Search results for "Octahedron"
showing 10 items of 405 documents
Modulation of the magnetic anisotropy of octahedral cobalt(ii) single-ion magnets by fine-tuning the axial coordination microenvironment
2019
Two mononuclear cobalt(II) complexes, with the formulas [Co(2,6-dfba)2(bpp)2(H2O)2]n (1) and [Co(2,6-dfba)2(bpe)2(H2O)2]n (2) (2,6-Hdfba = 2,6-difluorobenzoic acid, bpp = 1,3-bis(4-pyridyl)propane, bpe = 1,2-bis(4-pyridyl)ethylene), have been synthesized by combining Co(II) ions with benzoate derivatives and two homogeneous N-donor ligands, respectively. Constrained by the analogous CoN2O4 coordination spheres, the discretely hexa-coordinated Co(II) cores in both complexes display stretched octahedral geometries. The equatorial environments in both complexes are identical, whereas the axial sites are finely modulated by the different chemical natures of the terminal N-donor ligands. The com…
Nanoscale oxide growth on Al single crystals at low temperatures: Variable charge molecular dynamics simulations
2006
We investigate the oxidation of aluminum low-index surfaces [(100), (110), and (111)] at low temperatures (300-600 K) and three different gas pressure values. We use molecular dynamics (MD) simulations with dynamic charge transfer between atoms where the interaction between atoms is described by the Es+ potential composed of the embedded atom method (EAM) potential and an electrostatic contribution. In the considered temperature range and under different gas pressure conditions, the growth kinetics follow a direct logarithmic law where the oxide thickness is limited to a value of ∼3 nm. The fitted curves allow us to determine the temperature and the pressure dependencies of the parameters i…
X-ray absorption and Raman spectroscopy studies of tungstates solid solutions ZncNi1-cWO4 (c=0.0-1.0)
2020
G.B. acknowledges the financial support provided by the State Education Development Agency for project No. 1.1.1.2/VIAA/3/19/444 (agreement No. 1.1.1.2/16/I/001) realized at the Institute of Solid State Physics, University of Latvia. A.K. and A.K. would like to thank the support of the Latvian Council of Science project No. lzp-2019/1-0071. Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.
Cubic metamaterial crystal supporting broadband isotropic chiral phonons
2021
Chiral metamaterials can support chiral phonons leading to acoustical activity, the acoustical counterpart of optical activity. However, the properties of early metamaterial designs have been very highly anisotropic, and chiral acoustical phonons occurred only for selected high-symmetry directions. The authors propose a novel chiral metamaterial based on ``twisting'' a truncated octahedron in a simple-cubic unit cell. Not supported by crystal symmetry alone but rather by a tuned degeneracy, chiral phonons and large broadband acoustical activity are obtained for all phonon propagation directions in 3D. This result is notable because even isotropic achiral acoustical phonons are rare for crys…
Three-dimensional subnanoscale imaging of unit cell doubling due to octahedral tilting and cation modulation in strained perovskite thin films
2019
Determining the three-dimensional (3D) crystallography of a material with subnanometer resolution is essential to understanding strain effects in epitaxial thin films. A scanning transmission electron microscopy imaging technique is demonstrated that visualizes the presence and strength of atomic movements leading to a period doubling of the unit cell along the beam direction, using the intensity in an extra Laue zone ring in the back focal plane recorded using a pixelated detector method. This method is used together with conventional atomic resolution imaging in the plane perpendicular to the beam direction to gain information about the 3D crystal structure in an epitaxial thin film of La…
Synthesis and crystal structure of the α polytype of HfNBr
2002
Abstract α-HfNBr has been prepared at 760 °C in a sealed evacuated fused silica tube by reaction between NH4Br and Hf followed by purification through chemical vapour transport under a temperature gradient. The crystal structure of this compound at room temperature has been determined for the first time by Rietveld refinement of X-ray powder diffraction data, electron diffraction and high resolution electron microscopy. It crystallises in the orthorhombic space group Pmmn with the unit cell parameters a=4.1165(2), b=3.5609(2), c=8.6440(3) A. α-HfNBr is isotypic to FeOCl and is built from layers of composition BrHfNNHfBr stacked along c that are separated by a Van der Waals gap. The haf…
Effect of thermal treatment on the structural evolution of 3:2 and 2:1 mullite monophasic gels
2005
Abstract Single phase mullite gels with composition 3Al2O3 · 2SiO2 (3:2) and 2Al2O3 · SiO2 (2:1) were prepared by the semialkoxide method using aluminum nitrate nonahydrate and tetraethylorthosilicate as reagents. The influence of the thermal processing conditions on the reaction sequence was investigated. Both 3:2 and 2:1 mullite gels formed orthorhombic mullite under fast heating (40 °C/min) at 900 °C. Slow heatings delayed the formation of mullite in both gel compositions but no formation of spinel was detected between 1000 °C and 1100 °C. The slow transformation of pentacoordinated aluminum to octahedral and tetrahedral arrangements probably depends on the larger size and extent of the …
Oxygen K-edge x-ray absorption near-edge structure in crystalline and amorphous molybdenum trioxides
2004
X-ray absorption near-edge structure (XANES) signals at the oxygen K edge in polycrystalline α-MoO3 and amorphous a-MoO3 thin film were analysed within the full-multiple-scattering (FMS) formalism. Significantly different XANES signals were found for non-equivalent oxygen atoms in low-symmetry layered-type α-MoO3 structure. The obtained results are in agreement with the experimental data and allow us to interpret all XANES peaks for α-MoO3. Besides, the FMS XANES simulations, performed for several fragments of α-MoO3 structure, allowed us to explain the O K-edge XANES in amorphous a-MoO3 thin film. We found that although the crystallographic structures of α-MoO3 and a-MoO3 are strongly diff…
Aluminum and Gallium Substitution in Yttrium and Lutetium Aluminum–Gallium Garnets: Investigation by Single-Crystal NMR and TSL Methods
2016
The work reports the results on 71Ga and 27Al NMR investigation of the gallium and aluminum ions distribution over tetrahedral and octahedral positions in the Y3Al5−x GaxO12:Ce single crystals and Lu3Al5−xGaxO12:Ce single-crystalline epitaxial films. The gallium content x varies between 0 and 5 in crystals and between 0.3 and 2 in films.. We find that in both the Y- and Lu-based solid solutions the larger gallium ions are preferably located at the tetrahedral position while the smaller aluminum ions prefer the octahedral position of the garnet host. Based on NMR data, the dependence of fractional occupation parameters of the tetrahedral site of Ga and Al ions on the Ga content is determined…
Magneto-structural diversity of Co(ii) compounds with 1-benzylimidazole induced by linear pseudohalide coligands
2020
We report the preparation, spectroscopic characterisation, crystal structure determination and cryomagnetic investigation of three cobalt(II) complexes of formula trans-[Co(bim)4(NCS)2] (1), [Co(bim)2(NCO)2] (2) and [Co(bim)2(N3)2]n (3) (bim = 1-benzylimidazole). The structure of 1 is made up of neutral [Co(bim)4(NCS)2] mononuclear units, where the cobalt(II) ion is six-coordinate with four monodentate bim ligands in equatorial positions and two N-thiocyanato groups in the axial sites building a slightly compressed octahedron. In contrast to 1, each cobalt(II) ion in 2 is four-coordinate with two imidazole-nitrogen atoms from two bim molecules and two N-cyanato ligands describing a slightly…