Structural, transport, magnetic, and thermal properties ofEu8Ga16Ge

Abstract

Eu 8 Ga 16 Ge 30 is the only clathrate known so far where the guest positions are fully occupied by a rare-earth element. Our investigations show that, in addition to the previously synthesized Eu 8 Ga 16 Ge 30 modification with clathrate-I structure, there exists a second modification with clathrate-VIII structure. Polycrystalline samples of both phases behave as local-moment ferromagnets with relatively low Curie temperatures ͑10.5 and 36 K͒. The charge-carrier concentrations are rather small ͑3.8 and 12.5ϫ10 20 cm Ϫ3 at 2 K͒ and, together with the low Curie temperatures, point to a semimetallic behavior. Both the specific heat and the thermal conductivity are consistent with the concept of guest atoms ''rattling'' in oversized host cages, leading to low thermal conductivities ͑''phonon glasses''͒. However, the electron mobilities are quite low, which, if intrinsic, would question the properties of an ''electron crystal'', commonly presumed in ''filled-cage'' materials. The dimensionless thermoelectric figure of merit reaches values of 0.01 at 100 K.

Thermal stability and thermoelectric properties of p-type Ba8Ga16Ge30 clathrates

“…3. ͑Color online͒ Thermopower as a function of temperature for Ba 8 Ga 16 Ge 30 samples described in Table I collected from room temperature to 700°C.…”

“…Stoichiometric amounts was used for n-doped Ba 8 cess Ga ͑four times the stoichiometric amount͒ was used as flux. Mixing of the elements was performed in a glove box ͑high-purity Ar atmosphere͒.…”

“…The amount of work published on n-doped clathrates greatly outweighs the work on p-doped clathrates, and the reason for this is that the structure disfavors bonds between trivalent ͑Ga͒ atoms. 28 In the case of, e.g., Ba 8 30 , but when high temperature measurements were conducted, loss of gallium from the samples was observed. 36 The stability of p-type Ba 8 Ga 16 Ge 30 and its usefulness in an application can thus be questioned.…”

“…A large number of clathrate compounds have now been studied, and most of them are n-doped type-I structures, with the most prominent of the type-I clathrates being Ba 8 Ga 16 Ge 30 with a ZT of 1.35 at 900 K. 27 All TE devices need both n-and p-type materials, which should be compatible with each other in terms of, e.g., thermal expansion, and it is therefore beneficial to find p-type clathrates of equal potential and stability. The amount of work published on n-doped clathrates greatly outweighs the work on p-doped clathrates, and the reason for this is that the structure disfavors bonds between trivalent ͑Ga͒ atoms.…”

Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms

“…[4][5][6] The weak interaction between guests and host facilitates tuning electronic, optical and dynamical properties of clathrates; 7 as a result, they exhibit many intriguing physical properties, such as glasslike thermal conductivity, 8 superconductivity in sp 3 covalent bonded solids 9,10 and magnetism. 11,12 Recently, Si clathrate materials have attracted intense research interest for applications including lithium ion batteries, 13 thermoelectrics, 14 photovoltaic (PV) cells and optoelectronics, [15][16][17] etc.…”