Tetragonal Centers Research Articles

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.

Spin noise of magnetically anisotropic centers

Spin noise spectroscopy, as a sort of magnetic resonance technique, uses, for detection of spin precession, spontaneous fluctuations of magnetization revealed as a peak in the Faraday-rotation (FR) noise spectrum at Larmor frequency. In the model of precessing magnetization, the FR noise signal should be the greatest in the Voigt geometry (with magnetic field aligned across the light propagation), and should vanish in the Faraday geometry (with the field along the probe beam). This reasoning employs, implicitly or explicitly, the so-called Van Vleck theorem that establishes, within the limits of certain assumptions, a direct relation between the FR and magnetization of the spin system. We show that violation of these assumptions in crystals with anisotropic paramagnetic centers may qualitatively change the conventional laws of spin noise detection, making, in particular, the FR noise detectable in the Faraday geometry. These conclusions are confirmed by experimental studies of spin-noise spectra of ${\mathrm{CaF}}_{2}$ crystals with tetragonal centers of ${\mathrm{Nd}}^{3+}$ ions.

Парамагнитные центры Mn-=SUP=-2+-=/SUP=-, Gd-=SUP=-3+-=/SUP=- и Cu-=SUP=-2+-=/SUP=- в легированном хромом монокристалле Li-=SUB=-2-=/SUB=-CaSiO-=SUB=-4-=/SUB=-

In the Li2CaSiO4 crystal, in addition to intense axial centers of Cr4+, the EPR spectra of impurity ions Mn2+ (S = 5/2), Gd3+ (S = 7/2), and Cu2+ (S = 1/2) have been discovered and studied. Manganese and gadolinium ions exhibit spectra of tetragonal symmetry, copper ions are represented by spectra of both axial and triclinic symmetry. The parameters of the spin Hamiltonians of tetragonal centers are determined. It was shown that Mn2+ and Gd3+ ions substitute for calcium ions with an eightfold oxygen environment, copper ions are localized in lithium positions with a tetrahedral environment. The reasons for the appearance of Cu2+ triclinic centers are discussed.

Study of Yb3+ Optical Centers in Fluoride Solid Solution Crystals CaF2–SrF2–YbF3

A concentration series of Ca(1 − X)SrXF2:4 mol % YbF3 crystals with Х varying from 0 to 1 has been grown. The absorption spectra of Yb3+ ions in these crystals are measured at temperatures of 300 and 77 K. It is shown that the intensity ratio of the absorption lines of clusters and tetragonal centers in the congruent crystals is higher than that in the other CaF2–SrF2 solid solution crystals. Analysis of the absorption profiles of Yb3+ optical centers in the concentration series of the solid solution crystals shows that it is energetically favorable for the Sr2+ ion to enter the local environment of both the tetragonal center and the cluster.

EPR study of fluorite containing different concentration of ceria

The variation of EPR spectra has been investigated in CaF2 single crystals doped with ceria depending on the content of CeO2 and methods of the crystal growth. The concentration of CeO2 changed from 0.5% CeO2 up to 15% CeO2. For the low concentrations of CeO2 the spectra are mainly due to the usual tetragonal or trigonal EPR centers. For the sample with 5 at% CeO2 grown in argon atmosphere by the Bridgman technique the alternative tetragonal center with g|| = 0.73, and g┴ = 2.6 prevails. The possible models of EPR centers are briefly described.

Superhyperfine Structure of the EPR Spectra of Nd3+ Impurity Ions in Fluorite CaF2

EPR spectra of a CaF2 single crystal that was grown from melt containing a small addition of NdF3 were studied. Signals corresponding to tetragonal centers of Nd3+ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd3+ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd3+ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd3+ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.

Суперсверхтонкая структура спектров ЭПР примесных ионов Nd-=SUP=-3+-=/SUP=- во флюорите CaF-=SUB=-2-=/SUB=-

AbstractEPR spectra of a CaF_2 single crystal that was grown from melt containing a small addition of NdF_3 were studied. Signals corresponding to tetragonal centers of Nd^3+ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd^3+ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd^3+ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd^3+ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.

Formation of new Tm3+ tetragonal symmetry optical centers in CaF2 hot-formed laser ceramics

By means of time-resolved site-selective spectroscopy the formation of new Tm3+ optical centers with modified local environment and presumably tetragonal local symmetry in CaF2 hot-formed laser quality ceramics is observed. The spectroscopic properties of these new Tm3+ optical centers are investigated and shown to differ strongly from that for regular tetragonal optical centers.

