Structure and thermoelectric characterization of Ba₈Al ₁₄Si₃₁

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba₈Al₁₄Si₃₁. Single-crystal neutron diffraction data for Ba₈Al₁₄Si₃₁ were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba₈Al₁₄Si₃₁ confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state ²⁷Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba₈Al ₁₄Si₃₁, with the general formula, Ba₈Al _xSi_42-3/4x[]_4-1/4x ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.