Wide Thermal Hysteresis Loop Research Articles

Scalable room-temperature dielectric switchable composites with giant contrast ratio and great cycle stability

Room-temperature dielectric switching materials are highly desirable for many potential industrial applications in smart electronic/electrical engineering fields, but they suffer from key limitation of poor integration of remarkable dielectric switching properties, promising scalability, and low cost. Here, a general solution to solve this problem is demonstrated to attain an excellent cost/performance balance. By mixing ionic liquids (ILs) with polyethylene glycol (PEG), an all-organic composite enables an ultrahigh dielectric switching ratio above 320.6 at 34.2 Hz, almost 100% dielectric constant retention after 500 cycles, the highest attainable to a ∼35 °C wide thermal hysteresis loop, and tunable dielectric transition temperature. Also, the PEG/ILs can defeat the conflict of dielectric switching properties versus cost, namely impressive dielectric switching properties are achieved through simple mixing of commercialized components without solvent. The findings offer promise for highly efficient, scalable, low-cost, and environmentally friendly preparation of high-performance room-temperature dielectric switching materials.

Ultra‐Tough Room‐Temperature Dielectric Switching Ionic Gels with Long‐Cycle Stability

AbstractAs a favorable candidate for the next‐generation smart electronic devices, thermo‐responsive dielectric materials are faced with two crucial challenges: insufficient mechanical toughness and lack of the combination of promising dielectric switching properties, desired mechanical properties, and long‐cycle stability. Herein, a new scalable strategy is proposed for designing thermo‐responsive dielectric switching materials that simultaneously integrate the individual features, such as promising dielectric switching properties, outstanding mechanical properties, and great cycle stability, into one gel, based on a new dual dielectric switching mechanism induced by interfacial structure evolution. The ionic gel can readily achieve a superb combination of distinct reversible dielectric bistability, a high dielectric switching ratio above 150, a ≈15 °C wide thermal hysteresis loop, and tunable room‐temperature dielectric transition behavior, impressive high ductility, desirable high mechanical strength, record‐high stability of at least 1000 cycles. Such an all‐in‐one design enhances the adaptability to multiple application scenarios, durability, and a lifetime of the dielectric switching gels. Together with the facile fabrication process and recyclable thermoplastic system, thereby contributing to cost and energy saving, this research provides a feasible and sustainable strategy for constructing highly desirable thermo‐responsive dielectric switching materials.

Preparation and characterization of spin crossover thin solid films

Iron(II) spin crossover complexes display a reversible transition from low-spin (LS) state to high-spin (HS) state by e.g. variation of temperature, pressure or by irradiation with light. Therefore, these systems are promising candidates for information storage materials. In view of practical device applications thin films of these materials are needed. The SCO-compound [Fe(Htrz)2(trz)] (BF4) (1) switches between the LS and the HS state with a 50 K wide thermal hysteresis loop above room temperature. We have prepared thin films of 1 on a SiO2 substrate by spin coating. The spin states of the films have been characterized by Mossbauer spectroscopy in reflection mode using a MIMOS II spectrometer. A low quadrupole splitting (LS state) at 300 K and a high quadrupole splitting (HS state) at 400 K were found for the film, as well as for bulk powder of 1. This confirms that a spin crossover occurs above room temperature. Furthermore, synchrotron based nuclear resonance scattering measurements from 80 K to 400 K indicate that the hyperfine parameters are similar to those of the bulk powder of 1. DFT calculations reproduce the experimentally determined Fe-vibrational density of states of the bulk and of the thin film sample of 1. These results indicate that a higher fraction of HS Fe atoms is present in the film of 1. Therefore, we conclude different SCO properties of the thin film and the bulk material of 1.

Guest-Adaptable Spin Crossover Properties in a Dinuclear Species Underpinned by Supramolecular Interactions.

