Martensitic Phase Transformation of Isolated HfO2, ZrO2, and HfxZr1 – xO2 (0 < x < 1) Nanocrystals†

We previously reported that, during the reactions to make nanocrystals of HfO 2 and Hf-rich Hf x Zr 1 - x O 2 , 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 ZrO 2 and Zr-rich Hf x Zr 1 - x O 2 nanocrystals is attributed to the higher driving force needed in those instances compared to that needed for producing HfO 2 and Hf-rich Hf x Zr 1 - x O 2 nanocrystals. We also report here the post-synthesis, heat-induced phase transformation of Hf x Zr 1 - x O 2 (0nanocrystals. As temperature increases, all the tetragonal nanocrystals transform to the monoclinic phase accompanied by an increase in particle size (as evidenced by X-ray diffraction and transmission electron microscopy), which confirms that there is a critical size for the phase transformation to occur. When the monoclinic nanorods are heated above a certain temperature the grains grow considerably; under certain conditions a small amount of tetragonal phase appears.