T melt for bulk Hf is 2506 K and while NPs often melt below the bulk T melt , it is unlikely that melting could occur below 2100 K for a ~40 nm NP. More likely, the change in sublimation energetics reflects a solid-solid phase transition. Bulk Hf undergoes an hcp-to-bcc transition at temperatures that depend on the level of Zr (and O) contamination with a recommended corrected value of 2016 ± 20 K. The effects of Hf particle size on the phase diagram have also been reported, however, the size effects are small above ~15 nm, consistent with a field emission microscopy value of 2030 K, which pertains to the nanoscale field emission tip. Thus, for our ~40 nm NPs with a stated purity of >99.5% Hf, a value close to the recommended bulk transition temperature would be expected, consistent with the slope change observed between 2000 and 2100 K.