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In recent years, lead-free Bi 0.5 Na 0.5 TiO 3 (BNT) based piezoelectric ceramics with excellent strain properties have gained widespread recognition for their application in high-precision displacement actuators. The (1- x)(0.82Bi 0.5 Na 0.5 TiO 3 -0.18K 0.5 Bi 0.5 TiO 3)- x Ba(Fe 0.5 Sb 0.5)O 3 ceramics were fabricated through the die-pressing method in this work. The effect of BFS introduction on the dielectric, ferroelectric and microstructure characteristics of BNKT ceramics was systematically investigated. Among these compositions, the BNKT-0.015BFS ceramics exhibited a piezoelectric strain coefficient (d 33 *) of 658 pm/V at an electric field of 55 kV/cm, while achieving a strain response of 0.51 % at 85 kV/cm. The observed improvement in strain performance can be attributed to the evolution from non-ergodic relaxor state to ergodic relaxor state caused by BFS substitution. Furthermore, transmission electron microscopy (TEM) explicitly confirmed coexistence of rhombohedral and tetragonal phase and revealed the morphology of nanosized domains. This study adds to a deeper comprehension of the characteristics of strain response enhancement in BNT-based ceramics, opening up avenues for designing novel lead-free piezoelectric ceramics with outstanding electromechanical performance. [ABSTRACT FROM AUTHOR]

The lead-free piezoelectric ceramics used in piezoelectric actuators have attracted extensive attention owing to their substantial electric-field-induced strain. In this study, The (1- x)Bi 0.5 (Na 0.41 K 0.09)TiO 3 - x Sr(Hf 0.5 Sb 0.4)O 3 (BNKT- x SHS) piezoelectric ceramics were synthesized using the conventional solid-state reaction method. The impact of SHS substitution on the microstructure and electrical properties was systematically explored. Among them, the BNKT-0.015SHS sample, featuring ergodic relaxor states, demonstrates a high piezoelectric strain coefficient (d 33 *) of 785 p.m./V under a low electric field of 50 kV/cm(Strain = 0.392 %). It exhibits a repeatable strain value of 0.47 % under an 80 kV/cm electric field(d 33 * = 590 p.m./V). Moreover, the strain values remain around 0.4 % between room temperature and 100 °C, with variations within 90 %. The heightened repeatable strain response is considered to be linked to the electric field-induced reversible phase transition occurring within the ergodic relaxor state. The higher strain temperature stability is ascribed to the coexistence of quadrilateral polar nanoregions and rhombic polydomain nanoregions. This study provides an effective example for achieving significant strain responses, presenting a feasible candidate material for applications in piezoelectric actuators. [ABSTRACT FROM AUTHOR]

This article reports a novel lead‐free phase‐transition material (C8H10ClN)2SnCl6 (1), which undergoes a high‐temperature phase change at close to 377 K. The multi temperature powder X‐ray diffraction (PXRD) also confirmed the phase transition occurred in compound 1. Significantly, compound 1 produces light emission at 452 nm. Variable temperature fluorescence studies indicate that the spectral emission maximum of compound 1 remains substantially fixed at 452 nm with increasing temperatures, implying the presence of a certain degree of photoluminescence stability in compound 1. Furthermore, compound 1 is characterized by exceptional semiconductor attributes, possessing a band gap of 3.76 eV. This pioneering study presents an innovative option for optimizing the growth and practical implementation of Sn (IV)‐based organic‐inorganic hybrid perovskite phase‐transition materials, and offers a strategic avenue to create novel lead‐free, eco‐friendly materials. [ABSTRACT FROM AUTHOR]

High‐capacity O3‐type layered sodium oxides are considered one of the most promising cathode materials for the next generation of Na‐ion batteries (NIBs). However, these cathodes usually suffer from low high‐rate capacity and poor cycling stability due to structure deformation, native air sensitivity, and interfacial side reactions. Herein, a multi‐site substituted strategy is employed to enhance the stability of O3‐type NaNi0.5Mn0.5O2. Simulations indicate that the Ti substitution decreases the charge density of Ni ions and improves the antioxidative capability of the material. In addition, the synergistic effect of K+ and Ti4+ significantly reduces the formation energy of Na+ vacancy and delivers an ultra‐low lattice strain during the repeated Na+ extraction/insertion. In situ characterizations verify that the complicated phase transformation is mitigated during the charge/discharge process, resulting in greatly improved structure stability. The co‐substituted cathode delivers a high‐rate capacity of 97 mAh g−1 at 5 C and excellent capacity retention of 81% after 400 cycles at 0.5 C. The full cell paired with commercial hard carbon anode also exhibits high capacity and long cycling life. This dual‐ion substitution strategy will provide a universal approach for the new rational design of high‐capacity cathode materials for NIBs. [ABSTRACT FROM AUTHOR]

Abstract High‐capacity O3‐type layered sodium oxides are considered one of the most promising cathode materials for the next generation of Na‐ion batteries (NIBs). However, these cathodes usually suffer from low high‐rate capacity and poor cycling stability due to structure deformation, native air sensitivity, and interfacial side reactions. Herein, a multi‐site substituted strategy is employed to enhance the stability of O3‐type NaNi0.5Mn0.5O2. Simulations indicate that the Ti substitution decreases the charge density of Ni ions and improves the antioxidative capability of the material. In addition, the synergistic effect of K+ and Ti4+ significantly reduces the formation energy of Na+ vacancy and delivers an ultra‐low lattice strain during the repeated Na+ extraction/insertion. In situ characterizations verify that the complicated phase transformation is mitigated during the charge/discharge process, resulting in greatly improved structure stability. The co‐substituted cathode delivers a high‐rate capacity of 97 mAh g−1 at 5 C and excellent capacity retention of 81% after 400 cycles at 0.5 C. The full cell paired with commercial hard carbon anode also exhibits high capacity and long cycling life. This dual‐ion substitution strategy will provide a universal approach for the new rational design of high‐capacity cathode materials for NIBs.

