Crystal Field Components Research Articles

Optical Lines in Europium and Terbium-Activated Yttrium Tantalate Phosphor: Combined Experimental and Group-Theoretical Analysis

The rare-earth ions in crystals such as terbium (YTaO4:Tb3+) and europium (YTaO4:Eu3+)-activated yttrium tantalate phosphors have a number of attractive features that predetermine their crucial role in practical application in contemporary optoelectronic devices. In this article, we employ the group-theoretical arguments aimed to reveal the group-theoretical classification of the crystal field levels and selection rules for the allowed optical transition between the crystal field components of Tb3+ and Eu3+ of the low symmetry crystal field in the activated yttrium tantalate phosphors. We also establish possible polarization rules for the lines corresponding to the allowed transitions. We deduce the symmetry-assisted results for the selection rules in the optical transitions accompanied by the absorption/emission of the vibrational quanta. The selection rules for the vibronic satellites of the zero-phonon lines are expected to be useful for the identification of the lines in the spectra of rare-earth ions with a weak vibronic coupling. The results of the low-temperature measurements of photoluminescence under the 325 nm excitation are in compliance with the group-theoretical analysis. The aim of the paper is to establish symmetry-assisted results that are the background of the quantitative crystal field theory based on the quantum-mechanical consideration.

Transformation of 5I8→5F5 Ho absorption band in Ho0.75Nd0.25Fe3(BO3)4 crystal during spin-reorientation transition

Polarized absorption spectra of crystal Ho0.75Nd0.25Fe3(BO3)4 in the region of the 5I8→5F5 absorption band of Ho ions were investigated depending on temperature in the range of 5–20 K. Absorption spectra are decomposed into components of the Lorentz form. The spin-reorientation phase transition was detected at 6.9 K. The observed line position shifts during the reorientation transition were different for different lines, which indicated different local magnetic anisotropy of the Ho ion in its excited states. Significant changes in absorption line intensities as a function of temperature were observed during the reorientation transition and above. These were explained as a consequence of the change in even components of the local crystal field in excited states. These components mix the closely spaced 4f states. Distortion of local symmetry in the ground state was detected at one of the electron transitions.

The role of terminal ligands in the slow relaxation of magnetisation and luminescence thermometry of dinuclear NdIII complexes

Magneto- and opto-structural correlations are used as tools to help understand how changes of the crystal field components acting on NdIII influences the slow relaxation of magnetisation and luminescence thermometry of NdIII dinuclear complexes.

Polarized upconversion luminescence from a single LiLuF4:Yb3+/Er3+ microcrystal for orientation tracking

Polarized upconversion luminescence (UCL) of lanthanide-doped micro/nano-crystals has shown great promise in single-particle tracking and super-resolution bioimaging. However, because of the spectral line broadening and multiple sites of lanthanide in upconversion particles (UCPs), the crystal-field (CF) polarization components of UCL are usually undistinguishable. Herein, we report the linearly polarized UCL in LiLuF4:Yb3+/Er3+ single microcrystals with resolvable CF transition lines and a polarization degree up to 0.82. The CF levels and CF transition lines of Er3+, as well as their emission polarization anisotropy, are unraveled for the first time through low-temperature and high-resolution photoluminescence (PL) and UCL spectroscopies. By taking advantage of the well-resolved and highly-polarized CF transition lines of Er3+, we demonstrate the application of LiLuF4:Yb3+/Er3+ single microcrystals as anisotropic UCL probes for orientation tracking. These findings provide fundamental insights into the polarization anisotropy of UCL in lanthanide-doped single particles, thus laying a foundation for the future design of anisotropic luminescent probes towards versatile applications.

Random transverse crystal-field effects on the magnetic properties of the spin-[formula omitted] Blume–Capel model

The spin-3/2 Blume–Capel (BC) model under the effect of a random transverse crystal-field (RTCF) is studied on the square lattice within the mean-field approach. The crystal-field is either turned on or off randomly on the spin sites with probability p and 1−p, respectively. The probability p satisfies 0≤p≤1.0 and the magnetic bilinear interaction parameter, i.e. J>0. The thermal variations of the order parameters are examined to calculate the phase diagrams on the (Dz∕J,T∕J) and (Dx∕J,T∕J) planes for given values of the model parameters where Dx and Dz are respectively the transverse and longitudinal crystal-field components. It is found that the phase diagrams consist of second- and first-order phase transitions lines, in addition to the tricritical points and critical end-points that were observed. Negative values of Dz change fundamentally the topology of the system phase diagrams.

