Local Crystal Field Research Articles

Effect of Li+ ion concentration on upconversion emission and temperature sensing behavior of La2O3:Er3+ phosphors

The effects of Li+ co-doping concentration on the structure, upconversion luminescence and temperature sensing behavior of Er3+:La2O3 phosphors were investigated. X-ray diffraction and scanning electron microscopy observations reveal that Li+ ion co-doping can change the lattice parameter of La2O3 host and increase the particle size of the samples. The optical investigation shows that co-doping of Li+ ions can enhance the upconversion emission of Er3+ ions in La2O3 matrix effectively. Most importantly, the temperature sensing sensitivity of the samples is found to be dependent on Li+ co-doping concentration, when the emission intensity ratio of the (2H11/2 → 4I15/2) and (4S3/2 → 4I15/2) transitions of Er3+ is chosen as the thermometric index. Both of the optimum upconversion luminescence and temperature sensing sensitivity are obtained for 7 mol% Li+ co-doped sample. When the Li+ concentration is beyond 7 mol%, both the quenching in upconversion intensity and the degradation of temperature sensitivity are observed, which may be due to the serious distortion in local crystal field around Er3+ ions caused by the excess Li+ ions.

Low field magnetocaloric effect in bulk and ribbon alloy La(Fe0.88Si0.12)13

Low-field magnetocaloric effect occurred in itinerant metamagnetic materials is at core for magnetic cooling application. This works reports the magnetocaloric responses obtained at 1.35T for the silicon-doped iron-based binary alloy La(Fe0.88Si0.12)13 in the bulk and ribbon form. Both samples possess a same symmetry but with different crystallite sizes and lattice parameters. The ribbon sample shows a larger maximum entropy change (nearly 8.5 times larger) and a higher Curie temperature (5K higher) in comparison with that of the bulk sample. The obtained relative cooling power for the ribbon is also larger and very promising for application (RCP = 153J/kg versus 25.2J/kg for the bulk). The origin of the effect observed is assigned to the occurrence of negative magnetovolume effect in the ribbon structure with limit crystallization, caused by rapid cooling process at the preparation, which induced smaller crystallite size and large lattice constant at the overall weaker local crystal field.

Mixed symmetry of local crystal fields as a reason of ferromagnetism onset in pseudobinary PrNi5−xCux alloys

Numerical calculations of magnetic properties of Pr3+ ions in crystal field of the mixed (hexagonal+orthorhombic) symmetry in PrNi5−xCux pseudobinary alloys are presented. The molecular field theory and the crystal field theory in the point charge model are used in calculations. It is shown that the admixing of crystal field of orthorhombic symmetry to the original hexagonal crystal field of PrNi5 and PrCu5 compounds leads to an abrupt increase in their magnetic susceptibility followed by the onset of ferromagnetism, which was earlier observed experimentally in pseudobinary PrNi5−xCux alloys. The calculated dependences of magnetic moments of Pr3+ ions in crystal fields of the mixed symmetry on the magnetic field strength and temperature satisfactorily agree with the experimental results for PrNi5−xCux alloys.

Enhanced infrared-to-visible up-conversion emission and temperature sensitivity in (Er3+,Yb3+, and W6+) tri-doped Bi4Ti3O12 ferroelectric oxide

Strong up conversion (UC) luminescence at 527, 550, and 662 nm is compared under an excitation of 980 nm in single doped (Er3+), co-doped (Er3+/Yb3+), and (Er3+/Yb3+/W6+) tri-doped bismuth titanate (Bi4Ti3O12). For the co-doped system, the frequency (UC) emission intensity due to Er3+ ions is enhanced significantly in the green bands due to the efficient energy transfer from Yb3+ to Er3+ ions. Further increase in the emission intensity is seen with non-luminescent W6+ ions in the tri-doped system due to the modification in the local crystal field around the Er3+ ions, and is evidenced through a gradual change in the crystal structure of the host lattice with increasing W6+ content. The observed changes in the fluorescence lifetime and the associated energy transfer mechanisms are discussed. A progressive reduction of the lifetime of the 4S3/2 levels of Er3+ ions from 72 to 58.7 μs with the introduction of Yb3+ and W6+ dopant increases the transition probability and enhances the UC emission intensity. The efficiency of the energy transfer process (η) in the co-doped and tri-doped systems is found to be 9.4% and 18.6%, respectively, in comparison to the single doped system. Temperature sensing based on the fluorescence intensity ratio (FR) technique shows high sensitivity (0.0123 K−1) in the high temperature range (293 to 523 K) for an optimum content of Er3+, Yb3+, and W6+ with x = 0.03, y = 0.18, and z = 0.06 at. % in the tri-doped Bi4−x−yErxYbyTi3−zWzO12 ferroelectric composition, and is found useful for potential applications in optical thermometry.