Two-dimensional (2D) materials with reversible phase transformation are appealing for their rich physics and potential applications in information storage. However, up to now, reversible phase transitions in 2D materials that can be driven by facile nondestructive methods, such as temperature, are still rare. Here, we introduce ultrathin Cu9S5 crystals grown by chemical vapor deposition (CVD) as an exemplary case. For the first time, their basic electrical properties were investigated based on Hall measurements, showing a record high hole carrier density of ∼ 1022 cm−3 among 2D semiconductors. Besides, an unusual and repeatable conductivity switching behavior at ∼ 250 K were readily observed in a wide thickness range of CVD-grown Cu9S5 (down to 2 unit-cells). Confirmed by in-situ selected area electron diffraction, this unusual behavior can be ascribed to the reversible structural phase transition between the room-temperature hexagonal β phase and low-temperature β′ phase with a superstructure. Our work provides new insights to understand the physical properties of ultrathin Cu9S5 crystals, and brings new blood to the 2D materials family with reversible phase transitions. [ABSTRACT FROM AUTHOR]

Lead-free bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT) and related solid solutions are potential piezoelectric materials for such applications as actuators and transducers if their excellent strain responses and piezoelectric properties can be optimized. In this work, a large strain response of 0.61% is achieved in lead-free (0.94-x%)(Bi0.5Na0.5)TiO3-0.06BaTiO3-x%NaNbO3 (x = 0 –6, BNT-6BT-xNN) ceramics with the composition of x = 3.5 in a pseudo-cubic structure. Coexistence of ferroelectric (FE) and relaxor (RE) domain structures is observed in all the unpoled ceramics and the enhanced strain response is believed to be related to the evolution of the ergodic relaxor (ER) and non-ergodic (NR) states thanks to the substitution of antiferroelectric NN. BNT-6BT-3.5NN is a critical composition near the FE/NR/ER phase boundary close to room temperature (RT) and its high strain response arises from a synergistic combination of a reversible electric-field-induced phase transition and an active domain switching in the mixed NR/ER state. This work provides new insights into the dynamic interplay between mesoscopic domains and macroscopic electrical properties in the BNT-based piezoceramics.

• The co-doping of zirconium and titanium in lithium cobalt oxide significantly enhances the diffusion kinetics of lithium ions. • The homogeneous distribution of Zr/Ti effectively suppresses structure degradation. • Synergistic interaction of Zr and Ti stabilizes lattice oxygen. Lithium cobalt oxide (LCO) cathode materials could exceed 210 mAh g−1 with excellent volumetric energy density, when operated at voltages above 4.5 V. However, the progress in the advancement of high-voltage LCO cathodes is impeded by unstable lattice oxygen loss and rapid structural degradation. Herein, we propose a modification strategy to modulate the band structure and stabilize the crystal structure of LCO through Zr/Ti dual-doping. The modified Zr/Ti-LCO demonstrates stable cycling performance at elevated cut-off voltage of 4.6 V. Uniform co-doping with zirconium and titanium suppresses the severe H1-3/O1 deleterious phase transition of LCO. By fine-tuning TM-O interactions and modulated band structure mitigate oxygen release reactivity at high voltage, achieving a delicate balance between structural integrity and electrochemical stability. Consequently, the modified Zr/Ti-LCO half-cell achieves superb capacity retention of 87.7 % after 300 cycles at 3.0–4.6 V, while the Zr/Ti-LCO||graphite pouch full-cell retains up to 85.8 % capacity after 500 cycles at 3.0–4.5 V. This study offers critical perspectives for advancing the design of high-voltage LCO cathodes. [ABSTRACT FROM AUTHOR]

The use of pesticides to protect crops has long been an important measure to provide healthy and safe agricultural products, but excess pesticides flow into fields and rivers, causing environmental pollution. Earlier methods utilizing organic solvent liquid-liquid microextraction for pesticide residue detection were not environmentally friendly. Therefore, it is significant to find a greener and more convenient detection method to determine pesticide residues. A new method was established to detect three triazole fungicides (TFs), including myclobutanil, epoxiconazole and tebuconazole, in environmental water samples. And the determination was conducted using a high-performance liquid chromatography with the ultraviolet detector (HPLC-UV). The switchable deep eutectic solvent (SDES) can be reversibly switched between hydrophilic and hydrophobic states through temperature modulation. Additionally, the method exhibited excellent linearity for all target analytes within the concentration range of 10–2000 μg L−1, with satisfactory R2 values (≥0.9975). The limits of detection (LODs) ranged from 2.3 to 2.6 μg L−1, and the limits of quantification (LOQs) ranged from 7.8 to 8.7 μg L−1. The accuracy of the method was assessed through intra-day and inter-day precision tests, yielding relative standard deviations (RSDs) in the ranges of 2.8%–6.7% and 2.2%–7.5%, respectively. Density functional theory (DFT) results indicated that hydrogen bonding is a significant factor affecting the binding of DES with triazoles. Three different green assessment tools were used to prove that the SDES-HLLME method had good greenness and broad applicability. This is a homogeneous liquid-liquid microextraction (HLLME) method based on the upper critical solution temperature (UCST) type switchable deep eutectic solvent program, which can complete the extraction within a few minutes without dispersant. In terms of pesticide detection, the analytical method is simple and more conducive to environmental protection. [Display omitted] • The SDES-HLLME method was used to extract and determine triazole in water. • N-methyldiethanolamine/4-bromophenol DES exhibited excellent extraction performance. • H-bonding interaction was the main factor for separation from water. • The greenness and applicability of the method were evaluated by the AGREEprep metric, SPMS metric and BAGI metric. • The detection limit was found to be in the range of 2.3–2.6 μg L−1. [ABSTRACT FROM AUTHOR]

Crystalline vanadium oxide-titanium oxide (VO-TO) composite thin films of 197 nm are developed on quartz and silicon substrates by RF magnetron sputtering technique. A thorough structural investigation of deposited thin films is carried out by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. Differential scanning calorimetry (DSC) and temperature-based sheet resistance measurement techniques reveals the Smart (i.e., reversible) phase transition behaviour of VO-TO films at about 45 °C. Further selective depth analysis by XPS on the VO-TO films shows the variation of the Ti doping amount along the thickness. The top region of VO-TO thin film possess lower Ti species, while beyond 50 nm thickness from the surface possess comparatively rich Ti species. This graded Ti species along the depth also results in the variation of nanomechanical properties along the depth as confirmed by nanoindentation. The simulation through the finite element approach is also carried out to depict the experimental results of nanoindentation. The nanohardness and modulus of the presently developed VO-TO composite film were measured as 6.7 GPa and 158.7 GPa, respectively at 50 nm depth. Beyond 50 nm, nanohardness and modulus were 5.6 GPa and 116.6 GPa, respectively. Further, finite element simulation predicted yield stress value of 3.52 GPa at 50 nm depth whereas higher yield stress of 4.68 GPa was predicted beyond 50 nm. [ABSTRACT FROM AUTHOR]

