Charge Transfer Phase Transition Research Articles

Room-temperature bistability in a cobalt-octacyanidotungstate framework showing a charge-transfer phase transition with a red-blue color change

A cyanido-bridged Co–W assembly showing a charge transfer–induced phase transition realized room temperature bistability and red-blue drastic color change.

Pressure-Induced Structural and Charge-Transfer Phase Transitions for a Two-Dimensional Mixed-Valence Iron(II,III) Coordination Polymer

A two-dimensional (2D) mixed-valence iron(II,III) coordination polymer, (Ph4P)[FeIIFeIII(dto)3] (1; Ph4P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), was prepared, and its crystal structure was revealed. Compound 1 exhibited a 2D honeycomb structure and a rigid layered framework, due to a hydrogen bond between the cation and anion layers. The temperature dependence of the magnetic susceptibilities for 1 under high pressure showed a thermal hysteresis loop corresponding to a charge-transfer phase transition, although there was no charge-transfer phase transition under ambient pressure.

Effect of Transition Metal Substitution on the Charge-Transfer Phase Transition and Ferromagnetism of Dithiooxalato-Bridged Hetero Metal Complexes, (n-C3H7)4N[FeII1−xMnIIxFeIII(dto)3

The dithiooxalato-bridged iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato) undergoes a novel charge-transfer phase transition (CTPT) accompanied by electron transfer between adjacent FeII and FeIII sites. The CTPT influences the ferromagnetic transition temperature according to the change of spin configuration on the iron sites. To reveal the mechanism of the CTPT, we have synthesized the series of metal-substituted complexes (n-C3H7)4N[FeII1-xMnIIxFeIII(dto)3] (x = 0–1) and investigated their physical properties by means of magnetic susceptibility and dielectric constant measurements. With increasing MnII concentration, x, MnII-substituted complexes show the disappearance of CTPT above x = 0.04, while the ferromagnetic phase remains in the whole range of x. These results are quite different from the physical properties of the ZnII-substituted complex, (n-C3H7)4N[FeII1-xZnIIxFeIII(dto)3], which is attributed to the difference of ion radius as well as the spin states of MnII and ZnII.

Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C3H7)4N[FeIIFeIII(dto)3] with a Two-Dimensional Honeycomb Network.

The structural properties of the iron mixed-valence complex ( n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato, C2O2S2) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C3H7)4N[FeIIFeIII(dto)3] has two-dimensional (2D) honeycomb layers consisting of alternating FeII and FeIII arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C3H7)4N+ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at TC↑1/2 ∼ 120 K and TC↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the FeII and FeIII ions, accompanied by a spin-state change, FeII ( S = 2; HS)-O2C2S2-FeIII ( S = 1/2; LS) ↔ FeIII ( S = 5/2; HS)-O2C2S2-FeII ( S = 0; LS). During the CTPT, the intersheet [FeIIFeIII(dto)3] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS6 and FeO6 sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O2C2S2)3] and [Fe(S2C2O2)3] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C3H7)4N][FeIIFeIII(dto)3]}∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].

Theoretical prediction of a charge-transfer phase transition

Phase transition materials are attractive from the viewpoints of basic science as well as practical applications. For example, optical phase transition materials are used for optical recording media. If a phase transition in condensed matter could be predicted or designed prior to synthesizing, the development of phase transition materials will be accelerated. Herein we show a logical strategy for designing a phase transition accompanying a thermal hysteresis loop. Combining first-principles phonon mode calculations and statistical thermodynamic calculations considering cooperative interaction predicts a charge-transfer phase transition between the A–B and A+–B− phases. As an example, we demonstrate the charge-transfer phase transition on rubidium manganese hexacyanoferrate. The predicted phase transition temperature and the thermal hysteresis loop agree well with the experimental results. This approach will contribute to the rapid development of yet undiscovered phase transition materials.

