87 research outputs found
Monte Carlo simulation of pressure-induced phase transitions in spin-crossover materials
Pressure-induced phase transitions of spin-crossover materials were simulated
by a Monte Carlo simulation in the constant pressure ensemble for the first
time. Here, as the origin of the cooperative interaction, we adopt elastic
interaction among the distortions of the lattice due to the difference of the
molecular sizes in different spin states, i.e., the high spin (HS) state and
the low spin (LS) state. We studied how the temperature dependence of the
ordering process changes with the pressure, and we obtained a standard sequence
of temperature dependences that has been found in changing other parameters
such as strength of the ligand field (S. Miyashita et al., Prog. Theor. Phys.
\textbf{114}, 719 (2005)). Various effects of pressure on the spin-crossover
ordering process are examined from a unified point of view.Comment: 5 pages, 6 figure
Poly[[hexa-μ-cyanido-manganese(II)iron(III)] pentahydrate]
The structure of the title compound, MnII[FeIII(CN)6]2/3·5H2O, features a face-centered cubic –Mn—NC—Fe– framework with both Mn and Fe having site symmetry m
m. Since one-third of the [Fe(CN)6]3− units are missing for a given formula in order to maintain charge neutrality, each Mn atom around such a vacancy is coordinated not only by the N atoms of the CN groups but also by the O atoms of the ligand water molecules. In addition to ligand water molecules, two types of non-coordinated water molecules, so-called zeolitic water molecules, exist in the interstitial sites of the –Mn—NC—Fe– framework. The positions of the O atoms of the zeolitic water molecules are fixed by the linkage via hydrogen bonds between ligand water and zeolitic water molecules. The structure is related to a recently reported rubidium manganese hexacyanoferrate. Site occupancy factors for Fe, C, N are 0.67; for two O atoms the value is 0.83 and for one O atom is 0.17
Threshold Phenomena under Photo Excitation of Spin-crossover Materials with Cooperativity due to Elastic Interactions
Photo-induced switching from the low-spin state to the high-spin state is
studied in a model of spin-crossover materials, in which long-range
interactions are induced by elastic distortions due to different molecular
sizes the two spin states. At a threshold value of the light intensity we
observe nonequilibrium critical behavior corresponding to a mean-field spinodal
point. Finite-size scaling of the divergence of the relaxation time is revealed
by analysis of kinetic Monte Carlo simulations.Comment: 4 pages, 7 figure
Desolvation-induced highly symmetrical terbium(III) single-molecule magnet exhibiting luminescent self-monitoring of temperature
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
Low-pressure-responsive heat-storage ceramics for automobiles
The accumulated heat energy of a heat-storage material is typically released overtime. If a heat-storage material could preserve its accumulated heat energy for a prolonged period, the applicability of such materials would be expanded greatly. Herein we report a newly fabricated heat-storage material that can store latent heat energy for a long period and release the heat energy upon demand by applying an extremely low pressure. This material is a block-type lambda trititanium pentoxide (block-type lambda-Ti3O5). The block-type lambda-phase accumulates a large heat energy of 237 kJ L-1 and exhibits a pressure-induced phase transition to beta trititanium pentoxide. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar). Such a low-pressure-responsive heat-storage ceramic is effective to reuse excessive heat in automobiles or waste heat at industrial factories
Realization of the mean-field universality class in spin-crossover materials
In spin-crossover materials, the volume of a molecule changes depending on
whether it is in the high-spin (HS) or low-spin (LS) state. This change causes
distortion of the lattice. Elastic interactions among these distortions play an
important role for the cooperative properties of spin-transition phenomena. We
find that the critical behavior caused by this elastic interaction belongs to
the mean-field universality class, in which the critical exponents for the
spontaneous magnetization and the susceptibility are and , respectively. Furthermore, the spin-spin correlation function is a
constant at long distances, and it does not show an exponential decay in
contrast to short-range models. The value of the correlation function at long
distances shows different size-dependences: , , and
constant for temperatures above, at, and below the critical temperature,
respectively. The model does not exhibit clusters, even near the critical
point. We also found that cluster growth is suppressed in the present model and
that there is no critical opalescence in the coexistence region. During the
relaxation process from a metastable state at the end of a hysteresis loop,
nucleation phenomena are not observed, and spatially uniform configurations are
maintained during the change of the fraction of HS and LS. These
characteristics of the mean-field model are expected to be found not only in
spin-crossover materials, but also generally in systems where elastic
distortion mediates the interaction among local states.Comment: 13 pages, 16 figure
Cesium ion detection by terahertz light
Recent developments in terahertz technologies provide new tools for analysis, inspection, and nondestructive sensing. If a heavy atom is encapsulated in a cage of a porous material, the atom should vibrate slowly and resonate with a low-frequency terahertz light. From this perspective, a cyanide-bridged metal framework is a suitable system because it contains many cages that can adsorb Cs ions. Herein we show the vibration mode of a Cs ion in a cage of a cyanide-bridged metal framework. First-principles phonon mode calculations and terahertz time-domain spectroscopy (THz-TDS) measurements indicate that the vibration mode of a Cs ion in a cyanide-bridged manganese-iron framework is at 1.5 THz, which is significantly apart from other lattice vibrations. Taking advantage of this feature, we develop a THz-light detection method for Cs ions, which is useful for non-contact sensing of Cs ions in dangerous environments or harmful circumstances
Nanometer-size hard magnetic ferrite exhibiting high optical-transparency and nonlinear optical-magnetoelectric effect
Development of nanometer-sized magnetic particles exhibiting a large coercive field (Hc) is in high demand for densification of magnetic recording. Herein, we report a single-nanosize (i.e., less than ten nanometers across) hard magnetic ferrite. This magnetic ferrite is composed of ε-Fe2O3, with a sufficiently high Hc value for magnetic recording systems and a remarkably high magnetic anisotropy constant of 7.7 × 106 erg cm−3. For example, 8.2-nm nanoparticles have an Hc value of 5.2 kOe at room temperature. A colloidal solution of these nanoparticles possesses a light orange color due to a wide band gap of 2.9 eV (430 nm), indicating a possibility of transparent magnetic pigments. Additionally, we have observed magnetization-induced second harmonic generation (MSHG). The nonlinear optical-magnetoelectric effect of the present polar magnetic nanocrystal was quite strong. These findings have been demonstrated in a simple iron oxide, which is highly significant from the viewpoints of economic cost and mass production.UTokyo Research掲載「世界最小ハードフェライト磁石の開発に成功」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/the-worlds-smallest-hard-ferrite-magnet.htmlUTokyo Research "The world\u27s smallest hard ferrite magnet" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/the-worlds-smallest-hard-ferrite-magnet.htm
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