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Demonstrating the mechanism of a structural phase transition (SPT) is significantly important for guiding the precise design of new phase transition materials. Herein, an ion-pair compound [DPIm][Ni(mnt)2] (1, DPIm+ = 1,3-dipropylimidazolium and mnt2− = maleonitriledithiolate) has been obtained and thoroughly characterized through microanalysis, spectral, and thermal analysis techniques. The [Ni(mnt)2]− anions form columnar stacks, featuring one-dimensional (1D) S = ½ spin chains. 1 undergoes an SPT, associated with thermal, magnetic, and dielectric anomalies. The SPT results from the competition between antiferromagnetic (AFM) coupling interactions within anion stacks and elastic deformation of the lattice. The magnetic anomaly arises from changes in the magnetic coupling constant within an anion stack between two phases, while the dielectric anomaly is due to the volume-conserving motion of the imidazolyl ring in the cation. 1 exhibits high-κ characteristics, attributed to the electron polarization of both anions and cations, ion displacement, and dipole orientation polarization of cations. It is also demonstrated that a longer dialkyl chain inhibits the rotational freedom of the imidazolyl ring, resulting in the absence of a plastic crystal state in 1 compared to others containing [Ni(mnt)2]− and dialkylimidazolium. [ABSTRACT FROM AUTHOR]

A new haloargentate hybrid, [Et‐dabco]₂Ag₂Br₄ (1) (Et‐dabco+=1‐Ethyl‐1,4‐diazabicyclo[2.2.2]octan‐1‐ium), has been successfully synthesized by a solution diffusion method and characterized using microanalysis, single crystal X‐ray diffraction, variable‐temperature powder X‐ray diffraction (PXRD), differential scanning calorimetry (DSC), and dielectric spectra. The crystal structure of 1 at 273 K contains one‐dimensional (1D) anionic [Ag₂Br₄]2− chains, with the [Et‐dabco]+ cations embedded in the gaps between the inorganic chains. Hybrid 1 underwent a reversible structural phase transition (SPT) at around 464.5 K on heating, and the Pawley refinement revealed similar crystal structures in the low‐ and high‐temperature phases. The SPT endows 1 with switchable and bistable dielectric properties. Combined with previous studies, it was revealed that the self‐assembly of silver halide (AgX) with the lower rotational energy barrier of 1‐alkyl‐1,4‐diazabicyclo[2.2.2]octan‐1‐ium in solution probably achieves new phase transition materials. [ABSTRACT FROM AUTHOR]

Fe2P2O7 is a multifunctional material and has potential applications in a variety of fields but there exist controversies in the postulated space groups and studies on its phase transition and thermal expansion properties are still lacking. High-resolution synchrotron radiation x-ray diffraction, first-principles calculations, and specific heat capacity analyses are applied to solve these problems. The results reveal that Fe2P2O7 crystallizes in a triclinic structure with the C 1 ¯ space group at low temperatures (α phase) and undergoes successive phase transitions to an intermediate phase (α′) at about 346 K and a monoclinic structure with the B 2 1 / c space group (β phase) at about 456 K. Fe2P2O7 exhibits a low, giant negative, and near-zero thermal expansion in the regions of 100–325, 325–375, and 375–475 K, respectively. The Jahn–Teller effect of Fe2+ ions and their transition from the static to dynamic one by thermal activation are proposed to account for the unique thermal expansion/contraction properties. [ABSTRACT FROM AUTHOR]

In this study, three supramolecular compounds, namely [Na(18-crown-6)]ReO4 (1), [K(18-crown-6)]ReO4 (2) and [NH4(15-crown-5)2]ReO4 (3) were synthesized and characterized using microanalysis, thermogravimetric, differential scanning calorimetric and powder X-ray diffraction techniques, among others. Herein, 15-crown-5 and 18-crown-6 represent 1,4,7,10,13-pentaoxacyclopentadecane and 1,4,7,10,13,16-hexaoxacyclooctadecane, respectively. Both 1 and 2 experienced one-step reversible phase transition at ∼436 K and ∼360 K, respectively, while 3 underwent three steps of reversible phase transitions at ∼289 K, ∼352 K and ∼380 K. A dielectric anomaly occurred at ∼433 K (∼363 K) for 1 (2), consistent with the temperature obtained from DSC measurement. In the dielectric spectra of 3, anomalies were observed at ∼386 K and ∼433 K, corresponding to two thermal anomalies observed at higher temperatures in the DSC plot. However, there was an absence of dielectric response around 289 K where a thermal anomaly appeared in the DSC plot. The distinct crystal structures, phase transitions and dielectric properties in 1–3 stem from the degree of compatibility between the radii of M+ (Na+, K+ and NH4+) and the ring sizes of crown ethers (18-crown-6 and 15-crown-5). [ABSTRACT FROM AUTHOR]