Spectroscopic properties of long-lifetime Tm3+ optical centers in Ca–Sr–Ba fluorides in the form of single crystals and ceramics at the 1G4–3H5 magnetic dipole allowed transition

The spectroscopic properties of new long-lifetime Tm3+ tetragonal optical centers at low (77 K) temperature were investigated using a site-selective time-resolved technique. The absorption and excitation spectra at the 3H6–1G4 transition and the fluorescence spectra at the 1G4–3H5 transition were measured in CaF2, SrF2, and BaF2 single crystals. The appearance of additional weak lines in the excitation and fluorescence spectra in hot-formed ceramics produced from the same crystals was observed. These lines were attributed to the recently observed long-lifetime tetragonal optical centers with a modified local environment formed in fluoride ceramics.

A semi-empirical analysis of ZFS parameters for transition metal (TM) ions (Cr3+ and Mn2+) centers in K2MgF4

The local structure around the transition metal (TM) ions (Cr3+ and Mn2+) centers in K2MgF4, namely tetragonal (TE) center I, monoclinic (MO) center II and orthorhombic (OR) centers III, have been explored by semi-empirical calculations in the frame of superposition model (SPM). We proposed the most appropriate structural model for each center by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. It is shown that the tetragonality of MgF6 octahedra increases with the substitution of both Cr3+ and Mn2+ for Mg2+ sites as well as with a rather higher value for the latter one. A tilt angle of 12.51° is found for MO Cr3+ center, which is comparatively small, as compared to the other A2BF4 crystals such as K2ZnF4 and Tl2ZnF4. Almost three times larger relaxation of intervening F-ligand than that of other equatorial F-ligands is also indicated for OR Cr3+ center.

Selective laser spectroscopy of SrF2 crystal doped with Pr3+ ions

crystals doped with ions with concentrations corresponding to 0.5, 1.0 and 2.0 wt % of are studied by selective laser excitation. The absorption and luminescence spectra of ions are measured at a temperature of . Three types of tetragonal centres (C, E, S) with the point symmetry group and three types of cluster centres are found. Energy level diagrams of the and terms of ions in crystals are plotted for the tetragonal (C) and cluster optical centres. The lifetimes of the tetragonal C centre and cluster centres and are measured at for the first time.

Specific spectroscopic and laser properties of Tm3+ ions in hot-formed CaF2 laser ceramics

The formation of new Tm3+ tetragonal optical centers with spectroscopic properties different from those for regular ones in hot-formed CaF2 ceramics is observed. The influence of the new centers’ formation on the fluorescence and laser properties of the 2 μm 3F4–3H6 transition under ~795 nm laser diode excitation is studied.

Structural analysis of Fe3+ centers in Tl2MgF4 and Tl2ZnF4 fluorine compounds

Theoretical analysis of the Fe3+ centers observed in Tl2MgF4 fluorine crystals have been carried out by means of semi-empirical approaches. The most appropriate models are proposed by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. Compression on the MF6 octahedron of tetragonal (TE) center I is indicated in both Tl2MgF4 and Tl2ZnF4. A structural model for monoclinic (MO) center II and orthorhombic (OR) Fe3+ center IV in Tl2ZnF4 is proposed by assuming that the substitution of Fe3+ induces both ligand length and angular distortions.

Investigation of the Fe3+ centers in perovskite KMgF3 through a combination of ab initio (density functional theory) and semi-empirical (superposition model) calculations

Analyses of the local crystal and electronic structure in the vicinity of Fe3+ centers in perovskite KMgF3 crystal have been carried out in a comprehensive manner. A combination of density functional theory (DFT) and a semi-empirical superposition model (SPM) is used for a complete analysis of all Fe3+ centers in this study for the first time. Some quantitative information has been derived from the DFT calculations on both the electronic structure and the local geometry around Fe3+ centers. All of the trigonal (K-vacancy case, K-Li substitution case, and normal trigonal Fe3+ center case), FeF5O cluster, and tetragonal (Mg-vacancy and Mg-Li substitution cases) centers have been taken into account based on the previously suggested experimental and theoretical inferences. The collaboration between the experimental data and the results of both DFT and SPM calculations provides us to understand most probable structural model for Fe3+ centers in KMgF3.

Influence of Tm3+ optical centers on spectroscopic properties at the 3F4–3H6 2-μm laser transition

Individual low temperature (77K) fluorescence spectra of tetragonal (C4v), clustered, and long-lifetime optical centers of Tm3+ ions at the 3F4–3H6 transition are recorded and the lifetimes of these centers are measured. Analysis of the number and positions of the fluorescence lines of the long-lifetime optical center allows to suggest that it has tetragonal (C4v) local symmetry.

