Ab Crystal Plane Research Articles

In Situ Induction of Strain in Iron Phosphide (FeP2) Catalyst for Enhanced Hydroxide Adsorption and Water Oxidation

AbstractCarbon‐based materials have been widely used in heterogeneous catalysis because of their advantages of high surface area, thermal stability, and chemical inertness. However, their role in the catalysis is not fully understood although most studies conclude that the coupling between the carbon support and catalyst could reduce the charge transfer resistance and improve the kinetics of catalytic reactions such as water splitting. In this study, a carbon‐modified FeP2 electrocatalyst with a one‐step strategy is synthesized. The tensile strain is introduced in situ in the ab crystal plane of the FeP2 catalyst. This leads to charge redistribution between H and O atoms in the OH bonds and enhances the adsorption of reaction intermediates. In the water oxidation process, this results in a decrease in the energy barrier for the rate‐determining step, specifically, the chemical step of *OH adsorption preceded by one‐electron transfer. Benefiting from the optimized adsorption energy, the strained catalysts exhibit excellent oxygen evolution reaction (OER) activity with a low overpotential in addition to their increased stability. This study provides a new strategy for the introducing of strains in functional materials and provides new insights into the influence of carbon modification on OER activity.

Polarization of the spontaneous magnetic field and magnetic fluctuations in s+is anisotropic multiband superconductors

We show that multiband superconductors with broken time-reversal symmetry can produce spontaneous currents and magnetic fields in response to the local variations of pairing constants. Considering the iron pnictide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ as an example we demonstrate that both the point-group symmetric $s+is$ state and the C$_4$-symmetry breaking $s+id$ states produce in general the same magnitudes of spontaneous magnetic fields. In the $s+is$ state these fields are polarized mainly in ab crystal plane, while in the $s+id$ state their ab-plane and c-axis components are of the same order. The same is true for the random magnetic fields which are produced by the order parameter fluctuations near the critical point of the time-reversal symmetry breaking phase transition. Our findings can be used as a direct test of the $s+is/s+id$ dichotomy and the additional discrete symmetry breaking phase transitions with the help of muon spin relaxation experiments.

Syntheses, Structures, and Properties of Two Coordination Polymers Based on 2-Pyrimidineamidoxime

Two coordination polymers, {[Cd(L1)2(L2)] · 0.25H2O} n (I) and {[Cd(L1)(L3)H2O] · 2H2O} n (II) (L1 = 2-pyrimidineamidoxime, L2 = 4-sulfobenzoate dianion and L3 = 5-sulfosalicylate dianion), has been synthesized and structurally characterized by single-crystal X-ray diffraction (CIF files CCDC nos. 1565646 (I) and 1565728 (II)). Complex I crystallizes in monoclinic space group P21/n with a = 10.1462(3), b = 16.0152(5), c = 14.0349(5) A, β = 93.267(3)°, V = 2276.87(13) A3, C68H66N32O29S4Cd4, M = 2373.36, ρcalcd = 1.731 g/cm3, μ(MoKα) = 1.109 mm−1, F(000) = 1186, GOOF = 0.806, Z = 1, the final R1 = 0.0287 and wR2 = 0.0733 for I > 2σ(I). Complex II crystallizes in monoclinic space group P21 with a = 6.882(2), b = 17.138(2), c = 7.883(2) A, β = 103.83(3)°, V = 902.8(4) A3, C12H16N4O10SCd, M = 520.75, ρcalcd = 1.916 g/cm3, μ(MoKα) = 1.388 mm−1, F(000) = 520, GOOF = 1.047, Z = 2, the final R1 = 0.0739 and wR2 = 0.2041 for I > 2σ(I). Crystal structural analysis reveals that complex I possesses the corrugated 1D chain structure extending along the $$[\bar 101]$$ direction. However, complex II displays a 2D coordination network lying on the ab crystal plane, which can be simplified as a binodal 3-connected 63 topological network by considering Cd2+ ions and L3 ligands as 3-connected nodes. Their photoluminescent and thermal properties were also investigated.

Crystal and local structure refinement in Ca2Al3O6F explored by X-ray diffraction and Raman spectroscopy

We present a combined structural analysis on the powder of the Ca2Al3O6F phase using X-ray diffraction (XRD) and Raman spectroscopy techniques. The crystal structure of Ca2Al3O6F has been refined in the rhombohedral system, R3[combining macron] space group, a = 17.3237(7) Å, c = 7.00017(4) Å, V = 1819.38(2) Å(3), Z = 6. The Ca2Al3O6F phase consists of almost ideal AlO4 tetrahedrons linked through corners, Ca(2+) ions in voids, and F(-) ions disordered over 6 sites around the Ca2 ion. The two different Ca sites have also been verified by the photoluminescence spectrum and decay curves using Eu(2+) as the probe ion substituted onto the Ca(2+) sites. A lattice dynamics simulation based on the simplified version of the Born-Karman potential model has been produced. Calculated Raman phonon modes agree qualitatively well with the experimental data. The calculations show that the strong line at 538 cm(-1) (Ag) corresponds to the vibrational mode of a six-membered AlO4 tetrahedrons ring, and the line at 572 cm(-1) (Ag) corresponds to the full symmetric vibration of fluorine atoms in the ab crystal plane.

