Symmetric Crystal Field Research Articles

A highly efficient narrow-band blue phosphor of Bi3+-activated cubic borate Ba3Lu2B6O15 towards backlight display applications

A novel narrow-band blue emitting phosphor of Bi 3+ -activated cubic borate Ba 3 Lu 2 B 6 O 15 is synthetized by solid state reaction, which benefited from a highly symmetrical crystal structure. Ba 3 Lu 2 B 6 O 15 :Bi 3+ shows great application potential in LCD backlight display attributed to its extreme color purity of 97.76%, high internal quantum efficiency of 90.6% and weak chromaticity drift. • FWHM = 35.5 nm. • IQE = 90.6%. • Excellent thermal stability. • Energy level scheme. • 90.06% NTSC. Developing innovative efficient phosphors featured with a narrow-band emission is essential for present white light-emitting diodes (wLEDs) towards high-performance liquid–crystal display (LCD) backlights applications, especially for Bi 3+ -activated phosphors. Herein, we synthesized a near-ultraviolet (n-UV) excitable narrow-band blue-emitting Ba 3 Lu 2 B 6 O 15 :Bi 3+ (BLB:Bi 3+ ) phosphor. It exhibits a narrow band emission (full-width at half-maximum, FWHM = 35.5 nm), high color purity of 97.76%, surprisingly high internal quantum efficiency (IQE) which reached up to 90.6%, excellent thermal stability (85%@150 °C of the integrated emission intensity at room temperature) and reversibility and high consistency. The outstanding photoluminescence properties of Bi 3+ are attributed to a highly symmetrical crystal field environment and tightly connected polyhedral network of Ba 3 Lu 2 B 6 O 15 . In addition, the energy level scheme is constructed from the calculated electronic structure to explore the difference in luminescence stability of Ce and Bi doped BLB phosphors. The pc-wLED device fabricated using commercial green β -Sialon:Eu 2+ , red K 2 SiF 6 :Mn 4+ , and as-prepared blue BLB:Bi 3+ phosphor on a 365 nm n-UV chip covered 90.06% of the color space of National Television System Committee (NTSC) standard, suggesting great potential for industrial applications as LCD backlighting.

Two-Dimensional Fe8N Nanosheets: Ferromagnets and Nitrogen Diffusion.

We perform a first-principles study and identify two intriguing ferromagnets, hollow-Fe8N (H-Fe8N) and bridge-Fe8N (B-Fe8N) monolayers, by extensive structural searches. Both H-Fe8N and B-Fe8N nanosheets are buckled triangular lattices with a similar motif, but they are distinguishable by the positions of N atoms. The magnetic and electronic properties show that H-Fe8N is a low-spin ferromagnet; in contrast, B-Fe8N is a high-spin ferromagnet, which originates from the 3d orbital splitting of the Fe atom due to the low/high symmetric crystal field. Surprisingly, two stable Fe8N monolayers can be transferred to each other by N atom diffusion from the bridge position to the hollow position with the migration energy barrier of 1.5 eV. The energy barrier is affected by introduced Fe defects and rare earth metal dopants. These findings introduce a new tactic to regulate the 2D Fe-nitride monolayers at the atomic scale.

The doping site analysis and control of Eu3+ in ZnO:Eu crystal lattice

A new effective and convenient method is put forward to accurately analyze and control the doping sites of Eu3+ in ZnO:Eu nanocrystal. By analyzing the drift of XRD peaks, the splits of laser-stimulated fluorescence peak, intensity variation of E2 Raman scattering peaks and lattice distortion from HRTEM, the position that Eu3+ ions occupy in host lattice is accurately analyzed. There generally exists the symmetry broken regularity for disordered doped system such as ZnO:Eu — it is that Eu3+ is inclined to occupy the position with low symmetry for equilibrium growth crystal. At high annealing temperature, the crystallographic symmetry of Eu3+ is reduced from C3v to C2, which fully evidences that Eu3+ ions are separated out of host lattice. In the same time, the doping position of the Eu can be completely controlled by controlling annealing temperature. In the low symmetrical crystal field, 612 nm fluorescence peak splits to 4 splits, and the peak width of the strongest one is only about 4 nm. This nanomaterial with extremely narrow luminescence will have intensively potential applications in the fields of high quality light source with high contrast control and high signal to noise ratio such as fluorescence probe in high resolution bioimaging.

