Insights into Thermal Transport through Molecular π-Stacking.

π-Stacking, which is a ubiquitous structural motif in assemblies of aromatic compounds, is well-known to provide a transport pathway for charge carriers and excitons, while its contribution to thermal transport is still unclear. Herein, based on detailed experimental observations of the thermal diffusivity, thermal conductivity, and specific heat of a single-crystalline triphenylene featuring a one-dimensionally π-stacked structure, we describe the nature of thermal transport through the π-stacked columns. We reveal that acoustic phonons are responsible for thermal transport through the π-stacked columns, which exhibit crystal-like behavior. Importantly, the thermal energy stored as intramolecular vibrations can also be transported by coupling to the acoustic phonons. In contrast, in the direction perpendicular to the π-stacked columns, an amorphous-like thermal transport behavior dominates. The present finding offers deep insight into nanoscale thermal transport in organic materials, where the constituent molecules exist as discrete entities linked together by weak intermolecular interactions.

Defect Structure and Oxide Ion Conduction of Potassium Ion Substituted CaWO4.

We have prepared Ca1−xKxWO4−x/2 solid solutions with the Scheelite-type structure to investigate high-temperature electrochemical properties. Room-temperature X-ray diffraction suggested the solid solution range was x ≤ 0.2, since the second phase presumably of K2WO4 was detected for x = 0.3. For all the substituted samples up to x = 0.4, a large jump in conductivity has been observed around 500 °C. At higher temperatures, oxide ion conduction is found to be predominant even for x = 0.4, exceeding the solution limit estimated from the room-temperature XRD. The conductivity at high temperature is essentially proportional to the amount of substituted potassium ions up to x = 0.4, indicating that oxide ion conduction is associated with the formed oxide ion vacancy. High-temperature X-ray diffraction detected no apparent change in lattice parameters around 500 °C for x = 0.1, and the remaining second phase seems to be incorporated into the Scheelite lattice at high temperatures.

Two-Dimensional Superconductivity Emerged at Monatomic Bi(2-) Square Net in Layered Y2O2Bi via Oxygen Incorporation.

Discovery of layered superconductors such as cuprates and iron-based compounds has unveiled new science and compounds. In these superconductors, quasi-two-dimensional layers including transition metal cations play principal role in the superconductivity via carrier doping by means of aliovalent-ion substitution. Here, we report on a two-dimensional superconductivity at 2 K in ThCr2Si2-type layered oxide Y2O2Bi possessing conducting monatomic Bi(2-) square net, possibly associated with an exotic superconductivity. The superconductivity emerges only in excessively oxygen-incorporated Y2O2Bi with expanded inter-net distance, in stark contrast to nonsuperconducting pristine Y2O2Bi reported previously. This result suggests that the element incorporation into hidden interstitial site could be an alternative approach to conventional substitution and intercalation methods for search of novel superconductors.

Expansion of the hexagonal phase-forming region of Lu1-xScxFeO3 by containerless processing.

Hexagonal Lu1-xScxFeO3 (0 ≤ x ≤ 0.8) was directly solidified from an undercooled melt by containerless processing with an aerodynamic levitation furnace. The hexagonal phase-forming region was considerably extended compared to that of the conventional solid-state reaction (x ∼ 0.5). Synchrotron X-ray diffraction measurements revealed that the crystal structure of the hexagonal phase was isomorphous to hexagonal ferroelectric RMnO3 (R = a rare earth ion) with a polar space group of P63cm. As x increased, the a-axis lattice constant decreased linearly, strengthening the antiferromagnetic interaction between the Fe(3+) ions on the a-b plane. Accordingly, the weak ferromagnetic transition temperature increased from 150 K for x = 0 to 175 K for x = 0.7. These transition temperatures were much higher than those of hexagonal Lu1-xScxMnO3. The results indicate that hexagonal Lu1-xScxFeO3 is a suitable alternative magnetic dielectric for use at higher temperatures.

Negative differential resistance in MX- and MMX-type iodide-bridged platinum complexes.

Negative differential resistance (NDR) was discovered in MX- and MMX-type iodide-bridged platinum complexes for the first time. The low resistance of the complex observed under the large current cannot be explained only by the Joule heat. The intrinsic charge-ordering states are considered to play an important role in the NDR of these compounds.

Weak ferromagnetic transition with a dielectric anomaly in hexagonal Lu0.5Sc0.5FeO3.

Lu1-xScxFeO3 (0 ≤ x ≤ 1) was synthesized by a conventional solid-state reaction. The hexagonal phase appeared at 0.4 ≤ x ≤ 0.6, between the perovskite phase (0 ≤ x ≤ 0.3) and the bixbyite phase (0.7 ≤ x ≤ 1). Structural, magnetic, and dielectric properties of hexagonal Lu0.5Sc0.5FeO3 were investigated. Synchrotron X-ray diffraction measurements revealed that the crystal structure of Lu0.5Sc0.5FeO3 is isomorphic to hexagonal ferroelectrics RMnO3 (R = rare earth ion) with a polar space group of P63cm. A weak ferromagnetic transition with a dielectric anomaly occurred at a much higher temperature (162 K) than those in hexagonal RMnO3. Although remanent magnetization was observed below the transition temperature, it decreased to almost zero at 10 K. These results indicate a strong antiferromagnetic interaction between ground-state Fe(3+) ions on the triangular lattice.

Synthesis and characterization of the crystal structure, the magnetic and the electrochemical properties of the new fluorophosphate LiNaFe[PO4]F.

