Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y0.7_{0.7}Lu0.3_{0.3}MnO3_{3}

We use inelastic neutron scattering to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. In the undoped YMnO$_3$ and LuMnO$_3$, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, we find a much smaller in-plane (hexagonal $ab$-plane) single ion anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-y}$Lu$_{y}$MnO$_{3}$.Comment: 5 pages, 5 figure

Topological states on the breathing kagome lattice

We theoretically study the topological properties of the tight-binding model on the breathing kagome lattice with antisymmetric spin-orbit coupling (SOC) between nearest neighbors. We show that the system hosts nontrivial topological phases even without second-nearest-neighbor hopping, and that the weakly dispersing band of the kagome lattice can become topological. The main results are presented in the form of phase diagrams, where the $\mathbb{Z}_2$ topological index is shown as a function of SOC (intrinsically allowed and Rashba) and lattice trimerization. In addition, exact diagonalization is compared with effective low-energy theories around the high-symmetry points. We find that the weakly dispersing band has a very robust topological property associated with it. Moreover, the Rashba SOC can produce a topological phase rather than hinder it, in contrast to the honeycomb lattice. Finally, we consider the case of a fully spin polarized (ferromagnetic) system, breaking time-reversal symmetry. We find a phase diagram that includes systems with finite Chern numbers. In this case too, the weakly dispersing band is topologically robust to trimerization.Comment: 8 pages, 6 figures; published versio

Selective ethylene trimerization by titanium complexes bearing phenoxy-imine ligands: NMR and EPR Spectroscopic studies of the reaction intermediates

The catalyst systems (FI)TiCl₃/MAO (FI = phenoxyimine ligand with an additional aryl–O–CH₃ donor) display exceptionally high activity in selective ethylene trimerization. By means of NMR and EPR spectroscopy, the nature of the Ti species formed in the catalyst systems (FI)TiCl₃/MAO, (FI)TiCl₃/MMAO, and (FI)TiCl₃/AlR₃/[Ph₃C]⁺[B(C₆F₅)₄]⁻ (R = Me, Et, ⁱBu) has been studied. It was shown that outer-sphere ion pairs of the type [(FI)TiIVMe₂]⁺[A]⁻ ([A]− = [MeMAO]⁻, [MeMMAO]⁻, [B(C₆F₅)₄]⁻) are formed at the initial stage of the reaction of (FI)TiCl₃ with MAO, MMAO, and AlMe₃/[Ph₃C]⁺[B(C₆F₅)₄]⁻. These ion pairs further partially convert into TiIII and TiII species. In the systems (FI)TiCl₃/MAO and (FI)TiCl₃/AlMe₃/[Ph₃C]⁺[B(C₆F5)₄]⁻, complexes with the proposed structures (FI)TiIIIMe₂, (FI)TiIICl, and [(FI)TiII(S)]⁺[A]⁻ ([A]− = [MeMAO]⁻, [B(C₆F₅)4)]⁻, S = solvent, vacancy) were observed (concentrations of TiIII species was lower than those of the TiII congeners). In contrast, in the system (FI)TiCl₃/MMAO, the concentrations of TiIII species (ion pairs of the type [(FI)TiIII(μ-H)(μ-Cl)AlⁱBu₂]⁺[MeMMAO]⁻) were higher than those of the TiII counterparts (ion pairs [(FI)TiII(S)]⁺[MeMMAO]⁻). The system (FI)TiCl₃/MMAO displays lower activity and selectivity in 1-hexene formation, in comparison to (FI)TiCl₃/MAO, due to undesirable PE generation. Probably, TiII and TiIV ion pairs are those participating in ethylene trimerization

Electrostatic topology of ferroelectric domains in YMnO3_3

Trimerization-polarization domains in ferroelectric hexagonal YMnO$_3$ were resolved in all three spatial dimensions by piezoresponse force microscopy. Their topology is dominated by electrostatic effects with a range of 100 unit cells and reflects the unusual electrostatic origin of the spontaneous polarization. The response of the domains to locally applied electric fields explains difficulties in transferring YMnO$_3$ into a single-domain state. Our results demonstrate that the wealth of non-displacive mechanisms driving ferroelectricity that emerged from the research on multiferroics are a rich source of alternative types of domains and domain-switching phenomena

Letter. On the activation of [CrCl3{R-SN(H)S-R}] catalysts for selective trimerization of ethene: a freeze-quench Cr K-edge XAFS study

Homogeneous chromium catalysts for the selective conversion of ethene to hex-1-ene are formed from Cr(III) reagents, amino-thioether ligands of the type HN(CH2CH2SR)2, and aluminum reagents. In this study the early activation steps are investigated by EPR, UV-visible and Cr K-edge XAFS spectroscopy; rapid stopped-flow mixing and a freeze-quench allows good quality EXAFS analysis of a species formed in ~ 1 second of reaction. This is shown to involve reduction to Cr(II) and deprotonation of a NH group of the auxiliary ligand. This 4-coordinate metal-center may act as precursor for the coordination of ethene and subsequent selective oligomerization

Trimerization of aromatic nitriles

Triazine compounds and cross-linked polymer compositions were made by heating aromatic nitriles to a temperature in the range of about 100 C to about 700 C, in the presence of a catalyst or mixture of catalysts. Aromatic nitrile-modified (terminated and/or appended) imide, benzimidazole, imidazopyrrolone, quinoxaline, and other condensation type prepolymers or their precopolymers were made which were trimerized with or without a filler by the aforementioned catalytic trimerization process

Catalytic trimerization of aromatic nitriles for synthesis of polyimide matrix resins

Aromatic nitriles may be trimerized at moderate temperature and pressure with p-toluenesulfonic acid as catalyst. Studies were conducted to establish the effect of the reaction temperature, pressure, time, and catalyst concentration on yield of the trimerized product. Trimerization studies were also conducted to establish the effect of substituting electron donating or withdrawing groups on benzonitrile. Preliminary results of using the catalytic trimerization approach to prepare s-triazine cross-linked polyimide/graphite fiber composites are presented

Towards achieving low background levels in routine dating by liquid scintillation spectrometry

International radiocarbon intercalibration studies have revealed that radiometric laboratories using liquid scintillation (LS) spectrometry of benzene reported, on average, younger ages for near-background standards than either gas proportional counter (GPC) or accelerator mass spectrometry (AMS) laboratories. These studies suggested that the younger LS ages are probably related to the use of spectrophotometric benzene as a background standard. An analysis of successive 110-ka subfossil wood (Airedale Reef Ancient Wood: ARAW) standards shows that vacuum line memory effects occur in LS spectrometry and, consequently, must be corrected to obtain accurate ¹⁴C dates. ARAW standards, measured at monthly intervals in the Waikato laboratory, are used to provide blank corrections for both research and routine dating applications. The strong correlation between the ARAW Δ¹⁴C data and the sample activities that preceded the standards may provide an opportunity to obtain sample-specific blank corrections. Lithium carbide synthesis is likely to prove a source of contamination. This work suggests that reproducible background levels for routine dating of less than 0.1 pMC (55 ka ¹⁴C yr) are achievable