142 research outputs found
Magnetic excitation in a new spin gap compound CuScGeO: Comparison to CuFeGeO
The compound \CuScGeO is presented as a new member of the family of weakly
coupled spin chain and dimer compounds \CuMGeO. Magnetic susceptibility, heat
capacity, and neutron inelastic scattering measurements reveal that the
compound has the same spin dimer component as \CuFeGeO. The observed narrow
band excitation in bulk measurements is consistent with spin gap behavior. The
energy scale of the weakly coupled dimers in the Sc compound is perfectly
coincident with that in the Fe compound.Comment: 5 page
Spin gap behavior in CuScGeO by Sc nuclear magnetic resonance
We report the results of a Sc nuclear magnetic resonance (NMR) study
on the quasi-one-dimensional compound CuScGeO at
temperatures between 4 and 300 K. This material has been a subject of current
interest due to indications of spin gap behavior. The temperature-dependent NMR
shift exhibits a character of low-dimensional magnetism with a negative broad
maximum at 170 K. Below , the NMR shifts and
spin lattice relaxation rates clearly indicate activated responses, confirming
the existence of a spin gap in CuScGe% O. The experimental
NMR data can be well fitted to the spin dimer model, yielding a spin gap value
of about 275 K which is close to the 25 meV peak found in the inelastic neutron
scattering measurement. A detailed analysis further points out that the nearly
isolated dimer picture is proper for the understanding of spin gap nature in
CuScGeO.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
Li distribution in the ionic conductor Li1+xAlxTi2-x(PO4)3 with 0 ≤ x ≤ 0.6 studied by neutron diffraction
Li1+xAlxTi2-xP3O12 (LATP) is among the most widely investigated solid electrolyte materials for Li ion batteries because of its relatively high ionic conductivity, chemical stability and mechanical strength. Its ionic conductivity is induced by partial substitution of Ti with Al, in combination with the incorporation of additional Li ions into the structure. The electrical conductivity therefore depends strongly on the substitution parameter x. A series of Li1+xAlxTi2-x(PO4)3 powders with 0 ≤ x ≤ 0.6 were prepared by two different syntheses routes, a) solid state reaction and b) a novel sol-gel method leading to high phase purity, densification activity and conductivity. The highest ionic conductivity of 1.0 × 10-3 S cm-1 at 25 °C was obtained for Li1.5Al0.5Ti1.5(PO4)3 with nearly theoretical density.
We have studied the distribution of Li ions in this series by neutron diffraction analysis at different temperatures. The refinements revealed that excess Li ions are scattered throughout the crystal structure at temperatures above 600 K, making it difficult to precisely determine Li positions and non-stoichiometry. At low temperatures Li ions may occupy a set of different positions, namely the 6a site identified in our previous study or the 18e and 36f sites discussed in the literature [1-3]. A correlation between the synthesis method and the location of Li ions was observed, which is consistent with the ionic conductivity of these materials.
[1] E. Dashjav, F. Tietz, Z. Anorg. Allg. Chem., 2014, 640, 15, 3070
[2] D. Tran Qui, S. Hamdoune, J. L. Soubeyroux, E. Prince, J. Solid State Chem. 1988, 72, 309
[3] M. Catti, A. Comotti, S. di Blas, R. M. Ibberson, J. Mater. Chem., 2004, 14, 83
Development and research methods of compression of complex signals based on optimal and suboptimalnoy processing phase response
The article provides further study of information possibilities phase response of seismic signals to improve resolution records in a thin-layered environment. In the paper, new algorithm of compression of complex signals based on optimal and sub optimal treatment of their phase response of proposed
High-pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes
In situ high-pressure Raman spectra were collected on four pyroxenes, with composition CaCoSi2O6, CaMgSi2O6, CaCoGe2O6 and CaMgGe2O6, up to P = 7.6 and 8.3 GPa for silicates and germanates, respectively. The peak wavenumbers \ucf\u85i increase almost linearly with pressure; the slope d\ucf\u85i/dP is more pronounced for the modes at higher wavenumbers, and higher in germanates than in silicates. No phase transition or change in the compressional behaviour was observed within the investigated P-range. The strong dependence of the peak position with pressure of the high energy stretching modes is due to the high sensitivity of the vibrational frequencies probed by Raman spectroscopy to subtle changes in the tetrahedral deformation, which are overlooked by single-crystal X-ray diffraction
Structural Phase Transition in the 2D Spin Dimer Compound SrCu2(BO3)2
A displacive, 2nd order structural phase transition at Ts=395 K from space
group I`4 2 m below Ts to I 4/m c m above Ts has been discovered in the
two-dimensional spin dimer compound SrCu2(BO3)2. The temperature evolution of
the structure in both phases has been studied by X-ray diffraction and Raman
scattering, supplemented by differential scanning calorimetry and SQUID
magnetometry. The implications of this transition and of the observed phonon
anomalies in Raman scattering for spin-phonon and interlayer coupling in this
quantum spin system will be discussed.Comment: 13pages, 13 figure
Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry
In this paper we examine the role of crystal chemistry factors in creating
conditions for formation of magnetoelectric ordering in BiFeO3. It is generally
accepted that the main reason of the ferroelectric distortion in BiFeO3 is
concerned with a stereochemical activity of the Bi lone pair. However, the lone
pair is stereochemically active in the paraelectric orthorhombic beta-phase as
well. We demonstrate that a crucial role in emerging of phase transitions of
the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order
types belongs to the change of the degree of the lone pair stereochemical
activity - its consecutive increase with the temperature decrease. Using the
structural data, we calculated the sign and strength of magnetic couplings in
BiFeO3 in the range from 945 C down to 25 C and found the couplings, which
undergo the antiferromagnetic-ferromagnetic transition with the temperature
decrease and give rise to the antiferromagnetic ordering and its delay in
regard to temperature, as compared to the ferroelectric ordering. We discuss
the reasons of emerging of the spatially modulated spin structure and its
suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table
Excess Silica in Omphacite and the Formation of Free Silica in Eclogite
Silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite. However, mechanisms other than exsolution might produce free silica, and the possible mechanisms depend in part on the nature and definition of excess silica. ‘Excess’ silica may occur in both stoichiometric and non-stoichiometric pyroxene. Although the issue has been debated, we show that all common definitions of excess silica in non-stoichiometric clinopyroxene are internally consistent, interchangeable, and therefore equivalent. The excess silica content of pyroxene is easily illustrated in a three-component, condensed composition space and may be plotted directly from a structural formula unit or recalculated end-members. In order to evaluate possible mechanisms for the formation of free silica in eclogite, we examined the net-transfer reactions in model eclogites using a Thompson reaction space. We show that there are at least three broad classes of reactions that release free silica in eclogite: (i) vacancy consumption in non-stoichiometric pyroxene; (ii) dissolution of Ti-phases in pyroxene or garnet; (iii) reactions between accessory phases and either pyroxene or garnet. We suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility, but also of titanium solubility, and the possible roles of accessory phases and inclusions on the balance of free silica
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