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introduce structural stabilizing supramolecular interactions into a dinuclear material and henceforth demonstrate a dynamic structural and spin crossover property interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A = [FeII2( o-NTrz)5(NCS)4]; 4-( o-nitrobenzyl)imino-1,2,4-triazole). Relatively uncommon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a single-crystal to single-crystal transformation with affiliated phase transition resulting in a reversible transformation from one- to two-step spin crossover (SCO) transition character. We additionally present the water-saturated product (A·3H2O), which distinctly shows an abrupt one-step SCO character with a 22 K wide thermal hysteresis loop. Detailed structure-property analysis highlights that the substantial structural malleability and guest-adaptable SCO properties of this dinuclear species are afforded by the supportive, yet flexible, supramolecular interaction pathways derived from the ligand functionalization.

Large thermal hysteresis for iron(II) spin crossover complexes with N-(pyrid-4-yl)isonicotinamide.

A new series of iron(II) 1D coordination polymers with the general formula [FeL1(pina)]·xsolvent with L1 being a tetradentate N2O2(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], and pina being a bridging axial ligand N-(pyrid-4-yl)isonicotinamide, are discussed. The X-ray crystal structure of [FeL1(pina)]·2MeOH was solved for the low-spin state. The compound crystallizes in the monoclinic space group P21/c, and the analysis of the crystal packing reveals the formation of a hydrogen bond network where additional methanol molecules are included. Different magnetic properties are observed for the seven samples analyzed, depending on the nature of the included solvent molecules. The widest hysteresis loop is observed for a fine crystalline sample of composition [FeL1(pina)]·xH2O/MeOH. The 88 K wide thermal hysteresis loop (T1/2↑ = 328 K and T1/2↓ = 240 K) is centered around room temperature and can be repeated without of a loss of the spin transition properties. For the single crystals of [FeL1(pina)]·2MeOH, a 51 K wide hysteresis loop is observed (T1/2↑ = 296 K and T1/2↓ = 245 K) that is also stable for several cycles. For a powder sample of [FeL1(pina)]·0.5H2O·0.5MeOH a cooperative spin transition with a 46 K wide hysteresis loop around room temperature is observed (T1/2↑ = 321 K and T1/2↓ = 275 K). This compound was further investigated using Mössbauer spectroscopy and DSC. Both methods reveal that, in the cooling mode, the spin transition is accompanied by a phase transition while in the heating mode a loss of the included methanol is observed that leads to a loss of the spin transition properties. These results show that the pina ligand was used successfully in a crystal-engineering-like approach to generate 1D coordination polymers and improve their spin crossover properties.

Remarkable Scan Rate Dependence for a Highly Constrained Dinuclear Iron(II) Spin Crossover Complex with a Wide Thermal Hysteresis Loop

The abrupt [HS-HS] ↔ localized [HS-LS] spin crossovers of a new triazole-based diiron(II) complex result in a record-equaling thermal hysteresis loop width for a dinuclear complex (ΔT = 22 K by SQUID magnetometer in "settle" mode) and show a remarkable scan rate dependence of only the cooling branch, as revealed by detailed magnetic, DSC, and Mössbauer studies.

Disorder–Order Transformation and Significant Dislocation Motion Cooperating with a Surprisingly Large Hysteretic Magnetic Transition in a Nickel–Bisdithiolene Spin System

The compound [4'-CF3bzPy][Ni(mnt)2] (1) (where 4'-CF3bzPy = 1-(4'-(trifluoromethyl)benzyl)pyridinium and mnt(2-) = 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 significantly. 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]2(2-) dimers in the LT phase is much stronger than that within the [Ni(mnt)2]2(2-) dimer in the HT phase. The lattice reorganization during this phase transition is proposed to be responsible for the wide thermal hysteresis loop.

