Albert M. Ziatdinov

Phase transitions and incommensurate states in GIC C5nHNO3

Abstract ESR data of graphite intercalation compounds C 5 n HNO 3 ( n = 2, 4) point at change of quantity and mobility of spin carriers in compounds of different stages and at intercalate crystallization ( T

Synthesis and characterization of nanographites with chemically modified edges

Nanographites, the structural blocks of activated carbon fibers in which the predominant part of edge carbon atoms forms covalent bonds with a chosen halogen (fluorine or chlorine), were synthesized and studied using X-ray photoelectron spectroscopy, X-ray diffraction analysis, and electron paramagnetic resonance. It was found that the formation of these bonds leads to changes in the density of states at the Fermi level and also in the values of some parameters of the spin system of current carriers of the boundary π-electron states of nanographites.

Incommensurate and inhomogeneous phases in improper ferroelastic MgSiF6·6H2O: Mn2 studied by EPR

The results of EPR study of inhomogeneous and incommensurate phases in improper ferroelastic crystals MgSiF6·6H2O:Mn2+ are presented. On the basis of analysis of temperature and angular dependences of experimental parameters and numerical calculations the conclusion has been drawn that at Ti1=370±0.3 K considered crystals undergo transition to structurally incommensurate phase and the order parameter of this transition may be angle of Mg[H2O]62+ octahedra rotation around crystal C3 axis. From Ti1 to Ti2=343±0.3 K the incommensurate phase evolve according to classical complete scenario: origin of incommensurate structure with plane-wave modulation at Ti1, appearance of structural phase soliton lattice with temperature decrease, and, finally, abrupt decrease in soliton density at Ti2 down to values ≈0.1. The assumption was expressed that at Ti2 the crystals investigated undergo phase transition from incommensurate phase to another inhomogeneous phase, in which studied crystals consist of two types of regions characterized by homogeneous and inhomogeneous structural organization. The performed calculation of spectral density functions has shown that the structure of former regions almost does not change with temperature variation whereas that of latter ones considerably modifies when the temperature tends to the improper ferroelastic phase transition temperature TC=296.5±0.3 K.

EPR studies of phase transitions and incommensurate states in 3dn-ions doped MgSiF6 6H2O crystals

Abstract Electron paramagnetic resonance (EPR) of Mn2+, Ni2+, and Cu2+ in magnesium fluosilicate hexahydrate (MgSiF6.6H2O) crystals has been studied at X- and Q-band frequency in the temperature interval 77 ÷ 400 K. At cooling of crystals with natural abundance of 3dn-ions, at T i1 = 370 ∓ 0.5 K, T i2 = 344 ∓ 0.5 K, T c = 296 ∓ 0.3 K structural phase transitions of the second and the first (the latter two) order were observed. The transition at T i1 is of paraphase-incommensurate phase type, and up to T i2 the form of Ni2+ and Mn2+ EPR lines can be well described in the limit of plane-wave modulation of lattice displacements. At T → T i1, the amplitude of the trigonal modulation decreases according to the power law with a critical exponent β = 0.35 ∓ 0.02. Below T i2 up to T c the Mn2+ line shape was analyzed using the structural soliton model. At T → T c, a succession of step-wise changes in the slope of Mn2+ line shape parameter curves which are less significant than those at T i2 is observed. These ste...

EPR studies of phase transitions and incommensurate states in ferroelastic MgGeF6·6H2O doped with 3d-ions

Abstract The succession of phase transitions and incommensurate states has been found and investigated in MgGeF6·6H2O crystals doped with Mn2+(3d5, S=5/2, I=5/2) and Ni2+(3d8, S=1) by means of electron paramagnetic resonance (EPR) at X - and Q - band frequencies in the temperature interval from 77 to 430 K.

