Xiaojun Kuang

Non-Centrosymmetric RbNaMgP2O7 with Unprecedented Thermo-Induced Enhancement of Second Harmonic Generation

It is of great difficulty to obtain deep-UV transparent materials with enhanced second harmonic generation (SHG), mainly limited by the theoretically poor transparency of these materials in the deep-UV spectral region. Here we report a new noncentrosymmetric, deep-UV transparent phosphate RbNaMgP2O7, which undergoes a thermo-induced reversible phase transition (at a high temperature of 723 K) and correspondingly an evident SHG enhancement up to ∼1.5 times. The phase transition is aroused by the twist of [P2O7]4- dimers with deviation from the P-O-P equilibrium positions. Theoretical analyses reveal that the enhanced SHG can be ascribed to the thermo-induced collective alignment of SHG-active [P2O7]4- dimers along the polar axis of high-temperature phase. This work provides an unprecedented physical routine (to SHG-enhanced materials) that is distinguished from the traditional one by chemical design and synthesis.

Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

Tetrahedral units can transport oxide anions via interstitial or vacancy defects owing to their great deformation and rotation flexibility. Compared with interstitial defects, vacancy-mediated oxide-ion conduction in tetrahedra-based structures is more difficult and occurs rarely. The isolated tetrahedral anion Scheelite structure has showed the advantage of conducting oxygen interstitials but oxygen vacancies can hardly be introduced into Scheelite to promote the oxide ion migration. Here we demonstrate that oxygen vacancies can be stabilized in the BiVO4 Scheelite structure through Sr2+ for Bi3+ substitution, leading to corner-sharing V2O7 tetrahedral dimers, and migrate via a cooperative mechanism involving V2O7-dimer breaking and reforming assisted by synergic rotation and deformation of neighboring VO4 tetrahedra. This finding reveals the ability of Scheelite structure to transport oxide ion through vacancies or interstitials, emphasizing the possibility to develop oxide-ion conductors with parallel vacancy and interstitial doping strategies within the same tetrahedra-based structure type.Fast oxide ion conductors are the key materials for some technological devices. Here the authors report the creation and stabilization of oxygen vacancies in BiVO4 Scheelite with isolated tetrahedral anion structures for improved ionic conducting performance and understanding of the conduction mechanism.

8H–10H Stacking Periodicity Control in Twinned Hexagonal Perovskite Dielectrics

Isovalent substitution of Zr4+ for smaller Ti4+ was performed in the 8-layer twinned hexagonal perovskite (referred to as 8H) tantalate Ba8Ti3Ta4O24, which stabilizes a 10-layer twinned hexagonal perovskite (referred to as 10H). The formation of the 10H phase occurs at low substitution concentration ( x = 0.1) in Ba8Zr xTi3- xTa4O24 at 1300 °C and reverts back to the 8H phase upon heating at elevated temperatures. Such a 10H-to-8H phase transformation is suppressed at higher Zr-substitution contents ( x > 0.1). The approach combining simulated annealing and Rietveld refinement with compositional constrain indicates that the 10H Ba8Zr0.4Ti2.6Ta4O24 ( x = 0.4) composition adopts a simply P63/ mmc disordered structure with Zr cations preferably located in corner-sharing octahedral (CSO) sites compared to face-sharing octahedral (FSO) sites. This 8H-10H phase competition, dependent on the substitution of Zr4+ for Ti4+ and firing temperature, is discussed in terms of the FSO B-B repulsion controlled by the cationic size, as well as the stacking periodicity which affects the thermodynamic stability. Both 8H- and 10H-phase pellets of Ba8Zr xTi3- xTa4O24 exhibit comparable and poorer microwave dielectric properties than the parent 8H Ba8Ti3Ta4O24, which is characterized by cationic disorder and FSO B-B repulsion. The 8H and 10H Ba8Zr xTi3- xTa4O24 ceramics display electrical insulator behavior but with electrically heterogeneous microstructure on the bulk grains. This study demonstrates the opportunity to control the stacking periodicity for the twinned hexagonal perovskites via tuning the B-cationic size and the firing temperature.