Theory of an Fm3m→I4/m structural phase transition in an Rb2KScF6 crystal

An Rb₂KScF₆ crystal having an elpasolite structure undergoes a sequence of Fm3m→I4/m→P12₁/n1 structural phase transitions where the transition to the tetragonal phase is associated with “rotation” of the ScF₆ octahedron. An effective Hamiltonian is constructed to describe the Fm3m→I4/m transition using the approximation of a local mode for which we selected a “soft mode” whose eigenvector corresponds to the rotation of the octahedron. The effective Hamiltonian also includes the relationship between the local mode and the homogeneous elastic strains. The parameters of the effective Hamiltonian were determined using the generalized Gordon-Kim model of an ionic crystal which allows for the deformability and polarizability of the ions. The thermodynamic properties of a system with this model Hamiltonian were investigated using the Monte Carlo method. The calculated phase transition temperature of 250 K is almost the same as the experimental value (252 K). The tetragonal phase remains stable as far as T=0 K and a second transition (to the monoclinic phase) cannot be obtained using this effective Hamiltonian. This suggests that if the transition to the tetragonal phase is mainly associated with “rotations” of the octahedrons, in order to describe the phase transition to the monoclinic phase the effective Hamiltonian must allow for additional degrees of freedom mainly associated with the motion of rubidium ions.     

Efficient generation of blue light by frequency doubling of a Nd:YAG laser operating on 4 F 3/24 I 9/2 transitions

Efficient room-temperature operation of ⁴ F _3/2⁴ I _9/2 transitions in diode-end-pumped Nd:YAG lasers at 946 nm and 938.5 nm is reported. 7.0-W continuous-wave output power at 946 nm and 3.9 W at 938.5 nm have been obtained. An analytical model has been developed for the quasi-three-level laser including the influence of energy-transfer upconversion. Frequency doubling of these transitions in periodically poled KTP generated blue light at 473 nm and 469 nm. Both single-pass extra-cavity as well as intracavity schemes have been investigated. Received: 31 July 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +46-8/750-5430, E-mail: stefan.bjurshagen@acreo.se     

Oscillation conditions of cw simultaneous dual-wavelength Nd：YAG laser for transitions 4F3／2-4I9／2 and 4F3／2-4I11／2

The operating condition of a cw simultaneous dual-wavelength Nd:YAG laser for both {}^4F_{3/2}-{}^4I_{9/2} (quasi-three-level system) and {}^4F_{3/2}-{}^4I_{11/2}(four-level system) transitions is studied. According to this condition, the cw simultaneous dual-wavelength at 1.06μm and 946 nm in Nd:YAG crystal has been realized experimentally through an adaptive coating design of the cavity mirrors. A total power output of 2.5W of the dual-wavelength at 946nm and 1.06μm has been achieved with an optical conversion efficiency 14.3%. The experimental results are in good agreement with the theoretical analysis.     

Pressure broadening of the ground vibrational state J = 31→J = 32 transition of CH3F by coincidence laser spectroscopy

We have studied self-broadening and foreign gas broadening of the ground state J = 31→J = 32 transition of CH₃F using a coincidence with the 184.3 μm emission from an optically pumped CH₂F₂ laser. Experiments have been carried out with polar and non-polar species as well as the noble gases.This transition is in the region where the classical rotation period is of the same order as the duration of a collision. The measurements indicate an anomalously low value of 7.0 MHz/Torr for the self-broadening coefficient of CH₃F and an unusually large value for the CH₃OH broadening coefficient. The latter is believed to be due to the presence of a low energy OH torsional model in CH₃OH.     

Li+ transportation kinetics of FeF3 · 0.33H2O/C nanocomposite synthesized by one-step solid state method

A new cathode material for lithium ion battery FeF₃?·?0.33H₂O/C was synthesized successfully by a simple one-step chemico-mechanical method. It showed a noticeable initial discharge capacity of 233.9 mAh g^?1 and corresponding charge capacity of 186.4 mAh g^?1. A reversible capacity of ca.157.4 mAh g^?1 at 20 mA g^?1 can be obtained after 50 charge/discharge cycles. To elucidate the lithium ion transportation in the cathode material, the methods of electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were applied to obtain the lithium diffusion coefficients of the material. Within the voltage level of 2.05–3.18 V, the method of EIS showed that \( {D}_{{\mathrm{Li}}^{+}} \) varied in the range of 1.2?×?10^?13?~?3.6?×?10^?14 cm² s^?1 with a maximum of 1.2?×?10^?13 cm² s^?1 at 2.5 V. The method of GITT gave a result of 8.1?×?10^?14?~?1.2?×?10^?15 cm² s^?1. The way and the range of the variation for lithium ion diffusion coefficients measured by the GITT method show close similarity with those obtained by the EIS method. Besides, they both reached their maximum at a voltage level of 2.5 V.     