In this study, two polymorphs of the [1,1′-dibutyl-4,4′-bipyridinium][Ni(mnt)2] salt (1) were synthesized. The dark-green polymorph (designated as 1-g) was prepared under ambient conditions by the rapid precipitation method in aqueous solutions. Subsequently, the red polymorph (labeled as 1-r) was obtained by subjecting 1-g to ultrasonication in MeOH at room temperature. Microanalysis, infrared spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD) techniques were used to characterize the two polymorphs. Both 1-g and 1-r exhibit structural phase transitions: a reversible phase transition at ∼403 K (∼268 K) upon heating and 384 K (∼252 K) upon cooling for 1-g (1-r) within the temperature range below 473 K. Interestingly, on heating 1-r to 523 K, an irreversible phase transition occurred at about 494 K, resulting in the conversion of 1-r into 1-g. Relative to 1-r, 1-g represents a thermodynamically metastable phase wherein numerous high-energy conformations in butyl chains of cations are confined within the lattice owing to quick precipitation or rapid annealing from higher temperatures. Through variable-temperature single crystal and powder X-ray diffractions, UV-visible spectroscopy, dielectric spectroscopy, and DSC analyses, this study delves into the mechanism underlying phase transitions for each polymorph and the manual transformation between 1-g and 1-r as well. [ABSTRACT FROM AUTHOR]

The supramolecular crystals, Mn(15-crown-5)(MnCl4)(DMF), (1; 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane), were synthesized via a self-assembly strategy under ambient conditions. Comprehensive characterization of the crystals involved microanalysis for C, H, and N elements, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC) and single-crystal X-ray diffraction techniques. The results reveal that 1 undergoes a two-step thermotropic and isostructural phase transition at around 217 K and 351 K upon heating. All three phases belong to the same space group (P212121) with analogous cell parameters. These two phase transitions primarily involve the thermally activated ring rotational dynamics of the 15-crown-5 molecule, with only the transition at ca. 351 K being associated with a dielectric anomaly. 1 exhibits intense luminescence with a peak at ∼600 nm and a high quantum yield of 68%. The mechanisms underlying this intense luminescence are likely linked to low-symmetry ligand fields. Additionally, 1 displays phase transition-induced luminescence enhancement behavior, and the possible mechanism is further discussed. [ABSTRACT FROM AUTHOR]

Crown ethers demonstrate significant conformational flexibility and rotational symmetry, rendering them invaluable in the realms of supramolecular chemistry and crystal engineering. These unique natures facilitate the construction of supramolecular crystals of crown ethers, characterized by disorder‐order phase transitions, imparts unique properties that hold promise for diverse applications across multiple fields. In this study, four supramolecular compounds, namely [Na(15‐crown‐5)]BF4 (1), [Na(18‐crown‐6)]BF4⋅H2O (2), [K(15‐crown‐5)]BF4 (3) and [K(18‐crown‐6)]BF4⋅H2O (4) were synthesized and characterized by microanalysis, thermogravimetric analysis, differential scanning calorimetry and powder X‐ray diffraction techniques. Herein, 15‐crown‐5 and 18‐crown‐6 correspond to 1,4,7,10,13‐pentaoxacyclopentadecane and 1,4,7,10,13,16‐hexaoxacyclooctadecane, respectively. It was observed that the crystal structure, phase transition, and dielectric properties of these supramolecular compounds are significantly influenced by the size of the crown‐ether rings. The research extensively discussed the correlation between the coordination mode of metal ions of K+ or Na+ with crown ethers, the compatibility between metal ions and crown‐ether rings in terms of size, and the effects of crown‐ether disorder on dielectric permittivity during phase transitions. Our discoveries hold significant implications for the design and development of crown‐ether supramolecular functional materials. [ABSTRACT FROM AUTHOR]

Two haloargentate hybrids, [Me-dabco]Ag2X3 (Me-dabco = 1-methyl-1,4-diazabicyclo-[2.2.2]octan-1-ium, X = I (1) or Br (2)), with the same formula but different structures have been synthesized by a slow evaporation method and characterized by microanalysis, infrared spectroscopy, thermogravimetric, and powder X-ray diffraction techniques. Hybrid 1 consists of completely isolated [Ag4I6]2− clusters, while hybrid 2 exhibits a complicated one-dimensional (1D) chain structure formed by four different configurations of neutral chains and two dissimilar configurations of anionic chains. Hybrid 2 undergoes two reversible order–disorder phase transitions, while hybrid 1 displays one reversible and one irreversible structural phase transition. Both 1 and 2 displayed step-like dielectric anomalies in the vicinity of the phase transition temperature. The corresponding dielectric constants in the high dielectric states are approximately 13 and 6 times higher than those in the low dielectric states for 1 and 2, respectively. Interestingly, the subtle change of halides from I− to Br− significantly affects the aggregated structure of haloargentate, the phase transition, and dielectric behaviors, revealing the typical 'butterfly effect' with the ion radii of halides in these two haloargentate hybrids. [ABSTRACT FROM AUTHOR]