Lattice dynamics and electron‐phonon coupling in pentacene crystal structures

AbstractThe crystal structures and lattice phonons of pentacene are computed by the quasi harmonic lattice dynamics (QHLD) method. From the eigenvectors of the low frequency phonons we calculate the e‐ph coupling constants due to the modulation of the transfer integrals. The transfer integrals are computed by the Hückel method and by the INDO/S Hamiltonian for all the nearest neighbor pentacene pairs in the ab crystal plane.

Lattice dynamics and electron-phonon coupling in theβ−(BEDT−TTF)2I3organic superconductor

The crystal structure and lattice phonons of the $(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}$ superconducting $\ensuremath{\beta}$ phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the quasiharmonic lattice dynamics (QHLD) method. The empirical atom-atom potential is that successfully employed for neutral BEDT-TTF and for nonsuperconducting $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}.$ The rigid molecule approximation has to be relaxed by allowing mixing between lattice and low-frequency intramolecular vibrations. Such a mixing is found to be essential to account for the specific heat measurements, and also yields good agreement with the observed Raman and infrared frequencies. The crystal structure and its temperature and pressure dependence are also properly reproduced, though the effect of the mixing is less important in this case. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The charge transfer integrals are evaluated by the extended H\"uckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function ${\ensuremath{\alpha}}^{2}(\ensuremath{\omega})F(\ensuremath{\omega}),$ which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling constant $\ensuremath{\lambda}$ is found to be $\ensuremath{\sim}0.4.$

Large-Magnitude High-Spin Nuclear Parameters in a Ti3+Center from X-band EPR Measurements at 10 K

Precise 10 K X-band EPR measurements and subsequent spin-Hamiltonian analysis by direct matrix diagonalization methods are reported for a Ti3+(S= 1/2) center in tetragonal zircon (zirconium silicate, ZrSiO4). A special, and previously unobserved, feature of the supposedly uniaxial spectrum is the marked angular dependence of the titanium hyperfine lines in the perpendicular,abcrystal plane. As discussed, this can only arise from the presence of high-spin nuclear terms of dimensionBIk, SIk(k= 3, 5) in the spin Hamiltonian. Parameters arising from these terms were determined to have magnitudes very much larger than observed previously in first-row transition ions. The consequences of precise determination of these high-spin parameters are significant and several: a precise determination of the nuclear quadrupole tensor leading to a ratio47P/49Pin excellent agreement with the ratio derived from the corresponding nuclear quadrupole moments; an apparent anisotropy in the nuclear Zeeman interaction which can be identified with anisotropy in the chemical shielding “tensor”; a marked hyperfine anomaly. The origin and significance of these observations are discussed.

Detection of quantized flux penetration into very small 0-type Josephson linksinBi2Sr2Ca1Cu2Oysingle crystals using ESR spectrometry

Magnetic-flux trapping in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathrm{y}}$ single crystals is investigated using an electron-spin-resonance spectrometer to study the trapping mode of high-${\mathrm{T}}_{\mathrm{c}}$ superconductors. Several series of periodic microwave (MW) absorption spectra were observed at external fluxes ${\mathrm{\ensuremath{\Phi}}}_{\mathrm{x}}$=(k\ifmmode\pm\else\textpm\fi{}1/2)${\mathrm{\ensuremath{\Phi}}}_{0}$,nk=0,1,2,ldots . They are superposed on a broad MW spectrum ascribed to the surface impedance. The periodic spectrum is clear evidence for stepwise transitions (k\ensuremath{\rightarrow}k+1) between energy states of Josephson-junction links by microwave absorption and for stepwise penetration of flux quanta ${\mathrm{\ensuremath{\Phi}}}_{0}$. The periodicity with a half quantum (k\ifmmode\pm\else\textpm\fi{}1/2)${\mathrm{\ensuremath{\Phi}}}_{0}$ was shown to be due to 0-type Josephson-junction links. It was possible to follow the angular dependence of a few sets of periodic spectra and to determine the sizes and orientations of the intracrystal Josephson links. The link size and the number of quanta penetrating the links are very small, an order of 0.5--4 \ensuremath{\mu}${\mathrm{m}}^{2}$ and several to a few tens of quanta, and are about 10--${10}^{2}$ times smaller than those in Y-Ba-Cu-O and ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ crystals. Every link surface is found to be closely located in the ab crystal plane.

SYNTHESIS AND STRUCTURE OF (2-AMINO-5-BROMOPYRIMIDINE)BROMOCOPPER(I)

The reaction of CuBr2 with 2-amino-5-bromopyrimidine and HBr in aqueous solution gave as the major product (2-amino-5-bromopyrimidine)bromocopper(I) (1) in 58% isolated yield. Compound 1 crystallizes as small orange blocks in the rare space group 14mm. Crystal data for 1 are: tetragonal. space group: 14mm, a = b= 15.646(3), c = 6.705(3) Å, V = 1641.3(8) Å3 Z = 8. Dcale = 2.634 Mg/m3, μ = 12.334 mm−1, F(OOO) =1216, MoKa (λ. = 0.71073 A˚). R = 0.0322 for [|I| ⩾2σ(I)] and R = 0.0420 for all 343 unique reflections and 70 parameters. The structure exists as a firee-dimensional coordination polymer with sheets of pyrimidine bridged copper atoms, parallel to the ab-crystal plane, bridged by bromine atoms roughly parallel to the c-axis.

