Ferroelastic Domain Organization in Solution-Grown HfO2 Nanorods

Hafnia (HfO2) is a technologically important hard material with a relatively high dielectric constant and wide band gap [1]. However, the basic properties of HfO2 have been little studied in the literature due to its close resemblance to zirconia in chemical properties. In the bulk, the monoclinic (space group: P21/c) phase represents the thermodynamic minimum; phase transformation to a distorted fluorite tetragonal (space group: P42/nmc) phase is induced upon heating to 1720 o C. [2] The symmetry-lowering tetragonal to monoclinic phase transition in HfO2 closely parallels the better studied phase transition in ZrO2 and is believed to be Martensitic and athermal in nature. The transformation is thought to proceed through a diffusionless process wherein bond distances and angles are altered without disrupting the atomic connectivity within the lattice with preservation of a mirror plane symmetry element. Similar to ZrO2, this tetragonal to monoclinic phase transformation in HfO2 is strongly anisotropic being most pronounced along the aand c-axes and negligible along the crystallographic b-axis. This anisotropic expansion induces a substantial shear strain within the materials that can be variously accommodated. One approach by which this transformation strain can be relieved in this symmetry-lowering transition is by formation of periodic sequences of twin variants. The system seeks to establish a balance between compensating for macroscopic strain and the inevitable energetic penalty for establishing a new interface (the twin boundary). In other words, by forming a sequence of periodic domains separated by a coherent twin boundary, the structural integrity of the material can be retained and the macroscopic strain can be relaxed without extensive deformation or crack propagation. Evidence for formation of twinned domains has been reported in HfO2 polycrystalline samples [3] thin films [4] and nanorods [5]. However, periodic organization of twinned domains along 1D nanorods has not thus far been evidenced in either thin films or bulk ceramic grains. In this study, we demonstrate the organization of ferroelastic domains that create a nanoscopic stripe pattern within colloidal HfO2 nanorods.