Evolution of structure and transport properties of the Ba₈Cu₁₆P₃₀ clathrate-I framework with the introduction of Ga

Two type-I clathrates were synthesized by introducing Ga into the framework of the Ba₈Cu₁₆P₃₀ type-I clathrate. The introduction of minute amounts of Ga, 1.9% Ga/M_total (where M_total = Cu + Ga), resulted in the disturbance of the completely ordered Pbcn superstructure of Ba₈Cu₁₆P₃₀. Ba₈Cu_15.43(2)Ga_0.3P_30.26(3) crystallizes in a partially ordered orthorhombic Pmna clathrate-I superstructure with five out of 15 framework sites being jointly occupied by metal+phosphorus. Increasing the Ga content resulted in all framework sites being occupied by metal + phosphorus in the archetype cubic Pm$3¯$n clathrate-I crystal structure of Ba₈Cu_14.5(3)Ga_1.3P_30.2(4) with 8.2% Ga/M_total. A combination of energy dispersive x-ray spectroscopy, inductively coupled plasma mass spectroscopy, and single crystal x-ray diffraction was used to determine the structures alongside the compositions. The positional disorder was verified by ³¹P solid state NMR spectroscopy. Characterization of the transport properties indicated that the Ga-substituted samples exhibit higher Seebeck coefficients and electrical resistivities compared to its pristine counterpart, in line with the expected reduction of the hole concentration due to Ga/Cu substitution. Moderate improvements in the thermoelectric power factor and overall figure-of-merit were observed for samples with 6.9% and 3.8% Ga/M_total as compared to those for the pristine Ba₈Cu₁₆P₃₀ clathrate. Band structure calculations shed light on how Ga substitution affects the electronic structure and thermoelectric properties of studied clathrates.