Molecular and electronic structures and magnetic properties of multilayer graphene nanoclusters and their changes under the influence of adsorbed molecules

The results of investigations of the structures and properties of multilayer graphene nano-clusters (nanographites), structural blocks of activated carbon fibers, and their changes under the influence of adsorbed molecules are presented. The presence of specific edge p-electron-ic states in the nanographites and a reversible decrease in their density at the Fermi level upon the interaction of the graphite nanoparticles with adsorbed molecules of oxygen, chlorine, and water were found. The explanation of the discovered effect was proposed in the terms of the model of spin splitting of edge p-electronic states initiated by the transfer of a small fraction of the electron density from the nanographites to adsorbed molecules. The change in the sign of the temperature coefficient of current carrier spin relaxation rate in the presence of adsorbates can be accounted for by their interaction with edge spin-split (magnetically ordered) states. The preservation of peripheral p-electronic states of the nanographites of free (dangling) s-orbitals of edge carbon atoms at saturation with chlorine was substantiated.

Multi-Walled Carbon Nanotubes Synthesized by Methane Pyrolysis: Structure and Magnetic Properties

The structure and magnetic properties of multi-walled carbon nanotubes produced by catalytic pyrolysis of methane have been investigated by means of mutually complementary physical methods. The average sizes and number of carbon layers forming nanotubes, smearing of the density of states near the Fermi level, degeneracy temperature of gas of extrinsic current carriers, concentrations of localized spins and extrinsic two-dimensional current carriers have been determined. The conclusion has been drawn that ferromagnetic nanoparticles are present in the inner regions of nanotubes, including their tubular cavities. The difference in electronic structure near the Fermi level for carbon nanotubes and ordered graphite has been revealed. The possible reason is that the electronic states near zigzag-type sites of ends as well as edges of linear structural defects in nanotubes make greater contribution to the spectrum than that from similar sites of graphite.

Two-dimensional spectral-spatial image of highly oriented pyrolytic graphite

Experimentally observed electron paramagnetic resonance (EPR) spectra of highly oriented pyrolytic graphite samples can be considered as a sum of three signals coming from two adjacent faces and crystal edges. Contributions from faces can be analyzed analytically by Dyson’s theory based on an infinite flat conductive plate, while the contribution from edges is not described by this theory. Overlapping of these signals makes it difficult to get useful information about the sample from spectra observed. Implementation of two-dimensional spectral-spatial imaging technique proved to be helpful to solve this problem. It permits the characterization of the EPR spectrum from a selected flat spatial region located far from crystal edges where the model of the infinite flat conductive plate can be applied. By analyzing the EPR signal from spatial slices by Dyson’s equation we have obtained the values of the diffusion coefficient and the surface relaxation rate.

Electrical Conductivity and Conduction ESR in Incommensurate Phase of Graphite Intercalation Compounds C5nHNO3

Electrical conductivity and conduction ESR (CESR) for graphite intercalation compounds (GICs) with nitric acid C 5n HNO 3 (n = 2, 3) have been measured. In the incommensurate phase of second stage compounds (210 K < T < 250 K) absence of temperature dependence of the c-axis conductivity, but preservation of the metallic temperature dependence of the basal plane conductivity, has been found. In the third stage compounds the similar plateau is a less marked one. In both stages of GICs investigated at the incommensurate crystallization of intercalate subsystem, the CESR linewidth undergoes a stepwise increase. It was shown that in GICs studied the c-axis conductivity is realized by means of a non-band mechanism, which might be the mechanism of transport of free charge carriers along the c-axis through the defect-mediated narrow high-conductivity paths (channels).

In situ conduction ESR and theoretical studies of graphite intercalation by nitric acid

Results of an in situ conduction electron-spin resonance (CESR) study of HNO, molecule intercalation into highly oriented pyrolytic graphite (HOPG) plate with width being comparable with the graphite skin-depth governed by thec-axis conductivity are presented. The changes in the graphite CESR signal line shape, intensity and linewidth and the stepwise changes both of intensity and linewidth of CESR signal of intercalated sample were clearly detected during this reaction. Under the assumption that the graphite CESR signal evolution is caused by the advance of a boundary separating the intercalated and nonintercalated HOPG, the average value of spin reorientation probability during the collision of current carriers with this interface and the diffusion coefficient of nitric acid into the HOPG plate were extracted from experimental data. With the chemical potential versus intercalation time proposed by the authors for the experimental conditions, the stepwise changes of the CESR signal intensity of intercalated sample was calculated theoretically.