Films of ferroelectric hafnia have hitherto been deposited on electrodes with a non‐fluorite crystal structure. As a result, they are polycrystalline, contain fractions of non‐ferroelectric polymorphs or have poor crystal quality. Here, a strategy that circumvents all these limitations is shown. Seven nanometers‐thick epitaxial Hf_0.5Zr_0.5O₂ (HZO) films are deposited directly on yttria‐stabilized zirconia (YSZ) single‐crystals. The fluorite structure of the whole system enables coherent epitaxy, while the substrate orientation induces polymorph‐selective growth, being the HZO films orthorhombic on YSZ(111) and monoclinic on YSZ(001). Besides, the YSZ substrate can play the role of a buried floating electrode under the appropriate measuring conditions (temperature and frequency) thanks to its thermally‐activated oxygen conductivity. Indeed, out‐of‐plane ferroelectric switching is confirmed in the orthorhombic HZO samples at 185 °C and 0.01 Hz frequency. This original approach avoids the need to deposit conducting bottom layers, allowing high‐quality orthorhombic hafnia to be obtained directly on the substrate and its ferroelectric nature to be studied. Moreover, it constitutes a case of ion‐driven ferroelectric switching, and thus gives support to the recently proposed relationship between ionic conductivity, and ferroelectricity in fluorite systems.

Single-phase epitaxial Hf_0.5Zr_0.5O₂ films with non-centrosymmetric orthorhombic structure have been grown directly on electrode-free corundum (α-Al₂O₃) substrates by pulsed laser deposition. A combination of high-resolution X-ray diffraction and X-ray absorption spectroscopy confirms the epitaxial growth of high-quality films belonging to the Pca2₁ space group, with [111] out-of-plane orientation. The surface of a 7-nm-thick sample exhibits an atomic step-terrace structure with a corrugation of the order of one atomic layer, as proved by atomic force microscopy. Scanning transmission electron microscopy reveals that it consists of grains with around 10 nm lateral size. The polar nature of this film has been corroborated by pyroelectric measurements. These results shed light on the mechanisms of the epitaxial stabilization of the ferroelectric phase of hafnia.

Infrared (IR) reflectance spectra of monoclinic Hf0.5Zr0.5O2 (HZO) films with thickness in the range of 10–90 nm deposited on different cuts of Al2O3 and yttria-stabilized zirconia (YSZ) substrates were measured in the frequency region 30–4000 cm−1. Several phonons belonging to HZO were observed even for the 10 nm films. X-ray diffraction measurements were used to determine the crystal structure of the films and their orientation with respect to the substrates. The phonon spectra of HZO films grown on the anisotropic (11¯02) R-cut Al2O3 substrates exhibit strong anisotropy. Comparing them with the spectra of the films on isotropic YSZ substrates, it allows us to assign the symmetry and polarization of the phonons in HZO films. Fitting the IR spectra by the classical Lorentz model, we obtain the phonon parameters, their response functions, and their contribution to static permittivity. We also measured the IR reflectance of HfO2, ZrO2 and HZO ceramics and compared their phonon parameters with those of the corresponding thin films.

High‐quality purely c‐axis oriented epitaxial thin films of the Aurivillius phase Sr₂Bi₄Ti₅O₁₈ with n = 5 (Sr,Bi)TiO₃ perovskite‐like layers, are grown on SrTiO₃ substrates by pulsed laser deposition. The highest crystalline quality is obtained with a 20 wt.% Bi‐excess target and average stacking order values in the proximity of the ideal value n = 5 are attained for an optimum deposition temperature of 650 °C. Scanning transmission electron microscopy reveals regions with n ranging from 4 to 6 around an average thickness of n = 5, in agreement with the X‐ray diffraction analysis. Interdigital electrodes are used to probe the in‐plane polarization and survey the electrocaloric properties. A maximum adiabatic temperature change of ΔT ∼ 0.95 °C for an electric field of 150 kV cm⁻¹ is observed at ≈135 °C. Larger values are expected at higher temperatures around the ferroelectric Curie temperature, T_C. Since T_C of Sr₂Bi₄Ti₅O₁₈ can be tuned by codoping, the findings pave the way toward a large electrocaloric effect at ambient temperature.