It has been an enormous challenge to obtain double P - E loops and identify structure evolution in NaNbO 3 based ceramics. BiMg 2/3 Ta 1/3 O 3 (BMT) modified NaNbO 3 ceramics were fabricated by solid-state methods, showing slimmer double P - E loops compared with other reported work. Combined with the Raman and refined XRD results, we confirmed that the coexistence of anti-ferroelectric (AFE) P (Pbma) and ferroelectric (FE) Q (P2 1 ma) phases gradually transformed into the coexistence of AFE R (Pnma) and AFE P phases by adding BMT, and thus, the relaxor behavior and reversible phase transition can be enhanced. TEM analysis demonstrated the coexistence of the P and Q phases in pure NN, verified by the characteristic ¼ and ½ (010) type reflections in the selected area electron diffraction patterns, respectively. The characteristic antiphase boundaries are observed as well-parallel lines with a 6-fold modulation, interrupting the 4-fold P phase. After BMT modification, the high-temperature R phase was stabilized, evidenced by the 1/6 (001) type reflections. Moreover, the domain morphology changes dramatically, illustrated by the complicated network of APBs and the elongated band morphology comprising orientational nanodomains, which indicate a strong structural heterogeneity in the NN-BMT ceramics. [ABSTRACT FROM AUTHOR]

The crystal structure determination based on 90 K data of the title imine ligand, C18H10ClNO, revealed non-merohedral twinning with three twin domains. In our experience, this is an indication of an ordering phase transition. Consequently, the structure was redetermined with higher temperature data, and a reversible phase transition was discovered. The higher temperature phase is indeed an ordered structure. At the higher temperature, the 4-chloro-phenyl group has rotated by ca 7° into a crystallographic mirror plane. Warming the crystal from 90 K to 250 K changes the space group from triclinic P-1, to monoclinic P21/m. Diverse non-classical inter-actions are present in the crystal packing, and these are described for the phase change reported in this work. The crystal structure of the title imine ligand, measured at 100 K, has been reported on previously [Kovach et al. (2011 ▸). J. Mol. Struct.992, 33-38].

Bistable switches (electrical switching between "ON" and "OFF" bistable states) have gradually developed into an ideal category of highly intelligent materials, due to their significant applications in optical technology, signal processors, data storage and other switchable media applications in the field of electrical devices. Here, we successfully designed and synthesized [(FC6H4C2H4NH3)2MCl4]n(FC6H4C2H4NH3+)=deprotonated 4‐fluoro‐ phenethylamine; M=Cd (1), Mn (2)), which realized the coupling of thermo‐dielectric switching characteristics, semi‐conductor characteristics and photo‐luminescent properties. DSC (differential scanning calorimetry) and dielectric measurements show that 1 is a sensitive dielectric bistable switch between the high dielectric (ON) and low dielectric (OFF) states. The temperature‐variable single crystal structure shows that the both 1 and 2 undergo a high‐temperature reversible phase transition around 383 K/380 K, which is caused by the order‐disordered transformation of organic cations and the slight distortion of the inorganic framework. In particular, 1 shows outstanding switchable dielectric behavior and semiconducting properties. Further, 1 and 2 emit strong green and yellow luminescence at 527 and 595 nm, respectively. [ABSTRACT FROM AUTHOR]

Phase transition is common during (de)-intercalating layered sodium oxides, which directly affects the structural stability and electrochemical performance. However, the artificial control of phase transition to achieve advanced sodium-ion batteries is lacking, since the remarkably little is known about the influencing factor relative to the sliding process of transition-metal slabs upon sodium release and uptake of layered oxides. Herein, we for the first time demonstrate the manipulation of oxygen vacancy concentrations in multinary metallic oxides has a significant impact on the reversibility of phase transition, thereby determining the sodium storage performance of cathode materials. Results show that abundant oxygen vacancies intrigue the return of the already slide transition-metal slabs between O3 and P3 phase transition, in contrast to the few oxygen vacancies and resulted irreversibility. Additionally, the abundant oxygen vacancies enhance the electronic and ionic conductivity of the Na0.9Ni0.3Co0.15Mn0.05Ti0.5O2 electrode, delivering the high initial Coulombic efficiency of 97.1%, large reversible capacity of 112.7 mAh·g−1, superior rate capability upon 100 C and splendid cycling performance over 1,000 cycles. Our findings open up new horizons for artificially manipulating the structural evolution and electrochemical process of layered cathodes, and pave a way in designing advanced sodium-ion batteries. [ABSTRACT FROM AUTHOR]

Luminescent perovskite nanocrystals (NCs), possessing the advantages of low cost, easy detection, and excellent luminescence, are becoming more and more significant in the fields of information encryption and decryption. Most hydrochromic perovskite NCs for information encryption have moderate reversibility and are easily passively decrypted by water in the moist air, limiting their practical applications. Herein, a lyochromic material is synthesized based on reversible phase transition between luminescent CsPbBr 3 -HBr (pretreating CsPbBr 3 with HBr) and nonluminescent Cs 4 PbBr 6 , exhibiting excellent reversibility in 50 cycles triggered by CsBr solution. HBr treatment boosts the ion migration of NCs via diminishing surface ligands and passivating Br vacancy, assisting CsBr concentration acting as a crucial factor in dynamic ion exchange equilibrium between the trigger solution and CsPbBr 3 -HBr. By utilizing CsPbBr 3 -HBr as a safety ink, the CsBr-triggered photoluminescence switch has been demonstrated to be reproducible, stable, and reliable for information encryption and decryption.