Effects of the PPy layer thickness on Co–PPy composite films

Co–PPy composite films were electrodeposited on ITO substrate from two different solutions potentiostatically. Firstly, the PPy layers with the thicknesses changing from 20 to 5000nm were produced on ITO. Then Co was electrodeposited on these PPy/ITO substrates with a charge density of 1000mCcm−2. The electrochemical properties were investigated by the current density–time transients and the variation of the elapsed time for the Co deposition depending on the PPy layer thickness. X-ray photoelectron (XPS) spectra indicated the presence of both Co metal and its oxides on the surface. The weak reflections of the Co3O4, CoO and hcp Co were detected by the X-ray diffraction (XRD) technique. According to scanning electron microscopy (SEM) images, the thickness of the PPy layer strongly affects the Co nucleation. The composite films with the PPy layer thinner than 200nm and thicker than 2000nm have an isotropic magnetic behavior due to the symmetrical crystal field. The composite films with the PPy layer thicknesses between 200 and 2000nm have an anisotropic magnetic behavior attributable to the deterioration of this symmetrical crystal field by the PPy bubbles on the surface. All films are hard magnetic material, since the coercivities are larger than 125Oe.

Coherent manipulation of dipolar coupled spins in an anisotropic environment

We study coherent dynamics in a system of dipolar coupled spin qubits diluted in solid and subjected to a driving microwave field. In the case of rare earth ions, anisotropic crystal background results in anisotropic g tensor and thus modifies the dipolar coupling. We develop a microscopic theory of spin relaxation in transient regime for the frequently encountered case of axially symmetric crystal field. The calculated decoherence rate is nonlinear in Rabi frequency. We show that the direction of static magnetic field that corresponds to the highest spin g-factor is preferable in order to obtain higher number of coherent qubit operations. The results of calculations are in excellent agreement with our experimental data on Rabi oscillations recorded for a series of CaWO4 crystals with different concentrations of Nd3+ ions.

Local structure around rare-earth ions in B2O3 glass at high pressure

Melt quenching of B2O3 with less than 25 mol. % rare-earth oxide (RE2O3) at ambient pressure results in a milky white glass because of liquid-liquid phase separation into B2O3 and RE2O3·3B2O phases. In contrast, we have found that melt quenching under GPa-order pressure realizes a transparent RE-doped B2O3 glass. This study investigates the local structure around the RE ions in the B2O3 glass prepared at 3 GPa using optical measurements and electron-spin-echo envelope modulation spectroscopy. It is shown that the RE-rich microparticles disappear and the RE ions are isolated from each other in a highly symmetric crystal field formed by triangular and tetrahedral boron units. This result is consistent with that extrapolated from the data for RE-doped sodium borate glasses.

Formation of Eu(III) Nanoparticles on Borosilicate Sol−Gel Studied with Time-Resolved Luminescence Techniques

The formation of Eu(III) nanoparticles in borosilicate sol−gels and the glass formation heat treatment effect on those particles were studied using luminescence techniques. The presence of the particles was observed using transmission electron microscopy (TEM) images followed by analysis with energy dispersive X-ray spectroscopy (EDS). These experiments showed the presence of particles with a large quantity of europium and chlorine and only small amounts of oxygen with sizes ranging from 30 to 100 nm. Heat treatment at 400, 600, and 800 °C lead to glass samples in which those particles were no longer observed. Steady-state and time-resolved luminescence techniques allowed a detailed study of Eu(III) photophysics in sol−gel and glass samples. In sol−gel matrices, the 5D0 → 7F0 transition is very weak, hinting at Eu(III) species experiencing a rather symmetric crystal field. The 5D0 → 7F2 transition intensity is not very strong, which according to a Judd−Ofelt analysis indicates low interaction with the ani...

