Ions In Single Crystals Research Articles

Ultrahigh-frequency NMR of Tm 3+ ions in single crystals of thulium ethylsulfate at high magnetic fields

Resonant transitions predicted earlier between low-lying electron–nuclear sublevels of the Tm 3+ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet—thulium ethylsulfate (TmES) single crystal. It is shown that, due to the distortion of the 4f-electron shell of rare-earth ion in an applied magnetic field, the parameters of electron–nuclear interaction become field dependent. Experimental results of second moment of Tm 3+ enhanced NMR line in TmES are presented. The possible mechanisms of line broadening are considered, particularly as a result of the first excited doublet splitting.

Volume effects on the Raman frequencies of phosphate ions in fluorapatite crystals

Abstract The vibrations of phosphate (PO4−3) ions in single crystals of hexagonal Ba5(PO4)3F (B-FAP) have been investigated by means of polarized Raman scattering and compared to Raman frequencies observed in fluorapatite analogs Sr5(PO4)3F (S-FAP) and Ca5(PO4)3F (FAP). The Raman vibrational frequencies decrease upon substitution of Ba2+ for Sr2+ or Ca2+ in the lattice, asymtotically approaching the Raman frequencies for vibrations of the free ion. The observed splitting is in accord with predictions made by a group theory analysis of the factor group of the unit cell. The effects of the host lattice environment on the magnitude of the Davydov splittings and frequency shifts of phosphate ions within different crystals with the fluorapatite structure are demonstrated to be inversely proportional to the unit cell volume.

Electron paramagnetic resonance of Cr2+ and Cr4+ ions in CdGeAs2 crystals

Electron paramagnetic resonance (EPR) has been used to investigate chromium ions in single crystals of CdGeAs2 grown by the horizontal gradient freeze technique. Signals from Cr2+ and Cr4+ ions were observed near 12 K. The Cr2+ ions have the 3d4 configuration with S=2 and substitute at the cadmium sites, while the Cr4+ ions have the 3d2 configuration with S=1 and substitute at the germanium sites. As a result, each impurity represents a neutral center. Both EPR spectra display the tetragonal symmetry of the lattice, which suggests that the centers most likely are not perturbed by nearby defects. The ratio of Cr2+ ions to Cr4+ ions in a particular CdGeAs2 crystal depends on the growth conditions and stoichiometry.

Orbital order in the low-dimensional quantum spin system TiOCl probed by ESR

We present electron spin resonance data of Ti$^{3+}$ (3$d^1$) ions in single crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is predominantly occupied and owing to the occurrence of orbital order a linear spin chain forms along the crystallographic b axis. This result corroborates recent theoretical LDA+U calculations of the band structure. The temperature dependence of the parameters of the resonance signal suggests a strong coupling between spin and lattice degrees of freedom and gives evidence for a transition to a nonmagnetic ground state at 67 K.

EPR Study of Nd3☎ Ions in β-BaB2O4 Single Crystals

An Electron Paramagnetic Resonance (EPR) study of Nd3☎ ions in single crystals of the low-temperature phase of barium borate (β-BaB2O4 or BBO) is presented. The EPR spectra show the existence of a minimum of three different Nd3☎ centres. The g-matrices for three of the centres have been determined from the analysis of the angular dependencies of the spectrum in two planes of the crystal. This study allows us to conclude that neodymium is incorporated in the low-symmetry Ba2☎ site. Possible reasons for the appearance of various Nd3☎ centres are discussed.

Ultrahigh-frequency NMR of Tm3+ ions in single crystals of thulium ethylsulfate in high magnetic fields

Resonant transitions predicted earlier between low-lying electron-nuclear sublevels of the Tm3+ ground state were observed at frequencies up to 700 MHz in a dielectric Van Vleck paramagnet—thulium ethylsulfate single crystal. It is shown that, due to the distortion of the 4f-electron shell of a rare-earth ion in an applied magnetic field, the parameters of electron-nuclear interaction become field-dependent.

