Enhancing the stability of piezoelectric properties is essential for ensuring the reliability of high-temperature piezoelectric sensors. In this study, we have synthesized AlN piezoelectric crystals as representative materials and employed first-principles methods to investigate their temperature-dependent piezoelectric properties. By integrating the effects of lattice expansion and electron–phonon interactions, we accurately constructed the crystal structure of AlN across a wide temperature range and successfully predicted its piezoelectric behavior. Theoretical analysis reveals that ion polarization driven by lattice distortion and elastic softening of chemical bonds maintains the overall structural integrity of defect-free AlN single crystals, resulting in a stable piezoelectric coefficient d₃₃ with a deviation of only 8.55% at temperatures up to 1300 K. However, experimental results indicate that the stability of the piezoelectric performance of the grown AlN crystals is disrupted at temperatures above 870 K. This temperature limitation is attributed to point defects within AlN crystals, particularly those caused by oxygen-substituted nitrogen (O_N). These findings provide valuable guidance for enhancing the piezoelectric temperature stability of AlN crystals through optimized experimental conditions, such as oxygen atmosphere treatment and defect modification during crystal growth.

The Zintl compound EuMg₂Sb₂ is a promising thermoelectric material due to its inherently low lattice thermal conductivity and tunable electronic and thermal properties related to its multi-component nature. However, the large difference in electronegativity between Mg and Sb results in poor electronic transport properties, reducing its thermoelectric conversion efficiency and limiting its practical application. Thus, this study investigates a doping modification strategy for enhancing the thermoelectric performance of EuMg₂Sb₂ and the microscopic mechanism using the first-principle calculations combined with the Boltzmann transport theory. Indeed, the larger energy separation at the valence band maximum is the key factor affecting the electronic transport properties of EuMg₂Sb₂. The results demonstrate that Zn doping at the Mg site effectively increases the thermoelectric performance by promoting the valence band convergence owing to the close electronegativity to Sb and softening the phonon thus largely suppressing the lattice thermal conductivity. By optimizing the Zn doping concentration, the highest figure of merit (zT) value is significantly increased to 2.24 (2.66) in the x (z) direction at 800 K. The results suggest that the proposed modulation strategy and effect are of great significance for improving the thermoelectric performance of Zintl materials.

The exploration of two-dimensional antiferrovalley materials as potential candidates for valleytronics offers intriguing prospects to investigate exotic valley physics and develop next-generation nano-electronic devices. Achieving efficient anomalous valley Hall effect (AVHE) switching in antiferrovalley materials constitutes an important step towards their application, yet such advancement has been scarcely reported so far. In this study, we demonstrate, through first-principles calculations and model analysis, that the experimentally synthesized MnSe monolayer is a hitherto unexplored but exceptional antiferrovalley material with spontaneous valley polarization. And more importantly, by constructing a multiferroic MnSe/In₂Se₃ heterostructure, the desired nonvolatile on/off switching of the AVHE can be successfully realized through polarization reversal. This unique phenomenon, characterized by the emergence/annihilation of fully spin-polarized valley polarization, arises from the combined effect of strong magnetoelectric coupling-induced changes in magnetic anisotropy and PT symmetry breaking. Our findings provide a novel approach for achieving nonvolatile control of the AVHE in antiferrovalley materials, opening up significant opportunities for valleytronic applications.

Due to ever-increasing concerns about safety issues in using Li ionic batteries, solid electrolytes have extensively explored. The Li-rich anti-perovskite Li₃OBr has been considered as a promising solid electrolyte candidate, but it still suffers challenges to achieve a high ionic conductivity owing to the high intrinsic symmetry of the crystal lattice. Herein, we presented a design strategy that introduces various point defects and grain boundaries to break the high lattice symmetry of Li₃OBr crystal, and their effect and microscopic mechanism of promoting the migration of Li-ion were explored theoretically. It has been found that Li_i^· are the dominant defects responsible for the fast Li-ion diffusion in bulk Li₃OBr and its surface, but they are easily trapped by the grain boundaries, leading to the annihilating of the Frenkel defect pair V'_Li + Li_i^· and thus limits the V'_Li diffusion at the grain boundaries. The V_Br^· defect near the grain boundaries can effectively drive V'_Li across the grain boundary, thereby converting the carrier of Li⁺ migration from Li_i^· in the bulk and surface to V'_Li at the grain boundary, and thus improving the ionic conductivity in the whole Li₃OBr crystal. This work provides a comprehensive insight into the Li⁺ transport and conduction mechanism in the Li₃OBr electrolyte. It opens a new way of improving the conductivity for all-solid-state Li electrolyte material through the defect design.

The yttrium calcium oxyborate crystal (YCa₄O(BO₃)₃, YCOB) has been actively studied for high-temperature piezoelectric sensing applications. In this work, the stability of electric properties of YCOB crystal annealed in critical conditions (high-temperatures of 900–1100 ℃ with a low oxygen partial pressure of 4 × 10⁻⁶ atm for 24 h) was investigated and the recovery mechanism for the electrical resisitivity, dielectric permittivity and dielectric loss were studied, taking advantage of the X-ray photoelectron spectra and the first principle calculations. The electrical resistivity of the annealed YCOB crystal was slightly decreased when compared to the pristine counterpart, being (2–5) × 10⁷ Ω·cm at 850 ℃. The dielectric permittivity and dielectric loss were found to increase after annealing, showing recoverable behaviours after thermal treatment above 650 ℃ in air. The calculated vacancy formation energy indicates that the oxygen vacancy is the dominant defects in YCOB. The formation of oxygen vacancy weakens the chemical bonding strength between B (Ca or Y) and O atoms, introduces extra donor levels in the band gap, which excites the electrons to conduction band more easily thus enhances the electrical conductivity and dielectric loss. The recovered electrical properties are believed to be associated with the reduced vacancy defects at elevated temperatures in air.