268 research outputs found
Incommensurate interactions and non-conventional spin-Peierls transition in TiOBr
Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2
quantum magnet TiOBr shows that the phase transition at T_{c2} = 47.1 (4) K
corresponds to the development of an incommensurate superstructure. Below
T_{c1} = 26.8 \pm 0.3 K the incommensurate modulation locks in into a two-fold
superstructure similar to the low-temperature spin-Peierls state of TiOCl.
Frustration between intra- and interchain interations within the spin-Peierls
scenario, and competition between two-dimensional magnetic order and
one-dimensional spin-Peierls order are discussed as possible sources of the
incommensurability.Comment: 5 pages including 3 figures and 1 tabl
Novel B(Ar')2(Ar'') hetero-tri(aryl)boranes: a systematic study of Lewis acidity
A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids
Quantitative three-dimensional local order analysis of nanomaterials through electron diffraction
Structure-property relationships in ordered materials have long been a core principle in materials design. However, the introduction of disorder into materials provides structural flexibility and thus access to material properties that are not attainable in conventional, ordered materials. To understand disorder-property relationships, the disorder â i.e., the local ordering principles â must be quantified. Local order can be probed experimentally by diffuse scattering. The analysis is notoriously difficult, especially if only powder samples are available. Here, we combine the advantages of three-dimensional electron diffraction â a method that allows single crystal diffraction measurements on sub-micron sized crystals â and three-dimensional difference pair distribution function analysis (3D-ÎPDF) to address this problem. In this work, we compare the 3D-ÎPDF from electron diffraction data with those obtained from neutron and x-ray experiments of yttria-stabilized zirconia (Zr0.82Y0.18O1.91) and demonstrate the reliability of the proposed approach
Antiferrodistortive phase transition in EuTiO3
X-ray diffraction, dynamical mechanical analysis and infrared reflectivity
studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics.
Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal
I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in
Glazer notation). The phase transition is analogous to SrTiO3. However, some
ceramics as well as single crystals of EuTiO3 show different infrared
reflectivity spectra bringing evidence of a different crystal structure. In
such samples electron diffraction revealed an incommensurate tetragonal
structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with
modulated structure are activated in the IR spectra due to folding of the
Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly
influence the temperature of the phase transition to antiferrodistortive phase
as well as the tendency to incommensurate modulation in EuTiO3.Comment: PRB, in pres
Spin gap formation in the quantum spin systems TiOX, X=Cl and Br
In the layered quantum spin systems TiOCl and TiOBr the magnetic
susceptibility shows a very weak temperature dependence at high temperatures
and transition-induced phenomena at low temperatures. There is a clear
connection of the observed transition temperatures to the distortion of the
octahedra and the layer separation. Band structure calculations point to a
relation of the local coordinations and the dimensionality of the magnetic
properties. While from magnetic Raman scattering only a small decrease of the
magnetic exchange by -5-10% is derived comparing TiOCl with TiOBr, the
temperature dependence of the magnetic susceptibility favors a much bigger
change.Comment: 5 figures, 15 pages, further information see
http://www.peter-lemmens.d
New salicylaldoximato-borate ligands resulting from anion hydrolysis and their respective copper and iron complexes
Experimental charge density of LiBD4 from maximum entropy method
We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data)
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