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

Structural Phase Transition between γ‑Ti₃O₅ and δ‑Ti₃O₅ by Breaking of a One-Dimensionally Conducting Pathway

The phase transition between gamma-trititanium-pentoxide (γ-Ti₃O₅) and delta-trititanium-pentoxide (δ-Ti₃O₅) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic <i>I</i>2/<i>c</i> crystal structure of γ-Ti₃O₅ was found to switch to a monoclinic <i>P</i>2/<i>a</i> crystal structure of δ-Ti₃O₅ due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti₃O₅ behaves like a metallic conductor with a σ value of 4.7 S cm^–1 at 320 K, while δ-Ti₃O₅ shows a semiconductive property with a σ value of 2.5 × 10^–5 S cm^–1 at 70 K. Optical measurement also supports that γ-Ti₃O₅ is a metallic conductor, while δ-Ti₃O₅ is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti₃O₅ is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with one-dimensional linkage along the crystallographic <i>c</i>-axis. On the contrary, δ-Ti₃O₅ has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti₃O₅ and δ-Ti₃O₅ is caused by breaking of a one-dimensionally conducting pathway due to a Mott–Hubbard metal–insulator phase transition

Structural Phase Transition between γ‑Ti₃O₅ and δ‑Ti₃O₅ by Breaking of a One-Dimensionally Conducting Pathway

The phase transition between gamma-trititanium-pentoxide (γ-Ti₃O₅) and delta-trititanium-pentoxide (δ-Ti₃O₅) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic <i>I</i>2/<i>c</i> crystal structure of γ-Ti₃O₅ was found to switch to a monoclinic <i>P</i>2/<i>a</i> crystal structure of δ-Ti₃O₅ due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti₃O₅ behaves like a metallic conductor with a σ value of 4.7 S cm^–1 at 320 K, while δ-Ti₃O₅ shows a semiconductive property with a σ value of 2.5 × 10^–5 S cm^–1 at 70 K. Optical measurement also supports that γ-Ti₃O₅ is a metallic conductor, while δ-Ti₃O₅ is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti₃O₅ is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with one-dimensional linkage along the crystallographic <i>c</i>-axis. On the contrary, δ-Ti₃O₅ has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti₃O₅ and δ-Ti₃O₅ is caused by breaking of a one-dimensionally conducting pathway due to a Mott–Hubbard metal–insulator phase transition

Structural Phase Transition between γ‑Ti₃O₅ and δ‑Ti₃O₅ by Breaking of a One-Dimensionally Conducting Pathway

The phase transition between gamma-trititanium-pentoxide (γ-Ti₃O₅) and delta-trititanium-pentoxide (δ-Ti₃O₅) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic <i>I</i>2/<i>c</i> crystal structure of γ-Ti₃O₅ was found to switch to a monoclinic <i>P</i>2/<i>a</i> crystal structure of δ-Ti₃O₅ due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti₃O₅ behaves like a metallic conductor with a σ value of 4.7 S cm^–1 at 320 K, while δ-Ti₃O₅ shows a semiconductive property with a σ value of 2.5 × 10^–5 S cm^–1 at 70 K. Optical measurement also supports that γ-Ti₃O₅ is a metallic conductor, while δ-Ti₃O₅ is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti₃O₅ is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with one-dimensional linkage along the crystallographic <i>c</i>-axis. On the contrary, δ-Ti₃O₅ has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti₃O₅ and δ-Ti₃O₅ is caused by breaking of a one-dimensionally conducting pathway due to a Mott–Hubbard metal–insulator phase transition