Paleomagnetism carried by crystal inclusions: The effect of preferred crystallographic orientation

Randomly oriented phlogopite crystals from a pegmatite contain submicroscopic inclusions of magnetite. The inclusions have oblate magnetic fabrics, subparallel to the ab crystal plane. Alternating field demagnetization of individual crystals shows that the remanence is influenced by the crystal symmetry of the phlogopite host. The magnetizations isolated by alternating field demagnetization either cluster close to the a-axis or scatter in the ab plane. Coarse-grained igneous and metamorphic rocks can only record a paleofield direction accurately if there is a suitable orientation distribution of the rock-forming minerals that host the ferromagnets. An isotropic distribution of rock-forming minerals is preferable but it is not obligatory. Rock-forming minerals that have L-fabrics or S-fabrics with a strong L-component are also suitable if they contain oblate inclusion fabrics of ferromagnets.

Influence of molecular thermal motion on electronic excitation transport in anthracene crystals

Anisotropies in both the diffusion coefficient of triplet excitons and the mobilities of electrons and holes were numerically calculated in each direction of anthracene crystal axes at 95 and 290 K. Dynamic fluctuations of the molecular position, obtained by a molecular-dynamics simulation method, were used in ab initio calculations which determined the electronic excitation transport based on a hopping model. Compared with results from investigations considering only static perfect lattices, improvements were obtained in particular for the triplet exciton in the ab crystal plane and for the hole in all directions. However, as previously found in several studies, a discrepancy still remains between the theoretical and experimental anisotropies of the electron and triplet-exciton transport in the c' direction as compared with those in the ab plane. It is suggested this discrepancy is caused by static structural fluctuations, such as lattice defects in real crystals.

Charge-transfer transitions in crystalline anthracene and their role in photoconductivity

Electric-field modulated absorption spectra of polycrystalline anthracene layers delineate existence of a series of five charge-transfer bands that can be assigned to transitions within the ab crystal plane. The energy versus distance relationship is coulombic (e = 3.2) yielding an optical band gap E g opt = 4.4±0.05 eV. Absorption coefficients are about one order of magnitude lower than calculated by Bounds et al. and indicate a coupling constant A ∼ 0.15 for interaction between CT and Frenkel exciton states. Previous data for intrinsic free-carrier production, in particular the energy dependence of the “thermalization” distance, can be consistently interpreted in terms of dissociation of CT pairs if the assumption is made that the vibrational CT energy (0.3 eV) can fully or in part be used for additional separation of the electron-hole pair. The adiabatic (electrical) band gap is E g e1 = 4.1±1 eV.

THE CRYSTAL STRUCTURE OF TETRAMETHYLAMMONIUM trans-DIAQUOTETRACHLORORHODATE(III)

Abstract The crystal structure of tetramethylammonium trans-diaquotetrachlororhodate(III) has been solved by X-ray diffraction using counter techniques. The compound crystallizes in the orthorhombic system with a=7.961 (7) Å, b=8.439(7) Å, and c=9.222(9) Å. The space group is D 2h 13-Pmmn. The measured density of 1.82(3) g-cm−3 agrees with that of 1.90(1) g-cm−3 calculated for two formula units in the unit cell. A total of 353 independent reflections, up to (sin Θ)/λ of 0.50, and having I ≥ 3[sgrave](I) was used in solving the structure which refined to a conventional R-factor of 0.051 and a weighted R-factor of 0.072. The RhCl4(H2O)2 − anion lies at a site of mm symmetry in the crystal as does the (CH3)4N+ cation. There is slightly distorted, octahedral coordination about the rhodium ion with Rh-Cl distances of 2.300(7), 2.330(4) and 2.357(6) Å, and a Rh-O distance of 2.032(10) Å. There is apparently extensive hydrogen bonding between the chloroaquorhodate(III) anions in the crystal, with sheets of these anions being interleaved with sheets of cations parallel to the ab crystal plane.

Infrared Spectra in Polarized Light and Vibrational Assignment of the Infrared-Active Modes of Pyrene and Pyrene-d10

The infrared spectra of pyrene and pyrene-d10 in the crystalline state and in solution of various solvents were measured between 4000 and 400 cm-1. Spectra in polarized light with the electric vector of the incident light vibrating in the ab crystal plane were obtained from both single crystals and oriented polycrystalline samples. Spectra in polarized light with the electric vector in the ac crystal plane were also obtained from oriented films. The comparison of the experimental dichroism with the predictions based on the knowledge of the crystal structure allowed an almost complete assignment of the fundamental modes of vibrations active in the ir spectrum.