DC Stark addressing for quantum memory in Tm:YAG

We observed a linear DC Stark effect for 3 H 6 – 3 H 4 optical transition of Tm 3+ ions in Y 3 Al 5 O 12 . We observed that application of electric field pulse suppresses the two-pulse photon echo signal. If we then apply a second electric pulse of opposite polarity the echo signal is restored again, which indicates the linear nature of the observed effect. The effect is present despite the D 2 symmetry of the Tm 3+ sites that prohibits a linear Stark effect. Experimental data analysis shows that the observed electric field influence can be attributed to defects that break the local crystal field symmetry near Tm 3+ ions. Using this effect we demonstrate selective retrieval of light pulses in two-pulse photon echo.

Upconversion luminescence enhancement and temperature sensing behavior of F− co-doped Ba3Lu4O9:Er3+/Yb3+ phosphors

F− doping to enhance upconversion luminescence and temperature sensitivity of Ba3Lu4O9:Er3+/Yb3+ (EYBLO) phosphors is ascribed to the modification of local crystal field of activator ions and reduction of crystal site symmetry.

A novel anion doping strategy to enhance upconversion luminescence in NaGd(MoO4)2:Yb3+/Er3+ nanophosphors.

We propose a novel and efficient F- anion doping strategy for enhancing upconversion luminescence in upconversion nanophosphors. NaGd(MoO4)2:Yb3+/Er3+ nanophosphors doped with different F- contents are synthesized hydrothermally. Rietveld refinement results obtained from X-ray diffraction data indicate that the Gd-O bond length decreases and the O-Gd-O bond angle varies with increasing F- content, resulting in augmented local crystal field strength and distorted local site symmetry of the dopant lanthanide sites. Judd-Ofelt analysis suggests that the calculated radiative quantum efficiency of the 4S3/2 level and the radiative branching ratio of 4S3/2 → 4I15/2 transition in F--doped NaGd(MoO4)2:Yb3+/Er3+ nanophosphors are much greater than those in F- anion-free samples. It is inferred that F- anion doping helps to reduce the nonradiative transition probabilities based on the luminescence dynamics. Rietveld refinement results and Judd-Ofelt analysis confirm jointly that doping of interstitial F- anions could enhance local crystal field strength with odd parity and modify site symmetry of the lanthanide activator ions, leading to enhanced radiative transitions and inhibited nonradiative transitions. A maximum of 17-fold enhancement of total emission intensity is found in NaGd(MoO4)2:Yb3+/Er3+/F- nanophosphors compared with F- anion-free counterparts. The proposed F- anion doping strategy provides an alternative approach for enhancing upconversion luminescence efficiency and could be extended to other inorganic upconversion nanomaterials.

Topological engineering of doped photonic glasses

Abstract

Enhanced frequency upconversion in Ho3+/Yb3+/Li+:YMoO4 nanophosphors for photonic and security ink applications

The YMoO4 nanophosphors codoped with Ho3+/Yb3+/Li+ ions synthesized by the chemical coprecipitation method have been structurally characterized by using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), and Transmission Electron Microscopy (TEM) techniques. The TEM bright field imaging shows that the developed nanophosphors are crystalline in nature with particle size ∼45 nm. The upconversion (UC) emission spectra upon excitation at 980 nm of the nanophosphors at low pump power (<900 mW) show the emission peaks in the UV, green, and red regions, whereas at high pump power (≥900 mW) an intense broad band ranging from 400–900 nm along with a UV band has been observed. The enhancement of about ∼104 times corresponding to the green band in the Ho3+-Yb3+-Li+ codoped nanophosphors compared to that of the Ho3+ singly doped nanophosphors has been observed. This enhancement is caused by the energy transfer from the Yb3+ to Ho3+ ions and modified the local crystal field developed around the rare earth ions. A higher value of the slope (i.e., n ∼ 6.38) for broad band emission within the 944 mW-1200 mW pump power region in the Ho3+-Yb3+-Li+ codoped nanophosphors is found to be due to the involvement of the photon avalanche population process but it is not related to the black body radiation. The intense peak at ∼564 nm and ∼648 nm for the broad band emission is attributed to the charge transfer luminescence of codoped nanophosphors, which is related to the oxygen ion present in the MoO4 group and Yb3+ ion. The observations described in this paper may be of significant interest for developing the visible upconverters, security ink, and novel devices for displays in the low and high pump power region.

