Origin of Pyroelectricity in Ferroelectric HfO2

The pyroelectric response of materials is a promising phenomenon that is used in many applications. In this paper, we report first-principles lattice dynamics and ab initio molecular dynamics calculations to investigate the mechanisms of the pyroelectric effect in the Pca2₁ orthorhombic phase of HfO₂; this is an emerging pyroelectric material with promising technological applications. With first-principles lattice dynamics calculations, we demonstrate a large pyroelectric response that is intrinsic to the pure Pca21 orthorhombic phase. Unlike most conventional ferroelectric pyroelectrics, such large pyroelectricity arises unexpectedly from the secondary effect (induced by the negative longitudinal piezoelectric effect) that has not received due attention. Ab initio molecular dynamics simulations further demonstrate a giant pyroelectric response that is associated with an orthorhombic (polar) to tetragonal (nonpolar) structural phase transition. Upon the incorporation of Si dopants with increasing dopant concentrations, the phase transition persists, but occurs at reduced temperatures with enhanced pyroelectric responses. The present work highlights the importance of the secondary pyroelectric effect responsible for the pyroelectricity in HfO₂ and its promising potential for device applications.