Reversible Structural Phase Transition Research Articles

Second-Order Nonlinear Switching and Photoluminescence Properties of Cd-Based Hybrid Perovskite with High-Temperature Phase Transition.

Recently, organic-inorganic hybrid perovskites exhibiting facile structural phase transitions have accumulated significant attention due to their switchable second-order nonlinear optical (NLO) properties, which hold significant promise for next-generation intelligent optoelectronic devices. In this study, we present a novel one-dimensional hexagonal hybrid perovskite, (4-methoxypiperidinium)CdCl3, which undergoes a reversible high-temperature structural phase transition at 389 K. Notably, (4-methoxypiperidinium)CdCl3 demonstrates switchable second-order NLO and dielectric properties, accompanied by symmetry breaking from the centrosymmetric Pnma to noncentrosymmetric Pna21 space group. Variable-temperature structure analyses reveal that this transition is mainly driven by the order-disorder transformation of the 4-methoxypiperidinium cations. Furthermore, it also features a promising photoluminescence performance with blue-light emission and a long lifetime of 25.34 ns. It is anticipated that this study will expand the family of hybrid perovskites exhibiting high-temperature phase transitions and offer valuable guidance for the design of new NLO switching materials with superior optoelectronic properties.

Phase Transition Control in Molecular Solids via Complementarity of Hydrogen-Bond Strength.

Phase transitions in molecular solids involve synergistic changes in chemical and electronic structures, leading to diversification in physical and chemical properties. Despite the pivotal role of hydrogen bonds (H-bonds) in many phase-transition materials, it is rare and challenging to chemically regulate the dynamics and to elucidate the structure-property relationship. Here, four high-spin CoIIcom-pounds were isolated and systematically investigated by modifying the ligand terminal groups (X = S, Se) and substituents (Y = Cl, Br).S-ClandSe-Brundergo a reversible structural phase transition near room temperature, triggering the rotation of 15-crown-5 guests and the swing between syn- and anti-conformation of NCX-ligands, accompanied by switchable magnetism. Conversely,S-BrandSe-Clretain stability in ordered and disordered phases, respectively. H-bonds geometric analysis andab initiocalculations reveal that the electronegativity of X and Y affects the strength of NY-ap-H···X interactions. Entropy-driven structural phase transitions occur when the H-bond strength is appropriate; otherwise, the phase stays unchanged if it is too strong or weak. This work highlights a phase transition driven by H-bond strength complementarity - pairing strong acceptor with weak donor andvice versa, which offers a straightforward and effective approach for designing phase-transition molecular solids from a chemical perspective.

Dynamic strain coupling driven by structural phase transition in mixed-dimensional 2H-MoS2/VO2 van der Waals heterointerfaces

Mixed-dimensional van der Waals (vdW) heterostructures, integrated two-dimensional (2D) atomic crystals with three-dimensional (3D) functional materials, offer a powerful means to manipulating physical properties and generating unprecedented functionalities. Understanding interfacial couplings at those hetero (homo)-interfaces is indispensable for exploring new optical and electronic devices. Herein, we investigated dynamically phase-transition-driven strain coupling across a vdW heterointerface through integrating 2D layered 2H-MoS2 nanoflakes onto 3D phase-change VO2 epitaxial thin films. The Raman peak positions of the in-plane and out-of-plane vibration modes E2g1 and A1g from the 2H-MoS2 nanoflakes show a phonon softening and reversible hysteresis loop as a function of temperature in this mixed-dimensional vdW 2H-MoS2/(1¯11)-VO2/(11¯02)-Al2O3 heterostructure, originating from the co-action of temperature-dependent anharmonicity in 2H-MoS2 and reversible structural phase transition (SPT)-induced in-plane tensile strain from the VO2 thin film. Accordingly, the integrated Raman scattering intensity of these two feature peaks of the 2H-MoS2 nanoflakes increased (decreased) as the temperature increased (decreased), exhibiting a hysteresis loop in the SPT and metal–insulator transition region of VO2. Additionally, the peak integrated intensity enhancement ratio of the E2g1 and A1g vibration modes was approximately 2.3 and 2.8, respectively. These results indicate that the dynamically SPT-driven in-plane tensile strain from the bottom VO2 layer interfacially couples with the adjacent 2H-MoS2 nanoflakes and results in a reduction in the electronic transition energy, leading to an enhancement in the Raman scattering intensity of 2H-MoS2. Our work holds promise for dynamic strain control of lattice dynamics and electron–phonon interaction of 2D materials for functional electronic and photoelectronic devices.