A series of amphiphilic linear prepolymers for gelation, poly(poly(ethylene glycol) diglycidyl ether—monoethanolamine—poly(propylene glycol) diglycidyl ether), i.e. polyPEMPPs, were designed and synthesized by using poly(ethylene glycol) diglycidyl ether (PEGDGE), Poly(propylene glycol) diglycidyl ether (PPGDGE), and monoethanolamine (MEA) as main raw materials through nucleophilic addition/ring-opening reaction. The amphiphilic structure in the backbone of the prepolymers polyPEMPPs causes a phase transition in aqueous solution and has multiple stimuli responsiveness. The effects of hydrogen bonding and electrostatic interactions on phase transition behavior were discussed and analyzed structural reasons in detail. This has important guiding significance for further research on control sol–gel transition. The effects of polyPEMPPs concentration, content of PEGDGE and solution pH values on the phase transition temperature were examined. The experimental results show that increasing polyPEMPPs concentration promotes the phase transition, the transition temperature decreasing with increasing PEGDGE content. In the pH experiment, intra- and intermolecular association became difficult and the phase behavior will be inhibited, whether hydrochloric acid system or sodium hydroxide system. Moreover, we also experimented oscillatory phase transition, and the result exhibited a stably reversible phenomenon transition. [ABSTRACT FROM AUTHOR]

As a hydrogen storage alloy, the body-centered cubic (BCC) structure has been well studied due to its large hydrogen storage capacity. On this basis, the introduction of catalytic elements and an appropriate amount of C14 Laves phase can further improve the comprehensive hydrogen storage properties of hydrogen storage alloys. Based on this theory, (Ti 32.5 V 27.5 Zr 7.5 Nb 32.5) 1-x Ni x (x = 0.03, 0.06, 0.09) high entropy hydrogen storage alloys with an appropriate amount of catalytic element Ni are prepared in this study. The raise in Ni content is accompanied by grain refinement of the dendritic structure, an increase in the number of grains and grain boundaries, and a gradual increase in the proportion of C14 Laves phase. The introduction of catalytic element Ni reduces the dissociation energy of hydrogen molecules. The increasing grain and grain boundaries increase more hydrogen absorption sites with lower activation energy. The diffusion channel provided by the appropriate amount of C14 Laves phase makes it easier for hydrogen atoms to diffuse into the main phase BCC phase with strong hydrogen absorption capacity, and the hydrogen atoms are trapped by vacancy defects with large hydrogen binding energy. The synergistic effect of the three factors ensures the improvement of hydrogen absorption performance of high entropy hydrogen storage alloys. (Ti 32.5 V 27.5 Zr 7.5 Nb 32.5) 0.94 Ni 0.09 high entropy hydrogen storage alloy achieves excellent saturated hydrogen storage capacity of 2.78 wt % and 2.99 wt % at room temperature and 150 °C under 3 MPa hydrogen pressure, respectively. In addition, the introduction of Ni element reduces the thermal stability of metal hydrides, thereby improving the dehydrogenation performance of high entropy hydrogen storage alloys. The reversible phase transition occurs during the whole hydrogen absorption and desorption cycle. • The synergistic effect of the catalytic element Ni and the C14 Laves phase improves the properties of the alloy. • (Ti 32.5 V 27.5 Zr 7.5 Nb 32.5) 0.94 Ni 0.09 alloy achieves excellent saturated hydrogen storage capacity of 2.78 wt % and 2.99 wt % at room temperature and 150 °C under 3 MPa hydrogen pressure, respectively. • The reversible phase transition occurs during the whole hydrogen absorption and desorption cycle. [ABSTRACT FROM AUTHOR]

Doped and co-doped vanadium oxide (VO) thin films were deposited on quartz substrates with varying concentrations of doping elements by sol-gel spin coating technique. The VO thin films were characterized by the X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), cross-sectional FESEM and X-ray photoelectron spectroscopy (XPS) techniques. The optical properties, e.g., the average solar absorptance (α s), the average solar reflectance (ρ s) and the average solar transmittance (τ s) of the films deposited on quartz were evaluated by using a solar spectrum reflectometer. Based on the optical characterizations, an optimized doping concentration was identified. Further, a process optimized condition was utilized to grow both doped and co-doped vanadium oxide films on metallic Al6061 substrates; which are generally used in spacecraft. The phase transitions of all as grown vanadium oxide films on Al6061 were characterized by the differential scanning calorimetry (DSC) technique. The thin films showed reversible and repeatable phase transitions at sub-zero level of temperatures e.g., ∼ (̶ 23.3 °C). Thus, the VO thin films developed in the present work emerge out to be of paramount importance for the smart radiative device applications in spacecraft. [ABSTRACT FROM AUTHOR]

Highlights from the article: The rotation of protonated aniline cations and the order-disordered motions of both 18-crown-6 crown host molecules and perchlorate (tetrafluoroborate) guest anions induce the reversible phase transition. To reveal the source of the phase transition, we regard each Han cation and the related 18-crown-6 host molecule as well as the acid radical ClO SB 4 sb SP - sp as an independent entirety, and the N atoms in each Han cation acts as the center. In detail if we regarded the direction of the Han cations above the crown ethers as reference, all the 1-ClO SB 4 sb SP - sp and 3-ClO SB 4 sb SP - sp are located in the left front of the 1-Han and 3-Han cations, and the deflection angles of planes ( 1 and 3) are both about 3°, however, 2-ClO SB 4 sb SP - sp is located in the right front of the 2-Han cation. Meanwhile, the crown ether clathrate is also prone to displaying the structural phase resulted by the changes in the degree of disorder of some parts, such as crown ether rings, protonated ammonium cations or acid anions.

A series of new carbonyl dichromium complexes bearing aryl germanes as ligands were prepared using improved approaches. The thermal reaction of Cr(CO)6 (1) with Me3GeGePh3 (3) led to the formation of Me3GeGePh[(η6‐C6H5)Cr(CO)3]2 (3a). The lithiation of [(η6‐C6H6)Cr(CO)3] (2) with nBuLi followed by the addition of Me2GeCl2 (4) or ClGeMe2GeMe2Cl (5) gave Me2Ge[(η6‐C6H5)Cr(CO)3]2 (4a) and [(OC)3Cr(η6‐C6H5)]GeMe2GeMe2[(η6‐C6H5)Cr(CO)3] (5a), respectively. The molecular structures of 3a and 4a, in their crystal forms, were studied by X‐ray diffraction analysis. The crystals of oligogermane 3a have shown to undergo a fully reversible phase transition at 160 K without any sign of decomposition. The complexes synthesized were also studied by multinuclear NMR, IR and UV/Vis spectroscopy, DFT calculations and electrochemistry. The presence of a Cr(CO)3 group in a range of oligogermanes has shown to impact on the physical and chemical properties of the compounds. [ABSTRACT FROM AUTHOR]

