Proton transfer in layered hydrogen-bonded system γ-MOOH (M = Al, Sc): Robust bi-mode ferroelectricity and 1D superionic conductivity

Layered γ-MOOH, such as synthetic boehmite γ-AlOOH and γ-ScOOH, has been explored for various applications since 1950s. In this paper, based on first-principles calculations, we show the evidence of two proton transfer modes in their hydrogen-bonded network that give rise to extraordinary properties: (1) they, respectively, result in two distinct types of ferroelectricity with different switching mechanisms and polarizations, while the exhibiting mode under an electric field depends on various factors, including the field intensity and direction, the existence of vacancies, and temperature; and (2) the combination of two modes can lead to ultra-high proton conductivity along 1D channels. Their proton migration barriers ensure high ferroelectric Curie temperature, while still much lower compared with current proton conductors, giving rise to 1D superionicity with unprecedented protonic conductivity over 24 mS/cm. Those light weight nontoxic layered materials with high polarizations, Curie temperature, and ultra-high protonic conductivity should provide vast opportunities for various applications.