Crystal structure and transport properties of Ba8Ge43□3

Abstract

The single phase clathrate-I Ba(8)Ge(43)square(3) (space group Ia3d (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2-673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (kappa(l)(300 K) = 2.7 W m(-1) K(-1)) has been evidenced.

Refinement of the crystal structure of K8Ge44⬜2, an intermetallic clathrate I

“…The framework defects in K 8 Ge 44 £ 2 have been confirmed in recent works from powder diffraction data [12,13]. Moreover, the validity of the split atom model has been proven by the detection of superstructures with ordered defect arrangement (space group Ia3d )for the clathrate-I phases Rb 8Sn44£2 [14], Cs8Sn44£2 [15] and Ba 8Ge43£3 [9,16,17]. Interestingly, vacancies occur for the larger tetrel elemenst but not for the corresponding silicide.…”

“…The here presented redetermination of the crystal structure reveals the expected partial occupancy of the Ge1 site 6c of » 2/3, whereas the site 16i of Ge2 and the site 24k of Ge3 atoms are fully occupied. The composition of the clathrate phase was determined to be K 8 Ge 44.05(5) £ 1.95 (5) .The accumulation of defects at site 6c can be explained, because the Ge1 atoms connect always two planar Ge 6-rings and provide therefore an unfavorable environment for Ge atoms due to the strong deviation of the bond angles from the tetrahedral values [9]. Atoms at site 6c are only connected to atoms at site 24k,which means that the Ge3 atoms surround either aGe1 atom or adefect.…”

Unraveling the role of vacancies in the potentially promising thermoelectric clathratesBa8ZnxGe46−x−y□y

“…2, the number of vacancies, y, in the most stable composition decreases with increase in x. In the Zn-deficient condition, i.e., at x = 0, the most stable composition (Ba 8 Ge 43 3 ) is found to have three vacancies in the framework as observed experimentally [17,43]. Our results show that vacancies are present in compositions for 0 < x < 6, whereby y varies differently as y = 4 − x/2 and y = 3.5 − x/2 for the even and odd concentrations of x, respectively (cf.…”

“…In fact, a figure of merit (zT ) of ∼ 1.4 has been achieved in Ba 8 Ga 16 Ge 30 [9][10][11], which is to date reported to be the highest among intermetallic clathrates [12]. However, the presence of guests or framework substitution also leads to unexpected structural changes, such as the formation of vacancies in the host framework of the clathrates [13][14][15][16][17][18][19][20][21]. Simple concepts such as the Zintl's rule have been useful to some extent to rationalize these effects [13,14]; however, the exact understanding of the mechanisms and their impact on the electronic and transport properties are largely unknown.…”

Theoretical and Experimental Study on Thermoelectric Properties of Ba8TM x Ga y Ge46–x–y (TM = Zn, Cu, Ag) Type I Clathrates

“…8,21 Far deviation of the composition from charge balance towards excess electrons causes formation of framework atom vacancies, and at high vacancy concentrations, this leads to vacancy ordering with the formation of a superstructure. 22 Moreover, for a specific composition in the real material, different site occupancies are observed for different preparation methods, hence some dissimilarities of transport properties between different published data can be expected. All of these effects should be taken into account in band-structure calculations when appropriate, making reproduction of real material properties in a theoretical model quite complex.…”