Piezoelectric Polar Oxides

Piezoelectric polar oxides are of paramount importance due to their unique ability to convert mechanical stress into electrical energy, making them indispensable in various applications such as sensors, energy harvesting, and ultrasound imaging. Application examples of such sensors include trace chemical detection in harsh environments, biochemical detection/characterization in highly damping liquid environments, gas and temperature sensing in high-vibration automotive environments, and long-term structural-health monitoring in civil infrastructures with continuously variable temperature and stress. Additionally, the tunable properties and versatile nature of piezoelectric materials create opportunities for exciting progress in areas like electronics, telecommunications, medical equipment, environmental monitoring and many more. The current special issue of Physica Status Solidi A brings together ten articles of selected contributions presented at the Symposium V “Piezoelectric Polar Oxides,” held at the E-MRS 2023 Fall Meeting. This symposium provided a forum for discussing recent advances in material synthesis, crystallography, physical and chemical properties, and device applications related to piezoelectric materials. In particular, a special focus was placed on the results of the Research Unit 5044 (www.for5044.de), funded by the German Research Foundation, which explores lithium niobate-lithium tantalate (LiNbO3-LiTaO3) solid solutions. Those crystals are both widely employed for technological applications as piezoelectric systems as well as in other areas such as integrated photonics, nonlinear optics and so on. The mixed lithium niobate-lithium tantalate solid solution is therefore an extremely promising model system for the discovery and application of new fundamental phenomena in materials science and physics, due to the miscibility in the entire compositional range, entailing the possibility to modify a large number of physical properties, as well as the adjustable size of ferroelectric domains. The scientific contributions collected in this Special Issue are devoted to fundamentals or applications of lithium niobate-tantalate family materials. The research by K. Lengyel et al. (202300932) focuses on the spectroscopic properties of nanocrystalline lithium niobate and explores the effects of Yb3+ and Er3+ doping of LiNbO3 nanocrystals, produced by high-energy ball milling. This study provides insights into their potential for quantum nanophotonics applications. Further, in situ optical spectroscopy studies of single crystalline LiNbO3-LiTaO3 solid solution system are performed as a function of temperature by P. Gaczynski et al. (202300972). The authors show that the bandgaps of all studied compounds are found to decrease nearly linearly with the temperature by about 1.5 eV over the range from room temperature up to 1200 °C. The investigation by U. Yakhnevych et al. (202400129) examines the acoustic losses in bulk LiNb1–xTaxO3 resonators with varying Nb/Ta ratios and their correlation with charge transport processes at temperatures up to 900 °C and at reduced oxygen partial pressures. The authors have shown that losses in LiNb1–xTaxO3 are caused by a superposition of several physical mechanisms, including intrinsic phonon-phonon loss, point-defect relaxations and conductivity-related relaxations. E. Tichy-Racs et al. (202300962) investigate the impact of the lithium content on the electrical conductivity, acoustic properties and high-temperature stability of single crystalline LiNb1–xTaxO3 at high temperatures. The crystals grown from lithium-deficient melts were treated by the vapor transport equilibration (VTE) method to achieve near-stoichiometric composition. It is shown, that the VTE-treated specimens generally exhibit lower conductivity at temperatures below 800 °C, which is attributed to the reduced number of lithium vacancies in near-stoichiometric LiNb1–xTaxO3, provided that the lithium ion migration dominates the conductivity in this temperature range. Additionally, the changes in electrical conductivity near the Curie temperature in single crystalline LiNb1–xTaxO3 are studied in detail in the work by F. El Azzouzi et al. (202300966), providing a microscopic understanding of the structural phase transitions, which is critical for optimizing the high-temperature performance of the material. Notably, the findings of this study played an important role in supporting the work of U. Yakhnevych et al. (202400129), where conductivity was identified as a critical factor influencing acoustic losses in LiNb1–xTaxO3 at elevated temperatures. The study by M. Gonzalez et al. (202400129) is devoted to determination of key parameters such as annealing time, temperature, and equilibration powder concentration, required for preparation