Structural phase transitions and photoluminescence properties of oxonitridosilicate phosphors under high hydrostatic pressure.

Spectroscopic properties of a series of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) compounds has been studied under high hydrostatic pressure applied in a diamond anvil cell up to 200 kbar. At ambient pressure the crystal structures of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ≤ x ≤ 0.98) are related to the ratio of strontium to barium and three different phases exists: orthorhombic Pbcn(0.78 ≤ x ≤ 0.98), triclinic P1 (0 < x ≤ 0.65) and triclinic P1 (0.65 < x < 0.78). It was found that Eu2+ luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu2+ sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (Sr0.98-xBaxEu0.02)Si2O2N2 into orthorhombic structure. Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (Sr0.98-xBaxEu0.02)Si2O2N2 series. Additionally, very different pressure shifts of the Eu2+ luminescence in different phases of (Sr0.98-xBaxEu0.02)Si2O2N2:Eu from −40 cm−1/kbar to 0 cm−1/kbar have been observed. This effect is explained by different interaction of the Eu2+ 5d electron with the second coordination sphere around the impurity cations.

Effect of local fields on the Mg acceptor in GaN films and GaN substrates

The electron paramagnetic resonance (EPR) spectrum of the Mg acceptor is studied in a variety of GaN samples, including mm-thick free-standing substrates and sub-micron heteroepitaxial films. The former allows a view of the acceptor unique for EPR—in an environment with less than 107 cm−3 dislocations and doping densities ranging from 4 × 1016 to 6 × 1018 cm−3. By probing Mg in a broad range of samples in one study a new feature of the acceptor emerges. The EPR data reveal an anisotropic line shape that reflects a change in the crystal field in the vicinity of the Mg acceptor. This feature must be accommodated by any of the models proposed for the Mg acceptor. Here, we show that one such previously proposed model agrees well with the EPR data obtained from the wide variety of samples studied. The work implies that Mg-doped GaN contains a common Mg-related defect which can be affected by local crystal fields established during growth.

Crystal structure and partial Ising-like magnetic ordering of orthorhombicDy2TiO5

The structure and magnetic properties of orthorhombic $\mathrm{D}{\mathrm{y}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$ have been investigated using x-ray diffraction, neutron diffraction, and alternating current (ac)/direct current (dc) magnetic susceptibility measurements. We report a continuous structural distortion below 100 K characterized by negative thermal expansion in the [0 1 0] direction. Neutron diffraction and magnetic susceptibility measurements revealed that two-dimensional (2D) magnetic ordering begins at 3.1 K, which is followed by a three-dimensional magnetic transition at 1.7 K. The magnetic structure has been solved through a representational analysis approach and can be indexed with the propagation vector $k=[01/20]$. The spin structure corresponds to a coplanar model of interwoven 2D ``sheets'' extending in the [0 1 0] direction. The local crystal field is different for each $\mathrm{D}{\mathrm{y}}^{3+}$ ion (Dy1 and Dy2), one of which possesses strong uniaxial symmetry indicative of Ising-like magnetic ordering. Consequently, two succeeding transitions under magnetic field are observed in the ac susceptibility, which are associated with flipping each $\mathrm{D}{\mathrm{y}}^{3+}$ spin independently.

Upconversion Luminescence Properties of Er3+ Doped Yb2Ti20 Nanophosphor by Gd3+ Codoping.

