No abstract

Electron-density analyses of tetragonal and cubic 0.4 PbTiO3-0.6 BiFeO3 (PTBF) have indicated that the (Bi,Pb)-O bond in tetragonal PTBF exhibits strong covalency compared with that in cubic phase. Compared with Pb−O, the Bi−O exhibits stronger covalency, which would be a key factor of extremely large tetragonality and high T C of PTBF.

Calcium hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is the principal inorganic component of bone and teeth, one of the most important bioceramics, and a proton (H+) conductor with potential for energy conversion devices. The proton diffusion pathway in the HAp lattice is a key to understand the proton conduction mechanism and chemical reaction. Previous neutron-diffraction studies of HAp visualized the short-range proton diffusion pathway. In this work, we report the successful visualization of the long-range proton diffusion pathway in stoichiometric HAp at 923, 673, and 298 K through a combined technique of high-temperature neutron diffraction and bond valence method. We have visualized (1) one-dimensional proton diffusional pathways along the c-axis in the hexagonal channel and (2) two-dimensional proton migration pathway network on the ab-planes at z = 0 and 1/2. The proton diffusion and reorientation of hydroxide ions (OH–) are a complex sinusoidal process in the hexagonal channel along the c-axis, which is consistent with the anisotropic nuclear-density distribution of proton obtained by the neutron diffraction and maximum-entropy method.

K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al-O2) being higher than that between Al and equatorial oxygen O1 α(Al-O1) [α(Al-O2) = 2.41(18)α(Al-O1)]. The higher α(Al-O2) is attributed to the Al-O2 bond being longer and weaker than the Al-O1 bond. Thus, the minimum electron density and bond valence of the Al-O2 bond are lower than those of the Al-O1 bond. For Sr2TiO4, the Ti-O2 interatomic distance, d(Ti-O2), is equal to that of Ti-O1, d(Ti-O1) [d(Ti-O2) = 1.0194(15)d(Ti-O1)], relative to LaSrAlO4 [d(Al-O2) = 1.0932(9)d(Al-O1)]. Therefore, the bond valence and minimum electron density of the Ti-O2 bond are nearly equal to those of the Ti-O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B-O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B-O2)/d(B-O1) of A2(2.5+)B(3+)O4 compared with A2(2+)B(4+)O4, A2(2.5+)B(3+)O4 compounds have higher α(B-O2), and A2(2+)B(4+)O4 materials exhibit smaller α(B-O2), leading to the anisotropic thermal expansion of A2(2.5+)B(3+)O4 and isotropic thermal expansion of A2(2+)B(4+)O4. The "true" thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition.

Interfaces in nanocarbon materials are highly important, as they determine the properties of carbon-based devices. In terms of carrier and thermal transport properties, the interfacial features are often more important than the intrinsic characteristics. Herein, we describe how 1 min Joule annealing of carbon nanotube (CNT) yarns can convert the interfacial amorphous carbon into graphene fragments. After 1 min Joule annealing, we have obtained multiwalled CNT yarns with extremely high Seebeck coefficients (±100 μV/K) and high thermoelectric power factor (400 and 1000 μW/mK 2 ) at room temperature, both with or without polyethylenimine doping. Theoretical simulations and experimental measurements helped to determine the optimal annealing conditions in terms of a rapid transformation of the interfacial amorphous carbon between the bundled CNTs in the yarn into graphene fragments at ∼2000 K. The present approach represents significant progress in energy materials science, as it provides a guiding principle for the design of interfaces in nanocarbon materials with potential applications in energy-harvesting systems.

The experimental electron-density distribution of ferroelectric bismuth ferrite (BiFeO3) has been visualized using the synchrotron X-ray powder diffraction data and the maximum-entropy method. The present work has clearly revealed the partial covalency of the Bi-O bond, which is a key to understand the spontaneous polarization. Density functional theory (DFT) calculation has supported the validity of the experimental results.

The crystal structure, phase stability, and oxideion diffusion of nanocrystalline ceria−zirconia materials are unresolved important issues, in particular at high temperatures, where the ceria−zirconia catalysts work efficiently. Here, we report a high-temperature neutron diffraction study of nanocrystalline [10.1(7) nm], compositionally homogeneous, tetragonal Ce 0.5 Zr 0.5 O 2 . Contrary to the previous work, we have observed no tetragonal-to-cubic phase transition in the nanocrystalline Ce 0.5 Zr 0.5 O 2 up to 1176 K. The axial ratio c/a F and oxygen displacement along the c-axis from the fluorite regular 8c 1/4,1/4,1/4 position of the nanocrystalline Ce 0.5 Zr 0.5 O 2 are almost independent of temperature. It was found that the c/a F ratio and oxygen displacement in nanocrystalline Ce 0.5 Zr 0.5 O 2 are smaller than those of the bulk sample. It was shown that the refined atomic displacement parameter and spatial (maximum entropy method nuclear density) distribution of oxygen atom in nanocrystalline Ce 0.5 Zr 0.5 O 2 are larger than those in nanocrystalline CeO 2 , which are factors of the high bulk oxygen diffusivity and catalytic activity in nanocrystalline Ce 0.5 Zr 0.5 O 2 . Possible diffusion pathways of oxide ions along the fluorite ⟨100⟩ and ⟨110⟩ directions were visualized in the spatial distribution of bond valence sums calculated using the present refined crystal structure of nanocrystalline Ce 0.5 Zr 0.5 O 2 at 1023 K. The diffusion path is not straight but curved and forms a three-dimensional network.

No abstract