The influence of nd0 transition metal cations on the Eu3+ asymmetry ratio R=I(5D0−7F2)I(5D0−7F1) and crystal field splitting of 7F1 manifold in pyrochlore and zircon compounds

This paper examines the influence of transition metal cations with empty nd0 cores on the Eu3+ asymmetry ratio R=I(5D0−7F2)I(5D0−7F1) and the magnitude of crystal field splitting of the 7F1 manifold (ΔΕ; 7F1). Two families of compounds that crystallize with the pyrochlore and zircon structures are considered. The pyrochlores under consideration have the general formula La2M2O7 (M = Hf, Zr, Sn). Comparison of the Eu3+ optical properties indicate that R increases in order La2Sn2O7 < La2M2O7 (M = Hf, Zr) while (ΔΕ; 7F1) varies as La2M2O7 (M = Hf, Zr) < La2Sn2O7. The strength of M-O covalent bonding is responsible for these trends. The M-cations in La2M2O7 (M = Hf, Zr) have empty nd0 orbitals that participate strongly in covalent bonding with oxide anions, while the covalent contribution to the Sn4+-O2- bonding is weak due to filled Sn4+ 4d10 core. Mulliken charge calculations of the cations and anions in isostructural pyrochlore compounds are interpreted to indicate the presence of a stronger low-symmetry crystal field component that increases the asymmetry ratio in compounds with cations having empty ndo cores. This stronger covalent bonding is also responsible for decreasing (ΔΕ; 7F1). The second family of compounds under consideration crystallize in the zircon structure with the general formula YZO4 (Z = P, V). Comparison of Mulliken charges of cations and anions in these compounds indicates strong covalent bonding in the vanadate because it contains V5+ transition metal cation with empty 3d0 orbital. It leads to R(YPO4) < R(YVO4) and [YVO4: (ΔΕ; 7F1)] < [YPO4: (ΔΕ; 7F1)]. It is concluded that in isostructural family of compounds, the covalent bonding within the crystal framework determines R and (ΔΕ; 7F1).

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Chemical stabilization of Eu2+ in LuPO4 and YPO4 hosts and its peculiar sharp line luminescence

The efficient 4f65d→4f7 (d-f) luminescence from Eu2+ is well-known for host lattices where Eu2+ is substituted on a divalent (e.g. Ca2+, Sr2+) or monovalent (e.g. Na+, K+) cation site. Only recently some studies appeared concerning d-f emission from Eu2+ on a trivalent cation site. The stable Eu2+ emission was observed in X-ray irradiated LuPO4:Eu3+ crystals where Eu2+ is produced through trapping of conduction band electrons generated by X-ray absorption. Here, we demonstrated that Eu2+ can also be chemically stabilized in YPO4 and LuPO4 through co-doping with a tetravalent charge compensator: Hf4+. In orthophosphate crystals doubly doped with Eu and Hf, the characteristic Eu2+ emission is observed at low temperatures without the need for X-ray irradiation. The excitation and emission spectra show strong and narrow zero-phonon lines and a rich vibronic structure. In the excitation spectra a multitude of zero-phonon transitions to individual crystal field components of 7FJ multiplets in the 4f6(7FJ)5d excited state was observed for Eu2+ in both LuPO4 and YPO4. Analysis of the spectra is consistent with a slightly larger crystal field splitting for Eu2+ on the smaller Lu3+ site (compared to Y3+). The observation of many 4f6(7FJ)5d electronic origins make these systems to be model compounds for testing the validity of theoretical models for energy level calculations of 4f65d states of Eu2+.