Spin‐Crossover Behaviors Based on Intermolecular Interactions for Cobalt(II) Complexes with Long Alkyl Chains

AbstractA new family of terpyridine (terpy)‐based spin‐crossover cobalt(II) complexes, [Co(HO–Cn‐terpy)2](BF4)2 [n = 6 (1), 8 (2), 10 (3), 12 (4), and 14 (5)] that possesses a hydroxy‐appended long alkyl chain present on the terpyridine skeleton was synthesized and characterized by X‐ray structure analysis and temperature‐dependent magnetic susceptibility. In the solid state, the compounds 1·H2O, 4, and 5·H2O were crystallized in the triclinic P$\bar {1}$ space groups, and their central cobalt(II) ion was coordinated by six nitrogen atoms from two terpyridine units to form a pseudo‐octahedral symmetry. In the case of 1·H2O, one of the terminal hydroxy substituents was involved in hydrogen bonding with a water molecule. For compound 4, on the other hand, both of terminal hydroxy units form a short intermolecular contact with the BF4– anion. The temperature‐dependent magnetic behavior of 1 and 2 showed a gradual spin crossover, whereas 3 and 4 revealed an abrupt spin transition with a wide thermal hysteresis loop (ΔT = 21 K, T1/2↑ = 316 K, and T1/2↓ = 295 K for 3; ΔT = 40 K, T1/2↑ = 348 K, and T1/2↓ = 308 K for 4), and 5 exhibits “reverse spin transition” (ΔT = 17 K, T1/2↑ = 214 K, andT1/2↓ = 231 K). It has been suggested that compounds 3 and 4 form stronger intermolecular interactions through the fastener effect and hydrogen bonding than 1 and 2. On the basis of a comparison of the resulting magnetic behaviors of the compounds that contain long alkyl chains with a hydroxy group at the end, it was found that the long alkyl chains with the terminal hydroxy group could induce stronger intermolecular interactions between molecules. However, there are many short contacts through the counteranions for 5·H2O, but thermal motion of the long alkyl chains can be expected. Compound 5 exhibits “reverse spin transition” induced by a structural phase transition.

Unique spin transition and wide thermal hysteresis loop for a cobalt(ii) compound with long alkyl chain

The cobalt(II) compounds with long alkyl chains, [Co(C12-terpy)(2)](BF(4))(2)·EtOH·0.5H(2)O(1·EtOH·0.5H(2)O) and [Co(C12-terpy)(2)](BF(4))(2) (1) was synthesized and characterized. The compound 1·EtOH·0.5H(2)O exhibits a "re-entrant spin crossover". The compound 1 exhibits the reentrant spin crossover and multi phase transitions with a wide thermal hysteresis loop.

Cooperative spin transition in a lipid layer like system

A novel iron(II) mononuclear spin transition complex [FeL(py)(2)] displays an abrupt spin transition around 225 K accompanied by a very wide thermal hysteresis loop (∼50 K) that spreads out over 100 K. Crystal structure analysis in both low-spin and high-spin states reveals a lipid layer-like arrangement of the complex molecules and provides insights into the spin switching mechanism.

Effects of Metal Doping on the Spin-Crossover Properties of an Iron(II) Complex with Extended π-Conjugated Schiff-Base Ligand Having an N4O2 Donor Set

Abstract The spin-crossover (SCO) complex [Fe(qnal)2]·CH2Cl2 (1), (Hqnal: N-(8′-quinolyl)-2-hydroxy-1-naphthaldimine) with an N4O2 donor set, has been synthesized and characterized. Investigation of magnetic properties shows that the complex exhibits an abrupt and complete spin transition with a 5 K wide thermal hysteresis loop. The X-ray diffraction analysis of complex 1 reveals that the molecules are connected into a quasi one-dimensional chain through extended π–π interactions between aromatic rings of ligands. The effects of metal doping on SCO properties have been investigated in the mixed-metal system [Fe1−xMx(qnal)2]·CH2Cl2 (M = zinc(II) and nickel(II)). The results reveal that metal doping increases the gradual character of spin transition, and no marked differences found between zinc and nickel doping, which suggest the dominant effect of the doping-degree (concentration) rather than metal species on cooperativity. However, the metal doping shows different effects on critical temperature (T1/2), where a more pronounced descending of T1/2 is observed in response to increased Zn-doping than in Ni-doping, indicating the noticeable consequence of internal pressure due to the different radii of doping metal ions.