Raman scattering in FeSe-based superconductors

Raman scattering is one of the fundamental techniques in modern condensed matter physics. Its technical uniqueness allows to probe a variety of excitations in condensed matter and their couplings. FeSe-based superconductors have abundant structural phases like bulk, monolayer, Fe-deficient phases, and intercalated phases. Meanwhile, they exhibit rich electronic and magnetic phase diagrams and easily tunable superconducting transition temperatures ranged from a few Kelvins up to more than 40 K. This leads to the formation of an ideal research platform for superconductivity and has attracted a great deal of interest in the field. In this paper, we made a brief introduction to the structural phases, magnetism and electronic structures of FeSe-based superconductors. And we focused on the recent progress on Raman scattering studies of FeSe-based superconductors. For phononic Raman scattering, we introduced the Raman scattering measurement of the phonons in the Fe(Se,Te) system firstly. Next, in order to further explore the influence of doping on T c, we discussed the doping/substitution of A-sites and Fe-sites of A x Fe2− y Se2. Finally, we studied the correlation between superconductivity and various structures. We then discussed two-magnon and spin-phonon coupling in the materials. We discussed the relationship between magnetic order and superconductivity, and the effect of magnetic order on phonons. For electronic Raman response, we presented the discussions and analysis on electronic nematic phases. We introduced the observation of nematic phases by different measurement methods. And we discussed the origin of nematic phases: spin fluctuations or/and orbital (charge) fluctuations, different measurement methods have different conclusions. Finally, we introduced the studies in monolayer FeSe and some Raman scattering results there. We introduced the discovery of T c above 77 K in monolayer FeSe/SrTiO3. And we discussed many novel physical properties such as the opening of energy gap in 1UC FeSe/NbSTO and the charge transfer and ferroelectric phase transition between 1-UC thin film and NbSTO substrate.

Magnetic properties of V-substitution effect in an iron mixed valence complex

The iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto=C2O2S2) exhibits a novel type of phase transition according electron transfer between FeII and FeIII called charge transfer phase transition (CTPT). The existence of the CTPT highly depends on the 3d metal substitution with Zn, Mn, Cr, or Co for corresponding Fe site in (n-C3H7)4N[FeIIFeIII(dto)3]. Here, we prepared a series of magnetically diluted complexes (n-C3H7)4N[FeIIFeIII1−xVIIIx(dto)3] (x=0–1), and carried out investigation of magnetic properties. Judging from the magnetic susceptibility measurement, there is no suppression of the CTPT unlike other 3d metal substituted cases. Moreover, 57Fe Mössbauer spectroscopy revealed that about 60% of FeII sites changed their spin states from high spin to low spin in case of substitution rate for x=1.00. Since the difference of hardness and softness between metallic ions (FeII, FeIII and VIII) and coordination sites of ligand (O or S in dto), this behavior is explained by the linkage isomerization of bridging ligand in formation process of the complex.

Colossal negative thermal expansion in reduced layered ruthenate

Large negative thermal expansion (NTE) has been discovered during the last decade in materials of various kinds, particularly materials associated with a magnetic, ferroelectric or charge-transfer phase transition. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators. Here, we report the discovery of giant NTE for reduced layered ruthenate. The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volume change reported to date. We observed a giant negative coefficient of linear thermal expansion α=−115 × 10−6 K−1 over 200 K interval below 345 K. This dilatometric NTE is too large to be attributable to the crystallographic unit-cell volume variation with temperature. The highly anisotropic thermal expansion of the crystal grains might underlie giant bulk NTE via microstructural effects consuming open spaces in the sintered body on heating.

Charge-Transfer Phase Transition of a Cyanide-Bridged Fe(II) /Fe(III) Coordination Polymer.

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge-transfer phase transitions have not been reported for the simplest FeFe cyanide-bridged systems. A mixed-valence Fe(II) /Fe(III) cyanide-bridged coordination polymer, {[Fe(Tp)(CN)3 ]2 Fe(bpe)⋅5 H2 O}n , which demonstrates a thermally induced charge-transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high-spin state of the whole system does not change to a low-spin state. This result is in contrast to FeCo cyanide-bridged systems that exhibit charge-transfer-induced spin transitions.