Magnetic anisotropy in the Fe(II)Fe(III) bimetallic oxalates

Bimetallic oxalates are layered molecule-based magnets with transition metals $M(\text{II})$ and ${M}^{\ensuremath{'}}(\text{III})$ coupled by oxalate molecules $\text{ox}={\text{C}}_{2}{\text{O}}_{4}$ in an open honeycomb structure. Among the most interesting molecule-based magnets, Fe(II)Fe(III) bimetallic compounds with spins $S=2$ and ${S}^{\ensuremath{'}}=5/2$ ferrimagnetically order at a transition temperature ${T}_{c}$ that ranges from 30 to 48 K, depending on the organic cation between the layers. In small magnetic fields, several of these compounds exhibit ``giant negative magnetization'' below a compensation temperature of about $0.62{T}_{c}$. By studying the behavior of the low-energy orbital doublet produced by a ${C}_{3}$-symmetric crystal field, we construct a reduced Hamiltonian that contains both the exchange and spin-orbit interactions. This Hamiltonian is used to explain almost all of the important behaviors of the Fe(II)Fe(III) bimetallic oxalates, including the stability of magnetic order in weakly coupled layers and the magnetic compensation in compounds with high transition temperatures. In a magnetic field perpendicular to the bimetallic layers, a spin-flop transition is predicted at a field of about $3{J}_{c}/{\ensuremath{\mu}}_{B}\ensuremath{\approx}24\text{ }\text{T}$, where ${J}_{c}\ensuremath{\approx}0.45\text{ }\text{meV}$ is the nearest-neighbor antiferromagnetic exchange coupling. Holstein--Primakoff $1/S$ and $1/{S}^{\ensuremath{'}}$ expansions are used to evaluate the spin-wave spectrum and to estimate the spin-wave gap ${\ensuremath{\Delta}}_{\text{sw}}\ensuremath{\approx}1.65\text{ }\text{meV}$ in compounds that exhibit magnetic compensation. We predict that the negative magnetization can be optically reversed by near-infrared light. Breaking the ${C}_{3}$ symmetry about each of the Fe(II) ions through either a cation-induced distortion or uniaxial strain in the plane of the bimetallic layer is predicted to increase the magnetic compensation temperature.

InternalE5→T25transition ofFe2+in GaN

We present Fourier transform infrared transmission measurements on Fe-doped GaN. A set of freestanding high-quality GaN:Fe samples with varying Fe concentrations shows a rich absorption structure around $390\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ which is unambiguously assigned to the internal $^{5}E\ensuremath{\rightarrow}^{5}T_{2}$ transition of the ${\mathrm{Fe}}^{2+}$ center. Up to now, the $^{5}T_{2}$ state was believed to be degenerate with the conduction band. By means of temperature- and polarization-dependent measurements the observed lines can be assigned to single transitions between $^{5}E$ and $^{5}T_{2}$ sublevels. As a result a quantitative term scheme of the ${\mathrm{Fe}}^{2+}$ center, under the influence of the ${C}_{3V}$-symmetric crystal field, is established. Only a weak Jahn-Teller interaction seems to effect the Fe center.

Surface Eu3+ions are different than “bulk” Eu3+ions in crystalline doped LaF3nanoparticles

Distinct surface effects on the luminescence properties of highly crystalline LaF3:Eu nanoparticles have been demonstrated, by incorporating different ligands on their surface as well as forming core–shell nanoparticles (i.e. doped LaF3:Eu core with an undoped LaF3 shell). These studies have established that the surface Eu3+ ions have a less symmetric crystal field than the “bulk”, and that they are responsible for the ligand induced changes in the asymmetric ratio, which is determined from the relative intensity ratio of the 5D0 → 7F2 (612 and 618 nm) and 5D0 → 7F1 (591 nm) transitions. This surface effect can very effectively be reduced by forming core–shell particles. The multi-exponential luminescent decay curves observed for these nanoparticles have been fitted using a three-parameter model involving the radiative decay constant of the nine inner shells (kR), the radiative decay constant of the outermost shell (kR10), and a parameter C, which describes the quenching, and is a function of the particular luminescent lanthanide ion, the type and amount of quenchers on or near the surface. The model accounts well for the observed changes in the asymmetric ratio of luminescence of these nanoparticles brought about by the ligands used to stabilise them.