A thermochromic or mechanochromic material can switch between at least two stable states in response to changes in temperature or static pressure/strain. In this study, we investigated a Ni-dithiolene dianion salt, 1,1′-diheptyl-4,4′-bipyridinium bis(maleonitriledithiolato)nickelate (1), and found that its cations and anions stack alternately to form a uniform mixed stack. These mixed stacks then combine to form a molecular solid through Coulomb and van der Waals interactions. Upon heating, 1 undergoes a reversible phase transition at around 340/320 K during the first heating/cooling cycle, resulting in rapid thermochromism with a color change from green (stable state) to red (metastable state) within a few seconds. This is the first report of a crystal of bis(maleonitriledithiolato)nickelate(II) salt with green color. Additionally, 1 exhibits irreversible mechanochromism, intense near-IR absorbance, and a dielectric anomaly. The structural phase transition is responsible for these properties, as it induces alterations in the π-orbital overlap between the anion and cation within a mixed stack. The intense near-IR absorbance arises from the ion-pair charge transfer transition from [Ni(mnt)2]2− to 4,4′-bipyridinium. [ABSTRACT FROM AUTHOR]

Herein, we demonstrated the unique advantage of a mechanochemical reaction to prepare a salt with hard and soft acid and base ions concurrently by solution synthesis owing to the soft acid preferring to combine with the soft base and vice versa. We prepared Bu4N1−xLixMnxPb1−xI3 (x = 0.011−0.14) by mechanochemical synthesis. The doping induced a structural phase transition at ∼342 K and much enhancement of ionic conduction above 342 K for all co-doped hybrids regarding Bu4NPbI3 because of the voids around the Mn2+/Li+ ions by doping. [ABSTRACT FROM AUTHOR]

Stimuli-responsive smart materials have applications in a range of technologies. Herein, we present a hybrid (1), built from Me3EtN+ organic cations and {Cu2I3−}∞ inorganic chains with Cu⋯Cu metal⋯metal interactions. The two-step phase transition undergone in 1 on the first heating and the phase transition at a lower temperature show symmetry-broken features, leading to switchable dielectrics; the one at a higher temperature displays isomorphic characteristics. Besides the switchable dielectrics, 1 exhibited other multi-stimuli-responsive functionalities, including thermochromism and piezochromism. Combining temperature-dependent powder and single crystal X-ray diffraction, as well as variable-temperature UV-visible absorption spectrum and EPR spectrum analyses, it is demonstrated that the thermochromism is due to the synergy of anharmonic fluctuations with electron–phonon couplings, and the piezochromism arises from compression inducing a lattice distortion in 1. Our study provides insight into understanding the thermochromic and piezochromic mechanisms of cuprous halide-based hybrids, and paves a pathway for designing new multi-stimuli-responsive hybrid materials. [ABSTRACT FROM AUTHOR]

The supramolecular lead iodide perovskite crystals, {[NH4(18-crown-6)]PbI3}∞ (1), (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane), was successfully achieved by a facile solvent evaporation strategy using a DMF solution containing equal molar quantities of PbI2, NH4I and 18-crown-6. The supramolecular perovskite was characterized by microanalysis for C, H and N elements, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and single crystal X-ray diffraction techniques. DSC measurements demonstrated that 1 experiences a two-step thermotropic phase transition around 333 K and 383 K, respectively. The phase transition is relevant to the disorder–order transformation of the 18-crown-6 molecule at ∼333 K, while both breaking-symmetry and ordered-disordered transformation of the 18-crown-6 molecule occurred at ∼383 K. In addition, the sharp change of the PbI6 coordination octahedron distortion degree plays a synergistic role in the two-step phase transition. The dielectric relaxation occurs above 243 K in 1 and is mainly attributed to the displacement of the NH4+ ions relative to the ring of the 18-crown-6 molecule and {PbI3}∞ chain induced by an AC electrical field. [ABSTRACT FROM AUTHOR]