ESR of different Gd 3+ sites in CaB 6

The environment of Gd 3+ (4f 7, S= 7 2 ) ions in single crystals of Ca l− x Gd x B 6 (0.0001⩽ x⩽0.01) is studied by electron spin resonance (ESR). The spectra for x⩽0.001 show a split spectrum due to cubic crystal field effects (CFE). The line shape of each fine structure line is Lorentzian, indicating an insulating environment for the Gd 3+ ions. For 0.003⩽ x⩽0.01, the spectra show a single resonance ( g≈1.992(4), Δ H l/2 ≈60 Oe) with no CFE and Dysonian line shape indicating metallic environment for the Gd 3+ ions. For intermediate concentrations, a coexistence of spectra corresponding to insulating and metallic regions is observed. Most of the measured samples show the weak ferromagnetism (WF) as reported for Ca 0.995La 0.005B 6, but, surprisingly, this WF has no effect in our ESR spectra either for metallic or insulating environments. This result suggests that the WF in these systems might be isolated in clusters (defect-rich regions) and its relationship with metallicity is nontrivial.

Optical absorption and electron paramagnetic resonance of Fe ions in KDP crystals

Optical absorption and electron paramagnetic resonance (EPR) have been used to study the sites and the charge states associated with Fe ions in single crystals of KH2PO4 (KDP). Doping KDP with 5ppm of Fe introduced a broad charge-transfer absorption band near 270nm. At the same time, room-temperature EPR showed that two Fe3+ centers were present. Although their intensities are different, the two EPR centers have similar zero-field splittings. Spin-Hamiltonian parameters, including fourth order in crystal field, were determined for the dominant center. We suggest that these EPR-active Fe3+ ions substitute for K+ ions, presumably with charge-compensating cation vacancies (i.e., potassium or hydrogen) nearby. Crystals containing both the 270-nm absorption band and the Fe3+ EPR spectra were X-ray irradiated at room temperature and then subjected to a series of thermal anneals up to 150°C. These treatments caused changes in both the absorption band and the EPR spectra, but the behaviors of the optical and EPR features did not correlate. This can be explained if a portion of the Fe ions occupy a site in the KDP crystals other than the K+ site. We suggest that most of the optical absorption in the 200-to-300nm region is due to Fe ions substituting for phosphorus; i.e., (FeO4)2− molecular units are present in the KDP crystals and give rise to ultraviolet absorption.

Diffusion of Scandium, Gallium and Ytterbium lons into Single- and Poly-Crystalline Yttrium Aluminum Garnets.

Diffusion of scandium, gallium and ytterbium ions into single- and poly-crystalline yttrium aluminum garnets has been studied. Volume diffusion coefficients of gallium ions in single crystals were larger than those of scandium and gallium ions. On the other hand, the diffusion coefficients of scandium and ytterbium ions took almost the same values. The cations diffusion exhibited an extrinsic behavior. Cation ratio of Y/Al influenced the diffusivity of scandium and ytterbium diffusion. The yttrium-rich YAG had a smaller concentration of yttrium vacancies than that with a stoichiometric composition. The scandium and ytterbium ions preferentially diffused along grain boundaries.

Ion conduction in single crystals of the hollandite-type one-dimensional superionic conductor Na xCr xTi 8− xO 16 ( x=1.7)

Ion conduction of Na + ions in single crystals of a one-dimensional superionic conductor, Na–Cr–priderites Na x Cr x Ti 8− x O 16 ( x=1.7), has been studied by an impedance spectroscopy method. In the low-frequency region, a frequency-independent conduction process was observed. The conduction process was thermally activated and the value of the total activation energy was about 0.288 eV. The value of the frequency-independent ion conductivity was about 2×10 −5 S/cm at room temperature and 10 −2 S/cm at 200°C.