Investigation of the Cr3+ centers in Rb2ZnF4, Rb2CdF4, and Rb2MgF4 fluorine compounds: A semi-empirical analysis

Abstract The trivalent chromium centers, namely tetragonal (TE) center I and orthorhombic (OR) centers III and IV, have been explored by means of semi-empirical analyses in Rb2MF4 crystals. The local structures for all Cr3+ centers are comprehensively investigated through the zero-field splitting (ZFS) parameters (ZFSPs), which are calculated theoretically and modeled following several modeling approaches. Based on the correlation between the experimental and theoretical ZFSPs, the local structure distortions around the Cr3+ centers are obtained. This study shows that the semi empirical calculations provide a powerful way to determine the local structure in the vicinity of all Cr3+ centers in Rb2MF4 crystals.

A comprehensive local structure analysis of Cr3+ centers in K2ZnF4 crystals: A semi-empirical approach

The local structure around Cr3+ impurities forming four Cr3+ centers of different symmetries has been investigated by means of semi-empirical calculations in K2ZnF4 crystal. Structural models for the all centers have been presented by analyzing their EPR data at various temperatures (297, 77, and 4.2K). A similar trend is found that Cr3+–F− distances of the axial ligands decrease whereas those of the equatorial ligands increase when Zn2+ is replaced by Cr3+. It is indicated that the distortions around the orthorhombic centers are quite slight with respect to those of the tetragonal and monoclinic centers. We have also found that the displacements of the axial and equatorial ligands for the monoclinic center II and the orthorhombic center III change uniformly when the temperature decreases, while both axial and equatorial ligands for the tetragonal center I displace less from 293 to 77K whereas more from 77 to 4.2K.

First-principles study of electronic structure of Ce3+ centres in alkaline-earth fluorides including spin-orbit and scalar relativistic effects

In this paper we present the results of ab initio calculations of Ce3+ impurity centres in CaF2 crystal. The cubic and tetragonal (interstitial fluorine compensated) configurations of centres were considered. The embedded cluster approach with polarizible crystalline lattice and CASSCF/CASPT2 methods were used to study the structural configurations and electronic properties of defects under consideration. Scalar relativistic corrections were taken into account at Douglas-Kroll-Hess level and spin-orbit effects were calculated with the RASSI approach. Calculated optical absorption spectra for cubic and tetragonal centres demonstrate good agreement with the experiment. Small symmetry breaking displacements of neighbouring ions were observed for cubic Ce3+ centre ground state.

Analysis of the local structure around Cr3+ centers in perovskite KMgF3 using both ab initio (DFT) and semi-empirical (SPM) calculations

The local structure around Cr3+ centers in perovskite KMgF3 crystal have been investigated through the applications of both an ab-initio, density functional theory (DFT), and a semi empirical, superposition model (SPM), analyses. A supercell approach is used for DFT calculations. All the tetragonal (Cr3+–VMg and Cr3+–Li+), trigonal (Cr3+–VK), and CrF5O cluster centers have been considered with various structural models based on the previously suggested experimental inferences. The significant structural changes around the Cr3+ centers induced by Mg2+ or K+ vacancies and the Li substitution at those vacancy sites have been determined and discussed by means of charge distribution. This study provides insight on both the roles of Mg2+ and K+ vacancies and Li+ ion in the local structural properties around Cr3+ centers in KMgF3.

Effect of Manganese on the Fe-Rich Phases in Al-22Si-2Fe Alloy by Cooling Slope

Hypereutectic Al-Si alloys have low thermal expansion coefficients, excellent wear resistance, and high hardness. To replace cast iron with hypereutectic Al-Si alloys, the hypereutectic Al-Si alloys must still have the originally desired mechanical properties. The addition of Fe and Mn to the hypereutectic Al-Si alloys can form Fe-rich phases, which can improve thermal stability. In this paper, the effect of manganese (0 %, 0.99 %, and 1.36 %) on the Fe-rich phases of hypereutectic Al-22Si-2Fe (% w/w) alloys was studied. The results showed that primary Si particles (PSPs), needle-like Fe phases, a coarser fishbone-shaped α-Al15(Fe, Mn)3Si2 phases, and eutectic Si could be refined by cooling slope. With such a CS process, the intermetallic compounds in the alloys with different Mn/Fe ratios were examined with an optical microscope, scanning electron microscope, and X-ray diffraction. Moreover, the blocky α-Al15(Fe, Mn)3Si2 phases were analyzed by transmission electron microscopy. Through the analysis of the SADP, the lattice structures of α-Al15(Fe, Mn)3Si2 phases were identified with tetragonal structure and body centre cubic.