Giant negative thermal expansion at the nanoscale in the multifunctional material G d5(Si,Ge ) 4

In this work the thermal expansion of G d₅S i_1.3G e_2.7 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 (β^LT∼-32.2 ppm K^-1) , and within the room temperature 255-340 K, where a giant NTE was observed (-69 ppm K^-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.