The effect of Al-substitution on superconducting type-I clathrate Ba8Si46

Abstract

A series of samples with the chemical formula Ba8Si46−xAlx (x=2, 3, 5, 6, 7 and 8) were prepared by arc melting, ball milling and washing with diluted HCl. The lattice parameter of Ba8Si46−xAlx increases linearly with the increase of nominal Al content x. The composition analysis by energy-dispersive X-ray spectroscopy (EDS) shown that the actual Al contents in clathrates are lager than the nominal compositions because the dilute Al-contained impurity phases were washed out. The experimental results show that the minimum incorporation of Al into clathrate structure is expected to be about 3 at ambient pressure, which is in agreement with a first-principle simulation. The Al substitution for Si results in the decrease of superconducting transition temperature TC, which can be explained on the BCS theoretical frame. The electron density of state at Fermi level N(EF) decreases with the increment of x except for an abnormal increase for the sample x=6. Such sample has a higher spatial symmetry of the structure in which all the six Si atoms at 6c sites were substituted by Al atoms. Its higher N(EF) causes to a higher TC. In addition, we calculated the phonon-dispersion relations and vibrational density of states for Al-doped silicon clathrates. The high frequency acoustic branch has a red shift from 430cm−1 to 420cm−1 with the doping of Al. The decreased frequency of bond-stretching vibration modes is another reason for the suppression of TC induced by Al substitution.