Abstract Reactions of 3-fluoro-N-methylbenzylamine (FBMA) with PbBr 2 in concentrated HBr aqueous solution afforded a two-dimensional (2D) single layered organic-inorganic hybrid material [(FBMA) 2 PbBr 4 ], which was confirmed by X-ray single crystal diffraction, infrared (IR) spectroscopy and powder X-ray diffraction (PXRD). The thermodynamic DSC analysis revealed it has a pair of reversible phase transition peaks with large thermal hysteresis exhibiting a first-order type phase transition. The variable-temperature single crystal diffraction proved the phase transition is originated from the order and disorder of organic-inorganic hybrid structures at room and high temperatures. Simultaneously compound [(FBMA) 2 PbBr 4 ] exhibited a strong emission peak at 443 nm when under an excitation wavelength of 380 nm. Graphic abstract A new two-dimensional (2D) single layered organic-inorganic hybrid material (FC 6 H 4 CH 2 NH 2 CH 3) 2 [PbBr 4 ] showed a first-order type phase transition, which was originated from the order-disorder transition of organic-inorganic hybrid structures at room and high temperatures. fx1 Highlights • A new layered organic-inorganic-hybrid perovskite is designed, synthesized, and characterized. • The relationship between phase transitions and structural changes is addressed. • The optical property of (FC 6 H 4 CH 2 NH 2 CH 3) 2 [PbBr 4 ] is studied. [ABSTRACT FROM AUTHOR]

[Display omitted] • A phenylalanine derivative is synthesized as gelator and self-assembled to the CSH. • The CSH is composed of nanofibers and exhibits reversible phase transition behavior. • The CSH can be used for the selective loading of chiral drug naproxen. • The delivery of S-naproxen from the CSH is pH-sensitive. • The findings indicate that the proposed CSH is of great importance for nanomedicine. A chiral supramolecular hydrogel (CSH) is designed for selective loading of chiral drugs and pH-responsive drug delivery. A derivative of phenylalanine is first synthesized as the gelator, which can be self-assembled to the CSH via intermolecular hydrogen bonding. The resultant CSH is composed of spiral nanofibers and exhibits reversible phase transition behavior (sol–gel) depending on heating/cooling process and pH values. More importantly, the CSH can be used for the selective loading of chiral drug naproxen (NPX) due to the stronger attractive interactions between CSH and S-NPX resulted from their same optical activity. Interestingly, the delivery of S-NPX from the CSH is pH-sensitive and the cumulative release of S-NPX is significantly enhanced in alkaline solutions (pH = 10.0), which might be ascribed to the broken hydrogen bonding and the consequently ruined nanostructure of the CSH under alkaline conditions. [ABSTRACT FROM AUTHOR]

Smooth, uniform and crystalline vanadium oxide thin films were deposited on quartz by spin coating technique with four different rpm i.e., 1000, 2000, 3000 and 4000 and subsequently post annealed at 350, 450 and 550 °C in vacuum. Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) techniques were utilized for microstructural characterizations and phase analysis, respectively, for vanadium oxide powder and deposited film. Nanorods were observed to be grown after vacuum annealing. X-ray photoelectron spectroscopy (XPS) technique was utilized to study the elemental oxidation state of deposited vanadium oxide films. Thermo-optical and electrical properties such as solar transmittance (τ s ), reflectance (ρ s ), absorptance (α s ), infrared (IR) emittance (ε ir ) and sheet resistance (R s ) of different thin films were evaluated. Based on the optical characteristics the optimized condition of the film processing was identified to be spin coated at 3000 rpm. Subsequently, the nanoindentation technique was utilized to measure hardness and Young's modulus of the optimized film. The measured nanomechanical properties were found to be superior to those reported for sputtered vanadium oxide films. Finally, temperature dependent phase transition characteristics of optimized vanadium oxide films were studied by differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition was found to occur in the range of 44–48 °C which was significantly lower than the phase transition temperature (i.e., 68 °C) of bulk VO 2 . [ABSTRACT FROM AUTHOR]

Paracetamol [N‐(4‐hydroxyphenyl)acetamide, C8H9NO2] has several polymorphs, just like many other drugs. The most stable polymorphs, denoted Forms I and II, can be obtained easily and their crystal structures are known. Crystals of the orthorhombic, less stable, room‐temperature Form III are difficult to grow; they need a special recipe to crystallize and suffer from severe preferred orientation. A crystal structure model of Form III has been proposed and solved from a combination of structure prediction and powder X‐ray diffraction (PXRD) [Perrin et al. (2009). Chem. Commun.22, 3181–3183]. The final Rwp value of 0.138 and the corresponding considerable residual trace were reasons to check its validity. A new structure determination of Form III using new high‐resolution PXRD data led to a final Rwp value of 0.042 and an improvement of the earlier proposed model. In addition, a reversible phase transition was found at 170–220 K between the orthorhombic Form III and a novel monoclinic Form III‐m. The crystal structure of Form III‐m has been determined and refined from PXRD data to a final Rwp value of 0.059. [ABSTRACT FROM AUTHOR]

Conformal thin films of a cerium-manganese mixed oxide with a stoichiometry of CeMnO x were deposited on a high-surface-area γ-Al 2 O 3 support by Atomic Layer Deposition (ALD). Upon redox cycling, the CeMnO x film underwent a reversible phase transition between a reduced perovskite CeMnO 3 phase and an oxidized fluorite CeMnO 3.5 phase, as shown by X-ray diffraction (XRD) and High-Resolution Scanning/Transmission Electron Microscopy (HR-S/TEM). X-ray Photoelectron Spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS) revealed that Ce and Mn were in +3-oxidation states in the reduced perovskite phase and +4 and + 3 respectively in the oxidized fluorite phase. The composition of the film as a function of P(O 2) at 1073 K was measured by coulometric titration and showed that a transition between Ce3+ and Ce4+ occurred at a P(O 2) of 10−9 atm, a value that is much higher than that required for oxidizing either reduced bulk ceria or thin-film ceria on γ-Al 2 O 3. These properties make CeMnO x films an interesting candidate for various catalytic applications. Conformal thin films of a cerium-manganese mixed oxide CeMnO x were deposited on a high-surface-area γ-Al 2 O 3 support by Atomic Layer Deposition. These films can reversibly transition between a reduced perovskite Ce+3Mn+3O 3 phase and an oxidized fluorite Ce+4Mn+3O 3.5 phase upon redox cycling at 1073 K. [Display omitted] • CeMnO x films undergo a reversible perovskite-fluorite phase transition at 1073 K. • Ce cations transition between +3- and +4-oxidation states upon redox cycling. • Mn cations remain in the +3-oxidation state throughout redox cycling. • Mn cations remain +3 in CeMnO 3 film even at P(O 2) as low as 10−19 atm at 1073 K. [ABSTRACT FROM AUTHOR]

Materials that combine flexibility and open metal sites are crucial for myriad applications. In this article, we report a 2D coordination polymer (CP) assembled from CuII ions and a flexible meta-carborane-based linker [Cu2(L1)2(Solv)2]•xSolv (1-DMA, 1-DMF, and 1-MeOH; L1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane). 1-DMF undergoes an unusual example of reversible phase transition on solvent treatment (i.e., MeOH and CH2Cl2). Solvent exchange, followed by thermal activation provided a new porous phase that exhibits an estimated Brunauer-Emmett-Teller (BET) surface area of 301 m2 g−1 and is capable of a CO2 uptake of 41 cm3 g−1. The transformation is reversible and 1-DMF is reformed on addition of DMF to the porous phase. We provide evidence for the reversible process being the result of the formation/cleavage of weak but attractive B−H∙∙∙Cu interactions by a combination of single-crystal (SCXRD), powder (PXRD) X-ray diffraction, Raman spectroscopy, and DFT calculations.