Dynamical Behavior of the Charge Transfer Phase Transition in Dithiooxalato-Bridged Iron Mixed-Valence System

Dynamical Behavior of the Charge Transfer Phase Transition in Dithiooxalato-Bridged Iron Mixed-Valence System

Multifunctional Material: Bistable Metal–Cyanide Polymer of Rubidium Manganese Hexacyanoferrate

Abstract Since phase-transition phenomena in solid-state materials can change the magnetic, electrical, and optical properties with external stimuli, they are attracting widespread attention from the viewpoints of both fundamental science and practical applications. We have synthesized various cyano-bridged metal complexes possessing anomalous bistabilities that exhibit unprecedented phase transition phenomena. This paper focuses on rubidium manganese hexacyanoferrate, RbxMn[Fe(CN)6](x+2)/3·zH2O, which is a Prussian blue analogue. Here, the unique functionalities of rubidium manganese hexacyanoferrate are described: a charge-transfer-induced phase transition, control of the thermal hysteresis loop, and a huge thermal hysteresis with a hidden stable phase are presented as thermally-induced phase-transition phenomena, while a photoinduced phase collapse is described as a novel photoinduced phase-transition phenomenon. A one-shot-laser-induced charge-transfer phase transition is displayed as a photoinduced phase-transition phenomenon at ambient temperature. Moreover, second harmonic generation (SHG) and magnetization-induced second harmonic generation (MSHG) are provided as correlation effects between the optical and dielectrical properties. Finally, a one-shot-laser-induced photo-demagnetization and reversible photomagnetic switching between ferromagnetism and antiferromagnetism are demonstrated as photomagnetic effects.

Valence transitions in negative thermal expansion material SrCu₃Fe₄O₁₂.

The valence states of a negative thermal expansion material, SrCu3Fe4O12, are investigated by X-ray absorption and (57)Fe Mössbauer spectroscopy. Spectroscopic analyses reveal that the appropriate ionic model of this compound at room temperature is Sr(2+)Cu(~2.4+)3Fe(~3.7+)4O12. The valence states continuously transform to Sr(2+)Cu(~2.8+)3Fe(~3.4+)4O12 upon cooling to ~200 K, followed by a charge disproportionation transition into the Sr(2+)Cu(~2.8+)3Fe(3+)(~3.2)Fe(5+)(~0.8)O12 valence state at ~4 K. These observations have established the charge-transfer mechanism in this compound, and the electronic phase transitions in SrCu3Fe4O12 can be distinguished from the first-order charge-transfer phase transitions (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in Ln(3+)Cu(2+)3Fe(3.75+)4O12 (Ln = trivalent lanthanide ions).

Study on spin configuration in photoresponsive iron mixed-valence complexes by Mössbauer spectroscopy

We have investigated magnetic properties in a series of photoresponsive dithiooxalato (dto)-bridged iron mixed-valence complexes, (SP-R)[FeIIFeIII(dto)3] (SP-R = R-substituted pyridospiropyran cation; R = Me, Et, and Pr; abbreviated as 1 Me, 1 Et, and 1 Pr, respectively). As for our previous reports, 1 Me and 1 Et show two-step succeeding ferromagnetic transitions at T C = 25 & 8 K and 22 & 5 K, respectively. However, 1 Et has no hysteresis in the magnetic susceptibility, while 1 Me undergoes the charge transfer phase transition with thermal hysteresis around 75 K. To elucidate the two-step transitions of them, we measured 57Fe Mossbauer spectra of 1 Et. The spectra of FeII (S = 2) and FeIII (S = 1/2) in the HTP were observed in the magnetically ordered state as well as the paramagnetic state, and revealed that only HTP exists in a temperature range up to 5 K. The result is consistent with that of 1 Pr, where one ferromagnetic phase transition occurs at T C = 10 K. 57Fe Mossbauer spectroscopy is useful to clarify the origin of the succeeding magnetic transition for these systems.

