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

Abstract

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/ Mtotal (where Mtotal = Cu + Ga), resulted in the disturbance of the completely ordered Pbcn superstructure of Ba8Cu16P30. Ba8Cu15.43(2)Ga0.3P30.26(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[Formula: see text] n clathrate-I crystal structure of Ba8Cu14.5(3)Ga1.3P30.2(4) with 8.2% Ga/ Mtotal. 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/ Mtotal 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.