  HfO2, a simple binary oxide, holds ultra-scalable ferroelectricity integrable
into silicon technology. Polar orthorhombic (Pbc21) form in ultra-thin-films
ascribes as the plausible root-cause of the astonishing ferroelectricity, which
has thought not attainable in bulk crystals. Though, perplexities remain
primarily due to the polymorphic nature and the characterization challenges at
small-length scales. Herein, utilizing a state-of-the-art Laser-Diode-heated
Floating Zone technique, we report ferroelectricity in bulk single-crystalline
HfO2:Y as well as the presence of anti-polar Pbca phase at different Y
concentrations. Neutron diffraction and atomic imaging demonstrate (anti-)polar
crystallographic signatures and abundant 90o/180o ferroelectric domains in
addition to the switchable polarization with little wake-up effects.
Density-functional theory calculations suggest that the Yttrium doping and
rapid cooling are the key factors for the desired phase. Our observations
provide new insights into the polymorphic nature and phase controlling of HfO2,
remove the upper size limit for ferroelectricity, and also pave a new road
toward the next-generation ferroelectric devices.