Cation size effect on photomagnetism and charge transfer phase transition of iron mixed-valence complexes with spiropyrans

We have investigated the magnetic properties of a photoresponsive organic–inorganic hybrid system, (SP-R)[FeIIFeIII(dto)3] (SP=spiropyran; R=Me, Et, Pr; dto=C2O2S2), under hydrostatic pressures. At ambient pressure, (SP-Et)[FeIIFeIII(dto)3] and (SP-Pr)[FeIIFeIII(dto)3] undergo one and two ferromagnetic phase transitions corresponding to the high-temperature phase with the spin configuration of FeII(S=2)–FeIII(S=1/2), while (SP-Me)[FeIIFeIII(dto)3] undergoes two ferromagnetic phase transitions corresponding to the coexistence of the high-temperature phase and the low-temperature phase with the spin configuration of FeII(S=0)–FeIII(S=5/2). Under hydrostatic pressures, (SP-R)[FeIIFeIII(dto)3] (R=Et, Pr) undergo a new ferromagnetic phase transition corresponding to the low-temperature phase caused by the pressure-induced charge transfer phase transition.

A structural phase transition coupled to the Fe3+ spin-state crossover in anhydrous RbMn[Fe(CN)6

Linkage isomerism is the coexistence of iso-compositional molecules or solids differing by connectivity of the metal to a ligand. In a crystalline solid state, the rotation is possible for asymmetric ligands, e.g., for cyanide ligand. Here we report on our observation of a phase transition in anhydrous RbMn[Fe(CN)6] (nearly stoichiometric) and on the effect of linkage isomerism ensuing our interpretation of the results of Mossbauer study in which we observe the iron spin state crossover among two phases involved into this transition. The anhydrous RbMn[Fe(CN)6] can be prepared via prolonged thermal treatment (1 week at at 80 °C) of the as-synthesized hydrated RbMn[Fe(CN)6]·H2O. The latter compound famous for its charge-transfer phase transition is a precursor in our case. As the temperature is raising above 80 °C (remaining below 100 °C) we observe RbMn[Fe(CN)6] that inherited its F-43 m symmetry from RbMn[Fe(CN)6]·H2O transforming to a phase of the Fm-3 m symmetry. In the latter, more than half of Fe3 + ions are in high-spin state. We suggest a plausible way to explain the spin-crossover that is to allow the linkage isomerism by rotation of the cyanide ligands.

Zero Thermal Expansion Fluid and Oriented Film Based on a Bistable Metal-Cyanide Polymer

A zero thermal expansion (ZTE) material based on plate-shaped rubidium manganese hexacyanoferrate, Rb0.97Mn[Fe(CN)6]0.99·0.3H2O, is prepared using a polyethylene glycol monolaurate (PEGM) surfactant matrix. The prepared microcrystals show a charge transfer induced phase transition between the cubic MnII–NC–FeIII and tetragonal MnIII–NC–FeII phases. The MnIII–NC–FeII phase exhibits a small negative thermal expansion (NTE) along the aLT and cLT axes with a thermal expansion coefficient of α(aLT) = −1.40 ± 0.12 × 10–6 K–1 and α(cLT) = −0.17 ± 0.13 × 10–6 K–1 over a wide temperature range of 15 K – 300 K. Such small |α| materials are classified as ZTE materials. The far-infrared spectra show that NTE originates from the transverse modes δ(Fe–C≡N–Mn)of the transverse translational mode around 304 cm–1, and transverse librational modes at 253 and 503 cm–1, which are assigned according to first principle calculations. Molecular orbital calculations indicate that ZTE and the charge transfer phase transition both ...

Growth Dynamics of Photoinduced Phase Domain in Cyano-Complex Studied by Boundary Sensitive Raman Spectroscopy

Rubidium manganese hexacyanoferrate exhibits charge transfer phase transition from high temperature phase to low temperature phase at 230 K. This phase transition can also be triggered by light irradiation resonantly inducing charge transfer from Mn to Fe. In the present study, boundary sensitive Raman spectroscopy was performed for Rb0.94Mn[Fe(CN)6]0.98·0.2H2O in both cases of photoinduced and thermal phase transition. Since the frequencies of C≡N stretching vibration modes are very sensitive to the valence states of the adjacent metal ions, we can quantify the distribution of not only high and low temperature phase but also boundary configurations from the observed spectra. We obtained the time evolution of the fraction ratios of the valence states from the observed peak areas. In the case of photoinduced situation, the boundary increases up to 15% when high temperature phase diminishes to 55% of the initial fraction. This is quite different from the result in thermal phase transition where the boundary is created only 0.8% at the same high temperature phase fraction value. We conclude that many small domains are preferably created in photoinduced phase transition since the ratio of boundary is large, while large domains grow in thermal phase transition.