Spectral characteristics of solid solutions LiY1−x Lu x F4 doped by Ce3+ ions

During crystallization, the LiF-YF₃-LuF₃ compounds form a continuous series of solid solutions having a scheelite structure at any proportion of the YF₃ and LuF₃ components. It is established that the crystals conform to the Vegard law and the Rutgers rule. The spectral characteristics of the 5d–4f transitions of Ce³⁺ ions and color centers induced by ultraviolet radiation in these crystals were studied as a function of the Lu³⁺ concentration. It is shown that the active media based on LiF-YF₃-LuF₃: Ce³⁺ solid solutions are more efficient as compared to LiYF₄: Ce³⁺ and LiLuF₄: Ce³⁺ crystals in possible directional changes in the spectral-kinetic characteristics of impurity ions and the parameters of the losses induced by pumping radiation and in enhancement of energetic and spectral characteristics of lasers on their basis.     

Highly efficient continuous-wave intracavity frequency-doubled Nd:YVO4-LBO laser at 457 nm under diode pumping into the emitting level 4F3/2

The continuous-wave high efficiency laser emission of Nd:YVO₄ at the fundamental wavelength of 914 nm and its 457 nm second harmonic obtained by intracavity frequency doubling with an LBO nonlinear crystal is investigated under pumping by diode laser at 880 nm into emitting level ⁴F_3/2. 6.5 W at 457 nm with M ²=1.8 was obtained from a 5-mm-thick 0.4 at.% Nd:YVO₄ laser medium and a 15-mm-long LBO nonlinear crystal in a Z-type cavity for 18.6 W absorbed pump power. An optical-to-optical efficiency with respect to the absorbed pump power was 0.35. Comparative results obtained for the pump with diode laser at 808 nm, into the highly-absorbing level ⁴F_5/2, are given in order to prove the advantages of the 880 nm wavelength pumping.     

Subrecoil cooling of 6Li atoms by 2S→3P ultraviolet narrow transition

Laser cooling of atoms and molecules is crucial to exhibiting excellent features in the field of low-temperature physics.Cooling ⁶Li atoms to very low temperatures is difficult due to their partially resolved D₂ line of excited states.Here we report an observation of cooling ⁶Li atom samples to 16 μK with an ultraviolet(UV) laser in a magneto-optical trap,which breaks the Doppler cooling limit and approaches half of the photon-recoil limit.The essential mechanism...     

ZnS:ErF3电致发光薄膜2H11/2,4S3/2→4I15/2发光的猝灭过程

本文通过ZnS:ErF3交流电致发光薄膜发光光谱和发光衰减特性研究了Er3+离子2H11/2,3S3/2发光的猝灭过程,实验结果表明由于Er3+离子间多极矩相互作用,一个激发的Er3+离子从2H11/2态无辐射弛豫到4F9/2态,同时另一个激发的Er3+离子从4F7/2或4F5/2+4F3/2态跃迁到2H9/2或4G11/2态的交叉弛豫过程是2H11/2,4S3/2发光浓度猝灭的一个主要机制,交叉弛豫几率随着掺杂浓度的增加而增大,根据在固定掺杂浓度下2H11/2,4S3/2、4F9/2发光衰减时间不随激发密度而改变的实验结果,排除了处于激发的Er3+离子之间直接能量传递的可能性,这结果与扩散限制的能量传递理论相符。     

Lattice dynamics and statistical mechanics of the Fm3m → I4/m structural phase transition in Rb2KInF6

The static and dynamic properties of cubic Rb₂KInF₆ crystals with elpasolite structure are calculated using a nonempirical method. Calculations are performed within a microscopic ionic-crystal model taking into account the deformation and polarization of ions. The deformation parameters of ions are determined by minimizing the total energy of the crystal. The calculated equilibrium lattice parameters agree satisfactorily with the experimental data. It is found that in the cubic phase there are vibrational modes that are unstable everywhere in the Brillouin zone. The eigenvectors of the unstablest mode at the center of the Brillouin zone of the cubic phase are associated with the displacements of F ions and correspond to rotations of InF₆ octahedra. Condensation of this mode leads to a tetragonal distortion of the structure. In order to describe the Fm3m → I4/m phase transition, an effective Hamiltonian is constructed under the assumption that the soft mode whose eigenvector corresponds to octahedron rotation is local and coupled with homogeneous elastic strains. The parameters of the effective Hamiltonian are determined using the calculated crystal energy for the distorted structures due to soft-mode condensation. The thermodynamic properties of the system with this model Hamiltonian are investigated using the Monte Carlo method. The phase transition temperature is calculated to be 550 K, which is twice its experimental value (283 K). The tetragonal phase remains stable down to T=0 K; the effective Hamiltonian used in this paper thus fails to describe the second phase transition (to the monoclinic phase). Thus, the transition to the tetragonal phase occurs for the most part through octahedron rotations; however, additional degrees of freedom, first of all, the displacements of Rb ions, should be included into the effective Hamiltonian in order to describe the transition to the monoclinic phase.     