Structure and Magnetic Properties of Iron(II/III) Complexes with N2O22– Coordinating Schiff Base Like Ligands (Eur. J. Inorg. Chem. 4/2009)

AbstractThe cover picture shows the general formula of the iron complexes presented in the Microreview by B. Weber and E.‐G. Jäger on p. 465ff with single crystals of one of the complexes in the background. The application of this system ranges over model compounds for biological systems to molecular magnets or molecular switches. The examples show the reversible binding of the biomolecule nitric oxide, a spin‐crossover complex with a 70 K wide thermal hysteresis loop around room temperature and an example for spontaneous magnetic ordering below 10 K.

Graphical Abstract: Eur. J. Inorg. Chem. 4/2009

AbstractThe cover picture shows the general formula of the iron complexes presented in the Microreview by B. Weber and E.‐G. Jäger on p. 465ff with single crystals of one of the complexes in the background. The application of this system ranges over model compounds for biological systems to molecular magnets or molecular switches. The examples show the reversible binding of the biomolecule nitric oxide, a spin‐crossover complex with a 70 K wide thermal hysteresis loop around room temperature and an example for spontaneous magnetic ordering below 10 K.

Cooperative Iron(II) Spin Crossover Complexes with N4O2Coordination Sphere

Two new spin crossover complexes [FeL(py)(2)] (1) and [FeL(DMAP)(2)] (2) with L being a tetradentate N(2)O(2)(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], py = pyridine and DMAP = p-dimethylaminopyridine have been investigated using temperature-dependent susceptibility and thermogravimetric and photomagnetic measurements as well as Mössbauer spectroscopy and X-ray structure analysis. Both complexes show a cooperative spin transition with an approximately 9 K wide thermal hysteresis loop in the case of 2 (T(1/2) upward arrow = 183 K and T(1/2) downward arrow = 174 K) and an approximately 2 K wide thermal hysteresis loop in the case of the pyridine diadduct 1 (T(1/2) upward arrow = 191 K and T(1/2) downward arrow = 189 K). The spin transition was additionally followed by different temperature-scanning calorimetry and Mössbauer spectroscopy for 2, and a good agreement for the transition temperatures obtained with the different methods was found. Results from X-ray structure analysis indicate that the cooperative interactions are due to elastic interactions in both compounds. They are more pronounced in the case of 2 with very short intermolecular iron-iron distances of 7.2 A and several intense C-C contacts. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bit of the equatorial ligand, from 108 degrees in the high-spin state to 90 degrees in the low-spin state. The reflectivity measurements of both compounds give at low temperature indication that at the sample surface the light-induced excited spin state trapping (LIESST) effect occurs. In bulk condition using a SQUID magnetometer the complex 2 displays some photomagnetic properties with an photoexcitation level of 60% and a T(LIESST) value of 53 K.

Multi-Dimensional Crystal Structures and Unique Solid-State Properties of Heterocyclic Thiazyl Radicals and Related Materials

