National Institute Of Materials Physics - Romania

An Fe+-type center with axial (001) symmetry has been studied by electron nuclear double resonance (ENDOR) in the microwave X and Q bands. The Fe+(I) center is produced only after irradiation with x or gamma rays at 80 K of SrCl2:Fe2+ crystals grown in a Cl-2 atmosphere. It has a 3d(7) F-4 Gamma (8) ground state with spin S =3/2. As shown by the correlated analysis of the ENDOR data and electron paramagnetic resonance superhyperfine structure, the Fe+ ion is situated in the center of a tetragonally compressed cube of eight nearest Cl- ions. The simplest model of the Fe+(I) center, compatible with the magnetic resonance results, consists of an interstitial Fe+ ion with two substitutional monovalent cations, very likely K+ ions, in two opposite nearest-neighbor Sr2+ sites along the tetragonal axis of the center.

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.

The results of an in situ transmission electron microscopy study of the formation of Co-silicides on patterned (001) Si substrates are discussed. It is shown that the results of the in situ heating experiments agreed very well with the data based on standard rapid thermal annealing experiments. Fast heating rates resulted in better definition of the silicide lines. Also, better lines were obtained for samples that received already a low-temperature ex situ anneal. A Ti cap layer gave rise to a higher degree of epitaxy in the CoSi2 silicide.

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values B-4(0) = 0.0009(3) cm(-1), B2 = 0.0006(2) cm(-1) and 4 4 B-4(4) = 0.0017(3) cm(-1), to be considered together with the second-order parameters D = -1.1677(7) cm(-1) and E = -0.0135(6) cm(-1).

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI <5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-EN ponders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp(3), as in the case of c-BN, (C) 2001 Elsevier Science B.V. All rights reserved.

The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S = 1/2, local symmetry axis along one of the crystal [111] axes and principal g values g(parallel to) = 2.0032 +/- 0.0009 and g(perpendicular to) = 2.0094 +/- 0.0005 at T = 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.

Several Tl-0 (6s(2)6p(1))-type paramagnetic centers, produced by low temperature X-ray irradiation, were observed and studied by electron spin resonance (ESR) in the orthorhombic ferroelectric phase of thallium doped Rb2ZnCl4 crystals. The centers were formed by electron trapping at Tl+ ions localized substitutionally at Rb+ sites. The number and properties of the observed centers account for the tripling of the unit cell in the ferroelectric phase.

The alkoxide route and the spinning deposition were used to prepare sol gel ITO films. The morphology and crystalline structure were investigated by Cross-section Transmission Electron Microscopy and Atomic Force Microscopy. The inside of the ITO layer has three regions of different porosity and the basic crystalline structure is that of the In2O3 lattice. Three types of surface morphological structures were found from AFM images.

Dark-brown crystalline powders and selected single crystals of cubic boron nitride grown by the HP-HT method, with boron excess, have been studied in a broad temperature range by electron spin resonance (ESR) in the X and W microwave frequency bands, respectively. The X-band spectra consist of two superimposed lorentzian components attributed to two types of related paramagnetic defects called D1 and D2, respectively. According to the W-band data, the D I centre exhibits local axial < 111 > symmetry and ground spin state S = 1/2, with g(parallel to) = 2,0033 and g(perpendicular to) = 2.0094 at T = 10 K. The broad D2 line, observed only in the X-band, at g = 2.0084, at higher temperatures, seems to result from transitions inside the excited levels of another boron related defect.

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or gamma-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial [001] symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl- ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl- ions, compressed along an[001] direction.