Xiaobin Fu

Incommensurate modulation has been extensively investigated in inorganic oxides, representing an emerging frontier in aperiodic materials that enables the discovery of novel quantum phenomena and the manipulation of physical properties, whereas it remains largely unexplored in hybrid perovskites (HPs). Here, we present the driving of successive dynamic rotation of the interlayer dipoles in a two-dimensional HP ferroelectric (DFBZ)2(FA)Pb2I7 (DFBZ is 2,5-difluorobenzylammonium, FA is formamidinium), which enables an unusual incommensurate modulated structure between the paraelectric and ferroelectric phases. Specifically, (DFBZ)2(FA)Pb2I7 exhibits an incommensurate modulated state over a broad temperature range of 313–370 K, and transforms to commensurate modulation at 400–474 K. The incommensurate superstructure at 330 K is ∼6 times larger than its periodic prototype, and its structure can be solved using a (3 + 1)D superspace group with a modulation wavevector of q = (1, 0, 0.1578). The inorganic layers feature a pronounced undulation, while the interlayer organic cations are trapped in distinct rotational states. To the best of our knowledge, (DFBZ)2(FA)Pb2I7 is the first HP ferroelectric that exhibits such a long-range ordered incommensurate modulation. Moreover, its large spontaneous polarization (9.4 μC cm–2) and the near-room temperature ferroelectric-to-incommensurate phase transition result in a giant pyroelectric coefficient of 7.41 μC cm–2 K–1. This unusual incommensurate modulation in HP ferroelectrics offers new insights into the understanding of aperiodic materials and ferroelectric mechanisms.

Molecular antiferroelectrics (AFEs) with antiparallel dipole alignment are promising for energy‐storage capacitor applications. However, it is challenging to design new molecular AFEs with superior breakdown resistance, owing to the lack of knowledge on the atomic‐level origin regarding AFE orders. Here, we present stable breakdown resistance in 2D perovskite AFE, (2‑MBA) ₂ CsPb ₂ Br ₇ (2‑MBA = 2‑methylbutylammonium), involved with the confinement‐dependent atomic displacement. It shows antiferroelectricity with a large spontaneous polarization of 5.0 µC/cm ² . Particularly, the cage‑confined Cs ⁺ cations display atomic displacement to create stable antifatigue merits, including high breakdown field up to 175 kV/cm and the fatigue endurance beyond ∼10 ⁶ cycles, falling in the range of the highest level for molecular AFEs. Combination of energy barrier calculation and in situ solid‐state NMR spectroscopy was used to reveal the crucial role of displacive dynamics. Contrary to order‐disordering dynamics, it is the high energy barrier ( E _a = 2.91 eV) of cage‐confined Cs ⁺ cation displacement that leads to the increase in Curie temperature ( ∼ 327 K) and forward coercive field (∼56.8 kV/cm). Such attributes allow for AFE switching under stronger external stimuli, thus endowing stable fatigue resistance even at higher breakdown fields. This work provides a feasible principle of delicately manipulating cage‐confined dynamics to design new electric‐ordered candidates.

Molecular antiferroelectrics (AFEs) with antiparallel dipole alignment are promising for energy‐storage capacitor applications. However, it is challenging to design new molecular AFEs with superior breakdown resistance, owing to the lack of knowledge on the atomic‐level origin regarding AFE orders. Here, we present stable breakdown resistance in 2D perovskite AFE, (2‑MBA) ₂ CsPb ₂ Br ₇ (2‑MBA = 2‑methylbutylammonium), involved with the confinement‐dependent atomic displacement. It shows antiferroelectricity with a large spontaneous polarization of 5.0 µC/cm ² . Particularly, the cage‑confined Cs ⁺ cations display atomic displacement to create stable antifatigue merits, including high breakdown field up to 175 kV/cm and the fatigue endurance beyond ∼10 ⁶ cycles, falling in the range of the highest level for molecular AFEs. Combination of energy barrier calculation and in situ solid‐state NMR spectroscopy was used to reveal the crucial role of displacive dynamics. Contrary to order‐disordering dynamics, it is the high energy barrier ( E _a = 2.91 eV) of cage‐confined Cs ⁺ cation displacement that leads to the increase in Curie temperature ( ∼ 327 K) and forward coercive field (∼56.8 kV/cm). Such attributes allow for AFE switching under stronger external stimuli, thus endowing stable fatigue resistance even at higher breakdown fields. This work provides a feasible principle of delicately manipulating cage‐confined dynamics to design new electric‐ordered candidates.