Er3+ doped Yb2Ti207 nanophosphors by Gd3+ codoping with nominal composition of (Er0.05Yb0.95-xGd3+)2Ti2O7 (x = 0, 0.2, 0.4, 0.6, 0.8, and 0.95) have been prepared by sol-gel method. Er3+-Gd3+ codoped Yb2Ti2O7 was characteristic of a typical face-centered cubic crystal phase, and the unit cell parameter increased linearly with the increase of Gd3+ concentration. Under a 976 nm laser diode excitation, both green and red upconversion emissions were observed and the upconversion emissions were enhanced significantly by Gd3+ codoping, showing the strongest green and red emissions at 80 mol% Gd3+ codoping. The intensity ratio of green to red emissions (Igreen/Ired) increased monotonously with the increase of Gd3+ concentration. The energy transfer between Yb3+ and Er3+ and the variation of local crystal field symmetry of Er3+ by the substitution of Yb3+ by Gd3+ ions led to the improvement of upconversion properties of Er3+-Gd3+ codoped Yb2TiO7 nanophosphors.

Plasmon-photon conversion to near-infrared emission from Yb(3+): (Au/Ag-nanoparticles) in tungsten-tellurite glasses.

This manuscript reports on the interaction between 2F5/2→2F7/2 radiative transition from Yb3+ ions and localized surface plasmon resonance (from gold/silver nanoparticles) in a tungsten-tellurite glass. Such an interaction, similar to the down-conversion process, results in the Yb3+ emission in the near-infrared region via resonant and non-resonant energy transfers. We associated such effects with the dynamic coupling described by the variations generated by the Hamiltonian HDC in either the oscillator strength, or the local crystal field, i.e. the line shape changes in the emission band. Here, the Yb3+ ions emission is achieved through plasmon-photon coupling, observable as an enhancement or quenching in the luminescence spectra. Metallic nanoparticles have light-collecting capability in the visible spectrum and can accumulate almost all the photon energy on a nanoscale, which enable the excitation and emission of the Yb3+ ions in the near-infrared region. This plasmon-photon conversion was evaluated from the cavity’s quality factor (Q) and the coupling (g) between the nanoparticles and the Yb3+ ions. We have found samples of low-quality cavities and strong coupling between the nanoparticles and the Yb3+ ions. Our research can be extended towards the understanding of new plasmon-photon converters obtained from interactions between rare-earth ions and localized surface plasmon resonance.

Clarifying the preferential occupation of Ga3+ ions in YAG:Ce,Ga nanocrystals with various Ga3+-doping concentrations by nuclear magnetic resonance spectroscopy

Preferential substitution of Ga3+ ions for octahedral Al3+ ions in YAG:Ce,Ga nanophosphors enhances the luminescence properties of Ce3+ activators.

Mn concentration-dependent tuning of Mn(2+) d emission of Zn1-xMnxTe nanocrystals grown in a glass system.

We studied the effect of Mn concentration on the optical, morphological and magnetic properties of Zn1-xMnxTe NCs grown in a glass matrix produced by the fusion method. The physical properties of these materials were determined by optical absorption (OA), transmission electron microscopy (TEM), atomic/magnetic force microscopy (AFM/MFM) and photoluminescence (PL). An analysis of the OA spectra, based on the crystal field theory (CFT), showed strong evidence that Mn(2+) ions were substitutionally incorporated into the Zn1-xMnxTe NCs until reaching the solubility limit (concentration, x = 0.100). Above this nominal concentration, TEM showed the onset of Mn-related phases, such as MnO and α-MnO2, in the PZABP glass system. AFM images showed that NC density on the surface of the glass matrix decreased as x-content increased. It is probable that MnO and MnO2 NCs would outnumber Zn1-xMnxTe NCs at higher concentrations - a conclusion that was corroborated by the OA spectra and TEM images. MFM images revealed that samples with Mn(2+) ions responded to magnetization from an MFM probe. This implied that Mn(2+) ions were incorporated within the Zn1-xMnxTe NCs and gave rise to the diluted magnetic semiconductor (DMS) structure. The PL spectra not only confirmed the evidence obtained by OA, CFT, TEM and AFM/MFM, but also showed that Mn(2+) concentration could be used to tune (4)T1((4)G) → (6)A1((6)S) emission energy.