Organic ionic plastic crystals (OIPCs) have been increasingly drawing attention recently owing to their many practical applications. In this study, seven organic salts [Cat1][Cat2][Ni(mnt)2] were prepared, where mnt2− = maleonitriledithiolate, and Cat1+ and Cat2+ represent globular quaternary ammonium cations (Me4N+, Et4N+, Pr4N+ and Bu4N+). Each salt is labeled, according to its cations, as Me, Et, Pr, Bu, MeEt, MeBu and EtBu, respectively. These salts were characterized by microanalysis, IR spectroscopy, thermogravimetry (TG), differential scanning calorimetry (DSC), and single crystal and powder X-ray diffraction techniques. Although the seven salts show analogous packing structures, consisting of alternating anion layers and cation layers, only Bu and MeBu are OIPCs, and, moreover, have multiple plastic crystal phases. Another striking feature in this family is that only cocrystals (MeEt, MeBu and EtBu) were obtained, while MePr, EtPr and PrBu were not achievable. The calculation of lattice formation energy per formula unit (ELatt/f.u.) revealed that the cocrystal is metastable relative to its two parent salts, but achievable if the ELatt/f.u. of the cocrystal is comparable to the average ELatt/f.u. of its two parent salts. Besides multiple phase transitions, the critical temperature of the initial phase transition and melting points in the cocrystal shift towards low temperature compared to those of its two parent salts, and the reason was discussed. Both Bu and MeBu in plastic crystal phases display high ionic conduction (σ > 10−3 S cm−1) and the other salts show ionic conductivities in the scope of 10−12–10−5 S cm−1 at 338–473 K. [ABSTRACT FROM AUTHOR]

Open-framework zinc phosphate (NMe4)(ZnP2O8H3) undergoes irreversible phase transformation. Structural transformation with α (NMe4·Zn[HPO4][H2PO4] the low-temperature phase) and β (NMe4·ZnH3[PO4]2 the high-temperature phase) (Tc = 149 °C) and conduction properties were investigated by single-crystal X-ray diffraction, differential scanning calorimetry, and alternating current (ac) impedance. The open-framework material was sensitive to humidity and β proton conductivity was higher than 10−2 S cm−1 at room temperature and 98% relative humidity (RH). Given that the high proton conductivity of the open-framework material can compete with that of many advanced proton conductors based on metal–organic frameworks (MOFs), it has broad application prospects in various electrochemical devices. [ABSTRACT FROM AUTHOR]

In this study, a multi-step phase transition hybrid composed of (Pr-dabco)2Ag4I6 clusters (Pr-dabco+ = 1-propyl-1,4-diazabicyclo[2.2.2]octan-1-ium) has been prepared and characterized by microanalysis, IR and UV-vis spectroscopy, TG and DSC techniques, etc. This hybrid is thermally stable up to ∼486 K with five phases in the temperature region below 486 K. The phase transition shows symmetry breaking (SB) character between phases II (space group P21/c) and III (space group Pa3¯), while inverse symmetry breaking (ISB) between phases II and I (space group Pbca), and it is rather exceptional for matter to exhibit simultaneously SB and ISB nature in two successive phase transitions. Most importantly, each phase transition is associated with a dielectric anomaly, and phase V appears to be a plastic crystal with extra high ac conductivity (>10−2 S cm−1). Our work opens up new avenues to find a multi-phase transition material in silver halide hybrids. [ABSTRACT FROM AUTHOR]

A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, respectively. On going from the low‐ to intermediate‐temperature phase, the space group changes from P21/c to Cmca, and the crystallographic axis perpendicular to the layers is doubled with the order–disorder transformation of (CH3)4N+ cations between the interlayers. The intermediate‐ and high‐temperature phases are isomorphic with similar cell parameters and packing structure; their main difference concerns the disorder degree of the (CH3)4N+ cations between the interlayers. The two‐step structural phase transitions lead to dielectric anomalies around the corresponding Tc. Interestingly, 1 shows multiband emission, originating from the recombination of exciton and emission of defects. Moreover, 1 exhibits divergent thermochromic luminescent features around the Tc on the intermediate to low temperature transition. Thermochromic luminescence: A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, leading to dielectric anomalies around the corresponding Tc and divergent thermochromic luminescent features. [ABSTRACT FROM AUTHOR]

A supramolecular crystal, built from a 1 : 1 molar ratio of potassium tetraphenylboron (KBPh4) with 1,4,7,10,13,16-hexaoxacyclooctadecane (18-Crown-6), contains an organometallic half-sandwich superstructure unit {K(18-Crown-6)(ηn-C6H5B(C6H5)3)} (1 with n = 1–6), and shows a reversible phase transition at ∼211 K. The analysis of crystal structures at 173 K in the low-temperature phase, and 293 and 423 K in the high-temperature phase revealed that the breaking-symmetry phase transition is associated with the order–disorder transformation of [K(18-Crown-6)]+ and the change of the coordination mode of the phenyl ring to K+. A dielectric anomaly appears at ca. 212 K and dielectric relaxation occurs above 375 K in 1. The dielectric and thermal anomaly temperatures are close to each other, and the dielectric relaxation is relevant to the relative displacement of [K(18-Crown-6)]+ and the tetraphenylboron anion. [ABSTRACT FROM AUTHOR]