Hydrothermal Synthesis of Monoclinic VO2 Microparticles without Use of Hazardous Reagents: A Key Role for the W-Dopant.

Monoclinic vanadium dioxide (VO2 (M)) is a promising material for various applications ranging from sensing to signature management and smart windows. Most applications rely on its reversible structural phase transition to rutile VO2 (VO2 (R)), which is accompanied by a metal-to-insulator transition. Bottom-up hydrothermal synthesis has proven to yield high quality monoclinic VO2 but requires toxic and highly reactive reducing agents that cannot be used outside of a research lab. Here, we present a new hydrothermal synthesis method using nontoxic and safe-to-use oxalic acid as a reducing agent for V2O5 to produce VO2 (M). In early stages of the process, polymorphs VO2 (A) and VO2 (B) were formed, which subsequently recrystallized to VO2 (M). Without the presence of W6+, this recrystallization did not occur. After a reaction time of 96 h at 230 °C in the presence of (NH4)6H2W12O40 in Teflon-lined rotated autoclaves, we realized highly crystalline, phase pure W-doped VO2 (M) microparticles of uniform size and asterisk shape (ΔH = 28.30 J·g-1, arm length = 6.7 ± 0.4 μm, arm width = 0.46 ± 0.06 μm). We extensively investigated the role of W6+ in the kinetics of formation of VO2 (M) and the thermodynamics of its structural phase transition.

Structural Phase Transitions in the Double Salts (NH4)2PO3F·NH4NO3 and (NH4)2XO4·3NH4NO3 (X = Se, Cr)

In the context of investigating isostructural relationships between sulfates and monofluorophosphates, crystals of the double salts (NH4)2PO3F·NH4NO3 (AFP·AN) and (NH4)2XO4·3NH4NO3 (AX·3AN; X = Se, Cr) were grown from aqueous solutions and structurally characterized using X-ray diffraction and thermal analysis. Whereas the high-temperature forms of the two AX·3AN double salts are in fact isostructural with the sulfate analogue, AFP·AN crystallizes with a reduced amount of NH4NO3 and thus has a unique crystal structure. Both AFP·AN and the two AX·3AN compounds exhibit reversible structural phase transitions. Upon cooling, the monofluorophosphate double salt transforms from the monoclinic room-temperature polymorph (I; P21/n, Z = 4) to the intermediate triclinic polymorph (II; P1, Z = 4) that in turn transforms to the monoclinic low-temperature polymorph (III; P21/n, Z = 4). The two phase transitions (I) → (II) and (II) → (III) are characterized by a significant increase of the unit cell volumes upon cooling. The two AX·3AN double salts transform upon cooling from a disordered monoclinic crystal structure (P21, Z = 2) to a monoclinic polymorph with a doubled unit cell (P21/c, Z = 4). Such a phase transition is not observed for the sulfate analogue. All molecular moieties are fully ordered at −93 °C for the selenate double salt, whereas one of the nitrate anions remains disordered for the chromate double salt even at −173 °C. In all AFP·AN and AX·3AN crystal structures, the nitrate anions play a crucial role during the phase transitions, and an extensive network of N–H···O hydrogen-bonding interactions is responsible for the cohesion of the crystal.

Pressure-induced reversible structural phase transitions and metallization in GeTe under hydrostatic and non-hydrostatic environments up to 22.9 GPa

The pressure-dependent vibrational and electrical transport properties of GeTe have been investigated in a diamond anvil cell through in-situ Raman spectroscopy and electrical conductivity measurements under hydrostatic and non-hydrostatic environments up to 22.9 GPa. Upon compression, two structural transformations from rhombohedral to cubic NaCl-type to orthorhombic GeTe occurred at 3.2 GPa and 12.3 GPa under non-hydrostatic condition. Similarly, two corresponding phase transitions were detected at much higher pressures of 5.0 GPa and 15.4 GPa under hydrostatic condition. Additionally, a 3.3 GPa of electronic transition accompanying by the rhombohedral to cubic NaCl-type transition was characterized by the variable-temperature electrical conductivity experiments. Upon decompression, the recoverable Raman spectra and resumable electrical conductivity suggested that the structural and electronic transitions of GeTe were reversible. The reversibility was further confirmed by the microscopic structural observations from high-resolution transmission electron microscopy, fast Fourier transform and atomic force microscopy.

