Superconductivity in Al-substituted Ba8Si46 clathrates

Abstract

There is a great deal of interest vested in the superconductivity of Si clathrate compounds with sp3 network, in which the structure is dominated by strong covalent bonds among silicon atoms, rather than the metallic bonding that is more typical of traditional superconductors. A joint experimental and theoretical investigation of superconductivity in Al-substituted type-I silicon clathrates is reported. Samples of the general formula Ba8Si46−xAlx, with different values of x were prepared. With an increase in the Al composition, the superconducting transition temperature TC was observed to decrease systematically. The resistivity measurement revealed that Ba8Si42Al4 is superconductive with transition temperature at TC = 5.5 K. The magnetic measurements showed that the bulk superconducting Ba8Si42Al4 is a type II superconductor. For x = 6 sample Ba8Si40Al6, the superconducting transition was observed down to TC = 4.7 K which pointed to a strong suppression of superconductivity with increasing Al content as compared with TC = 8 K for Ba8Si46. Suppression of superconductivity can be attributed primarily to a decrease in the density of states at the Fermi level, caused by reduced integrity of the sp3 hybridized networks as well as the lowering of carrier concentration. These results corroborated by first-principles calculations showed that Al substitution results in a large decrease of the electronic density of states at the Fermi level, which also explains the decreased superconducting critical temperature within the BCS framework. The work provided a comprehensive understanding of the doping effect on superconductivity of clathrates.

Distribution of Al atoms in the clathrate-I phase Ba8AlxSi46−x at x = 6.9

“…were in reasonable agreement with recently reported optimized structure data, 22 taking into account the application of different calculation procedures. For the theoretical calculations on Ba 7 Si 46 the clathrate-I crystal structure of Ba 8 Si 46 was transformed to the space group Pm3, allowing for a split of Ba1 (site 2a) into Ba11 (site 1a) and Ba12 (site 1b).…”

RbB3Si3: An Alkali Metal Borosilicide that is Metastable and Superconducting at 1 atm

“…On the other hand, several metal silicide clathrates, largely based on Ba 8 Si 46 ( T c = 8 K), including (Na, Ba) x Si 46 ( T c = 4 K), and Ba 8 Al 6 Si 40 ( T c = 4.7 K) that are superconducting and stable at 1 atm have been synthesized. Because these assume the clathrate I structure type, we wondered whether metal silicides that adopt a sodalite-like structure, of which none are currently known, could be stabilized under pressure and quenched to atmospheric conditions, resulting in superconducting candidate structures analogous to the metal hexa-hydrides?…”

Low-density silicon allotropes for photovoltaic applications

“…Since the structure of clathrates is characterized by face-sharing cage-like units of host atoms enclosing guests, its properties can be tuned through tailored substitution of the host or guest elements. Superconductivity was observed in clathrate compounds [17,18] with transition temperatures as high as 8 K in the barium-encapsulated type I silicon clathrate Ba 8 Si 46 synthesized at high pressure [19]. Furthermore, the properties induced by the vibrational "rattling" modes of the guest atoms in the large host cages leads to reduced thermal conductivity at high electronic conductivity (the phonon glass-electron crystal concept introduced by Slack [20]), such that clathrate have been studied in the context of thermoelectric applications [21][22][23][24][25][26].…”