Sensitivity of a solid Eu(III) complex towards acetonitrile vapor: Structural and spectroscopic characterization

The Eu(III) nitrate complex of the meso- N,N′-bis(2-pyridylmethylene)-1,2-(R,S)-cyclohexanediamine ligand was synthesized and characterized by single crystal and powder X-ray diffraction. The crystal lattice of the complex is capable of absorbing and desorbing selectively acetonitrile molecules, at 293 K upon an acetonitrile vapor pressure of ∼0.1 × 105 Pa. This process, which is partially reversible, can be easily followed by both powder X-ray diffraction (P-XRD) and Eu(III) luminescence spectroscopy. The acetonitrile molecule, located in the outer coordination sphere of the metal ion, does not affect the radiative transition probability of 5D0 level of Eu(III) and also it does not activate further non-radiative channels from this level. On the other hand, this molecule is capable of affecting the energy position and intensities of the crystal field components of the 5D0→7F2 transition. The complex in solid form can be considered a promising material for the optical sensing of acetonitrile vapors.

Influence of low-symmetry component of crystal field on gemstones colors: Cr3+ in ruby and emerald

The origin of difference in the colors of two gemstones – red ruby Al2O3:Cr3+ and green emerald Be3Al2(SiO3)6: Cr3+ is explained by analyzing the crystal field effects. It is shown that the stronger crystal field in ruby and the significantly greater low-symmetry component of crystal field in emerald are the main sources of the observed color difference. Calculated energy levels of the Cr3+ ions in both materials agree well with the experimental results.

Understanding the Optical and Magnetic Properties of Ytterbium(III) Complexes

The absorption and emission spectra of three Yb3+ complexes possessing D3, D2, and C2 symmetries were analyzed with the aid of ab initio calculations based on Complete Active Space (CAS) self-consistent field wave functions (CAS(13,7)). The absorption spectra present contributions from both cold and hot bands, involving thermally populated excited sublevels of the 2F7/2 manifold. The high-resolution emission spectrum of the tris-picolinate complex [Yb(DPA)3]3- recorded at 77 K presents four components, while the complexes with macrocyclic ligands show both cold and hot emission bands, resulting in more than four components for the 2F5/2 → 2F7/2 transition. The combined information provided by the absorption and emission spectra allowed to identify most of the crystal field sublevels of the 2F5/2 and 2F7/2 states. The energies of these crystal field components are well-reproduced by the ab initio calculations, with deviations typically lower than 100 cm-1. The crystal field splitting is very sensitive to subtle changes of the Yb3+ coordination environment. The magnetic anisotropy of [Yb(DPA)3]3- obtained with ab initio calculations was found to be extremely sensitive to changes in the twist angle of the upper and lower faces of the tricapped trigonal prismatic coordination polyhedron. Ab initio ligand field theory provides a straightforward chemical justification for the changes in magnetic anisotropy, which are responsible for the observed pseudocontact shifts in the NMR spectra.

The structure and luminescence of boron nitride doped with Ce ions

Highly luminescent boron nitride (BN) was synthesized by pyrolysis of molecular adduct prepared by chemical mixing of boric acid and melamine with a small amount of cerium nitrate. After pyrolysis, Ce ions were substitutionally incorporated to B3N3 hexagonal layer of turbostratic BN at a very low doping level which was estimated to be less than 7 × 1018 atoms/cm3. Ordered stacking of hexagonal layers along c-axis of BN or transformation of turbostratic BN to hexagonal BN was induced by addition of Ce ions even though the amount of Ce ions surpassed substitutional doping limits. The intense photoluminescence (PL) spectrum of Ce-doped BN was detected at 390 nm by excitation of UV light (λ = 314 nm) and PL intensity was improved with increasing level of Ce addition following the enhanced crystallinity of turbostratic BN. The origin of UV-emission was ascribed to the transition from the crystal field components of the 5d level to the 4f ground states of the Ce3+ ion.