Abstract The crystal structures and magnetic properties of heterocyclic thiazyl radicals and related materials have been examined. TTTA (=1,3,5-trithia-2,4,6-triazapentalenyl) exhibited a first-order phase transition between a paramagnetic high-temperature (HT) phase and a diamagnetic low-temperature (LT) phase, with a wide thermal hysteresis loop over the temperature range 230–305 K. The phase control of TTTA was achieved by pressure and by light irradiation. BDTA (=1,3,2-benzodithiazolyl) also exhibited a diamagnetic–paramagnetic phase transition above room temperature. However, fresh samples always exhibited a superheating of the LT phase that resulted in a double melting (melt–recrystallization–melt process) and supercooling of the HT phase, which in turn led to an antiferromagnetic ordering at 11 K. The molecular compounds of thiazyl radicals were prepared; TTTA formed a coordination polymer structure in the TTTA·[Cu(hfac)2 (=bis(hexafluoroacetylacetonato)–copper(II))] crystal, where a ferromagnetic coupling was found between the organic and inorganic species. The cation radical salts, [BBDTA (=benzo[1,2-d:4,5-d′]bis[1,3,2]dithiazole)]·MCl4 (M = Ga and Fe), exhibited ferromagnetic ordering at 7 K and ferrimagnetic ordering at 44 K after evaporation of crystal solvents. We also grew crystals of M–TTDPz (TTDPz = tetrakis(thiadiazole)porphyrazine and M = H2, Fe, Co, Ni, Cu, and Zn) and performed their structural analyses. Their crystal structures were found to depend strongly on the central metal ion and could be classified into three forms: α, β, and γ.

Ab initio study on geometrical structures of the TTTA molecular crystal

The crystal made of organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), exhibits a large first-order magnetic and structural phase transition between a paramagnetic high-temperature (HT) phase and a diamagnetic low-temperature (LT) phase, with a surprisingly wide thermal hysteresis loop over room temperature. We investigate theoretically the crystal structures for the two phases by means of the ab initio structural optimization method based on the local density approximation. We present necessary and sufficient atomic coordinates in this paper. The resulting structures are in good agreement with the recent X-ray diffraction experiment. In the HT phase, uniform one-dimensional stacking of the radical molecule appears, while, in the LT phase, strong dimerization along the stacking direction appears.

Room-temperature magnetic bistability in organic radical crystals: Paramagnetic-diamagnetic phase transition in 1,3,5-trithia-2,4,6-triazapentalenyl

An organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), exhibits a first-order phase transition between a paramagnetic high-temperature (HT) phase and a diamagnetic low-temperature (LT) phase, with a surprisingly wide thermal hysteresis loop over the temperature range 230--305 K. The crystal structure of the HT phase consists of a one-dimensional (1D) regular \ensuremath{\pi}-stacking column with short $\mathrm{S}\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}\mathrm{N}$ and $\mathrm{S}\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}\mathrm{S}$ intercolumn contacts. The intra- and intercolumn exchange coupling constants are estimated to be ${J/k}_{\mathrm{B}}=\ensuremath{-}320 \mathrm{K}$ and ${\mathrm{zJ}}^{\ensuremath{'}}{/k}_{\mathrm{B}}=\ensuremath{-}360 \mathrm{K},$ indicating a 3D magnetic interaction. The LT phase has a diamagnetic property, caused by a strong alternation in the stacking column. The differential scanning calorimetry measurements reveal an exothermic and an endothermic transition upon cooling and heating, respectively, with the hysteresis in agreement with static magnetic measurements. The observed transition entropy is larger than the maximum estimation of the spin contribution, $R\mathrm{ln}2,$ suggesting incorporation of the lattice system in this phase transition. The electron paramagnetic resonance measurements also demonstrate the paramagnetic-diamagnetic phase transition with the hysteresis. The reflection spectra suggest a change in dimensionality at the phase transition, which is consistent with the results of the intermolecular overlap-integral calculations.

Thermochromism and Magnetic Phase Transition in an Organic Radical 1,3,5-Trithia-2,4,6-Triazapentalenyl

Thermochromic and magnetic properties of an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal were investigated through polarized reflection and EPR measurements. TTTA shows a first-order magnetic phase transition with a wide thermal hysteresis loop near room temperature. The high temperature (HT) phase is paramagnetic due to a uniform one-dimensional stacking of the radical molecules, while the low temperature (LT) phase is diamagnetic due to strong dimerization along the stacking direction. The colors of the HT and LT phases are purple and green, respectively, and both phases show a strong dichroism. We also found that part of the LT phase changes to the HT phase by laser light irradiation. The experimental results suggest the possibility of optical control of magnetic bistabitity in TTTA crystal.