Single-crystal neutron diffraction experiments at 100 and 2.5 K have been performed to determine the structure of 3-(pyridin-4-yl)pentane-2,4-dione (HacacPy) with respect to its protonation pattern and to monitor a low-temperature phase transition. Solid HacacPy exists as the enol tautomer with a short intramolecular hydrogen bond. At 100 K, its donor···acceptor distance is 2.450 (8) Å and the compound adopts space group C2/ c, with the N and para-C atoms of the pyridyl ring and the central C of the acetylacetone substituent on the twofold crystallographic axis. As a consequence of the axial symmetry, the bridging hydrogen is disordered over two symmetrically equivalent positions, and the carbon-oxygen bond distances adopt intermediate values between single and double bonds. Upon cooling, a structural phase transition to the t2 subgroup P occurs; the resulting twins show an ordered acetylacetone moiety. The phase transition is fully reversible but associated with an appreciable hysteresis in the large single crystal under study: transition to the low-temperature phase requires several hours at 2.5 K and heating to 80 K is required to revert the transformation. No significant hysteresis is observed in a powder sample, in agreement with the second-order nature of the phase transition. [ABSTRACT FROM AUTHOR]

Abstract: Reversible phase transitions triggered by guest AX4 type metal structural anions such as AuCl4−, FeCl4− etc are sparely documented before. Here we reported a host–guest clathrate, 18‐crown‐6 potassium tetrachloroferrate, [K2(C12H24O6)]2+⋅[FeCl4]22–(1) which exhibits reversible phase transition around 135 K. This special phase transition behaviours were further confirmed by the heat and dielectric anomalies in the vicinity of 135 K. The structural transition mechanism can be attributed to the ordering‐disordering motion of the tetrachloroferrate anions. Its switchable dielectric behaviours around the phase transition point suggest it maybe a potential material for dielectric switches. [ABSTRACT FROM AUTHOR]

The crystal structure determination based on 90 K data of the title imine ligand, C18H10ClNO, revealed non-merohedral twinning with three twin domains. In our experience, this is an indication of an ordering phase transition. Consequently, the structure was redetermined with higher temperature data, and a reversible phase transition was discovered. The higher temperature phase is indeed an ordered structure. At the higher temperature, the 4-chlorophenyl group has rotated by ca 7° into a crystallographic mirror plane. Warming the crystal from 90 K to 250 K changes the space group from triclinic P1, to monoclinic P21/m. Diverse non-classical interactions are present in the crystal packing, and these are described for the phase change reported in this work. The crystal structure of the title imine ligand, measured at 100 K, has been reported on previously [Kovach et al. (2011). J. Mol. Struct. 992, 33-38]. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel thermochromic (3,3-difluoropyrrolidinium) 2 [CuCl 4 ] crystal was prepared. • It has two reversible structure phase transitions, ferroelectricity and ferroelasticity. • It has reversible green to yellow–brown color change at 351 K. • The initial application in anti-counterfeiting and encryption inks are demonstrated. The design and fabrication of sensitive and reversible thermochromics (TCs) remain challenging although they own enormous applications. Especially, there is a tough task to introduce multifold physical properties to them. Here, a temperature-response organic–inorganic metal halide hybrid (OIHM) (3,3-Difluoropyrrolidium) 2 CuCl 4 ((DFP) 2 CuCl 4) 1 is fabricated, utilizing a self-assembly method based on Cu2+ inorganic salt and fluoropyrrolidinium dexterously. It possesses two reversible phase transitions (PTs) comprising a ferroelectric to paraelectric PT at 194 K and a ferroelastic to paraelastic PT at 351 K. The ferroelastic and ferroelectric properties are mainly derived from the order–disorder transitions in the microstructure and the displacement of the charge center. Importantly, the high-temperature structural phase transition is accompanied by a reversible color change. Its crystals can be directly applied in reversible thermochromic materials, exhibit an obvious color change (green to yellow–brown at 351 K) with full reversibility, and are successfully used as inks for hand-painting. To the best of our knowledge, 1 is the first Cu(II)-based OIMH TC with both ferroelectricity and ferroelasticity. This work laid the foundation for further development of alternative TCs, ferroics and practical applications for TCs. [ABSTRACT FROM AUTHOR]

Two-dimensional In 2 Se 3 , an unconventional phase-change material, has drawn considerable attention for polymorphic phase transitions and electronic device applications. However, its reversible thermally driven phase transitions and potential use in photonic devices have yet to be explored. In this study, we observe the thermally driven reversible phase transitions between α and β' phases with the assistance of local strain from surface wrinkles and ripples, as well as reversible phase changes within the β phase family. These transitions lead to changes in the refractive index and other optoelectronic properties with minimal optical loss at telecommunication bands, which are crucial in integrated photonic applications such as postfabrication phase trimming. Additionally, multilayer β'-In 2 Se 3 working as a transparent microheater proves to be a viable option for efficient thermo-optic modulation. This prototype design for layered In 2 Se 3 offers immense potential for integrated photonics and paves the way for multilevel, nonvolatile optical memory applications.