The new compound LiNaFe[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined using neutron powder diffraction data. LiNaFe[PO(4)]F was characterized by (57)Fe Mössbauer spectroscopy, magnetic susceptibility, specific heat capacity, and electrochemical measurements. LiNaFe[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9568(6) Å, b = 6.3959(3) Å, c = 11.4400(7) Å, V = 801.7(1) Å(3) and Z = 8. The structure consists of edge-sharing FeO(4)F(2) octahedra forming FeFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The specific heat and magnetization measurements show that LiNaFe[PO(4)]F undergoes a three-dimensional antiferromagnetic ordering at T(N) = 20 K. The neutron powder diffraction measurements at 3 K show that each FeFO(3) chain along the b-direction is ferromagnetic (FM), while these FM chains are antiferromagnetically coupled along the a and c-directions with a non-collinear spin arrangement. The galvanometric cycling showed that without any optimization, one mole of alkali metal is extractable between 1.0 V and 5.0 V vs. Li(+)/Li with a discharge capacity between 135 and 145 mAh g(-1).

Synthesis and characterization of the crystal structure and magnetic properties of the new fluorophosphate LiNaCo[PO4]F.

The new compound LiNaCo[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO(4)]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) Å, and Z = 8. The structure consists of edge-sharing CoO(4)F(2) octahedra forming CoFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie-Weiss behavior above 60 K with θ = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO(4)]F undergoes a three-dimensional magnetic ordering at T(mag) = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO(3) chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations.

Coexistence of light and heavy carriers associated with superconductivity and antiferromagnetism in CeNi0.8Bi2 with a Bi square net.

We found that the ZrCuSiAs-type crystal CeNi(0.8)Bi(2) with a layered structure composed of alternate stacking of [CeNi(x)Bi(1)](δ+) and Bi(2)(δ-) exhibits a superconductive transition at ∼4 K. The conductivities, magnetic susceptibilities, and heat capacities measurements indicate the presence of two types of carriers with notable different masses, i.e., a light electron responsible for superconductivity and a heavy electron interacting with the Ce 4f electron. This observation suggests that 6p electrons of Bi(2) forming the square net and electrons in CeNi(x)Bi(1) layers primarily correspond to the light and heavy electrons, respectively.

The boson peak of silicate glasses: the role of Si-O, Al-O, and Si-N bonds.

The role of Si-O, Al-O, and Si-N bonds on the boson peak of silicate glasses has been investigated from a study of amorphous Si, SiO(2), and two calcium aluminosilicates with 0 (Ca28-O) and 4.4 (Ca28-N) mol % Si(3)N(4). The low-frequency part of the vibrational density of states g(omega) has been calculated from inversion of literature data and new heat capacity measurements. As defined by g(omega)/omega(2), the boson peak correlates with the excess heat capacity observed with respect to Debye T(3) limiting law. That libration of SiO(4) tetrahedra represents the main source of low-frequency excitations in silica glass is illustrated by the strong difference between the anomalies of amorphous Si and SiO(2) glass and the marked decrease observed for SiO(2) phases of increasing density. When Al substitutes for Si, libration of AlO(4) tetrahedra appears hampered by the presence of a charge-compensating cation. Rigidification of the silicate network resulting from substitution of N for O causes the boson peak of Ca28-N to be smaller than that of Ca28-O and shifted toward higher frequencies as increased cross-linking hinders libration of SiO(4) or AlO(4) tetrahedra. In agreement with their universal phenomenology, the calorimetric boson anomalies of Ca28-O and Ca28-N plot on the master curve defined previously by SiO(2) and alkali silicate glasses.

Rotational sublevels of an ortho-hydrogen molecule encapsulated in an isotropic C60 cage.

From specific heat measurements in high quality H2@C60 samples performed over a broad temperature range, we obtain the smallest yet observed splitting of rotational energy sublevels of encapsulated single H2 molecules, 0.1-0.2 meV, in the nearly spherical potential well provided by highly isotropic C60 cages. Additionally, we find evidence of the quantized oscillation state of isolated H2 in the C60 cage. The minuscule splitting indicates that H2@C60 provides unprecedented opportunities to study free-molecule quantum dynamic properties.

Calorimetric and dielectric study of organic ferroelectrics, phenazine-chloranilic acid, and its bromo analog.

The heat capacities of single crystals of organic ferroelectric complexes phenazine-chloranilic acid (Phz-H(2)ca) and phenazine-bromanilic acid (Phz-H(2)ba) were measured. At temperatures below those of the reported ferroelectric phase transitions, heat capacity anomalies due to successive phase transitions were found in both complexes. Excess entropies involved in the low-temperature successive phase transitions are much larger than those due to the ferroelectric phase transitions. The temperature dependence of the complex dielectric constants showed the existence of multiple dielectric relaxation modes in both complexes and their deuterated analogs (Phz-D(2)ca and Phz-D(2)ba). We discuss the possibility of concerted hopping of neighboring protons within a hydrogen-bonded chain while taking into account the one-dimensional nature of the chain.

Specific-heat anomaly caused by ferroelectric nanoregions in Pb(Mg(1/3)Nb(2/3))O3 and Pb(Mg(1/3)Ta(2/3))O3 relaxors.

The specific heat of typical relaxors, Pb(Mg(1/3)Nb(2/3))O3 (PMN) and Pb(Mg(1/3)Ta(2/3))O3 (PMT), was measured by adiabatic and relaxation methods between 2 and 420 K. A broad anomaly was found in the specific heat curve over the wide temperature range between 150 and 500 K for PMN, and between 50 and 400 K for PMT, which provides evidence for the formation of ferroelectric nanoregions (FNR) in the paraelectric matrix. The entropy of the anomaly was estimated as 3.3 J K(-1) mol(-1) and 2.9 J K(-1) mol(-1) for PMN and PMT, respectively, which implies an order-disorder-type mechanism for the formation of FNR.