D−dexcitations and interband transitions in MnO: A spin-polarized electron-energy-loss study

In MnO with its half-filled Mn $3d$ shell, all $d\ensuremath{-}d$ excitations are spin forbidden and give rise to only weak structures in optical-absorption spectroscopy. In electron-energy-loss spectroscopy, they are excellently observable due to the possibility of electron exchange in the excitation process and the occurrence of strong resonances. We investigated $d\ensuremath{-}d$ excitations and transitions across the optical gap of MnO by spin-polarized electron-energy-loss spectroscopy with incident electrons of 26--130-eV primary energy. By use of the resonance primary energies and the possibility of separating the $d\ensuremath{-}d$ excitations from the dipole-allowed excitations across the gap in the spin-resolved spectra, nearly all sextet-quartet transitions of the ${d}^{5}$ configuration in the ${O}_{h}$-symmetric crystal field are found, and the excitation energies are determined. Several transitions across the gap are observed. The primary energy-dependent measurements reveal strong interactions between the $d\ensuremath{-}d$ excitations and some of these transitions: Beside the striking resonances of all $d\ensuremath{-}d$ excitations due to the simultaneous excitation of $\mathrm{O}2p--\mathrm{O}3p$ or $\mathrm{Mn}3s--\mathrm{Mn}3d$ transitions, a different interaction occurs at the $\mathrm{Mn}3p--\mathrm{Mn}3d$ threshold, leading to a preferential excitation of the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{A}_{1g}}}^{4}{T}_{2g}{(}^{4}D)$ transition up to \ensuremath{\approx}20 eV above this threshold.

Temperature dependence of Faraday rotation and magnetic susceptibility for Ce 3+ and Pr 3+ ions in fluorozirconate glass

The temperature dependencies of the Faraday rotation and magnetic susceptibility for fluorozirconate glass doped with cerium or praseodymium ions are reported. The glass compositions were 52% ZrF 4, 20% BaF 2, 10% CeF 3, 4% AlF 3, and 14% NaF for the cerium-doped glass, and 51% ZrF 4, 21% BaF 2, 8% PrF 3, 4% AlF 3, and 16% NaF for the praseodymium-doped glass; the samples were prepared by quenching from the melt at 1000°C and 960°C for Ce and Pr doped glass, respectively, and annealed at 240°C. Faraday rotations were measured using a modulated optical polarisation technique over a wavelength range of 320–850 nm, and a temperature range of 15–295 K. Magnetic susceptibilities were determined using a transformer-based method over a temperature range of 85–295 K. The paramagnetic part of the Faraday rotation and the magnetic susceptibility both show deviations from Curie's law, and are not simply proportional to each other as frequently assumed. For cerium-doped glass, the temperature dependence of the magnetic susceptibility can be fitted with an expression based on an axially symmetric crystal field appropriate to a square anti-prism site for the cerium ion. The fourth order crystal field parameters are estimated from earlier superposition-model analyses of crystal fields for cerium ions in crystalline fluorides, while the second order parameter is determined by the least squares fitting process.

STRUCTURAL AND MAGNETIC PROPERTIES OF Pt1-xCux/Co MULTILAYERS

The structural and magnetic properties of Pt1-xCux/Co multilayers have been investigated. Increasing the concentration of Cu in the Pt spacer layers results in monotonic decrease of the anisotropy and remanence, in a range of x from 0.04—0.30 except in a special range of 0.10—0.15. The reduction of these parameters is related to the growth of random anisotropy because a random distribution of Cu atoms in the Pt spacer layers causes a local distortion of crystal symmetry of Pt. In the special range, an order CuPt phase may appear. In such a case cobalt atoms are affected by a symmetric crystal field like that produced by Pt. These may be the possible explanation of the measured parameters after the doping of Cu in the Pt spacerlayers.