We successfully designed and synthesized a new low-dimensional cadmium-based hybrid metal halide reversible phase transition material, [(CH 2 CH 3) 3 NCH 2 Cl] 2 CdBr 4 , achieving reversible phase transition, semiconducting, and photoluminescence properties. [Display omitted] • Reversible phase transition at T c =421 K • High quality photoluminescence characteristic. • Semiconductor with 4.51eV band gap. Organic-inorganic hybrid metal halides have attracted considerable attention due to their interesting chemical variability and structure–property flexibility, with remarkable multifunctional properties, including tunable physical, chemical, optical, and electronic properties. However, with the continuous development and change of the social environment, the demand for multifunctional materials is more urgent. In this paper, a new low-dimensional cadmium-based hybrid metal halide multifunctional material with high temperature reversible phase transition is reported: [(CH 2 CH 3) 3 NCH 2 Cl] 2 CdBr 4 (1), which exhibits an ultrahigh temperature reversible phase transition at T c = 421 K variable, and has semiconductor and photoluminescence multifunctional materials. This will stimulate further interest in molecular materials with desirable multifunctional properties. [ABSTRACT FROM AUTHOR]

We investigated the adsorption behavior of Co(II)-5,10,15,20-tetrakis-(3,5-di- tert -butylphenyl)-porphyrin (CoTTBPP) on Cu(111) by scanning tunneling microscopy (STM). At room temperature (RT), the coverage dependent adsorption behavior follows an expected scheme: at low coverage step decoration is found, which evolves into supramolecular domains with a hexagonal order at higher coverage. Interestingly, upon cooling the sample to 180 K the occurrence of a clearly distinguishable coexisting herringbone phase is observed. Upon heating to RT again, the herringbone phase vanishes. Thus a temperature dependent, fully reversible phase transition was observed. High resolution STM micrographs allow for the determination of the intramolecular conformations which are different for the two supramolecular arrangements. In addition, we studied the bias voltage dependent appearance of the molecule in STM and assigned a dominant contribution of the central Co at negative bias voltages close to the Fermi edge to the occupied d z 2 orbital. Interestingly, the herringbone phase, which dominates at 180 K, exhibits a significantly higher molecular density than the monomodal hexagonal arrangement at RT, which is in line with the “normal” behavior of freezing substances. [ABSTRACT FROM AUTHOR]

P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 represents a promising cathode for Na-ion batteries, but it suffers from severe structural degradation upon storing in a humid atmosphere and cycling at a high cutoff voltage. Here we propose an in situ construction to achieve simultaneous material synthesis and Mg/Sn cosubstitution of Na 0.67 Ni 0.33 Mn 0.67 O 2 via one-pot solid-state sintering. The materials exhibit superior structural reversibility and moisture insensitivity. In-operando XRD reveals an essential correlation between cycling stability and phase reversibility, whereas Mg substitution suppressed the P2-O2 phase transition by forming a new Z phase, and Mg/Sn cosubstitution enhanced the P2-Z transition reversibility benefiting from strong Sn-O bonds. DFT calculations disclosed high chemical tolerance to moisture, as the adsorption energy to H 2 O was lower than that of the pure Na 0.67 Ni 0.33 Mn 0.67 O 2 . A representative Na 0.67 Ni 0.23 Mg 0.1 Mn 0.65 Sn 0.02 O 2 cathode exhibits high reversible capacities of 123 mAh g -1 (10 mA g -1 ), 110 mAh g -1 (200 mA g -1 ), and 100 mAh g -1 (500 mA g -1 ) and a high capacity retention of 80% (500 mA g -1 , 500 cycles).

Great attention is paid to that stimulus response of materials, which is a prerequisite for energy harvesting applications. Driven by advances in nanotechnology, the atomic motions, involved in phase transitions and associated with stimulus responses, require detailed investigation on the nanoscale. Recently, strain engineering of BiFeO 3 (BFO) has become the subject of broad research interest due to its promising potential in energy conversion applications, such as piezoelectric, pyroelectric and shape memory effect (SME) devices. In this study, an excellent pyroelectric response is associated with reversible phase transitions in mixed-phase BFO films using thermal stimuli. Using an in situ high-resolution transmission electron microscope (HRTEM), we observed that phase transition between rhombohedral-like (R-like) and tetragonal-like (T-like) BFO involved the migration of the phase boundary, which is a prerequisite for the growth of the T-like phase and requires an intermediate phase. Moreover, the origin of the phase transition is attributed to competition between thermodynamic stability and substrate-induced strain, as suggested by phase-field simulations. The results provide a fundamental understanding of the atomic processes that underlie the stimulus response of the strained BFO films and demonstrate the potential of this extraordinary material for novel energy harvesting applications. [ABSTRACT FROM AUTHOR]

Lead-free 0.99[(Bi 0.5 Na 0.5 ) 0.935 Ba 0.065 Ti (1– x ) Nb x O 3 ]–0.01SrZrO 3 (BNBTNb100 x –SZ, with Nb100 x = 0–1) ceramics were prepared by the conventional mixed oxide route. X-ray diffraction and Raman scattering was utilized for the structural evolution of Nb-modified BNBT–SZ ceramics at average and short-scale localized structure. Temperature dependent dielectric properties showed ferroelectric–ergodic relaxor (FE–ER) transition in Nb-modified BNBT–SZ ceramics by producing a significant disruption of the long-range FE order. A giant normalized strain of 825 pm/V at 4 kV/mm was achieved at Nb1.0. Interestingly, at a relatively low applied field of 3 kV/mm, the Nb0.75 sample displayed a large electric field-induced strain (EFIS) response of 0.20%, which is highest value obtained in non-textured lead-free BNT-based ceramics at such low driving field. The structural distortion induced by doping and electric poling is correlated with the dielectric, ferroelectric and EFIS response, and the evolution of giant strain was ascribed to reversible field induced phase transition from ER–FE phase. [ABSTRACT FROM AUTHOR]

The title salt, C6H6NO2+·ClO4−·C6H5NO2, was crystallized from an aqueous solution of equimolar quantities of perchloric acid and pyridine-2-carboxylic acid. Differential scanning calorimetry (DSC) measurements show that the compound undergoes a reversible phase transition at about 261.7 K, with a wide heat hysteresis of 21.9 K. The lower-temperature polymorph (denoted LT; T = 223 K) crystallizes in the space group C2/ c, while the higher-temperature polymorph (denoted RT; T = 296 K) crystallizes in the space group P2/ c. The relationship between these two phases can be described as: 2 aRT = aLT; 2 bRT = bLT; cRT = cLT. The crystal structure contains an infinite zigzag hydrogen-bonded chain network of 2-carboxypyridinium cations. The most distinct difference between the higher (RT) and lower (LT) temperature phases is the change in dihedral angle between the planes of the carboxylic acid group and the pyridinium ring, which leads to the formation of different ten-membered hydrogen-bonded rings. In the RT phase, both the perchlorate anions and the hydrogen-bonded H atom within the carboxylic acid group are disordered. The disordered H atom is located on a twofold rotation axis. In the LT phase, the asymmetric unit is composed of two 2-carboxypyridinium cations, half an ordered perchlorate anion with ideal tetrahedral geometry and a disordered perchlorate anion. The phase transition is attributable to the order-disorder transition of half of the perchlorate anions. [ABSTRACT FROM AUTHOR]

