Search results for "Trigonal crystal system"
showing 10 items of 21 documents
Combining Cyanometalates and Coordination Clusters: An Alternative Synthetic Route toward Original Molecular Materials
2013
International audience; With the discovery of molecules or molecule-based compounds that can display blocked magnetization, magnetic ordering or switchable magnetic bistability, the research efforts devoted to molecular magnetic materials have considerably increased over the past two decades, fully exploiting the advantages of the bottom-up approach. 1-4 This research field focuses on promising properties for potential technological applications such as information storage, quantum computing and spintronics at the molecular scale, but it also provides fundamental insights into original quantum phenomena. 5,6 Coordination chemists have developed efficient synthetic tools for the preparation …
Probing the Multiple Structures of Vaterite through Combined Computational and Experimental Raman Spectroscopy.
2014
First-principles Raman spectra have been computed for several new vaterite structural models that have been recently proposed, and compared with spectra recorded on a set of biogenic, geological and synthetic samples. This set includes new measurements collected on Herdamania momus spicules (Great Barrier Reef, Queensland, Australia), which are known to have purity and crystallinity that are higher than for other biogenic samples. Overall, due to the close structural connection between the various models, the computed Raman spectra are found to be broadly similar. However, the spectra obtained for the two most stable models (monoclinic C2 and trigonal P3221, corresponding to two different p…
The effect of Cu substitution on the A1g mode of La(0.7)Sr(0.3)MnO3 manganites
2003
We report on the first Raman data of Cu substituted La(1-y)Sr(y)Mn(1-x)Cu(x)O3 (0 < x < 0.10 and 0.17 < y < 0.3, accordingly in order to have the same Mn(4+)/[Mn(4+)+Mn(3+)] ratio), collected in the frequency range 100-900 cm-1 and at room temperature, with parallel and crossed polarizations of the incident and scattered light. Spectra were fitted with a Drude-Lorentz model, and peaks at 190-220 and 430 cm-1, together with two broad structures centered at near 500 and 670 cm-1, have been found. We also have observed that the A1g mode is substantially shifted with increasing Cu substitution. The A1g phonon shift is a linear function of the tolerance factor t and the rhombohedral …
Electronic structure of two crystallographic forms ofBaRuO3
2000
Electronic structure calculations have been performed to explain the difference in the electronic properties of two crystallographic forms of ${\mathrm{BaRuO}}_{3}.$ The calculations can explain the qualitatively different resistivities of isoelectronic $4H$- and $9R$-${\mathrm{BaRuO}}_{3}$ below 100 K. The difference in symmetry between the hexagonal four-layer ${\mathrm{BaRuO}}_{3}$ and the rhombohedral nine-layer compound allows the formation of a gap for the later. The electronic structure of these hexagonal perovskites is compared with the more familiar cubic perovskite ${\mathrm{CaRuO}}_{3}.$
Magnetic exchange interaction in a pair of orbitally degenerate ions: Magnetic anisotropy of [Ti2Cl9]−3
2001
The theory of the kinetic exchange in a pair of orbitally degenerate ions developed by the authors [J. Phys. Chem. A 102, 200 (1998)] is applied to the case of face-shared bioctahedral dimer (overall D3h-symmetry). The effective kinetic exchange Hamiltonian is found for a 2T2–2T2 system taking into account all relevant transfer pathways and charge-transfer crystal field states. The influence of different transfer integrals involved in the kinetic exchange on the energy pattern and magnetic properties of the system is examined. The role of other related interactions (trigonal crystal field, spin–orbit coupling) is also discussed in detail. Using the pseudoangular momentum representation and …
Quasinodal lines in rhombohedral magnetic materials
2021
A well-established result in condensed matter physics states that materials crystallizing in symmetry groups containing glide reflection symmetries possess nodal lines on the energy bands. These nodal lines are topologically protected and appear on the fixed planes of the reflection in reciprocal space. In the presence of inversion symmetry, the energy bands are degenerate and the nodal lines on the fixed plane may hybridize or may cross. In the former case, the crossing is avoided, thus producing lines on reciprocal space where the energy gap is small, and in the latter, the nodal lines will endure, thus producing Dirac or double nodal lines. In addition, if the material crystallizes in a …
Thermal expansion of LiZr2(PO4)3: Water inclusion influence
1989
Abstract Lattice thermal expansion has been measured on three samples of composition LiZr 2 (PO 4 ) 3 , prepared by (i) a ceramic method, (ii) a gel-route and (iii) a nonstoichiometric ceramic. The first sample is monoclinic (?), with a transition at 50°C to rhombohedral. The second is clearly monoclinic with lattice parameters depending on the calcination temperature (700 to 1200°C. The third kind of synthesis yielded a new PO 4 -deficitary rhombohedral structure. X-ray diffraction measurements in a high temperature camera have been made from room temperature up to 1100°C. Lattice parameters, as well as their dependence on temperature are different for the three samples. While thermal expa…
Phase transition in NaSn2(PO4)3 and thermal expansion of NaMIV2 (PO43; MIV = Ti, Sn, Zr
1991
Abstract NaSn2(PO4)3 presents a fast and reversible second order phase transition about 575°C. Both phases above and below the transition point are rhombohedral. The high temperature phase is isostructural with NaTi2(PO4)3 and NaZr2(PO4)3, NZP structure. The lattice thermal expansion of these three compounds has been determined from x-ray diffraction data at different temperatures ranging from room temperature up to 1000°C. Differences in behaviour are discussed in relation to the structure.
Precursor effects of the Rhombohedral-to-Cubic Phase Transition in Indium Selenide
2002
We report on the observation of precursor effects of the rhombohedral-to-cubic phase transition in Indium Selenide (InSe) with several experimental techniques. The pressure at which these precursor defects are first observed depends on the sensitivity of the experimental technique. In transport measurements, which are very sensitive to low defect concentrations, precursor effects are observed 5 to 6 GPa below the phase transition pressure whereas in X-ray diffraction measurements precursor effects are only observed 2 GPa below the phase transition pressure. We report optical absorption measurements, in which the precursor effects are shown by the growth and propagation of dark linear defect…
Interpretation of the Co K-edge EXAFS in LaCoO3 using molecular dynamics simulations
2011
Abstract Temperature dependent (180–400 K) Co K-edge EXAFS spectra from perovskite-type rhombohedral ( R 3 ¯ c ) LaCoO 3 have been successfully interpreted using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering (MS) theory. The method allowed us to account entirely for thermal disorder and to interpret reliably the contribution from the coordination shells beyond the first one into the total EXAFS spectrum taking into account many-body effects. The best agreement between experimental and configuration-averaged EXAFS spectra was obtained for pure ionic La 3+ and partially ionic Co 1.35+ and O 1.45− charges indicating the mixed ionic-covalent character …