A supramolecular crystal, [Na(15-crown-5)][BPh4] (1), (15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane, NaBPh4 = sodium tetraphenylboron), has been obtained by mixing the ethanol solution of 15-crown-5 and NaBPh4 in the molar ratio of 1 : 1. The crystal structure was determined at 293 K, revealing that two [Na(15-crown-5)]+ cations form a supramolecular dimer via sharing one side-edge of coordination pentagonal pyramids; also, there are significant H-bonding interactions between anions and supramolecularly dimeric cations. Differential scanning calorimetry (DSC) showed that 1 undergoes a reversible first-order phase transition at ca. 391 K (Tc) upon heating, with a thermal hysteresis of 19 K. ΔH and ΔS were estimated to be 6.9 kJ mol−1 and 17.7 J mol−1 K−1, respectively, in the heating run. The variable-temperature powder X-ray diffraction and dielectric spectra were collected, and both disclosed no significant difference between the low- and high-temperature phases. These results suggest that the phase transition is an ordered–disordered type, which probably involves the change of anion configuration. [ABSTRACT FROM AUTHOR]

Co-crystal [4,4ʹ-bipyridine]2[1,4-dihydroxybenzene] (1) was prepared by evaporating methanol solution of 4,4ʹ-bipyridine with 1,4-dihydroxybenzene at 4°C.1shows a fascinating dielectric anomaly with a maximum at ~210 K, which is independent on ac frequency. However, the absence of thermal anomaly and structural phase transition is related to the dielectric anomaly. The co-crystal [4,4ʹ-bipyridine]2[1,4-dihydroxybenzene-d2] (2) was further obtained via H+/D+exchange in1and the deuteration gives rise to the maximum shifting to 254 K in the dielectric spectrum of2. The dielectric anomaly corresponds to disorder-to-order transformation of H+/D+in the O–H/D···N H-bond in1and2. [ABSTRACT FROM AUTHOR]

An iodoplumbate-based hybrid, [C7-Apy][PbI3] (1), where C7-Apy+ = 1-heptyl-4-aminopyridinium, was prepared using a simple solution process. Three sequential phase transitions occur in the range of 402–443 K. In both the lowest and highest temperature phases, hybrid crystal 1 is composed of discrete [Pb2I6]∞ twin chains surrounded by C7-Apy+ cations. The connectivity between PbI6 octahedra within a [Pb2I6]∞ twin chain and the arrangement of cations are quite difference between the lowest and highest temperature phases. Hybrid crystal 1 shows switchable ion conductivity due to the structural phase transition and white light emission attributed to the broad band semiconductor emission of the twin chain. The former functionality has potential application in ion conductor devices; the single-phase white light emitter is a useful material in low-cost, easily-made, high-efficiency white light-emitting diodes. [ABSTRACT FROM AUTHOR]

Tuning the lattice degree of freedom in nanoscale functional crystals is critical to exploit the emerging functionalities such as piezoelectricity, shape-memory effect, or piezomagnetism, which are attributed to the intrinsic lattice-polar or lattice-spin coupling. Here it is reported that a mechanical probe can be a dynamic tool to switch the ferroic orders at the nanoscale multiferroic phase boundaries in BiFeO3 with a phase mixture, where the material can be reversibly transformed between the 'soft' tetragonal-like and the 'hard' rhombohedral-like structures. The microscopic origin of the nonvolatile mechanical switching of the multiferroic phase boundaries, coupled with a reversible 180° rotation of the in-plane ferroelectric polarization, is the nanoscale pressure-induced elastic deformation and reconstruction of the spontaneous strain gradient across the multiferroic phase boundaries. The reversible control of the room-temperature multiple ferroic orders using a pure mechanical stimulus may bring us a new pathway to achieve the potential energy conversion and sensing applications. [ABSTRACT FROM AUTHOR]

A one-dimensional (1-D) magnetic chain compound, [(CD3)2C=N-Py][Ni(mnt)2], where (CD3)2C=N-Py+ = 1-(propan-d6-2-ylideneamino)pyridinium and mnt2− = maleonitriledithiolate, was synthesized and characterized. This compound undergoes two structural phase transitions, at around 267 K and 325 K. The three phases are sequentially labeled as α (below 267 K), β (between 267 and 325 K) and γ (above 325 K). The [Ni(mnt)2]− anions form irregular stacks, with two values of neighboring Ni…Ni distances. The cations align in a straight and regular arrangement along the crystallographic a-axis direction in the β phase. The asymmetric unit switches from one anion–cation pair to two anion–cation pairs in the transition from the β to the α phase. This leads to a doubling of the crystallographic axis length, parallel to the anion stack. It also results in the irregular anion stack showing three non-equivalent neighboring Ni…Ni distances, as well as making the straight cation arrangement irregular. The β and γ phases are isostructural and exhibit quite similar packing structures. The anion stack shrinks a little with increasing temperature, with all crystallographic axes showing small reduction from the β to γ phase. The two observed magnetic phase transitions are driven by these various structural phase transitions. A thermal hysteresis loop, with a separation of ∼8 K, appears in the transition between the β and α phases. However, this is absent in the transition between the β and γ phases. A dielectric anomaly appears across the phase transition between the β and γ phases but not between the β and α phases. [ABSTRACT FROM AUTHOR]

