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1.
The GaN growth mechanism using Li3N flux was reinvestigated by designing a new experiment that allowed us to grow GaN simultaneously under varying Ga/Li3N molar ratios. The results confirm the two-step reactions involved in the Ga–Li3N system: Li3N+Ga→Li3GaN2+Li (1) and Li3GaN2+Ga→GaN+Li (2). It is found that reaction (2) is the main cause leading to small GaN crystals in the products. Larger GaN crystals, however, can grow by a different pathway. The growing process is concerned with the formation of Li–Ga–N melt by dissolving Li3GaN2 in Li–Ga melt after reaction (1). GaN crystals up to 3 mm grow from the Li–Ga–N melt upon cooling on the GaN particles by reaction (2). Our results suggest that it is necessary to restrain reaction (2) by choosing proper Ga/Li3N molar ratios so as to obtain GaN crystals in larger size. PACS 81.05.Ea; 81.10.Dn  相似文献   

2.
The results of measurements of the ionic conductivity σ in Li3M2(PO4)3 (M=Fe, Sc) single crystals along various crystallographic directions are analyzed. Possible causes of the different behavior of σ in the isostructural crystals are discussed: a jump of the conductivity in the transition to the superionic phase in Li3Sc2(PO4)3 and its absence in Li3Fe2(PO4)3; the existence of a conductivity maximum in different crystallographic directions (along the c axis in Li3Sc2(PO4)3 and along the a axis in Li3Fe2(PO4)3). Fiz. Tverd. Tela (St. Petersburg) 39, 83–86 (January 1997)  相似文献   

3.
Abstract

The effect of chemical composition of the crystallization medium in synthesis on and some properties of cubic boron nitride (cBN) has been studied. Cubic BN crystals with the lowest free boron content and a boron-nitrogen ratio close to the stoichiometric one are grown in a multicomponent crystallization medium in a Li3N-BN system. These crystals are characterized by a higher strength and thermal stability.  相似文献   

4.
Abstract

The composition range of lithium tantalate was found between 46.5 and 50.0 mole% Li. The congruent melt composition, from which homogeneous single crystals can be grown, was determined at about 48.5 mole% Li2O. Due to Li2O evaporation during the reaction of Li2CO3 and Ta2O5 the starting composition depends on the reaction conditions. The lattice constants change with the composition comparable to LiNbO3.  相似文献   

5.
Abstract

(Li1?Kx)2SO4 mixed crystals were prepared by the precipitation technique where x = 0.5, 0.7, 0.9 and 0.99. The phase transformations of the mixed crystals have been analyzed by the DSC technique. The DSC curves of (Li1?xKx)2SO4 mixed crystals reveal that as the potassium content increases the first high temperature phase of the intermediate LiKSO4 phase at T = 436°C disappears and a two- phase mixture of LiKSO4 and K2SO4 is found for x = 0.7 and 0.9. It is observed from the electrical conductivity measurements of (Li1?xKx)2SO4 mixed crystals that the electrical conductivity increases as the K2SO4 concentration increases with average activation energy of 1.04 eV. The enhancement in the electrical conductivity is primarily a result of the presence of two ionically conducting constituents and the formation of a diffuse space charge layer at the interface region between these two phases.  相似文献   

6.
The x-ray diffraction spectra of Li2B4O7 single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)  相似文献   

7.
Copper pair centers, which could be of interest for obtaining quantitative information about exchange interactions in superconductors based on cuprate perovskites, are observed in crystals with the perovskite structure by the ESR method. Such centers are investigated in KTaO3:Cu and K1−x LixTaO3:Cu crystals. A model consisting of a chain of two equivalent Cu2+ ions and three oxygen vacancies, extending along the 〈 100〉 axis, is proposed for the centers. The exchange interaction in the pairs is ferromagnetic. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 890–894 (25 June 1999)  相似文献   

8.
This paper reports on a study of the kinetics of electron tunneling transport between electron and hole centers in Li2B4O7 and LiB3O5 lithium borate crystals under the conditions where the mobility of one of the partners in the recombination process is thermally stimulated. A mathematical model has been proposed to describe all specific features in the relaxation kinetics of the induced optical density observed in Li2B4O7 (LTB) and LiB3O5 (LBO) nonlinear optical crystals within a broad time interval of 10−8−1 s after a radiation pulse. The results of calculations have been compared with experimental data on transient optical absorption (TOA) of LTB and LBO crystals in the visible and ultraviolet spectral regions. The nature of the radiation defects responsible for TOA and the dependence of the TOA decay kinetics on temperature, excitation power, and other experimental conditions have been discussed.  相似文献   

