Giant negative thermal expansion at the nanoscale in the multifunctional material Gd₅(Si, Ge)₄

In this work the thermal expansion of Gd5Si1.3Ge2.7 magnetic nanogranules was studied. A crossover from positive (macro- and microscale in the literature) towards a negative thermal expansion (at nanoscale) is observed. The negative thermal expansion (NTE) behavior was found in two temperature windows: 90-160 K (βLT∼-32.2ppmK-1), and within the room temperature 255-340 K, where a giant NTE was observed (-69ppmK-1). The key atomic mechanism driving the NTE is identified as an atomic triplet-chain flexibility. The results suggest that the NTE behavior emerges as a size-reduction effect caused by the intrinsic nanoparticles surface pressure (estimated at 11 kbar), leading to the positive thermal expansion-NTE crossover at the nanoscale.