Observation of Ferroelectricity in Paramagnetic Copper Octacyanomolybdate

We report the observation of ferroelectricity in a copper octacyanomolybdate-based paramagnet, Cu2[Mo(CN)8]·8H2O (CuII, S = 1/2; MoIV, S = 0). This compound has a freezing point for the fixation of hydrogen bonding at 150 K. Around this temperature, an enhancement in the ferroelectricity and an increase in the dielectric constant are observed. The ferroelectricity of this system is classified into amorphous ferroelectrics; i.e., the electric poling effect induces an electric polarization. The electric polarization is maintained by the structural local disorder of hydrogen bonding and the three-dimensional CN network. In this ferroelectricity, the crystal structure is a polar group of C∞v after application of an electric field

Magnetic Dimensional Crossover from Two- to Three-Dimensional Heisenberg Magnetism in a Cu–W Cyano-Bridged Bimetal Assembly

In this work, we synthesized a cyano-bridged Cu–W bimetal assembly, [Cu^II(pyrimidine)₂]₄­[Cu^II(H₂O)₂]₂[W^V(CN)₈]₄·4H₂O (<b>1</b>), which has a monoclinic crystal structure (<i>P</i>2₁/<i>n</i> space group, <i>a</i> = 15.7365(3) Å, <i>b</i> = 21.1555(4) Å, <i>c</i> = 27.1871(5) Å, β = 91.8630(7)°, and <i>Z</i> = 4). In this compound, Cu and W sites form two-dimensional (2-D) layers along the <i>ab</i> plane, while the other Cu sites are bridged between the 2-D layers, constructing a three-dimensional (3-D) structure. The magnetic susceptibility measurement showed that ferromagnetic interaction operates in the magnetic spins of the present compound. The field-cooled-magnetization (FCM) curve indicates that the magnetization gradually increases in the temperature range of ca. 40–8 K, and the spontaneous magnetization appears at a Curie temperature of 8 K. To understand the anomalous magnetization increase in the temperature range of ca. 40–8 K, we measured the magnetic heat capacity (<i>C</i>_mag). The <i>C</i>_mag vs <i>T</i> plots have a broad peak around 18 K and a sharp peak at 8 K. Such a type of <i>C</i>_mag vs <i>T</i> plots indicates a dimensional crossover from a 2-D to a 3-D Heisenberg magnetic model. This is because <b>1</b> has a pseudo 2-D network structure; that is, the magnitude of the intralayer superexchange interaction is much larger than that of the interlayer superexchange interaction. Such a magnetic dimensional crossover is a rare and intriguing issue in the field of magnetic substances

Magnetic Dimensional Crossover from Two- to Three-Dimensional Heisenberg Magnetism in a Cu–W Cyano-Bridged Bimetal Assembly

In this work, we synthesized a cyano-bridged Cu–W bimetal assembly, [Cu^II(pyrimidine)₂]₄­[Cu^II(H₂O)₂]₂[W^V(CN)₈]₄·4H₂O (<b>1</b>), which has a monoclinic crystal structure (<i>P</i>2₁/<i>n</i> space group, <i>a</i> = 15.7365(3) Å, <i>b</i> = 21.1555(4) Å, <i>c</i> = 27.1871(5) Å, β = 91.8630(7)°, and <i>Z</i> = 4). In this compound, Cu and W sites form two-dimensional (2-D) layers along the <i>ab</i> plane, while the other Cu sites are bridged between the 2-D layers, constructing a three-dimensional (3-D) structure. The magnetic susceptibility measurement showed that ferromagnetic interaction operates in the magnetic spins of the present compound. The field-cooled-magnetization (FCM) curve indicates that the magnetization gradually increases in the temperature range of ca. 40–8 K, and the spontaneous magnetization appears at a Curie temperature of 8 K. To understand the anomalous magnetization increase in the temperature range of ca. 40–8 K, we measured the magnetic heat capacity (<i>C</i>_mag). The <i>C</i>_mag vs <i>T</i> plots have a broad peak around 18 K and a sharp peak at 8 K. Such a type of <i>C</i>_mag vs <i>T</i> plots indicates a dimensional crossover from a 2-D to a 3-D Heisenberg magnetic model. This is because <b>1</b> has a pseudo 2-D network structure; that is, the magnitude of the intralayer superexchange interaction is much larger than that of the interlayer superexchange interaction. Such a magnetic dimensional crossover is a rare and intriguing issue in the field of magnetic substances

Extremely Gradual Spin-Crossover Phenomenon in a Cyano-Bridged Fe−Mo Bimetallic Assembly

