Unravelling the complex formation mechanism of HfO$_2$ nanocrystals using in situ pair distribution function analysis

Hafnia, HfO$_2$ , which is a wide band gap semiconducting oxide, is much less studied than the chemically similar zirconia (ZrO$_2$ ). Here, we study the formation of hafnia nanocrystals from hafnium tetrachloride in methanol under solvothermal conditions (248 bar, 225–450 °C) using complementary in situ powder X-ray diffraction (PXRD) and Pair Distribution Function (PDF) analysis. The main structural motif of the precursor solution (HfCl4 dissolved in methanol) is a Hf oxide trimer with very similar local structure to that of m-HfO$_2$ . Different measurements on precursor solutions show large intensity variation for the Hf–Cl correlations signifying different extents of HCl elimation. A few seconds of heating lead to a correlation appearing at 3.9 Å corresponding to corner-sharing Hf-polyhedra in a disordered solid matrix. During the next minutes (depending on temperature) the disordered structure rearranges and the nearest neighbour Hf–Hf distance contracts while the Hf–O coordination number increases. After approximately 90 seconds (at T = 250 °C) the structural rearrangement terminates and 1–2 nm nanocrystals of m-HfO$_2$ nucleate. Initially the m-HfO$_2$ nanocrystals have significant disorder as reflected in large Hf atomic displacement parameter (ADP) values, but as the nanocrystals grow to 5–6 nm in size during extended heating, the Hf ADPs decrease toward the values obtained for ordered bulk structures. The nanocrystal growth is not well modelled by the Johnson–Mehl–Avrami expression reflecting that multiple complex chemical processes occur during this highly nonclassical nanocrystal formation under solvothermal conditions