Study on the pressure induced charge transfer phase transition in (C 5H 11) 4N(Fe IIFe III(C 2O 2S 2) 3) by means of μSR spectroscopy

We have investigated the magnetic properties of iron mixed-valence complexes, ( n-C n H 2 n +1) 4N[Fe IIFe III(dto) 3] (dto = C 2O 2S 2, n = 3, 5), in which not only a ferromagnetic transition but also a novel charge transfer phase transition (CTPT) take place [1]. This CTPT can be observed under ambient pressure for n = 3, while it appears abruptly above 0.5 GPa for n = 5 [2]. Recently, we have measured the muon spin relaxation (μSR) for the CTPT of n = 3, which revealed the dynamical process of electron-transfer between Fe II and Fe III and its frequency was estimated at about 0.1 MHz [3]. To investigate the pressure induced CTPT for n = 5, we carried out the μSR measurement for n = 5 at 150 K between 0.30 and 0.64 GPa with the 4He gas-operated pressure system. The asymmetry of the muon spin relaxation for n = 5 with Cu–Be pressure cell was almost constant up to 0.55 GPa, while it rapidly decreased with increasing pressure above 0.60 GPa. This result shows that the applied pressure causes the spin fluctuation due to the CTPT, which induces the decrease of the asymmetry of muon spin relaxation. This experiment can correctly decide the phase transition pressure from the absence to the appearance of the CTPT for n = 5.

Novel magnetic functionalities of Prussian blue analogs

Cyano-bridged bimetal assemblies demonstrate novel magnetic functionalities, particularly Prussian blue analogs, which have unique properties. In this perspective, we describe a charge-transfer phase transition, reversible photomagnetism, second harmonic generation and magnetization-induced second harmonic generation, ferroelectric ferromagnetism, humidity-sensitive magnetism, high ionic conductivity, and a coupling effect (which we named spin-ionics) between ionic conduction and magnetic ordering with Prussian blue analogs.

Progress of Multi Functional Properties of Organic-Inorganic Hybrid System, A[FeIIFeIIIX3] (A = (n-CnH2n+1)4N, Spiropyran; X = C2O2S2, C2OS3, C2O3S)

In the case of mixed-valence systems whose spin states are situated in the spin crossover region, new types of conjugated phenomena coupled with spin and charge are expected. From this viewpoint, we have investigated the multifunctional properties coupled with spin, charge and photon for the organic-inorganic hybrid system, A[FeIIFeIIIX3](A = (n-CnH2n+1)4N, spiropyran; X = dto(C2O2S2), tto(C2OS3), mto(C2O3S)). A[FeIIFeIII(dto)3] and A[FeIIFeIII(tto)3] undergo the ferromagnetic phase transitions, while A[FeIIFeIII(mto)3] undergoes a ferrimagnetic transition. In (n-CnH2n+1)4N [FeIIFeIII(dto)3](n = 3,4), a new type of phase transition called charge transfer phase transition (CTPT) takes place around 120 K, where the thermally induced charge transfer between FeII and FeIII occurs reversibly. At the CTPT, the iron valence state dynamically fluctuated with a frequency of about 0.1 MHz, which was confirmed by means of muon spin relaxation. The charge transfer phase transition and the ferromagnetic transition for (n-CnH2n+1)4N[FeIIFeIII(dto)3] remarkably depend on the size of intercalated cation. In the case of (SP)[FeIIFeIII(dto)3](SP = spiropyran), the photoinduced isomerization of SP under UV irradiation induces the charge transfer phase transition in the [FeIIFeIII(dto)3] layer and the remarkable change of the ferromagnetic transition temperature. In the case of (n-CnH2n+1)4N[FeIIFeIII(mto)3](mto = C2O3S), a rapid spin equilibrium between the high-spin state (S = 5/2) and the low-spin state (S = 1/2) at the FeIIIO3S3 site takes place in a wide temperature range, which induces the valence fluctuation of the FeS3O3 and FeO6 sites through the ferromagnetic coupling between the low spin state (S = 1/2) of the FeIIIS3O3 site and the high spin state (S = 2) of the FeIIO6 site.