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Reversible upconversion luminescence (RUCL) through defect engineering holds great promise for various applications, including photoswitches, information recording, and storage devices. However, the reversible photoluminescence in inorganic crystals is rarely reported. Crucially, the underlying mechanism of reversible luminescence remains unclear, which challenges the design of high‐performance reversible luminescence inorganic materials. Herein, an anomalous RUCL in a class of β‐Ba2ScAlO5:Yb3+/RE3+ (RE = Er, Ho, Tm, Tb, Sm, or Eu) phosphors is reported. By alternating annealing environments (air or hydrogen), a robust and repeatable cycle of luminescence and quenching is observed. The first‐principles calculations reveal that porous β‐Ba2ScAlO5:Yb3+/RE3+ can host oxygen interstitials (during annealing in air) with a low formation energy. The interstitial oxygen state, close to the Yb 4f state, leads to luminescence quenching. Removal of oxygen interstitials (during annealing in hydrogen) can restore the luminescence by initiating radiative energy transfer. Experimentally, the existence of interstitial oxygen is confirmed with neutron diffraction technology. It is worth noting that the strong chemical bonds in crystals make adding and removing anion vacancies challenging, unlike the more "free" interstitials. The findings offer new insights into manipulating upconversion luminescence in inorganic crystals by defect engineering and provide motivation for future research in this field. [ABSTRACT FROM AUTHOR]

Solid-state refrigeration based on the caloric effect is viewed as a promising efficient and clean refrigeration technology. Barocaloric materials were developed rapidly but have since encountered a general obstacle: the prominent caloric effect cannot be utilized reversibly under moderate pressure. Here, we report a mechanism of an emergent large, reversible barocaloric effect (BCE) under low pressure in the hybrid organic–inorganic layered perovskite (CH3–(CH2)n−1–NH3)2MnCl4 (n = 9,10), which show the reversible barocaloric entropy change as high as ΔSr ∼ 218, 230 J kg−1 K−1 at 0.08 GPa around the transition temperature (Ts ∼ 294, 311.5 K). To reveal the mechanism, single-crystal (CH3–(CH2)n−1–NH3)2MnCl4 (n = 10) was successfully synthesized, and high-resolution single-crystal X-ray diffraction (SC-XRD) was carried out. Then, the underlying mechanism was determined by combining infrared (IR) spectroscopy and density function theory (DFT) calculations. The colossal reversible BCE and the very small hysteresis of 2.6 K (0.1 K/min) and 4.0 K (1 K/min) are closely related to the specific hybrid organic–inorganic structure and single-crystal nature. The drastic transformation of organic chains confined to the metallic frame from ordered rigidity to disordered flexibility is responsible for the large phase-transition entropy comparable to the melting entropy of organic chains. This study provides new insights into the design of novel barocaloric materials by utilizing the advantages of specific organic–inorganic hybrid characteristics.Solid-state coolants: Hybrid crystals deliver big chills at low pressures Materials that can absorb and release heat under low mechanical pressure hold promise for high-efficiency refrigeration technology. Recent studies have shown that significant compression-induced cooling effects at low pressures can be achieved using crystals known as perovskites containing layers of organic chains and metal cations. Yihong Gao from the Chinese Academy of Sciences in Beijing and colleagues have now uncovered the mechanism underlying the thermal response of layered perovskites. Using a combination of x-rays, spectroscopy and theoretical calculations, the team discovered how the crystal structure changes at different temperatures. Their experiments revealed a low-energy phase transition where organic chains transform from rigid states to highly flexible conformations held in place by metallic layers. The large entropy change associated with this transition and its reversible nature could aid in the design of other organic–inorganic solid-state coolants.For the emergent colossal, reversible barocaloric effect in organic–inorganic perovskite hybrids (CH3–(CH2)n−1–NH3)2MnCl4 (n = 9, 10), we successfully grew a single crystal, and the underlying mechanism was determined by high-resolution SC-XRD, IR spectroscopy and DFT calculations. The drastic transformation of organic chains confined to the metallic frame from ordered rigidity to disordered flexibility is responsible for the large phase-transition entropy, which is comparable to the melting entropy of organic chains. The result provides new insights into designing novel barocaloric materials by utilizing the disordering of organic chains of organic–inorganic hybrid materials. [ABSTRACT FROM AUTHOR]

Donor-doped melilite materials with interstitial oxygen defects in the structure are good oxide ion conductors with negligible electronic conduction and show great potential in the ceramic electrolyte of intermediate-temperature solid oxide fuel cells (IT-SOFC). However, the parent melilite-structured materials with stoichiometric oxygen are usually insulators. Herein, we reported high and pure oxide ion conduction in the parent K 2 ZnV 2 O 7 material with a melilite-related structure, e.g., ∼1.14 × 10 -3 S/cm at 600 °C, which is comparable to that of the state-of-the-art yttrial-stabilized ZrO 2 applied in practical fuel cells. Neutron diffraction data revealed the interesting thermally induced formation of oxygen vacancies at elevated temperatures, which triggered the transformation of the material from electronically conducting to purely and highly oxide ion-conducting. The VO 4 tetrahedron with non-bridging terminal oxygen in K 2 ZnV 2 O 7 was proved to be the key structural factor for transporting oxygen vacancies. The molecular dynamic simulation based on the interatomic potential approach revealed that long-range oxide ion diffusion was achieved by breaking and re-forming the 5-fold MO 4 (M = Zn and V) tetrahedral rings. These findings enriched our knowledge of melilite and melilite-related materials, and creating oxygen vacancies in a melilite-related material may be a new strategy for developing novel oxide ion conductors., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Modifying the crystal structure and corresponding functional properties of complex oxides by regulating their oxygen content has promising applications in energy conversion and chemical looping, where controlling oxygen migration plays an important role. Therefore, finding an efficacious and feasible method to facilitate oxygen migration has become a critical requirement for practical applications. Here, we report a compressive-strain-facilitated oxygen migration with reversible topotactic phase transformation (RTPT) in La 0.5 Sr 0.5 CoO x films based on all-solid-state electrolyte gating modulation. With the lattice strain changing from tensile to compressive strain, significant reductions in modulation duration (∼72%) and threshold voltage (∼70%) for the RTPT were observed, indicating great promotion of RTPT by compressive strain. Density functional theory calculations verify that such compressive-strain-facilitated efficient RTPT comes from significant reduction of the oxygen migration barrier in compressive-strained films. Further, ac-STEM, EELS, and sXAS investigations reveal that varying strain from tensile to compressive enhances the Co 3d band filling, thereby suppressing the Co-O hybrid bond in oxygen vacancy channels, elucidating the micro-origin of such compressive-strain-facilitated oxygen migration. Our work suggests that controlling electronic orbital occupation of Co ions in oxygen vacancy channels may help facilitate oxygen migration, providing valuable insights and practical guidance for achieving highly efficient oxygen-migration-related chemical looping and energy conversion with complex oxides.