Electron paramagnetic resonance of Co2+ compounds (abstract)

In cubic crystal field, the ground state of d7 configuration is 4T1 which is an orbital degenerate state. Even though it is in low symmetric crystal field, the orbital degeneracy of 4T1 is not completely quenched. So it makes it difficult to study its ground-state properties, such as the electron spin resonance g factors and the splitting of ground state. Usually, the method of ‘‘T-P’’ equivalence which is not convenient to investigate the physical processes was used in the previous works. In the present work, the formulae of d-d transition, the splitting of ground state, and the electron paramagnetic resonance parameter, which relate these quantities to the crystal structure as the d orbit of Co2+ is employed, are derived. The present theory shows that g∥>g⊥ and g∥<g⊥ are due to the different distortion of ligands along C3 axis. On the basis of ab initio, the formulae obtained are applied to deal with the magnetic properties of some Co2+ compounds. The agreement of theory with the experiment shows that the present theory is reasonable.

The effect of RKKY interactions on the magnetization of dilute magnetic alloys with hcp hosts

The magnetization due to the presence of magnetic impurities dissolved dilutely in nonmagnetic H.C.P. host metals has been calculated assuming a fine structure splitting by an axially symmetric crystal field and an RKKY interaction between the impurities. To compute the RKKY contribution, the interaction between the impurities is described by a randomly oriented effective internal exchange field H with a probability distribution P(H↘) = Δ/π2(Δ2+H2)2. Comparison of the calculated magnetization with the measured magnetization of a 5 ppm single crystal of MgMn and several polycrystalline ZnMn samples of varying concentration, over the temperature range 2K to 10mK, yields estimates for Δ, the most probable value of the magnitude of the effective exchange field, as well as the dependence of Δ on impurity concentration.

Mean field theory of the orientational properties of (J = 1) hydrogen molecules on the surface of Grafoil

A mean field theory is developed which is applicable to the orientational behavior of (J = 1) hydrogen molecules on the surface of Grafoil. The hydrogen molecules are assumed to form a triangular lattice in a weak, axially symmetric crystal field, Vc. perpendicular to the substrate and to interact via the electric quadrupole–quadrupole interaction with nearest-neighbor coupling constant Γ. Free energy calculations for the various possible ordered phases are presented, and a Landau expansion is used to investigate the phase boundary between the disordered and ordered phases. A computer calculation, based on the iterative method of James, is used to determine the lines of transition between ordered phases. For almost all values of Vc < 22.5Γ, the disordered to ordered state phase transition is continuous. Four distinct types of ordered phases are found — a four-sublattice phase, two two-sublattice phases, and a one-sublattice phase which resembles an X–Y model. Comparison is made to the recent nmr data of Kubik and Hardy for H2 and D2 on Grafoil.

Ion-lattice interactions in rare earth compounds

A study is made using a variational method, of the interaction of a dysprosium ion in an axially symmetric crystal field with a lattice distortion which gives a coupling of type epsilon Q(J+2+J-2). It is shown that a low energy configuration can be obtained in which the dysprosium moment lies in, or near, the perpendicular plane is accompanied by static strain in the lattice. Several orientations of moment plus strain have equal energies. It is then shown that in a magnetically concentrated crystal there will be mutual energies coming from the strains, and it is suggested that the unusual properties of dysprosium vanadate can be understood as arising from ordering of the lattice strains at low temperatures.

Pressure effects on zero‐field resonance. II. Gd3+ in various zircon structure compounds

AbstractA systematic investigation of the influence of hydrostatic pressure on the zero‐field resonance of trivalent gadolinium has been made in a set of nine isomorphic crystals. The pressure coefficients of the rotational symmetric crystal field parameters (which are reliably determined by our experiment) do not only vary markedly in the absolute value with the compounds but also change sign.