9.
The dielectric permittivity of Ni-doped Li2Ge7O15 crystals was studied in the vicinity of the ferroelectric phase transition. Introduction of Ni has been shown to suppress the dielectric anomaly and to reduce substantially the transition temperature. A temperature hysteresis in ɛ (T) has been observed in nominally pure and Ni-doped Li2Ge7O15 crystals near the transition point. Measurements performed under cooling from the paraphase reveal dispersion of dielectric permittivity at Debye relaxation frequencies of the order of 104–105 Hz at T c . It is proposed that the hysteresis phenomena and the low-frequency dispersion are caused by residual defects (of the type of random local fields), which become polarized in the ferroelectric phase and become disordered above T c . Fiz. Tverd. Tela (St. Petersburg) 40, 2198–2201 (December 1998)  相似文献   

10.
Abstract

The experimental values of the off-axis angle of FA(II) dipoles in KCl:Li+, RbCl:Li+ and KF:Li+ are in good agreement with those obtained from theoretical estimates of the off-centre displacements of the Li+ ion calculated with the shell model potentials. On this basis, one can quantitatively predict the off-axis behaviour of FA(II) centres from the calculated off-centre positions of small impurity ions in alkali halides.  相似文献   

11.
Proton-exchanged optical waveguides in lithium niobate crystals exhibit a rich variety of structures and phases. It is established that seven HxLi1−x NbO3 crystalline phases with a lithium niobate structure may form under various conditions of proton exchange and post-exchange annealing. A method is proposed to determine the proton concentration in the various phases identified. Relationships are established between the structural parameters, the proton concentration, and the ordinary and extraordinary refractive indices of various HxLi1−x NbO3 crystalline phases. The results can explain various optical phenomena observed in proton-exchanged waveguides and permit prediction of the characteristics of light-guide structures. Zh. Tekh. Fiz. 69, 47–57 (March 1999)  相似文献   

12.
The experimental data on the transient optical absorption of wide-band-gap optical crystals of lithium borates Li2B4O7, LiB3O5, and Li6Gd(BO3)3 and potassium (KH2PO4 (KDP)) and ammonium (NH4H2PO4 (ADP)) dihydrogen phosphates in the visible and ultraviolet spectral regions are analyzed using the theory of diffusion-controlled tunneling recombination. A nanosecond pulsed radiation action on these crystals is shown to form defect pairs, such as polaron-type hole centers and electron centers based on interstitial cations. The relaxation kinetics of these centers over a wide time range of 10?8?10 s is described by a proposed model of tunneling electron transfer between antimorphous defects in the cation sublattice under the thermally stimulated mobility of recombination partners. The numerical values of the kinetic parameters are determined and the time dependences of the reaction rate constants are calculated for all crystals under study. As a result, the dynamics of change in the optical properties of these crystals under a pulsed radiation action can be simulated.  相似文献   

13.
This paper reports on the results of the experimental and theoretical investigations of thermally stimulated recombination processes in crystals of the lithium borates Li2B4O7 and LiB3O5. For both types of crystals, the measurements of thermally stimulated luminescence curves, spectra, and temperature dependences of the intensities of steady-state X-ray luminescence have been performed in a single experimental cycle. In the framework of a unified model for the Li2B4O7 and LiB3O5 crystals, the thermally stimulated recombination processes have been calculated and the obtained results have been interpreted talking into account all available experimental data.  相似文献   

14.
Diffusion of lithium cations in C60 single crystals driven by an electric field has been studied. It was found that the stoichiometry of C60 single crystals can be changed with respect to lithium by injection of Li+ ions through the heterojunction Li7SiPO8|C60 and electrons through the heterojunction C60| Graphite. The double charge injection changes the stoichiometry of lithium in a C60 single crystal and increases both the lithium ionic and electronic conductivity. The electronic conductivity in LixC60 crystals is non-metallic in nature. The temperature dependence of the electronic conductivity lithium doped C60 single crystals was investigated. It was found that electrons occupied the lowest singlet exited states (LUMO) and that this leads to the appearance of an intensive EPR signal. The temperature dependence of the concentration of paramagnetic centers was investigated. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.  相似文献   

