type-I Clathrate Structure Research Articles

Crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba–Ga–Ge system

The crystal structure and thermoelectric properties of type-I clathrate compounds in the Ba–Ga–Ge system have been investigated as a function of Ga content. The solid solubility of Ga in the type-I clathrate compounds is determined to be X=16 when expressed with the formula of Ba8GaXGe46−X. As the Ga content increases, the crystal structure changes from a superlattice structure to the normal type-I clathrate structure with the transition occurring at X=3.5–5. The density of Ge vacancies in the type-I clathrate phase decreases as the Ga content increases. The absolute values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases with the increase in the Ga content. The changes in electrical resistivity and Seebeck coefficient are explained in terms of the number of excess electrons, while the change in lattice thermal conductivity is explained in terms of the extent of the rattling motion of Ba atoms encapsulated in the cage structure.

Crystal structure and thermoelectric properties of the type-I clathrate compound Ba8Ge43 with an ordered arrangement of Ge vacancies

The crystal structure of the type-I clathrate compound Ba8Ge43 has been investigated by x-ray diffraction and transmission electron microscopy. The thermoelectric properties of Ba8Ge43 have also been investigated. The crystal structure of Ba8Ge43 is different from that reported for the usual type-I clathrate compounds with the space group of Pm3¯n but is a superlattice structure based on the usual type-I clathrate structure due to the ordering of Ge vacancies in half the 6c sites of the usual type-I clathrate structure. The crystal structure of Ba8Ge43 belongs to the space group of Ia3¯d and Ge vacancies exclusively occupy the 24c sites. The thermoelectric properties of Ba8Ge43 are not particularly good, as exemplified by the rather low ZT value of 0.057 because of the high value of electrical resistivity, which may arise from the existence of Ge vacancies.

Crystal structure refinement of a type-I clathrate compound Ba 8Ge 43 with an ordered arrangement of germanium vacancies

The crystal structure of a type-I clathrate compound Ba 8Ge 43 with an ordered arrangement of vacancies has been investigated by synchrotron X-ray diffraction. Ba 8Ge 43 possesses a large cubic unit cell with a lattice parameter twice that of the usual type-I clathrate structure due to an ordered arrangement of Ge vacancies. The space group is Ia 3 ¯ d , which is different from that of the usual type-I clathrate structure ( Pm 3 ¯ n ) . Ge vacancies occupy exclusively the 24c sites, which correspond to half the 6c sites in the usual parent type-I clathrate structure. The atomic coordinates and isotropic atomic displacement parameters for the Ba and Ge atoms are determined through Rietveld refinement with synchrotron X-ray data.

New Cationic Clathrate: Synthesis and Structure of [Si40P6]I6.5.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Unusually high chemical compressibility of normally rigid type-I clathrate framework: synthesis and structural study of Sn(24)P(19.3)Br(x)I(8)(-)(x) solid solution, the prospective thermoelectric material.

A novel tin phosphide bromide, Sn(24)P(19.3(2))Br(8), and Sn(24)P(19.3(2))Br(x)()I(8)(-)(x) (x = 0-8) solid solution have been prepared and structurally characterized. All compounds crystallize with the type-I clathrate structure in the cubic space group Pmn (No. 223). The clathrate framework of the title solid solution shows a remarkable chemical compressibility: the unit cell parameter drops from 10.954(1) to 10.820(1) A on going from x = 0 to x = 8, a feature that has never been observed for normally rigid clathrate frameworks. The chemical compressibility as well as non-Vegard dependence of the unit cell parameter upon the bromine content is attributed to the nonuniform distribution of the guest halogen atoms in the polyhedral cavities of the clathrate framework. The temperature-dependent structural study performed on Sn(24)P(19.3(2))Br(8) has shown that, in contrast to the chemical compressibility, the thermal compressibility (linear contraction) of the phase is similar to that observed for the Group 14 anionic clathrates. The tin phosphide bromide does not undergo phase transition down to 90 K, and the atomic displacement parameters for all atoms decrease linearly upon lowering the temperature. These linear dependencies have been used to assess such physical constants as Debye temperature, 220 K, and the lattice part of thermal conductivity, 0.7 W/(m K). Principal differences between the title compounds and the group 14 anionic clathrates are highlighted, and the prospects of creating new thermoelectric materials based on cationic clathrates are briefly discussed.

New cationic clathrate: synthesis and structure of [Si 40P 6]I 6.5

The [Si40P6]I6.5 cationic clathrate possessing the type-I clathrate structure with a minimal known unit-cell volume and a remarkably high vacancy concentration in the guest sublattice was synthesised.

Structure and stability of Ba–Cu–Ge type-I clathrates

We have prepared samples of nominal type Ba8CuxGe46−xby induction melting and solid state reaction. Analysis shows that thesematerials form type-I clathrates, with a copper content between x = 4.9 and5.3, nearly independent of the starting composition. We used x-ray powder diffractionand single-crystal electron diffraction to confirm the cubic type-I clathrate structure,while electron microprobe measurements confirmed the stability of the x ≈ 5composition. This result differs from the corresponding Ag and Au clathrates andwas not known previously due perhaps to the similar Cu and Ge form factors inx-ray diffraction. The observed composition adheres very tightly to avalence-counting scheme, in agreement with a Zintl-type stability mechanism.This implies a gap in the electronic density of states, also in contrast to themetallic behaviour of the Au and Ag analogues. Magnetization measurementsshowed a large diamagnetic response in the Ba–Cu–Ge clathrate. Thisbehaviour is consistent with semiconducting or semimetallic behaviourand is similar to that of a number of intermetallic semiconductors.

Structural analysis and thermoelectric properties of Type-I clathrate compounds in the Ba-Ge-Ga system

ABSTRACTThe crystal structure and thermoelectric properties of Type-I clathrate compounds in the Ba-Ge-Ga system (Ba8GaXGe46-X) have been investigated as a function of Ga content. While all ternary compounds containing more than 3 Ga exhibit a crystal structure of normal Type-I clathrates with the space group Pm 3 n, the binary compound possesses a composition Ba8Ge43, which is leaner in Ge than the stoichiometric composition, and has a crystal structure different from that of normal Type-I clathrates because of the ordering of Ge vacancies in 6c sites of the parent normal Type-I clathrate structure. The temperature dependence of electrical resistivity of binary Ba8Ge43 is of semiconductor while that for all the ternary compounds is metallic. The value of Seebeck coefficient increases with the increase in the Ga content. The value of dimensionless figure of merit (ZT) increases with the increase in the Ga content, exhibiting a high value of 0.76 at 700°C for Ba8Ga16Ge30.