We investigate possible phase transitions among the different quantum anomalous Hall (QAH) phases in single-layer graphene under the influence of the exchange field. The effective tight-binding Hamiltonian for graphene is made up of the hopping term, the Kane-Mele and Rashba spin-orbit couplings as well as the Haldane orbital term. We find that the variation of the exchange field results in bulk gap-closing phenomena and phase transitions occur in the graphene system. If the Haldane orbital coupling is absent, the phase transition between the chiral (antichiral) edge state v = +2 (v = -2) and the pseudoquantum spin Hall state (v = 0) takes place. Surprisingly, when the Haldane orbital coupling is taken into account, an intermediate QSH phase with two additional edge modes appears in between phases v = +2 and v = -2. This intermediate phase is therefore either the hyperchiral edge state of high Chern number v = +4 or antihyperchiral edge state of v = -4 when the direction of exchange field is reversed. We present the band structures, edge state wave functions, and current distributions of the different QAH phases in the system. We also report the critical exchange field values for the QAH phase transitions. [ABSTRACT FROM AUTHOR]

Abstract: A nonmagnetic compound, [NO2BzPy][Cu(mnt)2] (mnt2− =maleonitriledithiolate; NO2BzPy+ =1-(4′-nitrobenzyl)pyridinium), is isostructural with [NO2BzPy][Ni(mnt)2], which is a quasi-1D spin system and exhibits a spin-Peierls-like transition with J =192K in the gapless state and spin energy gap=738K in the dimerization state, respectively. Further, five nonmagnetic impurity doped compounds [NO2BzPy][Cu x Ni1− x (mnt)2] (x =0.04–0.74) were prepared, and their crystal structures as well as magnetic properties were investigated. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 139(13)K/percentage of dopant concentration, and the transition collapse is estimated at around x >0.5. [Copyright &y& Elsevier]

Quantum critical behaviors induced by a putative quantum phase transition are vigilantly investigated, which separates a d -wave superconducting state and d -wave superconducting+ X state below the superconducting dome of the d −wave superconductors with tuning the non-thermal doping variable. Within the framework of renormalization group approach, we start with a phenomenological effective theory originated from the Landau-Ginzburg-Wilson theory and practice one-loop calculations to construct a set of coupled flows of all interaction parameters. After extracting related physical information from these coupled evolutions, we address that both fermion velocities and critical temperatures exhibit critical behaviors, which are robust enough against the initial conditions due to strong quantum fluctuations. At first, the evolution of Yukawa coupling between X -state order parameter and nodal fermions in tandem with quantum fluctuations heavily renormalize fermion velocities and generally drive them into certain finite anisotropic fixed point at the lowest-energy limit, whose concrete value relies upon the very quantum phase transition. In addition, these unique properties of fermion velocities largely reshape the fate of superfluid density, giving rise to either an enhancement or a dip of critical temperature. Moreover, we find that fermion-fermion interactions bring non-ignorable quantitative corrections to quantum critical behaviors despite they are subordinate to quantum fluctuations of order parameters. [ABSTRACT FROM AUTHOR]

Two deuteriumed quasi-one-dimensional (quasi-1D) spin-Peierls-type compounds, 4-X-benzylpyridinium-d5bis(maleo-nitriledithiolato)nickelate (the substituent X = Br or Cl), were structurally characterized. Compared with the corresponding non-deuteration compounds, the transition temperature TCshifts to higher temperature. The isotopic effect of countercations on TCis probably related to the change of phonon frequency ω0and ‘chemical pressure’ resulted from the substitution of pyridine by pyridine-d5. [ABSTRACT FROM AUTHOR]

A new haloargentate hybrid, [Et‐dabco]₂Ag₂Br₄ (1) (Et‐dabco+=1‐Ethyl‐1,4‐diazabicyclo[2.2.2]octan‐1‐ium), has been successfully synthesized by a solution diffusion method and characterized using microanalysis, single crystal X‐ray diffraction, variable‐temperature powder X‐ray diffraction (PXRD), differential scanning calorimetry (DSC), and dielectric spectra. The crystal structure of 1 at 273 K contains one‐dimensional (1D) anionic [Ag₂Br₄]2− chains, with the [Et‐dabco]+ cations embedded in the gaps between the inorganic chains. Hybrid 1 underwent a reversible structural phase transition (SPT) at around 464.5 K on heating, and the Pawley refinement revealed similar crystal structures in the low‐ and high‐temperature phases. The SPT endows 1 with switchable and bistable dielectric properties. Combined with previous studies, it was revealed that the self‐assembly of silver halide (AgX) with the lower rotational energy barrier of 1‐alkyl‐1,4‐diazabicyclo[2.2.2]octan‐1‐ium in solution probably achieves new phase transition materials. [ABSTRACT FROM AUTHOR]

