the paper discusses the stabilization of pure tetragonal zirconia (t-ZrO2) nanocrystallites using a supercritical-based synthesis route. the authors show how the crystallization pathways and kinetics of the target compound can be dramatically changed by varying the synthesis medium and the ratio of metal nitrate to the complexing agent. they demonstrate that using small amounts of complexing agent leads to a crystalline precursor that first becomes amorphous at 613 k, then crystallizes into biFeO3 at 706 k, and subsequently transforms into bi2Fe4O9 at 815 k. on the other hand, larger amounts of complexing agent produce an amorphous precursor that directly crystallizes into bi2Fe4O9 at 762 k. using pdf, they reveal the importance of the initial gel structure in the overall crystallization behavior of the system. the paper also presents a synthesis of quasi pure t-ZrO2 nanocrystals in supercritical fluids using borderline nonhydrolytic sol-gel (bnhsg) conditions. the term borderline is used to express the nuance from nonhydrolytic sol-gel (nhsge) reactions when small amounts of water molecules are generated in situ from solvent (alcohols) decomposition to trigger hydrolysis/condensation. this amount of water being controlled, the reaction kinetics can be tuned and slowed down. this enabled the authors to perform in situ total scattering measurements to catch the various stages of the nanocrystal formation under different conditions (temperature, nature of the alcohols, and presence of surfactants). these observations were combined with various ex situ characterizations (raman, pdf, exafs, hr-tem, xrd, etc.) to discuss the existence of a critical size below which pure t-ZrO2 is formed, as reported in literature, and to propose a formation mechanism in this specific synthesis route.the paper discusses the stabilization of pure tetragonal zirconia (t-ZrO2) nanocrystallites using a supercritical-based synthesis route. the authors show how the crystallization pathways and kinetics of the target compound can be dramatically changed by varying the synthesis medium and the ratio of metal nitrate to the complexing agent. they demonstrate that using small amounts of complexing agent leads to a crystalline precursor that first becomes amorphous at 613 k, then crystallizes into biFeO3 at 706 k, and subsequently transforms into bi2Fe4O9 at 815 k. on the other hand, larger amounts of complexing agent produce an amorphous precursor that directly crystallizes into bi2Fe4O9 at 762 k. using pdf, they reveal the importance of the initial gel structure in the overall crystallization behavior of the system. the paper also presents a synthesis of quasi pure t-ZrO2 nanocrystals in supercritical fluids using borderline nonhydrolytic sol-gel (bnhsg) conditions. the term borderline is used to express the nuance from nonhydrolytic sol-gel (nhsge) reactions when small amounts of water molecules are generated in situ from solvent (alcohols) decomposition to trigger hydrolysis/condensation. this amount of water being controlled, the reaction kinetics can be tuned and slowed down. this enabled the authors to perform in situ total scattering measurements to catch the various stages of the nanocrystal formation under different conditions (temperature, nature of the alcohols, and presence of surfactants). these observations were combined with various ex situ characterizations (raman, pdf, exafs, hr-tem, xrd, etc.) to discuss the existence of a critical size below which pure t-ZrO2 is formed, as reported in literature, and to propose a formation mechanism in this specific synthesis route.