of stoichiometric LiTaO3 by VTE method. It is shown, that homogeneous stoichiometric LiTaO3 crystals could be obtained by treatment for at least 36 h at 1250 °C in Li2O rich atmosphere (>54 mol% of Li2O). Further, C. Kofahl et al. (202300959) studied the oxygen tracer self-diffusion in LiNb1–xTaxO3 single crystals at temperatures up to 1050 °C using an 18O2 stable tracer isotope with subsequent analysis of the diffusion-depth profiles by secondary ion mass spectrometry. The authors suggest that oxygen diffusion in LiNb1–xTaxO3 is governed by oxygen vacancies. The work by B. Koppitz et al. (202300967) focuses on poling and structuring of ferroelectric domains in lithium-niobate family materials. The authors used hysteresis measurements to determine the poling properties (spontaneous polarization, forward and reverse coercive fields) of congruent and near stoichiometric LiNbO3, LiTaO3 as well as their magnesium-doped analogues. The obtained results serve as the fundamental input for the poling of single crystalline LiNb1–xTaxO3. This information was subsequently applied in the studies by U. Yakhnevych et al. (202400129) and by E. Tichy-Racs et al. (202300962), as poling was a necessary preparatory step before investigating acoustic losses in the material. The study by F. Bernhardt et al. (202300968) presents a joint experimental-theoretical approach to elucidate the vibrational properties of LiNb1–xTaxO3 solid solutions. Through a systematic comparison of experimentally determined data and first-principles calculations within the framework of density functional theory, the authors successfully assigned the spectral features of LiNb1–xTaxO3 measured by Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy to specific phononic eigenvectors. Lastly, the photoelastic and piezo-optic properties of lithium niobate under mechanical stress are calculated by M. Pionteck et al. (202300970), adding valuable data on the material's optical response under mechanical stress for precision device engineering. We hope that this Special Issue will inspire further innovation and collaboration in the field of piezoelectric materials, driving forward applications that will meet the demands of the future. We extend our sincere appreciation to the authors and participants of the E-MRS Symposium Vfor their invaluable contributions. Éva Tichy-Rács is a Postdoctoral Researcher at the Clausthal University of Technology. She earned her PhD in 2022 from Eötvös Loránd University in Budapest (Hungary). Her career includes over a decade at the Wigner Research Centre for Physics in Budapest, where she led the Crystal Processing Laboratory for six years. Her research spans the synthesis, growth, and optical characterization of various materials, including doped LiNbO3 crystals. Since 2023, she has been based in Goslar, Germany, focusing on the optical, electrical, and acoustic properties of piezoelectric materials for high temperature sensing applications. Marco Bazzan earned his Physics degree in 2001 and his PhD in Materials Science in 2004 at the University of Padova, where he has been an Associate Professor in the Department of Physics and Astronomy since 2022. His research primarily focuses on solid-state physics and optics, with a particular emphasis on lithium niobate, a versatile crystal material widely used in nonlinear optics and photonics. His work spans its growth, structural analysis using high-resolution X-ray diffraction, photorefractive nonlinear optics and related microscopic phenomena, as well as the fabrication of integrated optical circuits. As a member of INFN, he is also actively involved in the Virgo experiment, which aims to detect gravitational waves from Earth. Simone Sanna earned his Physics degree from the University of Cagliari, Italy, in 2003 and completed his doctorate in Theoretical Physics jointly at the University of Bremen and the University of Paderborn in 2007. In 2008, he joined the University of Paderborn, where he served as a Postdoctoral Researcher (2008–2014) and later as a Junior Professor (2015–2017). Since 2017, he has been a Professor of Theoretical Physics at Justus-Liebig-University Giessen, leading the Solid State Spectroscopy group. His research focuses on ab initio theory of ferroelectric materials and defect structures in polar oxides. Yuriy Suhak studied physics at Lviv Polytechnic National University, Ukraine, where he earned his M.Sc. in 2006 and his PhD in Solid State Physics in 2010. In 2014, he joined the High-Temperature Sensors group at Clausthal University of Technology in Goslar, Germany, as a Postdoctoral Researcher. His research focuses on the study and characterization of piezoelectric materials for high-temperature sensing applications, with an emphasis on transport phenomena, as well as their electromechanical and optical properties.