  Hafnia (HfO2) is a promising material for emerging chip applications due to
its high-k dielectric behaviour, suitability for negative capacitance
heterostructures, scalable ferroelectricity, and silicon compatibility. The
lattice dynamics along with phononic properties such as thermal conductivity,
contraction, and heat capacity are under-explored, primarily due to the absence
of high quality single crystals. Herein, we report the vibrational properties
of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY,
where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry
analysis and lattice dynamics calculations. We untangle the effects of Y by
testing our calculations against the measured Raman and infrared spectra of the
cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal
the signature modes of polar orthorhombic hafnia. This work provides a
spectroscopic fingerprint for several different phases of HfO2 and paves the
way for an analysis of mode contributions to high-k dielectric and
ferroelectric properties for chip technologies.
