The Smaller the Better Hosting Trivalent Rare-Earth Guests in Cu-P Clathrate Cages

International audienceThe most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba8-xRxCu16P30, with the smallest pentagonal dodecahedral cages encapsulating La3+ and Ce3+. Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance