187 research outputs found
To superspace and beyond
The significance of an algorithm developed by H. T. Stokes & B. J. Campbell [Acta Cryst. (2017), A73, 4-13] is discussed. The algorithm promises to be a key tool for understanding the structure-property relationships of the many technologically important materials that display incommensurate modulations in their atomic and/or magnetic structure
An investigation into the unusual linkage isomerization and nitrite reduction activity of a novel tris(2-pyridyl) copper complex
The copper-containing nitrite reductases (CuNIRs) are a class of enzymes that mediate the reduction of nitrite to nitric oxide in biological systems. Metalâligand complexes that reproduce the salient features of the active site of CuNIRs are therefore of fundamental interest, both for elucidating the possible mode of action of the enzymes and for developing biomimetic catalysts for nitrite reduction. Herein, we describe the synthesis and characterization of a new tris(2-pyridyl) copper complex ([Cu1(NO2)2]) that binds two molecules of nitrite, and displays all three of the common binding modes for NO2â, with one nitrite bound in an asymmetric quasi-bidentate Îș2-ONO manner and the other bound in a monodentate fashion with a linkage isomerism between the Îș1-ONO and Îș1-NO2 binding modes. We use density functional theory to help rationalize the presence of all three of these linkage isomers in one compound, before assessing the redox activity of [Cu1(NO2)2]. These latter studies show that the complex is not a competent nitrite reduction electrocatalyst in non-aqueous solvent, even in the presence of additional proton donors, a finding which may have implications for the design of biomimetic catalysts for nitrite reduction
First-principles investigation of the magnetoelectric properties of Ba7Mn4O15
Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of Ba7Mn4O15 which has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics. When the constraints on the magnetic moments are removed, the spins self-consistently relax into a canted antiferromagnetic ground state configuration where two magnetic modes transforming as distinct irreducible representations coexist. While the dominant magnetic mode matches well with the previous experimental observations, the second mode is found to possess a different character resulting in a non-polar ground state. Interestingly, the non-polar magnetic ground state exhibits a significantly strong linear magnetoelectric coupling comparable to the well-known multiferroic BiFeO3, suggesting strategies to design new linear magnetoelectrics
Charge, orbital and magnetic ordering in transition metal oxides
Neutron and x-ray diffraction has been used to study charge, orbital and magnetic ordering in
some transition metal oxides. The long standing controversy regarding the nature of the
ground state (Verwey structure) of the canonical charge ordered material magnetite (Fe3O4)
has been resolved by x-ray single crystal diffraction studies on an almost single domain
sample at 90 K. The Verwey structure is confirmed to have Cc symmetry with 56 unique
sites in the asymmetric unit. Charge ordering is shown to be a useful first approximation to
describe the nature of the ground state, and the conjecture that Verwey made in 1939 has
finally been confirmed. However, three-site distortions which couple to the orbital ordering
of the Fe2+ ordered states (trimerons) are shown to provide a more complete description of
the low temperature structure. Trimerons explain the rather continuous distribution of the
valence states observed in magnetite below Tv, anomalous shortening of Fe-Fe distances and
the off-centre distortions resulting in ferroelectricity. DFT+U electronic structure
calculations on the experimental coordinates support the conclusion of this crystallographic
study, with the highest electron densities calculated for those Fe-Fe distances predicated to
participate in the trimeron bonds.
The 6H-perovskites of the type Ba3ARu2O9 have been reinvestigated by high resolution
neutron and x-ray power diffraction. The charge ordered state of Ba3NaRu2O9 has been
characterised at 110 K (P2/c, a =5.84001(2) Ă
, b = 10.22197(4) Ă
, c = 14.48497(6) Ă
, ÎČ =
90.2627(3) °) and shown to consist of a structure with near integer charge ordering of
Ru5+
2O9 / Ru6+
2O9 dimers. The ground state has been shown to be very sensitive to external
perturbations, with a novel melting of charge ordering observed under x-ray irradiation
below 40 K (C2/c, a =5.84470(2) Ă
, b = 10.17706(3) Ă
, c = 14.45866(5) Ă
, ÎČ =
90.2151(3)-° at 10 K). High pressure studies reveal that the Ru-Ru intra-dimer distance may
dictate the response of the system to pressure. Empirical trends in the Ba3ARu2O9 series of
compounds have shown that change in âchemical pressureâ in these systems may be
rationalised in terms of Coulombâs law. In A = La and Y the magnetic ordering is shown to
be FM within the Ru2O9 dimers (1.4(2) ÎŒB and 0.5(1) ÎŒB, respectively per Ru), representing
the first case of intra dimer FM coupling reported in a system containing face-sharing RuO6
octahedra . The overall AFM coupling of the dimers implies an as yet unobserved breaking
of the parent symmetry. In A = Nd, a complex competition between the crystal field effect of
Nd3+ and the magnetic ordering of the Ru2O9 FM moments has been observed, leading first
vi
to FM order of Nd at 25 K (1.56(7) ÎŒB) followed by ordering of Ru moments (0.5(1) ÎŒB) and
a spin reorientation transition of Nd moments at 18 K. In A = Ca, the formation of a singlet
ground state is observed in Ru2O9 rather than the expected AFM coupling and below 100 K
Ba3CaRu2O9 is diamagnetic. All five systems indicate that the Ru2O9 dimer is the physically
significant unit in these systems when considering structural trends and the ordering of
charge, spin and orbital degrees of freedom
Hybrid Local-Order Mechanism for Inversion Symmetry Breaking
Using classical Monte Carlo simulations, we study a simple statistical
mechanical model of relevance to the emergence of polarisation from local
displacements on the square and cubic lattices. Our model contains two key
ingredients: a Kitaev-like orientation-dependent interaction between nearest
neighbours, and a steric term that acts between next-nearest neighbours. Taken
by themselves, each of these two ingredients is incapable of driving long-range
symmetry breaking, despite the presence of a broad feature in the corresponding
heat capacity functions. Instead each component results in a "hidden"
transition on cooling to a manifold of degenerate states, the two manifolds are
different in the sense that they reflect distinct types of local order.
Remarkably, their intersection---\emph{i.e.} the ground state when both
interaction terms are included in the Hamiltonian---supports a spontaneous
polarisation. In this way, our study demonstrates how local ordering mechanisms
might be combined to break global inversion symmetry in a manner conceptually
similar to that operating in the "hybrid" improper ferroelectrics. We discuss
the relevance of our analysis to the emergence of spontaneous polarisation in
well-studied ferroelectrics such as BaTiO and KNbO.Comment: 8 pages, 8 figure
Longitudinal Versus Lateral Estuarine Dynamics and Their Role in Tidal Stratification Patterns in Lower South San Francisco Bay
The dynamics of shoalâchannel estuaries require consideration of lateral gradients and transport, which can create significant intratidal variability in stratification and circulation. When the shoalâchannel system is strongly coupled by tidal exchange with mudflats, marshes, or other habitats, the gradients driving intratidal stratification variations are expected to intensify. To examine this dynamic, hydrodynamic data were collected from 27 January 2017 to 10 February 2017 in Lower South San Francisco Bay, a small subembayment fringed by extensive shallow vegetated habitats. During this deployment, salinity variations were captured through instrumentation of six stations (arrayed longitudinally and laterally) allowing for mechanisms of stratification creation and destruction to be calculated directly and compared with observed time variability of stratification at the central station. We present observationâbased calculations of longitudinal straining, longitudinal advection, lateral straining, and lateral advection. The time dependence of stratification was observed directly and calculated by summing measured longitudinal and lateral mechanisms. We found that the stratification dynamics switch between being longitudinally dominated during the middle of ebb and flood tides to being laterally dominated during the tidal transitions. This variability is driven by the interplay between tidally variable lateral density gradients and turbulent mixing. Relatively constant alongâestuary density gradients are differentially advected during flood and ebb tides, resulting in maximal lateral density gradients around tidal transitions. Simultaneous decrease in turbulent mixing at slack tides allows lateral densityâdriven exchange to stratify the estuary channel at the slack after flood. At the end of ebb, barotropic forcing drives negatively buoyant shoal waters toward the channel.Plain Language SummarySan Francisco Bay sits within a highly urbanized area. The dense population creates large wastewater effluent resulting in high nutrient levels. Scientists wonder why there have not been annual phytoplankton blooms like those observed in other estuaries with lower nutrient levels. Some have hypothesized it is due to high turbidity levels and tidal breakdown of stratification creating nonideal environments for phytoplankton growth. However, decadal trends show that the estuary is becoming less turbid, and with changes in climate patterns, there is potential for persistent stratification. We observed development of stratification over the ebb tide and destratification in two distinct events as the tide reverses over the flood tide. At the reversal of the tides, water in the shoals exchange with the water in the channel creating a pulse of salty water to the channel at the ebb to flood transition and a pulse of fresh water at the flood to the ebb transition. Destratification occurs in the early flood tide due to a pulse of saline water received from the shoals then due to the advection of less stratified water being pulled to the center channel of the estuary. Finally, stratification is destroyed completely due to longitudinal straining and turbulent mixing.Key PointsVertical stratification in shoalâchannel estuary is characterized by strong intratidal variabilityLateral circulation is a key driver of intratidal stratification dynamics at tide transitionsTiming and magnitude of longitudinal straining, advection, lateral straining, and advection set intratidal vertical stratification dynamicsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151865/1/jgrc23594_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151865/2/jgrc23594.pd
Improper Ferroelectric Polarisation in a Perovskite driven by Inter-site Charge Transfer and Ordering
It is of great interest to design and make materials in which ferroelectric
polarisation is coupled to other order parameters such as lattice, magnetic and
electronic instabilities. Such materials will be invaluable in next-generation
data storage devices. Recently, remarkable progress has been made in
understanding improper ferroelectric coupling mechanisms that arise from
lattice and magnetic instabilities. However, although theoretically predicted,
a compact lattice coupling between electronic and ferroelectric (polar)
instabilities has yet to be realised. Here we report detailed crystallographic
studies of a novel perovskite
HgMnMnO that is
found to exhibit a polar ground state on account of such couplings that arise
from charge and orbital ordering on both the A' and B-sites, which are
themselves driven by a highly unusual Mn-Mn inter-site charge
transfer. The inherent coupling of polar, charge, orbital and hence magnetic
degrees of freedom, make this a system of great fundamental interest, and
demonstrating ferroelectric switching in this and a host of recently reported
hybrid improper ferroelectrics remains a substantial challenge.Comment: 9 pages, 7 figure
Electronic Orders in the Verwey Structure of Magnetite
Electronic structure calculations of the Verwey ground state of magnetite,
Fe3O4, using density functional theory with treatment of on-site Coulomb
interactions (DFT+U scheme) are reported. These calculations use the
recently-published experimental crystal structure coordinates for magnetite in
the monoclinic space group Cc. The computed density distribution for minority
spin electron states close to the Fermi level demonstrates that charge order
and Fe2+-orbital order are present at the B-type lattice sites to a
first-approximation. However, Fe2+/Fe3+ charge differences are diminished
through weak bonding interactions of the Fe2+-states to specific pairs of
neighboring iron sites that create linear, three-B-atom trimeron units that may
be regarded as 'orbital molecules'. Trimerons are ordered evenly along most Fe
atom chains in the Verwey structure, but more complex interactions are observed
within one chain.Comment: 13 pages, 4 figures. Changes for version 2: Fig. 4 and corresponding
discussion extende
The use of classification and regression trees to predict the likelihood of seasonal influenza
Background Individual signs and symptoms are of limited value for the diagnosis of influenza. Objective To develop a decision tree for the diagnosis of influenza based on a classification and regression tree (CART) analysis. Methods Data from two previous similar cohort studies were assembled into a single dataset. The data were randomly divided into a development set (70%) and a validation set (30%). We used CART analysis to develop three models that maximize the number of patients who do not require diagnostic testing prior to treatment decisions. The validation set was used to evaluate overfitting of the model to the training set. Results Model 1 has seven terminal nodes based on temperature, the onset of symptoms and the presence of chills, cough and myalgia. Model 2 was a simpler tree with only two splits based on temperature and the presence of chills. Model 3 was developed with temperature as a dichotomous variable (â„38°C) and had only two splits based on the presence of fever and myalgia. The area under the receiver operating characteristic curves (AUROCC) for the development and validation sets, respectively, were 0.82 and 0.80 for Model 1, 0.75 and 0.76 for Model 2 and 0.76 and 0.77 for Model 3. Model 2 classified 67% of patients in the validation group into a high- or low-risk group compared with only 38% for Model 1 and 54% for Model 3. Conclusions A simple decision tree (Model 2) classified two-thirds of patients as low or high risk and had an AUROCC of 0.76. After further validation in an independent population, this CART model could support clinical decision making regarding influenza, with low-risk patients requiring no further evaluation for influenza and high-risk patients being candidates for empiric symptomatic or drug therap
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