Magnetic field induced ferromagnetism in pseudobinary PrAl2−xNix alloys

The magnetic properties of pseudobinary PrAl2−xNix intermetallics, 0≤x≤0.22 have been studied in stationary (up to 50kOe) and pulsed (up to 350kOe) magnetic fields in the temperature range of 2–300K. The magnetic state of alloys after melting and after annealing at 800°C is shown to be qualitatively different in the case of x=0.14 and 0.22. The observed peculiarities of magnetic properties of the pseudobinary alloys have been explained based on numerical calculations of the magnetic properties of Pr3+ ions in the composition of pseudobinary PrAl2−xNix alloys. Variable parameters were the constant λ of exchange interaction and the parameter of additional local crystal field created by alloying with nickel ions. It is shown that, depending on the value of λ, Pr3+ ions can be either in a state with magnetic ground level of the multiple J, or in a state with a non-magnetic ground level. In a certain range of λ values, a Pr3+ ion can stay in both the states, depending on the conditions of their realization. For example, application of a sufficiently high magnetic field brings the Pr3+ ion from a state with a nonmagnetic ground state level into that with the magnetic ground level. This process can be either hysteretic or no hysteretic. In the first case, we deal with the emergence of the magnetic-field induced ferromagnetic state as a result of a quantum transition of the first kind, i.e., a change of the ground state of J multiple.

Calculations of the local structure and EPR parameters for Cu2 + ions in xMgO–(30 − x)Na2O–69B2O3 glasses at different composition x

The local structures and electron paramagnetic resonance (EPR) parameters as well as their composition (x) dependences for Cu2+ in xMgO·(30−x)·Na2O·69B2O3(5≤x≤17mol%) glasses are theoretically studied by analyzing these parameters for tetragonally elongated 3d9 complexes. The nonlinear variations of the EPR spectra with modifier MgO composition x can be ascribed to the approximately parabolic relationships between cubic field parameter Dq, orbital reduction factor k, local relative tetragonal elongation ratio ρ, core polarization constant κ and reduction factor H with x. The [CuO6]10− complexes may experience the slight relative elongations of about 0.4% due to the Jahn–Teller effect. The theoretical d–d transition bands and EPR parameters for the various composition x are in good agreement with the observed results. The above composition dependences of the relevant quantities are illustrated as the modifications of the local crystal-fields and electronic states in the vicinity of the Cu2+ dopants with x.

Effects of Interatomic Coupling on Magnetic Anisotropy and Order of Spins on Metallic Surfaces

Both quantum and classical behavior of single atomic spins on surfaces is determined by the local anisotropy of adatoms and their coupling to the immediate electronic environment, yet adatoms seldom reside on surfaces alone, and it is generally acknowledged that substrated-mediated interactions can couple single spins among each other impacting their magnetic behavior. Here we show that also magnetic anisotropy, which is usually considered to be a constant determined by the local crystal field, can be extremely sensitive to such interactions. By the example of Co dimers on Cu(001) and Pt(001) surfaces we highlight the intricate interplay of exchange coupling and magnetic anisotropy, providing a much sought possibility to tune the latter through deliberate adjustment of the adatoms’ separation. As a technologically relevant implication we demonstrate the impact of such emergent nonlocal anisotropy on the hysterectic properties of single-atom magnetization curves.

Optical, Structural and Paramagnetic Properties of Eu-Doped Ternary Sulfides ALnS₂ (A = Na, K, Rb; Ln = La, Gd, Lu, Y).

Eu-doped ternary sulfides of general formula ALnS2 (A = Na, K, Rb; Ln = La, Gd, Lu, Y) are presented as a novel interesting material family which may find usage as X-ray phosphors or solid state white light emitting diode (LED) lighting. Samples were synthesized in the form of transparent crystalline hexagonal platelets by chemical reaction under the flow of hydrogen sulfide. Their physical properties were investigated by means of X-ray diffraction, time-resolved photoluminescence spectroscopy, electron paramagnetic resonance, and X-ray excited fluorescence. Corresponding characteristics, including absorption, radioluminescence, photoluminescence excitation and emission spectra, and decay kinetics curves, were measured and evaluated in a broad temperature range (8–800 K). Calculations including quantum local crystal field potential and spin-Hamiltonian for a paramagnetic particle in D3d local symmetry and phenomenological model dealing with excited state dynamics were performed to explain the experimentally observed features. Based on the results, an energy diagram of lanthanide energy levels in KLuS2 is proposed. Color model xy-coordinates are used to compare effects of dopants on the resulting spectrum. The application potential of the mentioned compounds in the field of white LED solid state lighting or X-ray phosphors is thoroughly discussed.