Structures, Phase Equilibria, and Properties of HfO2

Abstract Hafnium dioxide (HfO 2 ) has long been known as a refractory material due to its high melting temperature (~ 2800°C) and low thermal conductivity (1.5 W/m K) [1] , [2] . Refractories are thermally insulating materials known to withstand high temperatures without being degraded and are used for high-temperature applications to reduce heat losses [3] . Due to the higher dielectric permittivity of HfO 2 relative to SiO 2 , HfO 2 was studied as a high- k gate dielectric [4] , [5] , [6] . Decades of research have been dedicated to understanding the dielectric, electrical, mechanical, and thermal properties in these applications. Since the discovery of ferroelectricity in HfO 2 -based thin films in 2011 [7] , further research surrounding the dielectric, supercapacitive, piezoelectric, pyroelectric, and electrocaloric properties has emerged. Understanding the mechanisms underpinning these functionalities requires a basic understanding of the HfO 2 crystal structures and HfO 2 -metal oxide phase equilibria. This chapter reviews these fundamentals, including a discussion on the role of dopants and phase stability of doped HfO 2 .