Local structure of Tm<Superscript>2+</Superscript> and Yb<Superscript>3+</Superscript> impurity centers in <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Ca,Sr, Ba) fluoride crystals

The local structure of Tm²⁺ and Yb³⁺ cubic impurity centers in MeF₂: Tm²⁺ and MeF₂: Yb³⁺ (Me = Ca, Sr, Ba) fluoride crystals, as well as Yb³⁺ trigonal and tetragonal impurity centers in MeF₂: Yb ³⁺ crystals, is calculated within the shell model in the pair potential approximation.     

Low-temperature time-resolved VUV luminescence spectroscopy of SrF<Subscript>2</Subscript>: Er<Superscript>3+</Superscript> crystals

Time-resolved excitation and emission spectra of SrF₂: Er³⁺ upon selective excitation with synchrotron radiation in the VUV and ultrasoft x-ray ranges at T = 8 K were studied. The VUV luminescence of SrF₂: Er³⁺ derives from high-energy interconfiguration 4f¹⁰5d-4f¹¹ transitions in the Er³⁺ ion. The VUV emission spectrum revealed, in addition to the 164.5-nm band (millisecond-range kinetics), a band at 146.4 nm (with a decay time of less than 600 ps). The formation of excitation spectra for the f-f and f-d transitions in the Er³⁺ ion is discussed.     

The upconversion luminescence and magnetism in Yb<Superscript>3+</Superscript>/Ho<Superscript>3+</Superscript> co-doped LaF<Subscript>3</Subscript> nanocrystals for potential bimodal imaging

Biocompatible upconversion nanoparticles with multifunctional properties can serve as potential nanoprobes for multimodal imaging. Herein, we report an upconversion nanocrystal based on lanthanum fluoride which is developed to address the imaging modalities, upconversion luminescence imaging and magnetic resonance imaging (MRI). Lanthanide ions (Yb³⁺ and Ho³⁺) doped LaF₃ nanocrystals (LaF₃ Yb³⁺/Ho³⁺) are fabricated through a rapid microwave-assisted synthesis. The hexagonal phase LaF₃ nanocrystals exhibit nearly spherical morphology with average diameter of 9.8 nm. The inductively coupled plasma mass spectrometry (ICP-MS) analysis estimated the doping concentration of Yb³⁺ and Ho³⁺ as 3.99 and 0.41%, respectively. The nanocrystals show upconversion luminescence when irradiated with near-infrared (NIR) photons of wavelength 980 nm. The emission spectrum consists of bands centred at 542, 645 and 658 nm. The stronger green emission at 542 nm and the weak red emissions at 645 and 658 nm are assigned to ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The pump power dependence of luminescence intensity confirmed the two-photon upconversion process. The nanocrystals exhibit paramagnetism due to the presence of lanthanide ion dopant Ho³⁺ and the magnetization is 19.81 emu/g at room temperature. The nanocrystals exhibit a longitudinal relaxivity (r ₁) of 0.12 s^?1 mM^?1 and transverse relaxivity (r ₂) of 28.18 s^?1 mM^?1, which makes the system suitable for developing T2 MRI contrast agents based on holmium. The LaF₃ Yb³⁺/Ho³⁺ nanocrystals are surface modified by PEGylation to improve biocompatibility and enhance further functionalisation. The PEGylated nanocrystals are found to be non-toxic up to 50 μg/mL for 48 h of incubation, which is confirmed by the MTT assay as well as morphological studies in HeLa cells. The upconversion luminescence and magnetism together with biocompatibility enables the adaptability of the present system as a nanoprobe for potential bimodal imaging.     

Thermal quenching of luminescence of BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals activated with Er<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er³⁺ and Tm³⁺ ions in BaY₂F₈ crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er³⁺ and Tm³⁺, respectively. It is shown that quenching of 5d-4f luminescence of Tm³⁺ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.     

Electron paramagnetic resonance of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> centers in (Cr,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> synthetic forsterite