Thermal and Optical Switching of Iron (III) Complexes

Binuclear iron (III) spin-crossover complexes, [Fe2 (salten)2(az)] (BPh4)2 (1) and [Fe2(salten)2(cc)](BPh4)2 (2) (H2salten= 2, 2'-[iminobis(3, 1-propanediylnitrilomethylidyne)]bis-phenol, az=4, 4'-azobis-pyridine, and cc=4, 4'-(1, 2-ethenediyl)bis-pyridine), and mononuclear iron (III) spin-crossover complexes, [Fe(pap)2]ClO4·H2O (3) (Hpap = 2-(2-pyridylmethyleneamino)phenol) and [Fe(qsal)2]NCSe·CH2Cl2 (4) (Hqsal = 2-[(8-quinolinylimino)methyl]-phenol), were synthesized and characterized by single-crystal X-ray diffraction, Mössbauer spectra, magnetic susceptibilities, and electronic spectra. The structures of 1 and 2 were determined in the low-spin states at 100 K and the high-spin states at 298 K. The complexes 1 and 2 exhibited the spincrossover behavior; gradual and rapid spin interconversion was observed by means of Mössbauer spectroscopy at 293 K. The structure of 3 in the high-spin state was determined at 293 K. The complex 3 exhibited the abrupt spin transition with thermal hysteresis (T1/2↑=180 K and T1/2↓=165 K). The time dependence of the magnetism and the frozen-in effect were observed for the complex 3. Furthermore, light-induced excited spin state trapping effect was observed for the first time for iron (III) complexes. The structure of 4 in the low-spin state was determined at 200 K. The complex 4 exhibited a wide thermal hysteresis loop of 180 K (T1/2↑ = 392 K and T 1/2↓ = 212 K) in the first cycle, although the hysteresis loop observed for the first cycle is apparent ones. Following the first loop, it shows a two-step spin transition in warming mode (T1/2 (S1) ↑ = 215 K and T1/2 (S2) ↑ = 282 K) and a one-step spin transition in cooling mode (T1/2 ↓ = 212 K).

Iron(III) spin-crossover compounds with a wide apparent thermal hysteresis around room temperature.

The magnetic properties of the spin-crossover compounds, [Fe(qsal)2]NCSe-MeOH (1) and [Fe(qsal)2]NCSe-CH2Cl2 (2), have been measured. We have discovered that both compounds 1 and 2 exhibit a wide thermal hysteresis loop of 140 K (T(1/2) upward arrow = 352 K and T(1/2) downward arrow = 212 K) and 180 K (T(1/2) upward arrow = 392 K and T(1/2) downward arrow = 212 K), respectively, in the first cycle. Thermogravimetric analysis shows that solvent molecules escape from compounds 1 and 2 around 340 and 395 K, respectively. This means that the hysteresis loops observed for the first cycle are only apparent ones. Following the first loop, they show a two-step spin-crossover in warming mode. The so-called "step 1" and "step 2" are centered around T(1/2(S1)) upward arrow = 215 K and T(1/2(S2)) upward arrow = 282 K, respectively. On the other hand, a one-step spin-crossover occurs at T(1/2) downward arrow = 212 K in cooling mode. The hysteresis widths can be estimated to be 3 K (step 1) and 70 K (step 2), assuming that the widths in steps 1 and 2 are defined as the differences between T(1/2(S1)) upward arrow and T(1/2) downward arrow, and T(1/2(S2)) upward arrow and T(1/2) downward arrow, respectively. The hysteresis width of 70 K in step 2 is one of the widest values reported so far for spin-crossover complexes. It is thought that the cooperativity operating in the complexes arises mainly from the intermolecular pi interactions between quinoline and phenyl rings. Using a previously reported model, we are able to simulate the hysteresis loop with a two-step spin-crossover in warming mode and a one-step transition in cooling mode.