• The reversible hexagonal close-packed (HCP) structure to face-centered cubic (FCC) structure phase transformations occur at the { 10 1 ¯ 1 } twin tip caused by the successive slip of Shockley partial dislocations on { 0001 } HCP and { 111 } FCC planes in pure Ti, respectively. • The nucleation of Shockley partial dislocations stems from the dissociation of twinning dislocations (TDs) b ⇀ 4 at the { 10 1 ¯ 1 } twin tip, and these dislocations can be absorbed by twin tip boundaries by reacting with residual dislocations to form new TDs b ⇀ 4. • The HCP→FCC phase transformation is due to the stress concentration caused by the plugging of the periodically distributed TDs b ⇀ 4 at the twin tips, and its inverse transformation is caused by the relaxation of internal stress at the twin tip. • The periodic serrated feature of the (10 1 ¯ 0) T // (10 1 ¯ 3) M facets at the { 10 1 ¯ 1 } twin tip were discovered via atomistic simulation. In this paper, by using high-resolution transmission electron microscopy (HRTEM) observations and molecular dynamics (MD) simulations, the mechanisms of hexagonal close-packed (HCP) structure to face-centered cubic (FCC) structure transition and its inverse transition at the { 10 1 ¯ 1 } twin tip in pure titanium were investigated. The HCP→FCC phase transition at the { 10 1 ¯ 1 } twin tip in cold-rolled pure Ti was experimentally observed for the first time. On the other hand, under electron beam irradiation, the FCC lamella shrank gradually and eventually recovered to the HCP structure. After the HCP ↔ FCC phase transformations, the interfacial structures of the twin tip changed from the (10 1 ¯ 0) T // (10 1 ¯ 3) M facets to a common Basal-Pyramidal (BPy) facets. MD simulations indicated that the reversible HCP ↔ FCC phase transitions were caused by successive slip of Shockley partial dislocations on { 0001 } HCP and { 111 } FCC planes, respectively. The nucleation of Shockley partial dislocations stemmed from the dissociation of twinning dislocation (TD) with the Burgers vector of b ⇀ 4 = 4 γ 2 − 9 3 + 4 γ 2 < 10 1 ¯ 2 ¯ > at the twin tip. The Shockley partial dislocations could be absorbed by twin tip boundaries through reacting with the residual dislocations to form new TDs b ⇀ 4 during the FCC→HCP phase transition. The periodic serrated feature of the (10 1 ¯ 0) T // (10 1 ¯ 3) M facets of the twin tip boundaries in pure Ti was discovered via atomistic simulations. The interfacial structure evolution of the twin tip was caused by a series of dislocation reactions and generation of phase transformation at the twin tip. The transformation from the HCP structure to the FCC structures was due to the stress concentration caused by the plugging of the periodically distributed TDs b ⇀ 4 at the twin tips in the case of external loading. The shrinkage of FCC lamella was caused by the relaxation of internal stress at the twin tip during the thermal relaxation by electron beam. [Display omitted] [ABSTRACT FROM AUTHOR]

Crystals of hexa- tert-butyldisilane, C24H54Si2, undergo a reversible phase transition at 179 (2) K. The space group changes from Ibca (high temperature) to Pbca (low temperature), but the lattice constants a, b and c do not change significantly during the phase transition. The crystallographic twofold axis of the molecule in the high-temperature phase is replaced by a noncrystallographic twofold axis in the low-temperature phase. The angle between the two axes is 2.36 (4)°. The centre of the molecule undergoes a translation of 0.123 (1) Å during the phase transition, but the conformation angles of the molecule remain unchanged. Between the two tri- tert-butylsilyl subunits there are six short repulsive intramolecular C-H...H-C contacts, with H...H distances between 2.02 and 2.04 Å, resulting in a significant lengthening of the Si-Si and Si-C bonds. The Si-Si bond length is 2.6863 (5) Å and the Si-C bond lengths are between 1.9860 (14) and 1.9933 (14) Å. Torsion angles about the Si-Si and Si-C bonds deviate by approximately 15° from the values expected for staggered conformations due to intramolecular steric H...H repulsions. A new polymorph is reported for the crystal structure of 1,1,2,2-tetra- tert-butyl-1,2-diphenyldisilane, C28H46Si2. It has two independent molecules with rather similar conformations. The Si-Si bond lengths are 2.4869 (8) and 2.4944 (8) Å. The C-Si-Si-C torsion angles deviate by between −3.4 (1) and −18.5 (1)° from the values expected for a staggered conformation. These deviations result from steric interactions. Four Si-C( t-Bu) bonds are almost staggered, while the other four Si-C( t-Bu) bonds are intermediate between a staggered and an eclipsed conformation. The latter Si-C( t-Bu) bonds are about 0.019 (2) Å longer than the staggered Si-C( t-Bu) bonds. [ABSTRACT FROM AUTHOR]

Phase instability is one of the major obstacles to the wide application of formamidinium (FA)-dominated perovskite solar cells (PSCs). An in-depth investigation on relevant phase transitions is urgently needed to explore more effective phase-stabilization strategies. Herein, the reversible phase-transition process of FA 1- x Cs x PbI 3 perovskite between photoactive phase (α phase) and non-photoactive phase (δ phase) under humidity, as well as the reversible healing of degraded devices, is monitored. Moreover, through in situ atomic force microscopy, the kinetic transition between α and δ phase is revealed to be the "nucleation-growth transition" process. Density functional theory calculation implies an enthalpy-driven α-to-δ degradation process during humidity aging and an entropy-driven δ-to-α healing process at high temperatures. The α phase of FA 1- x Cs x PbI 3 can be stabilized at elevated temperature under high humidity due to the increased nucleation barrier, and the resulting non-encapsulated PSCs retain >90% of their initial efficiency after >1000 h at 60 °C and 60% relative humidity. This finding provides a deepened understanding on the phase-transition process of FA 1- x Cs x PbI 3 from both thermodynamics and kinetics points of view, which also presents an effective means to stabilize the α phase of FA-dominated perovskites and devices for practical applications., (© 2022 Wiley-VCH GmbH.)