Excitation spectra and structure of luminescence centers of manganese ions in single crystals of zinc sulfide

We investigate the photoluminescence excitation spectra in ZnS:Mn single crystals at room temperature and at the temperature of liquid nitrogen with a different concentration of Mn2+ ions. The strongest bands peaking at 557, 578, 600, and 637 nm are associated with a different position of the Mn ion in the lattice of the crystals under investigation. The difference obtained in the excitation spectra can be explained by the resonance transfer of energy between the Mn ions.

Anisotropic hyperfine structure in the EPR spectrum of impurity ions in single crystals

Expressions are obtained for the hyperfine splitting in EPR spectra of impurity ions with electron spin 1/2 and arbitrary nuclear spin for arbitrary anisotropy of the g-factor and hyperfine structure constants. These results generalize the previously obtained results of Weil for the case of weakly anisotropic g-factors and hyperfine structure constants.

Raman spectroscopic study of barium fluorapatite

The vibrations of phosphate (PO −3 4) ions in single crystals of hexagonal Ba 5(PO 4) 3F (B-FAP) have been investigated by means of polarized Raman scattering and compared to Raman frequencies observed in fluoroapatite analogs Sr 5(PO 4) 3F (S-FAP) and Ca 5(PO 4) 3F (FAP). The Raman vibrational frequencies decrease upon substitution of Ba 2+ for Sr 2+ or Ca 2+ in the lattice, asymtotically approaching the Raman frequencies for vibrations of the free ion. Splitting of the vibrational Raman frequencies is also observed. The observed splitting is in accord with predictions made by a group theory analysis of the factor group of the unit cell. The effects of the host lattice environment on the magnitude of the Davydov splittings and frequency shifts of crystals within the fluorapatite structure are demonstrated to be inversely proportional to the unit cell volume and proportional to the crystal field in the host crystal.

Optical spectroscopy of the Ag+ ion in NaF: Experimental results and analysis of manifestations of the Jahn–Teller effect

The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+.

Luminescence and stimulated radiation of neodymium in rare earth sulphate crystals

Various absorption transitions of Nd 3+ ion in single crystals of neodymium sulphate have been identified and their experimental oscillator strengths calculated by numerically integrating the absorption bands. The experimental oscillator strengths are utilised in evaluating the three phenominological Judd-Ofelt intensity parameters. Various radiative parameters such as the electric dipole linestrengths, radiative transition probabilities, radiative life times, fluorescence branching ratios and stimulated emission cross-sections for the fluorescence transitions from 4F 3 2 excited manifolds of Nd 3+ ion have been evaluated. Quantum efficiency and optical gain of the most efficient 4F 3 2 → 4I 11 2 fluorescent transition have been computed with the help of the fluorescence data. Some information regarding the excited state absorption intensities from the 4F 3 2 level are also presented.

Electron paramagnetic resonance investigation of ions in single crystals

An X-band electron paramagnetic resonance study at 5 K of ions in single crystals is reported. The spectra have been attributed to a unique centre of located at the site. The spin-Hamiltonian parameters have been determined for the even isotopes of Dy, as well as for the isotopes and . Moreover, a detailed g-factor analysis has been carried out by obtaining the ground doublet wavefunction. To this purpose, we have employed an interpolated set of crystal-field parameters for which was estimated from the fitted sets of and in . It is found that the interpolated set yields a satisfactory approach to the experimental g-factor values, which makes it feasible to predict the energy-level scheme for the ground manifold of in this host.

