Structural properties and thermodynamic stability of Ba-doped silicon type-I clathrates synthesized under high pressure - art. no. 045206

We present a joint experimental and theoretical study of the stability and structural properties of Ba-doped silicon type-I clathrates Ba8Si46 synthes ized under high pressures. The thermodynamic stability of Ba8Si46 under hig h pressure has been discussed from the total energy calculations of some ba rium silicides within the local density approximation (LDA). We have theore tically found that pressure favors the formation of the clathrate phase as experimentally observed. We have also per-formed a synchrotron x-ray-diffra ction experiment of Ba8Si46 prepared under high pressures. Some of the miss ing endohedral Ba elements in the small cage of Si(20) have been observed b y x-ray crystallography, while big cages of Si(24) are found to be complete ly occupied by Ba elements. The stabilization energies of Ba atoms in the e ndohedral sites estimated within the present LDA calculation suggest that t his is presumably attributed to the energetical site preference of Ba atoms between d(6) and a(2) sites. In addition, the isothermal parameter of Ba i n the big cage of Si(24) has been found to be larger than that in the small Si(20) unit, which is consistent with some theoretical predictions in earl ier works.