Synthetic single crystals of chromium-and lithium-doped forsterite, namely, (Cr,Li): Mg₂SiO₄, are studied using electron paramagnetic resonance spectroscopy. It is revealed that, apart from the known centers Cr³⁺(M1) and Cr³⁺(M2) (with local symmetries C_i and C_s, respectively), these crystals involve two new types of centers with C₁ symmetry, namely, Cr³⁺(M1)′ and Cr³⁺(M2)′ centers. The standard parameters D and E in a zero magnetic field [zero-field splitting (ZFS) parameters expressed in GHz] and principal components of the g tensor are determined as follows: D=31.35, E=8.28, and g=(1.9797, 1.9801, 1.9759) for Cr³⁺(M1)′ centers and D=15.171, E=2.283, and g=(1.9747, 1.9769, 1.9710) for Cr³⁺(M2)′ centers. It is found that the lowsymmetric effect of misalignment of the principal axes of the ZFS and g tensors most clearly manifests itself (i.e., its magnitude reaches 19°) in the case of Cr³⁺(M2)′ centers. The structural models Cr³⁺(M1)-Li⁺(M2) and Cr³⁺(M2)-Li⁺(M1) are proposed for the Cr³⁺(M1)′ and Cr³⁺(M2)′ centers, respectively. The concentrations of both centers are determined. It is demonstrated that, upon the formation of Cr³⁺-Li⁺ ion pairs, the M1 position for chromium appears to be two times more preferable than the M2 position. Reasoning from the results obtained, the R₁ line (the ²E → ⁴A₂ transition) observed in the luminescence spectra of (Cr,Li): Mg₂SiO₄ crystals in the vicinity of 699.6 nm is assigned to the Cr³⁺(M1)′ center.     

Spectroscopy of NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> and NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> crystals as advanced laser materials

Single crystals of double sodium-containing lanthanum and gadolinium molybdates doped with Tm³⁺ ions were synthesized by the Czochralski method. The spectroscopic properties of these crystals were investigated from the viewpoint of their use as active media in diode-pumped lasers. The polarized spectra of absorption on the ³ H ₄ and ³ F ₄ levels and the polarized spectra of luminescence due to the ³ F ₄-³ H ₆ laser transition were recorded, and the lifetimes of the ³ H ₄ and ³ F ₄ excited states of the Tm³⁺ ions were determined. The luminescence cross sections were calculated using the Füchtbauer-Ladenburg formula. The simulation of the decay curve of the ³ H ₄ excited state according to the Golubov-Konobeev-Sakun method revealed that, in the crystals under investigation, the interaction between Tm³⁺ ions predominantly occurs through the dipole-dipole mechanism.     

Study of absorption spectra of Er<Superscript>3+</Superscript> in KTaO<Subscript>3</Subscript> crystals

The first results of the study of optical absorption spectra of KTaO₃: Er³⁺ crystals are presented. In the 350–660-nm region, lines are observed deriving from intraconfigurational electronic transitions from the ⁴ I _15/2 ground state to levels of the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2(⁴ F _3/2), ² G _9/2, and ⁴ G _11/2 excited states of the Er³⁺ ions. A comprehensive study of transitions to the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, and ⁴ F _7/2 levels at 77 K is carried out. The number of lines observed for the above transitions fits the theoretically possible number for ?-? electronic transitions in Er³⁺ ions in the cubic crystal field. In the case of a differently charged substituted ion, this situation occurs only under nonlocal impurity charge compensation. The energies of the excited state levels for the transitions under study are determined.     

Laser spectroscopy of Eu<Superscript>3+</Superscript> cubic centers in the CaF<Subscript>2</Subscript> bulk single crystal

The optical transitions ⁵ D _{0, 1} → ⁷ F _J (J = 0, 1, ..., 6) of Eu³⁺ cubic centers in the CaF₂ single crystal are investigated using combined excitation and emission spectroscopy at different time delays after the excitation pulse. The energies of the Stark sublevels of the ⁷ F _J ground states are determined.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

Experimental Study and Analysis of Absorption Spectra of Ni<Superscript>2+</Superscript> Ions in Nickel Orthoborate Ni<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>2</Subscript>

The results of studies of the absorption spectra of nickel orthoborate Ni₃(BO₃)₂ in the range of electronic d–d-transitions are reported. The obtained data are analyzed in the framework of the crystal field theory. The Ni²⁺ ions are located in two crystallographically nonequivalent positions 2a and 4f with point symmetry groups C_2h and C₂, respectively, surrounded by six oxygen ions forming deformed octahedra. The absorption spectra exhibit three intense bands corresponding to spin-resolved transitions from the ground state of nickel ion ³A_2g (³F) to the sublevels of the ³T_2g (³F), ³T_1g (³F) and ³T_1g (³P) triplets split by the spinorbit interaction and the rhombic component of the crystal field. At temperatures below 100 K, the spectra exhibit a thin structure, in which phonon-free lines can be distinguished. Comparison of the calculated frequencies of the zero-phonon transitions with the experimental data allows estimating parameters of the crystal field acting on the nickel ions in the 2a- and 4f-positions, as well as the parameters of electrostatic interaction between the 3d electrons and spin-orbit interaction constants.     