Rashba-Dresselhaus (RD) spin splitting provides a crucial physical basis for realizing various advanced spintronic applications. Nevertheless, rationally regulating the RD splitting coefficient (α_RD) continues to be a difficult task owing to the still unclear structure–property relationship. In this work, we propose a molecular dipole engineering, the core of which involves introducing halogen atoms with different electronegativities into organic cations, aiming to design and synthesize two-dimensional (2D) ferroelectric semiconductors with strong RD spin splitting. By substituting the organic spacer cations in the parent compound (PMA)₂PbCl₄ (PMA = benzylammonium), we obtained two new 2D ferroelectric semiconductors, namely, (2F4ClPMA)₂PbCl₄ (2F4ClPMA = 2-fluoro-4-chlorobenzylammonium) and (2F4BrPMA)₂PbCl₄ (2F4BrPMA = 2-fluoro-4-bromobenzylammonium). Both variants exhibit significantly enhanced RD splitting coefficients. In particular, (2F4ClPMA)₂PbCl₄ possesses a large α_RD value of 1.878 eV·Å and a persistent spin texture region, which helps amplify its circular photogalvanic effect (CPGE). Through the spin-selective optical transition rule, the effect can enable the differentiation of carriers excited by circularly polarized light (CPL) in momentum space. A photodetector fabricated based on a (2F4ClPMA)₂PbCl₄ single crystal shows a high asymmetry factor of 0.53 under excitation by UV CPL at 325 nm. This work not only confirms the effectiveness of the molecular dipole engineering in tuning RD spin splitting, but also lays an important material foundation for the development of novel low-power, high-sensitivity spin-optoelectronic devices.

Heavy‐metal fluoride (HMF) glasses exhibit a combination of broad infrared transparency, low phonon energy, and potential fluoride‐ion conductivity, rendering them promising candidates for optical and electrochemical applications. However, the atomic‐scale environment of La ³⁺ ions, which governs these properties, remains inadequately characterized. In this work, we systematically examine NaF–LaF ₃ and LiF–LaF ₃ binary mixtures as model systems using a suite of solid‐state nuclear magnetic resonance (NMR) techniques. Our findings reveal markedly distinct behaviors of the alkali cations. NaF readily reacts with LaF ₃ to form a crystalline NaLaF ₄ phase, as unambiguously confirmed by ¹⁹ F and ²³ Na NMR, along with 2D ¹⁹ F– ²³ Na HETCOR and CP/MAS experiments. In contrast, LiF exhibits no evidence of forming Li–La–F coordination structures, instead persisting as a phase‐separated LiF/ LaF ₃ composite. This divergence is attributed to the stronger Li–F bonding and the limited coordination flexibility of Li ⁺ , which hinders disruption of the LaF ₃ lattice. These mechanistic insights highlight the critical influence of alkali cation identity on the structural evolution in mixed fluoride systems and offer valuable design principles for ZBLAN and related HMF glasses.

The physicochemical properties of molten fluorides stem from F ⁻ coordination (defined as fluoroacidity). This review links microscopic structure to macroscopic behavior, establishing a multi-scale framework for nuclear and metallurgical applications.

Salt-concentration effects on the structural dynamics of aqueous beryllium fluoride (BeF₂) must be elucidated to advance the separation and dehydration processes. This study systematically investigates BeF₂-solution concentration-dependent structures and dynamics using nuclear magnetic resonance and ultrafast spectroscopy. Be²⁺ primarily exists as the tetrahedral complex [BeF₂(H₂O)₂] in a BeF₂ solution, with small [BeF(H₂O)₃]⁺ and [BeF₃(H₂O)]^- complex contents. At BeF₂ concentrations exceeding 8.3 mol %, an extended Be-F network becomes prominent. Fourier transform infrared spectroscopy with a thiocyanate (SCN^-) vibrational probe reveals two SCN^- peaks: a bulk-water-associated peak and a Be²⁺-coordinated peak from the SCN^- substitution in [BeF_x(H₂O)₄-_x] complexes. The Be²⁺ peak full width at half-maximum exhibits nonlinear broadening at concentrations exceeding 8.3 mol %, consistent with network formation. Polarization-selective pump-probe measurements demonstrate that increased salt concentration accelerates vibrational energy relaxation and suppresses rotational diffusion. The rotational diffusion time is related to the macroscopic viscosity via the Stokes-Einstein-Debye model. The Be-F complexes cause non-Newtonian behavior of the BeF₂-solution viscosity. At concentrations exceeding 8.3 mol %, the formed network structures exponentially increase the viscosity, negating the classical Jones-Dole model. These macroscopic viscosity changes can be explained microstructurally. A coherent molecular mechanism linking the microscopic coordination structure to macroscopic transport properties is established.