicity. Meanwhile, the predicted κ L for Cs 8 Na 16 Al 24 Si 112 remains not sensitive to the Al arrangement on 96g Wyckoff sites.
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.
Physical Review B, 2011
Polycrystalline samples of the clathrate Ba 8 Au x Si 46−x were synthesized for 0.2 x 10. The homogeneity range of the type-I clathrate phase was determined to be 3.63 x 6.10 after annealing at 900 • C, while a lower Au concentration (x ≈ 2.2) was obtained by steel-quenching. Quasisingle phase materials were obtained for 4.10 x 6.10. In this composition range, thermoelectric properties, including electrical resistivity, thermopower, and thermal conductivity, were investigated between 2 and 350 K. These experiments were complemented by low-temperature specific heat and Hall-effect measurements (2-300 K). First-principles calculations were carried out to determine the evolution of the electronic structure as a function of x. Both theoretical and experimental results evidence a progressive evolution, with the Au content, from a metallic-like behavior towards a highly doped semiconducting state which develops around x = 5.43. At this concentration, a crossover from nto p-type conduction occurs, suggesting that the present system satisfies the Zintl-Klemm concept, which predicts a transition at x = 5.33. This crossover is traced by Hall-effect data indicating a dominant electronlike response for x 5.43, which turns into a holelike signal at higher x values. Analysis of the data based on a single-parabolic-band model under the assumption of a single scattering mechanism of the charge carriers proved to adequately describe the transport properties in the compositional range investigated. Interestingly, the temperature dependence of the lattice thermal conductivity is strongly influenced by the Au concentration: the typical behavior of crystalline insulators in the n-type compounds evolves into a glasslike dependence in the p-type samples. The series Ba 8 Au x Si 46−x thus provides an excellent testing ground for the interplay between crystal structure, electronic properties, and lattice thermal conductivity in type-I clathrates.
Density-functional investigation of Na 16 A 8 Ge 136 (A = Rb,Cs) clathrates
Journal of Physics: Condensed Matter, 2007
We have studied the electronic and vibrational properties of the filled Ge-based clathrates, Na 16 Rb 8 Ge 136 and Na 16 Cs 8 Ge 136. We have performed calculations using the generalized gradient approximation (GGA) to density-functional theory. Both materials are found to have metallic character, in agreement with experimental data. The predicted phonon dispersion curves show the low frequency, localized 'rattling' modes of the guest atoms. The calculated frequencies of these guests are in good agreement with experimentally estimated values. We have used the GGA calculated frequencies to estimate the temperature dependent isotropic atomic displacement parameter (U iso) of the different guest atoms. The temperature dependences of the estimated U iso are also consistent with experiment. We predict that the Rb guests in Na 16 Rb 8 Ge 136 have larger U iso than Cs in Na 16 Cs 8 Ge 136 , in agreement with experiment. This work shows that harmonic approximation based density-functional calculations may be used both to predict relatively accurate vibrational frequencies of the various guest atoms and to estimate the localized dynamic disorder created by such atoms at finite temperatures.
NMR and Computational Studies of Ba8Ga16Sn30 Clathrates
MRS Proceedings, 2010
We have synthesized type-VIII and type-I Ba8Ga16Sn30 clathrates by using different annealing treatments, confirmed with XRD and electron microprobe measurements. NMR lineshape measurements identified a broad resonance corresponding to first-order-shifted satellites. Simulations for the type I structure based on first principles calculations provided an excellent fit to the data, with the best agreement provided by the calculated lowest-energy configuration, having no Ga-Ga bonds. These results allow us to address local configurations within the random type-I alloy, as well as atomic displacements and bond-length distributions, which we compare to experiment.
Physical Review B - Condensed Matter and Materials Physics, 2012
To investigate the off-center rattling motion and its charge-carrier dependence in type-I clathrate compounds, we carried out ultrasonic measurements on type-I Ba 8 Ga 16 Sn 30 and a reference compound, K 8 Ga 8 Sn 38. We found elastic softening of C 44 originating from a lattice instability due to the off-center rattling motion of Ba atom in Ba 8 Ga 16 Sn 30. Elastic softening below 1 K suggests that the lattice instability remains at very low temperatures. We also found ultrasonic dispersion which has no mode selectivity. No-mode-selective ultrasonic dispersion in Ba 8 Ga 16 Sn 30 would be caused by a strong electron-phonon coupling. No charge-carrier dependence is observed between n-type and p-type Ba 8 Ga 16 Sn 30. The significant softening on the bulk modulus in Ba 8 Ga 16 Sn 30 contrasts to the continuous hardening in K 8 Ga 8 Sn 38 , indicating the central role of the rattling motion in the softening.
Journal of Applied Physics, 2003
Ba 8 Ga 10 Si 36 and Ba 8 Ga 16 Si 30 clathrates were prepared by the arc melting method and characterized for their structure, transport, and thermal properties by x-ray diffraction, electron probe microanalysis, electrical resistivity, and thermal conductivity measurements. The Rietveld refinement results gave the total occupancy of Ga/Si equal to 3.49/2.01, 0.99/14.01, and 5.51/17.99 for Ba 8 Ga 10 Si 36 and 3.99/2.01, 1.99/14.01, and 10.01/13.99 for Ba 8 Ga 16 Si 30 , respectively, for 6c, 16i, and 24k sites. From the refined isotropic thermal displacement parameters, Debye temperature, average velocity of sound, lattice thermal conductivity, and Einstein temperature values were estimated. The calculated lattice thermal conductivity values were in agreement with room temperature experimental values of 1.128 and 1.071 W m Ϫ1 K Ϫ1 , respectively, for two clathrates. The temperature variation of ͑300-850 K͒ electrical resistivity of Ba 8 Ga 16 Si 30 showed metallic like behavior, whereas that of Ba 8 Ga 10 Si 36 showed semiconducting behavior. The semiconducting nature was due to vacancies in the framework sites. High temperature thermal conductivity above Debye temperature was explained with anharmonic approximation.