Absorption spectroscopy ofR3+(R=Pr,Nd)ion pairs inCsCdCl3

The energy levels of ${R}^{3+}(R=\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d})$ ions replacing the divalent ${\mathrm{Cd}}^{2+}$ ions in single crystals of ${\mathrm{CsCdCl}}_{3}$ have been investigated by polarized absorption at 10 K. For charge balance, a majority of ${R}^{3+}$ ions in ${\mathrm{CsCdCl}}_{3}$ form pairs with a vacancy associated with it. The analysis reveals that ${R}^{3+}$ pair ions are in two unequal sites which result from two arrangements: (i) with a vacancy in between the two ${R}^{3+}$ ions and (ii) with a vacancy on any one side of the two ${R}^{3+}$ ions. Indeed two types of spectra are observed due to the different ${C}_{3v}$ crystal fields at these two sites. Thirteen levels of ${\mathrm{Pr}}^{3+},$ arising out of five multiplets, and thirty levels of ${\mathrm{Nd}}^{3+},$ arising out of twelve multiplets, have been identified along with their irreducible representations. The best free-ion and crystal-field parameters for ${\mathrm{Pr}}^{3+}$ have also been determined.

A Raman Scattering, Ion Channelling and Photoluminescence Study of Argon Ion Radiation Damage in Cu(Ga,In)Se2 - Dose Dependence and Dose Rate Effects

AbstractThe ternary chalcopyrite semiconductor CuInSe2 and related ternary compounds are promising materials for the production of high-efficiency thin film solar cells. In this paper we study the dose dependence of ion radiation damage produced by 30 keV and 80 keV Ar ions in single crystals and polycrystalline films of Cu(In,Ga)Se 2 over a wide dose range from 1012 to 1017 cm-2, using Raman spectroscopy and ion channeling measurements. For the first time, we also report on the dose rate dependence with a variation of the beam current density in the range 0.44 to 44 µcm-2. Even for low damage levels no significant dependence of the defect concentration or damage mechanism on the dose rate could be observed. From phonon correlation length considerations we estimate defect densities. They are in agreement with ion channeling data obtained in the 1015 to 1016 dose range, where the breakdown of the lattice structure occurs. In this dose range, the defect density is close to the concentration of implanted atoms. We conclude, that this high impurity concentration is responsible for the amorphization.

Dechanneling in single-crystals due to in-flight secondary decay: a method for measuring short nuclear lifetimes

The propagation of excited heavy ions in single crystals is complicated by their possible in-flight decay, and the subsequent random perturbation on their path (dechanneling). We develop here a theory of the dechanneling valid when the decay occurs at the very beginning of the ion path. This permits us to calculate analytically the distribution of initial conditions for ion motion, that is expressed in terms of elliptic integrals. A large sample of ion trajectories is then constructed by a vectorized Monte Carlo code not containing in-flight decay. These trajectories can be appropriately weighted, and used many times, to simulate dechanneling due to different decay functions. All this produces a variant of the blocking method of measuring short interaction times between nuclei. This new method can be applied when the composite system, formed by the interaction of an ion beam with a crystal target, emits primary fragments in a too short time to be measured by the standard blocking technique. In this case, the blocking dip is sensitive to the secondary time delay, i.e. the lifetime of the primary fragment that decays in flight. Times measurable in this way range from some 10 −18 s to 10 −16 s. As an example, we re-analyse the data of a previous blocking experiment on the 16O + 28Si system, that revealed unexpected long reaction times. We show that, if interpreted as delays associated to in-flight decay, such results do not contradict standard statistical model calculations.

A pseudo-Jahn - Teller vibronic model of ions in cubic crystals; the luminescence band shape and lifetimes of

A new model for the vibronic problem of the luminescence of impurity ions in the cubic environment is proposed. The approach developed is based on the numerical quantum-mechanical solution of the dynamic pseudo-Jahn - Teller vibronic problem for the excited states. The spin - orbit splitting of the term, and its mixing with , as well as the vibronic interaction with the full-symmetric local mode, are taken into account, resulting in the vibronic problem, and the and adiabatic problems. The effective dipole moment operator is built with due regard for a weak odd-parity trigonal crystal-field component. The vibronic wave-functions are used for the calculation of the band shape of the luminescence in the R line and U band, as well as for the evaluation of the temperature dependence of the lifetime decay emission of ions in single crystals. The proposed model provides a very good explanation of the experimental data.