Fe2P2O7 is a multifunctional material and has potential applications in a variety of fields but there exist controversies in the postulated space groups and studies on its phase transition and thermal expansion properties are still lacking. High-resolution synchrotron radiation x-ray diffraction, first-principles calculations, and specific heat capacity analyses are applied to solve these problems. The results reveal that Fe2P2O7 crystallizes in a triclinic structure with the C 1 ¯ space group at low temperatures (α phase) and undergoes successive phase transitions to an intermediate phase (α′) at about 346 K and a monoclinic structure with the B 2 1 / c space group (β phase) at about 456 K. Fe2P2O7 exhibits a low, giant negative, and near-zero thermal expansion in the regions of 100–325, 325–375, and 375–475 K, respectively. The Jahn–Teller effect of Fe2+ ions and their transition from the static to dynamic one by thermal activation are proposed to account for the unique thermal expansion/contraction properties. [ABSTRACT FROM AUTHOR]

The supramolecular lead iodide perovskite crystals, {[NH4(18-crown-6)]PbI3}∞ (1), (18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane), was successfully achieved by a facile solvent evaporation strategy using a DMF solution containing equal molar quantities of PbI2, NH4I and 18-crown-6. The supramolecular perovskite was characterized by microanalysis for C, H and N elements, thermogravimetric (TG) analysis, differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD) and single crystal X-ray diffraction techniques. DSC measurements demonstrated that 1 experiences a two-step thermotropic phase transition around 333 K and 383 K, respectively. The phase transition is relevant to the disorder–order transformation of the 18-crown-6 molecule at ∼333 K, while both breaking-symmetry and ordered-disordered transformation of the 18-crown-6 molecule occurred at ∼383 K. In addition, the sharp change of the PbI6 coordination octahedron distortion degree plays a synergistic role in the two-step phase transition. The dielectric relaxation occurs above 243 K in 1 and is mainly attributed to the displacement of the NH4+ ions relative to the ring of the 18-crown-6 molecule and {PbI3}∞ chain induced by an AC electrical field. [ABSTRACT FROM AUTHOR]

Two new one-dimensional (1-D) molecular solids, [CH 2 = CH-BzPy][Pt(mnt) 2 ] ( 1 ) and [CH 2 = CH-BzPy-d 5 ][Pt(mnt) 2 ] ( 2 ) where CH 2 = CH-BzPy-d 5 + = 1- N -(4-vinylbenzyl) pyridinium-d 5 and mnt 2− = maleonitriledithiolate, were synthesized and characterized. A structural phase transition undergoes with T C ≈ 335 K in the crystals of 1 and 2 , which were confirmed by variable-temperature magnetic susceptibility and DSC measurements. Crystals of 1 and 2 are isostructural in both high- and low-temperature phases. Besides the strong similarity in crystal structures, they show quite analogous magnetic and dielectric features. Above T C , the magnetic behavior is failed to follow the S = ½ Heisenberg linear chain model; below T C , a spin gap is opened for 1 and 2 . The thermally-assisted dielectric relaxation occurs above ca. 280 K for 1 and 2 , and this relaxation process is related to the dipole motion of the disordered CH 2 CH groups in the cation under an ac electrical field. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new one-dimensional molecular solid was achieved. [•] This molecular solid shows a hysteretic magnetic transition occurs at ca. 80K upon cooling with 5.6K thermal hysteresis. [•] Intramolecular vibrational modes cooperate with the magnetic phase transition. [•] There exists pre-transition phenomenon up until 100K in this molecular solid. [Copyright &y& Elsevier]

A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, respectively. On going from the low‐ to intermediate‐temperature phase, the space group changes from P21/c to Cmca, and the crystallographic axis perpendicular to the layers is doubled with the order–disorder transformation of (CH3)4N+ cations between the interlayers. The intermediate‐ and high‐temperature phases are isomorphic with similar cell parameters and packing structure; their main difference concerns the disorder degree of the (CH3)4N+ cations between the interlayers. The two‐step structural phase transitions lead to dielectric anomalies around the corresponding Tc. Interestingly, 1 shows multiband emission, originating from the recombination of exciton and emission of defects. Moreover, 1 exhibits divergent thermochromic luminescent features around the Tc on the intermediate to low temperature transition. Thermochromic luminescence: A direct band gap 2D corrugated layer lead chloride hybrid, [(CH3)4N]4Pb3Cl10 (1), shows analogous topology to the {Mg3F104−}∞ layer in Cs4Mg3F10, and with the (CH3)4N+ cations locating in the inorganic layer voids and between the interlayers. Two reversible structural phase transitions occur in 1 at 225/210 K and 328/325 K upon heating/cooling, leading to dielectric anomalies around the corresponding Tc and divergent thermochromic luminescent features. [ABSTRACT FROM AUTHOR]

