Order-disorder transition of a rigid cage cation embedded in a cubic perovskite

Atomic-Scale Observation of Grain Boundary Dominated Unsynchronized Phase Transition in Polycrystalline Cu2 Se

Phase transition is a physical phenomenon that attracts great interest of researchers. Although the theory of second-order phase transitions is well-established, their atomic-scale dynamics in polycrystalline materials remains elusive. In this work, second-order phase transitions in polycrystalline Cu₂ Se at the transition temperature are directly observed by in situ aberration-corrected transmission electron microscopy. Phase transitions in microcrystalline Cu₂ Se start at the grain boundaries and extend inside the grains. This phenomenon is more pronounced in nanosized grains. Analysis of phase transitions in nanocrystalline Cu₂ Se with different grain boundaries demonstrates that grain boundary energy dominates unsynchronized phase transition behavior. This suggests that the energy of grain boundaries is the key factor influencing the energetic barrier for initiation of phase transition. The findings advance atomic-scale understanding of second-order phase transitions, which is crucial for the control of this process in polycrystalline materials.

Sn1/3Na2/3Sn(OH)6 Perovskite with Sn4+/Na+ Disorder for Photocatalytic Degradation of 2,4-Dichlorophenol

Cation disorder in hydroxide-based perovskites remains relatively under-researched. In this work, novel hydroxide-based perovskite Sn_1/3Na_2/3Sn(OH)₆ was first fabricated by a direct hydrothermal method, and its ability to photodegrade 2,4-dichlorophenol was evaluated. The synthesized photocatalyst is isostructural with MSn(OH)₆ (M = Mg, Ca, Sr, Mn, Fe, Co, Ni, or Zn), where the M site is occupied by disordered Sn⁴⁺/Na⁺. Sn_1/3Na_2/3Sn(OH)₆ exhibits outstanding photocatalytic activity under ultraviolet light. Specifically, 99% of 2,4-DCP is photodegraded in 40 min, with approximately 94% of its total chlorine content converted to Cl^- anions. Radical trapping experiments indicated that superoxide radical anions (·O₂^-) play a critical role during the photocatalytic process. Finally, liquid chromatography-tandem mass spectrometry was conducted to monitor the photocatalytic intermediates. Overall, our findings demonstrate that hydroxide-based perovskites with cation disorder show promise for application in photocatalysis.

A series of dynamic single crystals of [MII(en)3]SO4 (M = Ni, Mn, and Cd) shows tunable dielectric properties and anisotropic thermal expansion

A series of dynamic single crystals with a chemical formula of [M^II(en)₃]SO₄ (en = ethylene and M^II = Ni^II, Mn^II, and Cd^II) was synthesized. As the temperature decreases, these materials exhibit dielectric switching in the vicinity of the phase transition point accompanied by anisotropic thermal expansion in the cell parameters as a consequence of the order-disorder structural change of SO2-4 in a cavity surrounded by five [M^II(en)₃]²⁺ complex cations. Because the variation of metal centers with different ionic radii changes the shape of the complex cation, which affects the distribution of hydrogen-bond interactions around the SO2-4, the dynamic motion of SO2-4 is substantially tuned. Correspondingly, the dielectric properties and anisotropic thermal expansion of materials were largely shifted, especially in the single crystals of [Mn^II(en)₃]SO₄, whose structural change is distinctly different from the crystals of Ni(II) and Cd(II). The detailed structural mechanism accounting for the different physical properties of these materials was discussed.