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We previously reported that, during the reactions to make nanocrystals of HfO₂ and Hf‐rich Hf_xZr_1 – xO₂, a tetragonal‐to‐monoclinic phase transformation occurs that is accompanied by a shape change of the particles (faceted spherical to nanorods) when the temperature at which the reaction is conducted is changed from 340 to 400 °C. We now conclude that this concomitant phase and shape change is a result of the martensitic transformation of isolated nanocrystals in a hot liquid, where twinning plays a crucial role in accommodating the shape‐change‐induced strain. That such change was not observed during the reactions forming ZrO₂ and Zr‐rich Hf_xZr_1 – xO₂ nanocrystals is attributed to the higher driving force needed in those instances compared to that needed for producing HfO₂ and Hf‐rich Hf_xZr_1 – xO₂ nanocrystals. We also report here the post‐synthesis, heat‐induced phase transformation …