We report a unique type of spin-crossover phenomenon in a three-dimensional (3-D) Fe−Mo network with a cubic structure, Fe2[Mo(CN)8]·(3-pyCH2OH)8·3H2O (3-py = 3-pyridyl). This compound exhibits an extremely gradual FeII spin-crossover over a wide temperature range, which is more gradual than the crossover according to the Boltzmann distribution. The electronic states at 320 and 50 K are represented as (FeIIhs)2[MoIV(CN)8]·(3-pyCH2OH)8·3H2O and (FeIIhs)0.48(FeIIls)1.52[MoIV(CN)8]·(3-pyCH2OH)8·3H2O, respectively, where hs and ls denote high spin (S = 2) and low spin (S = 0), respectively. The model calculation based on Slichter−Drickamer’s model suggests this extremely gradual spin-crossover can be explained by the contribution of 3-D alternating alignment of hs and ls sites, i.e., −hs−ls−hs−ls−. This system is a strongly correlated system of spin-crossover sites because the spin-crossover FeII sites are directly linked by −NC−Mo−CN− with a high symmetry (FeII sites have one type of symmetry). The elastic interaction due to the volume change in a spin-crossover site isotropically propagates in the whole crystal. Since the CN bridges cannot be disconnected during spin-crossover, a 3-D alternating order of hs and ls sites is considered to be preferable

Continuous Change of Second-order Nonlinear Optical Activity in a Cyano-bridged Coordination Polymer

In this work, we continuously control the second-order nonlinear optical activity by tuning the piezoelectric property in a series of rubidium manganese hexacyanoferrates, RbIxMnII[FeIII(CN)6](x+2)/3·zH2O. Above x = 0.7, second harmonic generation (SHG) is observed, and the SH light intensity (ISH) gradually increases with increasing x. The crystal structures of this series are analyzed using Rietveld analysis and the maximum entropy method. The crystal structural data shows that the difference between the existing probability of the Rb ion in interstitial site-1 (PRb1) and site-2 (PRb2), PRb1 − PRb2, gradually increases with increasing x. Because the difference between PRb1 and PRb2 produces a 4̅ rotoinversion, the PRb1 − PRb2 value is considered to be related to the magnitude of piezoelectricity or SH susceptibility (χSH). From the analysis of the χSH tensors elements, the observed x dependence of ISH can be explained by the PRb1 − PRb2 value. Such a tunable system of second-order nonlinear optical activity is very rare in condensed matters

Crystal Structure, Charge-Transfer-Induced Spin Transition, and Photoreversible Magnetism in a Cyano-Bridged Cobalt−Tungstate Bimetallic Assembly

This paper describes the crystal structure, magnetic properties, and photoreversible magnetic properties of CoII3[WV(CN)8]2(pyrimidine)4·6H2O. We found that complexes of this formula had two types of crystal structures ([{CoII(pyrimidine)2}2{CoII(H2O)2}{WV(CN)8}2]·4H2O (1) and [{CoII(pyrimidine)(H2O)}2{CoII(H2O)2}{WV(CN)8}2](pyrimidine)2·2H2O (2)). These two structures had similar metal−organic frameworks but differed in the coordination environment around Co1, i.e., Co1(NC)4(pyrimidine)2 in 1 and Co1(NC)4(pyrimidine)(H2O) in 2. In 1, a temperature-induced phase transition from the CoII (S = 3/2)−NC−WV (S = 1/2) [high-temperature (HT)] phase to the CoIII (S = 0)−NC−WIV (S = 0) [low-temperature (LT)] phase was observed due to a charge-transfer-induced spin transition. However, 2 did not exhibit such a phase transition. When the LT phase of 1 was irradiated by 840 nm light, ferromagnetism with a Curie temperature of 40 K and magnetic coercive field of 12 kOe were observed. UV–vis reflectance and infrared measurements suggested that the LT phase optically transited to the photoinduced (PI) phase, which had a similar valence state as the HT phase, through the metal-to-metal charge-transfer (WIV → CoIII) band. In contrast, when the back metal-to-metal charge transfer (CoII → WV) band of the PI phase was excited by 532 nm light, the reverse phase transition from the PI phase to the LT phase occurred, and the spontaneous magnetization decreased

Crystal Structure, Charge-Transfer-Induced Spin Transition, and Photoreversible Magnetism in a Cyano-Bridged Cobalt−Tungstate Bimetallic Assembly