The effect of Er3+-ion concentration on the Er3+ : 4I13/2 → 4I15/2 transition in tellurite glasses

The effect of Er³⁺ concentration on the Er³⁺?:?⁴I_13/2?→?⁴I_15/2 emission in tellurite glasses has been investigated. The full width at half-maximum increased with the increasing concentration of Er₂O₃ in tellurite glasses. The effect of local structure of Er³⁺ ions and related spectroscopic changes are taken into account to explain the line broadening. Inhomogeneous broadening due to the distribution of crystal field around the Er³⁺ ion has little effect on the absorption spectra. Highly efficient energy trapping between the ions was identified from the time-resolved analysis of the fluorescence decay and is found to be responsible for the extended lifetime at intermediate concentrations of Er³⁺ ions in tellurite glasses. The effect of temperature on spectral line shape has been determined for analysing the contribution of Boltzmann population on line broadening. The increased population of the overlying Stark sublevels at the ⁴I_13/2 energy level via direct pumping and/or interaction between Er³⁺ ions were also found to be significant for enhancement in spectral line shape at higher concentrations of Er³⁺ ions in tellurite glasses. It was observed that at higher concentrations of Er³⁺ ions in glass also enhance the local symmetry of ions, which is apparent from the absorption band of the Er³⁺?:?⁴I_15/2?→?²H_11/2 hypersensitive transition.     

Selective modulation of infrared radiation by an Al2O3 particles — liquid crystal system

We have developed a dual-frequency liquid crystal consisting of 4-n-pentyl-4′-cyanobiphenyl (5CB), 4-hexyloxyphenyl-4′-hexyloxy-3-nitrobenzoate (C2), and 4-butyl-4′-(hexyloxyphenyloxycarbonyl)phenyl carbonate (H22). We have measured the frequency dependences of the longitudinal and transverse components of the dielectric constant of the mixture and we have determined the frequency regions of positive and negative dielectric anisotropy. We show that for certain changes in the frequency of the applied electric field while the magnitude of the field remains the same, the transmission region of the Al₂O₃ particles — dual-frequency liquid crystal system switches from one wavelength to the other. The experimental results are explained by the optical homogeneity of the system in a narrow wavelength interval when the refractive indices of the particle material and the matrix are close, and also by reorientation of the liquid crystal molecules as the frequency of the applied voltage changes.     

Investigation on Nonradiative Decay of the Er^3＋^4I13/2 → ^4I15/2 Transition in Different Tellurite Glass Matrix

Three kinds of Er^3＋-doped tellurite glasses with different hydroxyl groups are prepared by the conventional melt-quenching method. Infrared spectra are measured to estimate the exact content of OH^- groups in samples. The maximum phonon energy in glasses are obtained by measuring the Raman scattering spectra. The strength parameters Ωt （t = 2, 4, 6） for all the samples are calculated and compared. The nonradiative decay rate of the Er^3＋ ^4I13/2 → ^4I15/2 transition are calculated for the glass samples with different phonon energy and OH^- group contents. Finally, the effect of OH^- groups on fluorescence decay rate of Er^3＋ is analysed, the constant KOH-Er of TWN, TZPL and TZL glasses are calculated to be 9.2 × 10^-19 cm^4s^-1, 5.9 × 10^-19 cm^4s^-1, and 3.5 × 10^-19 cm^4s^-1, respectively.     

Temperature effect on emission lines and fluorescence lifetime of the ~4F_(3/2) state of Nd:YVO_4

By measuring the absorption and fluorescence spectra and the fluorescence lifetime of 4F3/2 state of Nd3+ ions in YVO4 (2 at.-%) crystal at different temperature, the effects of temperature on the spectra and the lifetime of F3/2 state have been investigated. As the temperature is increased, the line width of the 4F3/2 -4I11/2 transitions is found to increase and the spectral line toward the longer wavelength, which are duo to the ion-phonon interaction. The variation fluorescence lifetime of the 4F3/2 state of Nd:YVO4 is found to be anomalous in the measured range 8 - 300 K. It is about 81 us at room temperature and decreases to 30 us at 8 K. The experimental results are explained by ascribing to the thermal mixing between the two Stark levels of 4F3/2 state with different lifetime.     

