Metastable polar orthorhombic local structure of hydrothermally grown HfO2 nanocrystals

Metastable polymorphs of periodic solids manifest atomic connectivities that represent local minima in free energy landscapes. Upon scaling to nanometer-sized dimensions, surface and stress energy can modify phase stabilities, providing a means of accessing metastable polymorphs that exhibit distinctive function. The recent discovery of the non-centrosymmetric orthorhombic phase of HfO2 in strained and doped thin films has generated considerable excitement, given that it may enable scalable ferroelectric devices for energy-efficient computing. In this work, we prepare HfO2 nanocrystals ca. 6 nm in diameter using a hydrothermal process. Total scattering and extended X-ray absorption fine-structure analyses are used to probe the local structure of the prepared nanocrystals. The measurements indicate that hydrothermally prepared HfO2 nanocrystals adopt a kinetically trapped orthorhombic local structure. The results demonstrate the rich landscape of crystal structures accessible for binary transition-metal oxides and the sensitivity of ultra-small nanocrystals to surface stresses and polarization.