Pressure-dependent compressibility of K2X(CO3)2 (X = Ca, Mg) under high pressure

The high-pressure phase transition behaviors and pressure-dependent compressibility are investigated on K2X(CO3)2 using first-principles calculation from ambient pressure to 30 GPa. The reversible structural phase transitions of K2X(CO3)2 are determined based on enthalpy difference and cell volume collapses. The pressure point of structural phase transitions of K2Ca(CO3)2 is obtained at around 3.3 GPa and 10.3 GPa, while that is found at about 6.2 GPa and 22.6 GPa for K2Mg(CO3)2. Compared with the high axial rigidity of C-O bonds in [CO3] groups, the Ca+ and Mg+ coordination polyhedron show much more compressible. The displacement behavior of the Ca+ and Mg+ coordination polyhedron occurs simultaneously with the rotation behavior of [CO3] groups during compression, and reverses induced by the structural phase transition.

Discovery of ferroelectricity in natural product androstane

Progesterone and its derivatives attracted widespread interest because of their applications in medicine, health care and birth control, which is the main active ingredient of contraceptive pills known as one of the five chemistry discoveries that changed human life. Although the research of pharmacological effects on contraceptive pill-related compounds has been around for decades, their ferroelectricity has long been overlooked. Here, we report that 4-androsten-3-one-5-ene-17-carboxylic acid, a derivative of progesterone, is an organic single-component ferroelectric, as confirmed by the polarization–electric field hysteresis loops. It crystallizes in the monoclinic space group P21 with a polar packing structure and undergoes a reversible structural phase transition at a high temperature of 489 K. Thermal analysis revealed that its ferroelectricity can persist up to 533 K, giving a wide working temperature range. As the first ferroelectric in steroid biomaterials, 4-androsten-3-one-5-ene-17-carboxylic acid shows great potential in applications for flexible devices, biomedical devices, bio-machines and so on.

Pressure-induced topological crystalline insulating phase in TlBiSe2 : Experiments and theory

We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_{2}$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_{2}$ show a systematic increase in frequencies under hydrostatic pressure up to $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, the linewidth of the ${A}_{1g}, N$, and ${E}_{g}$ phonon modes show clear anomalies at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\ensuremath{\lambda}$, anomalies in the Raman linewidth of ${E}_{g}$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number ${n}_{M}$ and ${\mathbb{Z}}_{2}$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number ${n}_{M}$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_{2}$ at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_{2}$ ternary chalcogenide $\mathrm{TlBi}{X}_{2}$ ($X=\mathrm{S}$, Se, Te) family.

Optical-vibration properties and pressure-induced phase transition in (In,Sc)2Ge2O7 pyrogermanates

Thortveitite-type (T-type) scandium and indium pyrogermanates were obtained through a solid-state reaction route. The crystal structures, morphologies, and chemical compositions of these compounds were investigated by X-ray diffraction and transmission electron microscopy techniques. The optical-vibration properties were evaluated by Raman scattering and infrared spectroscopy. Reflectivity far-infrared data were obtained for sintered Sc2Ge2O7 samples, which exhibited good surfaces after polishing, allowing the determination of the infrared optical functions, the complete polar phonon characteristics, the intrinsic dielectric constant and the unloaded quality factor. Because of the poor surface reflectivity of In2Ge2O7 samples, only mid-infrared absorbance data could be collected by using loose powders in an attenuated total reflectance accessory. By using polarized Raman spectroscopy on both ceramic materials, at room temperature and ambient pressure, all the 15 active Raman modes predicted by group theory for the monoclinic C2/m T-type structure were identified and assigned. Further, the phase stabilities of In2Ge2O7 and Sc2Ge2O7 were studied by high-pressure Raman spectroscopy. In the limit of pressures studied here (up to 10 GPa), a reversible structural phase transition (SPT) for In2Ge2O7 and Sc2Ge2O7 was induced by pressures of 4.2 GPa and 2.4 GPa, respectively. These would correspond to the reported C2/m (#12) ↔ P21/c (#14) SPT. Representative Raman spectra of the high-pressure phase for both materials were fitted and 24 first-order Raman modes were depicted for each material, a number greater than compatible with a proposed distorted and oxygen-deficient fluorite-like structure, but explained by an analysis of the gtroup-subgroup constraints of this SPT.

