Electric-field-induced charge-transfer phase transition: a promising approach toward electrically switchable devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb₀․₈Mn-[Fe(CN)₆]₀․₉₃•1.62H₂O and Co₃[W(CN)₈]₂(pyrimidine)₄•6H₂O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices

Thermal spin transition in [Fe(NH2-trz)3]Br2 investigated by spectroscopic ellipsometry

We investigated the thermal hysteresis of a pelleted sample of the spin-crossover compound [Fe(NH2-trz)3]Br2 by means of spectroscopic ellipsometry, in the temperature range 264–358 K. The ellipsometric parameters (psi,Delta) have been recorded in the optical range 240–1000 nm. The corresponding absorption and dispersion spectra show temperature-invariant isobestic points located at 240 and 291 nm, respectively. We found that the high-spin-fraction data, derived from the integrated absorption curves, are in excellent agreement with the magnetic data recorded on the same sample. We also investigated the consistency of the optical data by application of the Kramers-Kronig relations, which are well obeyed above ~450 nm. All these results demonstrate that the spectroscopic ellipsometry is well adapted to characterize the spin-crossover transition. We also expect that this nondestructive technique will be highly relevant to investigate the physical properties of thin films of switchable molecular solids, involving a change in the electronic properties upon a thermally or photoinduced phase transition

Temperature-dependent interactions and disorder in the spin-transition compound [FeII(L)2][ClO4]2·C7H8 through structural, calorimetric, cagnetic, photomagnetic, and diffuse reflectance investigations

The title compound [FeII(L)2][ClO4]2·C7H8 (L = 2-[3-(2′-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [FeII(L)2][ClO4]2, published earlier (Dalton Trans. 2003, 3392−3397). Magnetic susceptibility measurements, as well as Mössbauer and calorimetric investigations on polycrystalline samples of [Fe(L)2][ClO4]2·C7H8 revealed the occurrence of an abrupt HS (5T2) ↔ LS (1A1) transition with steep and narrow (2 K) hysteresis at ~232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale