Why clathrates are good thermoelectrics: A theoretical study of Sr8Ga16Ge30

Abstract

Recent measurements have shown that the inorganic clathrate Sr8Ga16Ge30 has good thermoelectric properties. This discovery has caused intense experimental activity to synthesize and test other compounds in this class. It has been conjectured that clathrates may be good thermoelectrics if they satisfy several conditions. The Sr atoms, trapped inside the clathrate cages, scatter phonons efficiently, leading to low thermal conductivity. Electric conductivity takes place mostly through the clathrate frame and the conduction electrons are not scattered by Sr vibrations. The compounds, being made of atoms that are semiconductors in the solid state, may have a high Seebeck coefficient. There has been no direct evidence, experimental or theoretical, for this scenario. By performing density functional calculations we show that these ideas are correct. The Sr atoms are weakly bound to the cage and do undergo large-amplitude motion. An analysis of conductivity shows that the largest contribution comes from a band in which the electrons are located on the clathrate frame. Bands originating from the Sr atoms contribute little to conductivity. There is very little charge transfer between the Sr atoms and the frame, and as a result, Sr vibrations are weakly coupled to the conduction electrons. The calculated Seebeck coefficient is in reasonable agreement with the measured one. We find that it is strongly affected by the positions of the Ga atoms in the frame and by doping.

Guest–Framework Interaction in Type I Inorganic Clathrates with Promising Thermoelectric Properties: On the Ionic versus Neutral Nature of the Alkaline‐Earth Metal Guest A in A8Ga16Ge30(A=Sr, Ba)

“…1.7 e) both in the smaller 20-atom and in the larger 24-atom cages, consistent with a Zintl-phase view of these type I clathrates. The assertion that the Sr atoms in the different cages have similar oxidation states is shown to be consistent with multiwavelength diffraction experiments on Sr 8 Ga 16 Ge 30 ; while the assertion of ionicity of the Sr center is supported by the observation that the adsorption edge lies close to that previously found in the Sr K-edge XANES spectra of Sr(OH) 2 ¥ 8 H 2 O. As such, this work contradicts previous experimental and theoretical studies that claim that the guest atoms are neutral.…”

“…Recently, experimental and theoretical studies have appeared on A 8 Ga 16 Ge 30 (A Sr, Ba) concerning their structure and stability, [7±10] band structure and thermoelectric properties, [8,11] and anisotropic vibrations of the metal guest. [3,8,10,12] A dramatic positive effect [13] on the figure of merit when an external pressure of 7.5 GPa was applied to n-doped Sr 8 Ga 16 Ge 30 has also been reported.…”

Superconducting state in the metastable binary bismuthide Rh3Bi14single crystals

“…Examples include covalently bonded IV group thermoelectric clathrates that are under influence of anharmonic lattice vibrations with electron -phonon coupling strength comparable to MgB 2 , alkali doped fullerenes and magnetic molecules. [1][2][3][4][5][6][7][8] Cluster units may substantially define physical properties whereas their size may considerably deviate from the length scales defined by the lattice parameters, even in intermetallic systems where delocalized metallic bonds are present. 9 Binary Rh-Bi intermetallic phase diagram has been studied for more than eight decades.…”

Synthesis and Crystal Structures of the Quaternary Zintl Phases RbNa8Ga3Pn6 (Pn = P, As) and Na10NbGaAs6

“…In the past 10-15 years, intermetallic clathrates have attracted widespread interest, largely due to their demonstrated potential in thermoelectrics development [1][2][3][4]. Having successfully synthesized Rb 7.3 Na 16 Ga 20 Si 116 and Cs 8 Na 16 Ga 21 Si 115 -the first clathrate-II compounds with mixed Ga and Si [5], as well as the first arsenic-based clathrates of type-I Rb 8 Zn 18 As 28 and Cs 8 Cd 18 As 28 [6], we extended our attention to the A-Ga-Pn systems (A = alkali metal or mixtures of alkali metals, Pn = pnictogen, i.e., group 15 element).…”