Application of the cluster variation method to an interstitial solid solution: Calculation of the γ ′-Fe 4 N 1-x -ε-Fe 2 N 1-z equilibrium

The cluster variation method has been applied to establish effective interaction potentials that describe both n'-Fe 4 N 1 mx - k-Fe 2 N 1 mz miscibility gaps in the Fe-N phase diagram. The calculated nitrogen distributions show that long-range order occurs in the n:'-Fe 4 N 1 mx phase and that short-range ordering as well as long-range ordering occurs in the k-Fe 2 N 1 mz phase. The calculated nitrogen distributions for the k-Fe 2 N 1 mz , pertaining to temperatures and concentrations at the n'- k-phase boundaries, are compared with available data obtained by Mössbauer spectrometry. Preferential occupation of specific interstitial sites occurs from about 16 at.% N onwards; at the highest concentration considered, about 25 at.% N, the occupation is that of Fe 3 N as proposed in the literature on the basis of diffraction data.     

Dielectric and infrared properties of SrTiO3 single crystal doped by 3d （V,Mn, Fe,Ni） and 4f （Nd,Sm, Er） ions

In this study, the effects of doping by 3d （V, Mn, Fe, Ni） and 4f （Nd, Sm, Er） ions on dielectric and infrared properties of SrTiO3 （STO） single crystals are investigated. It is well known that doping of the SrTiO3 can change the dielectric properties of the STO from an insulator to an n-type semiconductor, and even to a metallic conductor. Dielectric and infrared （IR） properties of the undoped STO and doped STO single crystals are analyzed using dielectric spectroscopy （80 kHz-5 MHz）, transmission （200 cm^-1-4000 cm^-1）, and reflection spectroscopy （50 cm^-1-2000 cm^-1）. It is found that doping by the 3d ions reduces the value of dielectric permittivity, but the trend of temperature dependence of the dielectric permittivity remains almost unchanged. On the other hand, dielectric spectroscopy measurements for samples doped by 4f ions show the anomalous behaviors of the dielectric permittivity at temperatures around the temperature of the structural phase transition. There are two fractures of temperature dependences of inverse dielectric permittivity εr^-1 （T）. Transmittance spectroscopy measurements show that there are differences in the shape of the spectrum in the mid-IR region between the undoped STO and the one doped by 4f ions. The differences in the reflectance spectrum between the STO：Nd and STO are analyzed in detail.     

CW one-micron (4F3/2-4I11/2) laser oscillation of Nd3+ ions in the melilite-type crystal Ca2MgSi2O7:Nd3+(Na+) at its incommensurate–commensurate phase transition

The spectroscopic and stimulated-emission (⁴F_3/2→⁴I_11/2) properties of the novel melilite-type laser crystal Ca₂MgSi₂O₇:Nd³⁺(Na⁺) were studied in a temperature range that covers its incommensurate–commensurate (IC↔N) phase transition. The phase transition temperatures of both undoped Ca₂MgSi₂O₇ and the doped crystal were ascertained at 346.6 K (undoped) and 341.3 K (doped) by means of differential scanning calorimetry (DSC). The temperature-dependent spectroscopic and laser experiments showed a significant decrease in CW output power and a strong distortion of the generated lasing beam in the region of the phase transition. The observed crystal field disorder of Nd³⁺ lasants in Ca₂MgSi₂O₇:Nd³⁺(Na⁺) is dominantly due to occupation of the position of Ca²⁺ by cations of different valency, while the influence of incommensurability is of minor importance.     

Magnetooptics of the 1D2 → 3F4 radiative transition in thulium:yttrium-aluminum garnet Tm3+:YAG

The spectra of luminescence and magnetic circular polarization of luminescence in the ¹ D ₂ → ³ F ₄ emission band in thulium:yttrium-aluminum garnet Tm³⁺:YAG (Tm³⁺:Y₃Al₅O₁₂) have been investigated at temperatures T = 78 and 293 K, respectively. Based on the analysis of the magnetooptical and optical spectra, the optical transitions between the Stark sublevels of the ³ F ₄ and ¹ D ₂ multiplets in Tm³⁺:YAG were identified. It is shown that the symmetry and energies of the Stark sublevels of the ³ F ₄, ¹ D ₂, and ³ H ₆ multiplets, found experimentally from magnetic and magnetooptical studies, confirm the results of theoretical calculations of the energy spectrum of the rare earth Tm³⁺ ion in YAG. Original Russian Text ? U.V. Valiev, J.B. Gruber, I.R. Gapdulkhakov, N.I. Juraeva, A.K. Mukhammadiev, Sh.A. Rakhimov, I.S. édel’man, 2009, published in Optika i Spektroskopiya, 2009, Vol. 106, No. 6, pp. 937–944.