Magnetic Switchability via Thermal-Induced Structural Phase Transitions in Molecular Solids

Magnetically switchable molecular solids with stimuli-responsive ON/OFF characteristics are promising candidates for smart switches and magnetic storage. In addition to conventional spin-crossover/charge-transfer materials whose magnetic responses arise from changes in the electronic structure of the metal centers, peripheral chemical entities that exhibit tunability provide an alternative and promising tactic for the construction of magnetic multi-stable materials. Temperature changes can trigger a reversible structural phase transition that can affect the coordination environment of a transition-metal center because of the thermal-induced motion of ligands, counterions, neutral guests, and/or changes in coordination number, thus potentially realizing magnetic bistability which can arise from a concomitant spin state change or the modulation of orbital angular momentum. Perspectives and challenges are also highlighted to provide insights into its development.

Dielectric switching in correlation with the structural phase transitions in tetrapropylammonium perchlorate.

The crystals of the tetrapropylammonium perchlorate ([(CH3CH2CH2)4N]ClO4, TePrAClO4) compound undergo two reversible phase transitions: at ca. T1 = 284 K and at ca. T2 = 445 K. The observed phase transitions and distinct dielectric and relaxation effects are due to the dynamic motions of the organic cations and anionic framework. The crystals become ordered at low temperatures, then disordered at room temperature (propyl chains of the organic part as well as perchlorate ions are disordered over the mirror plane at c = 1/4 and 3/4) and highly disordered at high temperatures. The comparable changes in the wavenumber and FWHM shifts (IR and Raman spectroscopy) in the case of tetrapropylammonium and perchlorate ions in the phase transition at T1 and slightly more significant changes for organic cations (juxtaposed with perchlorate ions) in the phase transition at T2 lead to a conclusion that the phase transition at T1 is equally driven by motions of the two ions, while the phase transition at T2 is more influenced by the motions of organic cations. The phase transition at T2 with its large entropy change resembles the behavior found in liquid crystals. The dielectric function values can be switched and tuned in the low- and high-dielectric states, which may indicate the potential application of this material in sensors or actuators.

Above room-temperature dielectric switching in a new Zn-halide hybrid crystal based on pyrrolidine-derivative

A new Zn-halide hybrid crystal based on pyrrolidine-derivative exhibits reversible phase transition at around 330 K/320 K (heating/cooling). • (HNBNMP) 2 (ZnCl 4 ) (HNBNMP = N-bromomethyl-N-methylpyrrolidinium cation, compound 1 ), shows a reversible phase transition at 330/320 K (heating/cooling) through differential scanning calorimetry (DSC). • Variable-temperature single-crystal structural analysis and powder X-ray diffraction patterns verify the structural phase transition. • Variable-temperature dielectric measurements show the dielectric switching behavior indicating the potential application in the dielectric functional materials. We have successfully synthesized compound 1 (HNBNMP) 2 (ZnCl 4 ) (HNBNMP = N-bromomethyl-N-methylpyrrolidinium cation) through the slow evaporation method at room temperature. Several studies (differential scanning calorimetry measurements, single crystal structural analysis, variable-temperature powder X-ray diffraction and dielectric measurements) show that compound 1 has a reversible structural phase transition at around 330 K/320 K (heating/cooling), respectively. Variable-temperature powder X-ray diffraction patterns further demonstrate that compound 1 undergoes a phase transition. Meanwhile, the unique phase transition behavior was also confirmed by its remarkable dielectric and thermal anomalies around the phase transition point. The step-like dielectric switching in compound 1 can be mainly ascribed to the motional changes of polar components between room-temperature and high-temperature phases. This finding should stimulate further synthesis for switchable functional materials.

Halogen tuning toward dielectric switch and band gap engineering in one-dimensional hybrid materials

Organic-inorganic hybrids with multiple tunable functions are very attractive for potential applications in sensors, smart switches, etc. To investigate the effect of halogen tuning on the properties of hybrid materials, two new multifunctional organic-inorganic hybrid compounds [3-hydroxy-azetidinecation]2SbX5 (1: X = Cl; 2: X = Br) are successfully synthesized here by replacing halogen anions. Reversible structural phase transitions accompanied by anomalous changes in dielectric constants are found when heating (T1 = 362 K; T2 = 373 K). Both two compounds have the same space group change from P212121 (Low Temperature Phase) to Pnma (High Temperature Phase). When replacing Cl with Br, the phase transition temperature increases by ca. 10 K and the band gap changes from 3.21 eV to 2.75 eV. Based on the above properties, this work could provide an effective molecular design strategy for the exploration and construction of temperature-tunable, lower bandgap semiconductor hybrid materials.