The compound [4'-CF3bzPy][Ni(mnt)2] (l) (where 4'-CF3bzPy = l-(4'-(trifluoromethyl)benzyl)pyridinium and mnt2- = maleonitriledithiolate) was synthesized and displays a magnetic bistability with a surprisingly large thermal hysteresis loop (~49 K). X-ray crystallographic studies reveal that in the high-temperature (HT) phase the anions and cations form mixed stacks, with alternating anion dimers (AA) and cation dimers (CC) in an ..AACCAACC... fashion along the crystallographic a + b direction, and disordered CF3 groups in the cations are aligned into a molecular layer parallel to the crystallographic (001) plane. However, in the low-temperature (LT) phase, the c-axis length of the unit cell is roughly doubled, and the asymmetric unit switches from one [4'-CF3bzPy][Ni(mnt)2] pair in the HT phase to two [4'-CF3bzPy] [Ni(mnt)2] pairs. Most interestingly, the CF3 group in the cations becomes ordered, and the conformation of one of two crystallographically different cations changes significandy. A dislocation motion between the neighboring molecular layers emerges as well. The analyses of the magnetic susceptibilities and the density functional theory calculations suggest that the antiferromagnetic exchange interaction within one of two types of [Ni(mnt)2]22- dimers in the LT phase is much stronger than that within the [Ni(mnt)2]22- dimer in the HT phase. The lattice reorganization during this phase transition is proposed to be responsible for the wide thermal hysteresis loop. [ABSTRACT FROM AUTHOR]

Abstract: An ion-pair compound 1,6-bis(1-methylimidazolium)hexane bis(maleonitrio-dithiolato)nickelate was synthesized and structurally characterized. This compound crystallizes in the triclinic space group P-1, and the anions form the alternating stacks in the crystal. Reversible three-step solid-to-solid phase transitions occur at ~210, ~320 and ~432K upon cooling. The entropy change of phase transition was estimated to be 25.4J⋅K−1⋅mol−1 and 9.97J⋅K−1⋅mol−1 for the transitions at ~320 and ~432K, respectively. The hysteresis effect was observed in the χm–T plot for the transitions at ~210 and ~320K. The mechanisms of transitions are discussed based on the crystal structure and thermal analyses. [Copyright &y& Elsevier]

Three crystalline phases of [4′-CH3-Bz-NH2Py][Pt(mnt)2] (1) (4′-CH3-Bz-NH2Py= 1-N-(4′-methylbenzyl)-4-aminopyridiniumand mnt2–= maleonitriledithiolate) were isolated,identified, and characterized. The pure 1α·solventand 1βphases can be obtained by the evaporationof 1in CH3CN/CH2Cl2(V:V = 1:5) and CH3CN/CH3OH (V:V = 1:1) mixedsolvents at ambient temperature, respectively; the pure 1γphase is easily formed via the evaporation of 1inCH3OH or mixed solvent of CH3OH/CH2Cl2at room temperature. 1α·solventand 1βphases crystallize in the triclinic spacegroup P1̅ and possess quite similar cell parametersand packing structures. The anions and cations form the segregatedand regular stacks in both 1α·solvent and 1βphases; moreover, the cations show the same orientationin a stack. The phase 1γwith monoclinic spacegroup P21/cexhibitsthe separated and non-equidistant stacks of anion and cation, andthe neighboring cations are aligned in a chair-type conformation ina stack. Charge-assisted intermolecular hydrogen-bonds exist betweenthe anions and cations in three different crystalline phases. Dramaticallydifferent magnetic properties were observed for three crystallinephases, a spin-Peierls-type transition occurred around 260 K for 1α·solvent, while it did not appear over the temperaturerange 1.8–300 K for 1β, although two crystallinephases show rather analogous crystal structures. The phase 1γshows the magnetic behavior of an S= 1/2Heisenberg alternating-exchange antiferromagnetic linearchain. [ABSTRACT FROM AUTHOR]

Review: this review is intended to give a self-consistent and logical account of strucural change occurring simultaneously with a ferromagnetic transition, which may provide new insight for developing highly magneto-responsive materials; 75 refs [ABSTRACT FROM AUTHOR]