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

A series of samples with the chemical formula Ba8Si46 Al-x(x) (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 Al-x(x) 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 T-C, which can be explained on the BCS theoretical frame. The electron density of state at Fermi level N(E-F) 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(E-F) causes to a higher T-C. 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 430 cm(-1) to 420 cm(-1) with the doping of Al. The decreased frequency of bond-stretching vibration modes is another reason for the suppression of T-C induced by Al substitution. (C) 2014 Elsevier B.V. All rights reserved.