Reversible disorder-order type structural phase transition of potassium dihydrogen phosphate bulk single crystals induced by dynamic shock waves

In this article, we report the reversible disorder-order type structural phase transition of potassium dihydrogen phosphate (KDP) bulk single crystal at dynamic shock pulses loaded conditions and the core results of the disorder-order type structural phase transitions have been authenticated by X-ray diffraction (XRD) and Raman spectroscopic results. In order to have further substantiation, we have also performed optical spectroscopic and microscopic analyses. As per the obtained data from the XRD, Raman and microscopic results, the control KDP crystal has the disordered type tetragonal structure whereas, at the 1st shocked condition, we observe a completely ordered tetragonal structure. Surprisingly, at the 2nd shocked condition, the disordered tetragonal structure has been retrieved. To the best of our knowledge, to date, this kind of reversible disorder-order type phase transition may be the first report on the research findings of KDP crystal.

Insights into the Molecular Dynamics of Quasi-Spherical (Chloromethyl)triethylammonium Confined in a Weakly Bound Ionic Cocrystal.

Here, we report a weakly bound ionic cocrystal, (Et3NCH2Cl)2[ZnCl4], which undergoes a reversible structural phase transition owing to the switched molecular dynamics of the quasi-spherical (Et3NCH2Cl)+ cation from static to dynamic. Interestingly, a unique rolling and moving mechanism is uncovered for such a cation in the high-temperature phase, where its two methylene groups exhibit different kinetic energy barriers. This study provides a meaningful insight into the solid-state molecular dynamics of large-size quasi-spherical molecules that contain both a rigid core and flexible shell.

Magnetic–dielectric bistabilities and magnetodielectric coupling effects in a new layered hybrid perovskite: (C6H5(CH2)4NH3)2[MnCl4

A new organic–inorganic layered perovskite, (C6H5(CH2)4NH3)2[MnCl4], undergoes three reversible structural phase transitions accompanying with magnetic–dielectric dual-bistabilities, and exhibits an intrinsic magnetodielectric effect during the spin-flop transition.

Temperature‐Induced Phase Transition in Cu4TiSe4

AbstractThe newly synthesized ultralow thermal conducting Cu4TiSe4 undergoes a temperature‐induced reversible structural phase transition at −92 °C. The room temperature structure exhibits cubic symmetry (space group P 3 m with lattice parameter a=5.6458(1) Å) and shows positional disorder in two Cu‐sites (4e and 1a Wyckoff sites). In this report, a structural study was carried out on the same sample by means of single‐crystal X‐ray diffraction data (nitrogen‐cryostat) collected at 131 K. At this temperature, Cu4TiSe4 crystallizes in rhombohedral symmetry, space group R3 m, with a=7.9538(5) Å and c=19.4828(17) Å. The low temperature structure is fourfold twinned. In the low temperature phase, copper atoms at 9b and 3a Wyckoff sites (originally 4e and 1a Wyckoff sites in the room temperature structure) order partially in comparison to room temperature structure. This temperature induced phase transition induce slight increase of the thermal conductivity of the material.

Phase transition and electrical properties of Cu2SnSe3 nanocrystals under high pressure

In this research, Cu2SnSe3 semiconductor nanocrystals with uniform size distribution and morphology were synthesized via a hot-injection approach. On the basis of the synthesis of nanomaterials, pressure effects on the structural evolution of Cu2SnSe3 nanocrystals were investigated. Cu2SnSe3 nanocrystals undergo a reversible structural phase transition from zinc blende structure to rock salt phase at about 8.3 GPa. By measuring electrical properties of Cu2SnSe3 nanocrystals under high pressure, discontinuous changes in resistance induced by the structural phase transition were found.

Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight into Competitive Intermolecular Interactions and Control of Stacking Patterns.

Control over the stacking patterns in 2D molecular assemblies is demonstrated using chemical modification. A target system is a hydrogen-bonded cocrystal (2:1) composed of 2-pyrrolidone (Py) and chloranilic acid (CA) (PyCA). X-ray crystallography showed that weak intersheet interactions give rise to a variety of metastable overlapping patterns comprised of the 2D assemblies mainly formed via hydrogen bonds, affording reversible and irreversible structural phase transitions. We prepared cocrystals of Py and anilic acids bearing different halogens, in which 2D assemblies isostructural with those observed in PyCA exhibit various overlapping patterns. The order of stability for each overlapping pattern estimated using calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results, which can be explained by considering the entropic effect: the molecular motion of Py as detected using nuclear quadrupole resonance spectroscopy.