Luminescence properties of translucent nano- and micro-crystalline LuPO4:Ce3+ films in the 20–700 K range of temperatures

Photoluminescent properties of nano- and micro-crystalline LuPO4:Ce3+ sintered materials were investigated in the temperature range of 20–700 K. It was shown that the nano-powders may be converted into highly translucent layers with good mechanical stability by heat-treatment at 1000 °C. Processing at higher temperatures caused grains growth and formation of a network of voids resulting in an increase of light scattering. Luminescent properties were analyzed in the 20–700 K range of temperatures. Electron-phonon coupling decreased when grains became smaller. As a result, the Stokes shift was smaller in the nano-ceramics (1010 cm−1) than in micro-ceramics (∼1580 cm−1). Energies of the crystal field components of 2F5/2 and 2F7/2, Stokes shift and kinetics of Ce3+ luminescence in the nano- and micro-size material over the 20–700 K range of temperatures were determined. It was shown that LuPO4:Ce does not suffer from thermal quenching up to about 650 K. Nano-ceramics appeared even more resistant to thermal quenching than micro-ceramics showing no indication of decay time shortening up to 700 K.

Direct determination of the zero-field splitting for a singleCo2+ion embedded in a CdTe/ZnTe quantum dot

When Co$^{2+}$ impurity is embedded in semiconductor structure, crystal strain strongly influences zero-filed splitting between Co$^{2+}$ states with spin projection $S_z = \pm 3/2$ and $S_z = \pm 1/2$. Experimental evidences of this effect have been given in previous studies, however direct measurement of the strain induced zero-field splitting has been inaccessible so far. Here this splitting is determined thanks to magneto-optical studies of individual Co$^{2+}$ ion in epitaxial CdTe quantum dot in ZnTe barrier. Using partially allowed optical transitions we measure strain induced zero-field splitting of Co$^{2+}$ ion directly on excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of $S_z = + 3/2$ and $S_z = - 1/2$ Co$^{2+}$ spin states in magnetic field, we determine axial and in-plane components of crystal field acting on Co$^{2+}$. Proposed technique can be applied for optical determination of zero-field splitting of other transition metal ions in quantum dots.

Intensities and spectral features of the – potential laser transition of Er3+ centers in CaF2–CeF3 disordered crystal**Project supported by Shanghai Engineering Research Center for Sapphire Crystals, China (Grant No. 14DZ2252500), the Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics Chinese Academy of Sciences (Grant No. 2008DP17301), the Fundamental Research Funds for the Central Universities of China, the National

A CaF2–CeF3 disordered crystal containing 1.06% of Er3+ ions was grown by the temperature gradient technique. Optical absorption and emission spectra recorded at room temperature and at 10 K, luminescence decay curve recorded at room temperature, and extended x-ray-absorption fine structure spectra were analyzed with an intention to assess the laser potential related to the transition of Er3+. In addition, the thermal diffusivity of the crystal was measured at room temperature. The analysis of room-temperature spectra revealed that the emission is long-lived with a radiative lifetime value of 5.5 ms, peak emission cross section of , and large spectral width pointing at the tunability of the emission wavelength in the region stretching from approximately 1480 nm to 1630 nm. The energies of the crystal field components for the ground and excited multiplets determined from low-temperature absorption and emission spectra made it possible to predict successfully the spectral position and shape of the room-temperature emission band. Based on the correlation of the optical spectra and dynamics of the luminescence decay, it was concluded that in contrast to Yb3+ ions in heavily doped CaF2 erbium ions in the CaF2–CeF3 crystal reside in numerous sites with dissimilar relaxation rates.

Spectroscopy of f−f transitions, crystal-field calculations, and magnetic and quadrupole helix chirality in DyFe3(BO3)4

Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on DyFe$_3$(BO$_3$)$_4$ using circularly polarized x rays [T. Usui et al., Nature Materials 13, 611 (2014)]. We show that this effect can be explained quantitatively by calculating the quadrupole moments of the Dy$^{3+}$ ions induced by the low-symmetry ($C_2$) crystal-field (CF) component. In this work, the CF parameters for the Dy$^{3+}$ ions in the $P3_121$ ($P3_221$) phase of DyFe$_3$(BO$_3$)$_4$ are obtained from CF calculations based on the analysis of high-resolution temperature-dependent optical spectroscopy data. We also consider the helix chirality of the single-site magnetic susceptibility tensors of the Dy$^{3+}$ ions in the paramagnetic $P3_121$ ($P3_221$) phase and suggest a neutron diffraction experiment to reveal it.