15.
Abstract

Electron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li3ThF7 single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D2h 22 phase to a monoclinic C2h (x) one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.  相似文献   

16.
The ionic conductivity of Li3N crystals doped with various metal ions (magnesium, copper and aluminum) or hydrogen has been investigated. The metal ions have a negative effect on the conductivity whereas hydrogen increases it. The intrinsic Li+ ionic conductivity of pure Li3N is (2·-4)×10-4Ω-1cm-1 at room temperature with an activation energy of 0.26?0.27 eV. Doping with hydrogen to a maximum level of 0.5?1.0 atom% results in a conductivity of 6×10-3Ω-1cm-1 and an activation energy which has been lowered to 0.20 eV. A model is proposed for the action of hydrogen whereby the Li-N bonds next to an NH2- group are weakened thereby facilatating the creation of Li+ Frenkel defects and the vacancy migration. Hydrogen-doped Li3N is termed an enhanced intrinsic conductor.  相似文献   

17.
Using vibrating sample magnetometery (VSM) 50 MeV Li3+ ion irradiation effects on magnetic properties of single crystals of SrGaxInyFe12−(x+y)O19 (where x=0, 5, 7, 9; y=0, 0.8, 1.3, 1.0), are reported. The substitution of Ga and In in strontium hexaferrite crystals decreases the value of magnetization sharply, which is attributed to shifting of collinear magnetic order to a non-collinear one. Reduction of magnetization is also explained to be as a result of the occupation of the crystallographic sites of Fe3+ by Ga3+ and In3+. The Li3+ ion irradiation decreases the value of magnetization, irrespective of whether the crystals are Ga–In substituted or unsubstituted crystals of SrFe12O19. The result is interpreted in terms of the occurrence of a paramagnetic doublet in crystals replacing magnetic sextuplet as a result of irradiation. Substitution of Ga–In in Strontium hexaferrite decreases the value of anisotropy constant. Irradiation with Li3+ ions increases the values of anisotropy field for both substituted as well as unsubstituted crystals. Substitution with Ga–In also decreases the Curie temperature (Tc) but the irradiation with Li3+ ions does not affect the curie temperature of either Ga–In substituted or pure SrFe12O19 crystals.  相似文献   

18.
Diffusion of lithium cations in C60 single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C60 single crystal” heterojunction has been suggested. It has been found that lithium doping of C60 single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in LixC60 crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T 1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm−1 to 1413 cm−1), which indicates that one electron is present at the C60 molecule, and this fact may be treated as evidence that the LiC60 phase is generated in a C60 crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)  相似文献   

19.
Lithium tetraborate (Li2B4O7) is a tissue equivalent material and single crystals of this material doped with Cu are promising for dosimetric applications. In the present study highly transparent single crystals of lithium tetraborate (Li2B4O7) doped with Cu (0.5 wt%) have been grown using the Czochralski technique. The Li2B4O7:Cu crystals were studied using photoluminescence, X-ray diffraction (XRD), UV-vis transmission, time resolved fluorescence and thermoluminescence (TL) techniques. The TL readout of Li2B4O7:Cu crystals showed two well-defined glow peaks at 402 K (peak-1) and 513 K (peak-2) for a 4 K/s heating rate. While the low temperature TL peak-1 fades completely within 24 h at room temperatures, the main dosimetric peak-2 remains the same. The TL sensitivity of the grown single crystal is found to be 3.3 times that of a conventional TL phosphor, TLD-100. The Li2B4O7:Cu crystals showed a linear TL dose-response in the range from 1 mGy to 1 kGy. The TL analysis using a variable dose method revealed first order kinetics for both the peaks. Trap depth and frequency factor for peak-1 were found to be 0.81 eV and 5.2×109 s−1, whereas for peak-2 the values were 1.7 eV and 1.7×1016 s−1, respectively.  相似文献   

20.
A Cr3+ EPR study of lithium heptagermanate crystals, Li2Ge7O15 (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T C as a result of emergence of two structurally nonequivalent positions for Cr3+ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998)  相似文献   

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