This paper describes the crystal structure, magnetic properties, and photoreversible magnetic properties of CoII3[WV(CN)8]2(pyrimidine)4·6H2O. We found that complexes of this formula had two types of crystal structures ([{CoII(pyrimidine)2}2{CoII(H2O)2}{WV(CN)8}2]·4H2O (1) and [{CoII(pyrimidine)(H2O)}2{CoII(H2O)2}{WV(CN)8}2](pyrimidine)2·2H2O (2)). These two structures had similar metal−organic frameworks but differed in the coordination environment around Co1, i.e., Co1(NC)4(pyrimidine)2 in 1 and Co1(NC)4(pyrimidine)(H2O) in 2. In 1, a temperature-induced phase transition from the CoII (S = 3/2)−NC−WV (S = 1/2) [high-temperature (HT)] phase to the CoIII (S = 0)−NC−WIV (S = 0) [low-temperature (LT)] phase was observed due to a charge-transfer-induced spin transition. However, 2 did not exhibit such a phase transition. When the LT phase of 1 was irradiated by 840 nm light, ferromagnetism with a Curie temperature of 40 K and magnetic coercive field of 12 kOe were observed. UV–vis reflectance and infrared measurements suggested that the LT phase optically transited to the photoinduced (PI) phase, which had a similar valence state as the HT phase, through the metal-to-metal charge-transfer (WIV → CoIII) band. In contrast, when the back metal-to-metal charge transfer (CoII → WV) band of the PI phase was excited by 532 nm light, the reverse phase transition from the PI phase to the LT phase occurred, and the spontaneous magnetization decreased

Direct Observation of Chemical Conversion from Fe₃O₄ to ε‑Fe₂O₃ by a Nanosize Wet Process

ε-iron oxide (ε-Fe₂O₃) has drawn attention from the viewpoints of high-density magnetic recording and high-frequency millimeter wave absorption. To date, chemical conversion from Fe₃O₄ (magnetite) to ε-Fe₂O₃ under wet process conditions have been difficult. Herein, we report that ε-Fe₂O₃ could be obtained from Fe₃O₄ using a nanosize wet process. In the present method, 10 or 16 nm sized Fe₃O₄ nanocrystals are used as the precursor. Fe₃O₄ nanocrystals are embedded in a silica matrix and subsequently sintered around 1000 °C in air, resulting in the chemical conversion from Fe₃O₄ to ε-Fe₂O₃ being confirmed. In the case of 10 nm sized Fe₃O₄ precursors, the sample consists of 16% ε-Fe₂O₃ and 84% γ-Fe₂O₃, whereas in the case of 16 nm sized Fe₃O₄ precursors, the sample consists of 24% ε-Fe₂O₃ and 76% γ-Fe₂O₃. The magnetic hysteresis loops of the samples are theoretically predicted using the large hysteresis loop of ε-Fe₂O₃ and the magnetization curve of super-paramagnetic γ-Fe₂O₃. The experimental and predicted hysteresis loops agree well. First-principles calculations suggest that Fe₃O₄ nanocrystals between 8 and 43 nm transform directly to ε-Fe₂O₃. Due to the strict size condition, the chemical conversion from Fe₃O₄ to ε-Fe₂O₃ is the first to be observed by a wet process. The nanosize wet process from Fe₃O₄ to ε-Fe₂O₃ should accelerate the development of highly functional hard magnetic ferrite ε-Fe₂O₃

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, Rb_0.97Mn­[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 Mn^II–NC–Fe^III and tetragonal Mn^III–NC–Fe^II phases. The Mn^III–NC–Fe^II phase exhibits a small negative thermal expansion (NTE) along the <i>a</i>_LT and <i>c</i>_LT axes with a thermal expansion coefficient of α₍<i>a</i>_LT) = −1.40 ± 0.12 × 10^–6 K^–1 and α₍c_LT) = −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 originate from the transverse mode. Additionally, an oriented film on SiO₂ glass is prepared using a microcrystal dispersive methanol solution and a spin-coating technique. This is the first example of a ZTE film that maintains a constant film thickness over a wide temperature range of 300 K

Spin-Flop Transition in a Nickel–Octacyanidotungstate Chain Magnet

Crystal engineering of molecular magnetic materials often leads to novel magnetic functionalities. In this study, we synthesized an ionic chain magnet, i.e., [NiII(imi)6]­{[NiII(imi)4]­[WV(CN)8]}2·4H2O (imi = imidazole; NiW) comprising anionic cyanido-bridged [NiII(imi)4WV(CN)8]nn– chains and cationic mononuclear [NiII(imi)6]2+ complexes. Ferromagnetic coupling between NiII and WV with S = 1 and 1/2, respectively, via a bridging cyanido ligand is found within the chain. However, the total magnetization of NiW is canceled out by a notable antiferromagnetic interchain interaction below a Néel temperature of 8.5 K. Such an antiferromagnetic interaction can be overcome by applying an external magnetic field of 0.9 T at 2 K, and a steep spin-flop transition is observed. From a crystal engineering perspective, we attribute this metamagnetic behavior to the isolated [NiII(imi)6]2+ complex that attracts the chains to be close to each other. Additionally, the complex operates as an independent paramagnetic spin source that offers an extra magnetization state of this compound