Template synthesis and luminescence of ordered Lu3Al5O12:Ce3+ nanowire arrays

Uniform Lu3Al5O12:Ce3+ nanowire arrays were successfully synthesized in Anodic Aluminum Oxide (AAO) templates by a sol–gel method. The as-prepared nanowires with ordered, homogenous and dense structure had a uniform diameter, corresponding to the pore diameter of the AAO template. The polycrystalline garnet structure of the as-prepared nanowires was found by X-ray diffraction and selected area electronic diffraction. AAO template has a confinement effect on the coalescence of Lu3Al5O12:Ce3+ nanocrystallites. Both photoluminescence and radioluminescence spectra of Lu3Al5O12:Ce3+/AAO composite consist of double peaks, which correspond to the electron transition of Ce3+ from the lowest crystal-field splitting component of 5d level to the 4f spin–orbit splitting 2F5/2 and 2F7/2 states.

Optical spectra and crystal field calculation for SrB4O7:Sm2+

Undoped and samarium-doped SrB4O7 polycrystalline samples were prepared by a solid state reaction method in air. XRD examination revealed that the undoped sample is consistent with the pure SrB4O7 phase but in samarium-doped sample in addition to the main tetraborate SrB4O7 phase there is a little amount of metaborate. Based on preliminary spectroscopic examination it was ascertained that Sm2+ ions are incorporated without measurable traces of Sm3+ admixture. High resolution luminescence spectra and decay curves of Sm2+ luminescence were measured as a function of temperature in the 5–550 K temperature region. Energies of crystal field components of the 7FJ (J = 1–5) multiplets at 5 K and those of the 5D1 multiplet at 300 K were determined from luminescence spectra and used subsequently as input data for the crystal field calculation. Performed crystal field calculation predicts the energy level scheme for the 4f6 configuration of Sm2+ in SrB4O7 up to 29000 cm−1 and makes it possible to assess the contribution of transitions from the excited states above the 5D1 to luminescence phenomena. Evaluated activation energy Ea ∼4600 cm−1 characterizing a steep decrease of the 5D0 lifetime of Sm2+ between 320 K and 550 K indicates strongly that electronic origin of the lowest energy 7F0 – 4f5d1 vibronic transitions is located around 19200 cm−1, slightly below the 7F0 – 5D3 transition energy.

Spectroscopic characterization of CaNb2O6 single crystal doped with samarium ions

Spectroscopic measurements of the calcium niobate CaNb2O6 single crystal doped with samarium ions were performed. Polarized absorption and emission spectra as well as luminescence decay curves of this material were recorded as a function of temperature in the 5–300K temperature region. Analysis of low temperature spectra made it possible to determine energies of crystal field components of Sm3+ multiplets involved in the excitation and luminescence phenomena. Number of crystal field levels derived implies that Sm3+ ions in the CaNb2O6 host are accommodated in several different sites. Radiative lifetime of the 4G5/2 metastable level determined based on the Judd–Ofelt analysis of room temperature absorption spectra amounts to 777μs, a value close to that of luminescence lifetime determined from luminescence decay curve. Energy transfer between the NbO6 group and samarium ions was observed in wide temperature range. Intense luminescence related to the 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions centered at 610nm and 660nm, respectively, combined with strong absorption band near 404nm that matches perfectly the pump light provided by commercial InGaN/GaN diode lasers points at a potential of CaNb2O6:Sm3+ for the design of all-solid-state visible lasers.

Photoluminescence Properties of Europium Complexes Encapsulated into MSBA-15

The Eu complexes-Eu (TTA)3phen (TTA: thenoyltrifluoroacetone, phen: 1,10-phenanthroline) were encapsulated, uniformly distributed into the channels of the modified SBA-15 (labeled with MSBA-15), and structurally characterized. The photoluminescence properties of the encapsulated complexes were systematically studied in contrast to the pure complexes. The results indicate that the excitation bands assigned to the π-π* electron transition of the ligands for Eu3+ complexes in encapsulated complexes were split into different components, and the 5D0-7F0 transitions became partly allowed. The emission lines for the 5D0-7F2 transitions became broader and the relative intensity for different crystal field components varied greatly in comparison to the pure complex. Most importantly, the photostability and thermostability of the emissions improved considerably.