Growing the Nearly Impossible: Routes to Stabilize Metastable Phases
Lauren Garten, Naval Research Laboratory
Friday, November 20, 2:40 PM
https://ucsc.zoom.us/j/92986339277?pwd=VkQvN0tpd2RqQ29Jbm5MNGdhNS9WZz09
Many technologically relevant materials are not in a true thermodynamic minimum but canstill be synthesized under the right conditions. Currently, the synthesizability of these metastable phases is difficult to predict a priori and targeting a particular metastable phase can be further complicated by the competition between energetically similar polymorphs. So how can we stabilize metastable materials? In this talk I will describe the processing methods we are developing to target metastable materials based on size, enthalpy, strain engineering, anddeposition rate. To highlight where these growth approaches do and do not work, I will discuss a model material system –TiO2, VO2, Mn(Se,Te), SrHfO3– for each growth approach. For example, when the desired phase has a larger volume than the ground state, heterostructural alloying has been employed to induce enthalpic stabilization, which was used in Mn(Se,Te), but if the desired phase has reduced dimensions, then substrate induce strain can be used, like in SrHfO3. To help provide guidance when substrates can be used in phase stabilization, we have developed a substrate picker tool to search through the Materials Project database for substrates that will preferentially stabilize a desired phase. Using this theory guided substrate epitaxy and beyond equilibrium deposition rates allowed us to realize the theoretically predicted piezoelectric P4mm phase of SrHfO3. I will also talk about how many of these metastable materials have functionalities that are enhanced or completely unique from the ground state, such as ferroelectricity in metastable P4mm SrHfO3 and piezoelectricity in metastable Mn(Se,Te), highlighting the importance of exploring metastable phase space.