Divalent rare-earth ions in LaF<Subscript>3</Subscript> crystals

The optical spectra and electric conductivity of LaF₃ crystals doped with 0.01, 0.1, and 0.3 mol % YbF₃, where Yb was partly or completely recharged to the divalent state, are studied. The long-wavelength absorption band of 370 nm is caused by electrons transitioning from state 4f ¹⁴ to the level of anion vacancies. The remaining bands at 300–190 nm are caused by 4f ¹⁴–5d ¹4f ¹³ transitions in Yb²⁺. The bulk electric conductivity and peaks of the dielectric losses of LaF₃–Yb²⁺ crystals are caused by Yb²⁺–anion vacancy dipoles. The activation energy of the reorientation of Yb dipoles is 0.58 eV. The optical and dielectric properties of Yb²⁺ centers are compared to those of Sm²⁺ and Eu²⁺ centers studied earlier in LaF₃ crystals.     

Correction of the band gap of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> phosphor

Submicron samples of Y₂O₃:Eu³⁺ phosphor with elevated photoluminescence (PL) efficiency and activator concentration of 9 at % obtained by the sol–gel method were investigated by diffuse reflection spectroscopy and PL spectroscopy. It is found that the diffuse reflection spectrum in the vicinity of the fundamental absorption edge (<300 nm) is distorted by the superposition of the PL of Eu³⁺ ions, as a result of which the calculated value of optical band gap E _g of the Y₂O₃ matrix is overestimated. An algorithm for eliminating the PL influence on the absorption edge is proposed, and the correct E _g values are found to be 4.61 ± 0.12 and 4.50 ± 0.12 eV for annealing at 700 and 1300°C, respectively.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

High-frequency EPR of Cr<Superscript>2+</Superscript> ions in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>

High-frequency broad-band (65–240 GHz) EPR is used to study impurity centers of bivalent chromium in a CdGa₂S₄ crystal. It is found that the EPR spectra correspond to tetragonal symmetry. The spin Hamiltonian H = βB · g · S + B ₂ ⁰ O ₂ ⁰ + B ₄ ⁰ O ₄ ⁰ + B ₄ ⁴ O ₄ ⁴ with the parameters B ₂ ⁰ =23659±2 MHz, B ₄ ⁰ =1.9±1 MHz, |B ₄ ⁴ |=54.2±2 MHz, g_‖=1.93±0.02, and g_⊥=1.99±0.02 is used to describe the observed spectra. It is concluded that chromium ions occupy one of the tetrahedrally coordinated cation positions.     

Theoretical study of photoionization of the isoelectronic sequence Rb<Superscript>+</Superscript>, Sr<Superscript>2+</Superscript>, and Y<Superscript>3+</Superscript>

The photoionization cross sections for the 4p shell of ions of the Kr isoelectronic sequence Rb⁺, Sr²⁺, and Y³⁺ are calculated. The configuration interaction theory and the perturbation theory are used to describe the many-electron effects. The relativistic effects are taken into account in the Pauli-Fock approximation. The calculated resonance structure of photoionization cross sections for the 4p shell in the region below the 4s threshold associated with the autoionization of the 4s-np singly excited states and the 4p4p-nln′l′ doubly excited states reproduces the results of recent measurements of total photoabsorption cross sections for the Rb⁺, Sr²⁺, and Y³⁺ ions. It is found that, as the nuclear charge in the isoelectronic sequence increases, the ratio between the direct and correlation parts of amplitudes of the 4s-(n/?)p transition changes and, as the consequence, the minimum of the photoionization cross section of the 4s shell shifts from the continuous spectrum to the region of states of discrete spectrum. This accounts for the strong changes in the shape of the 4s-np resonances in the photoionization cross sections for the 4p shell of Rb⁺, Sr²⁺, and Y³⁺, as well as the distinction between the shapes of the 4s-6p _1/2 mirror resonance in the partial 4p _1/2 and 4p _3/2 photoionization cross sections for the Y³⁺ ion which do not suppress each other in the total photoionization cross section, as is the case for similar resonances in Rb⁺ and Sr²⁺.     

Crystal field of Yb<Superscript>3+</Superscript> tetragonal centers in the YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> intermetallic compound

The spectra of electron paramagnetic resonance and inelastic neutron scattering in crystals of the heavy-fermion intermetallic compound YbRh₂Si₂ are interpreted. The phenomenological potentials of the crystal electric field of Yb³⁺ tetragonal centers and the parameter of the Hamiltonian for the spin-orbit interaction of electrons are determined from the experimental energy level schemes. A comparison of the results obtained from experimental data on electron paramagnetic resonance, inelastic neutron scattering, and Mössbauer spectroscopy shows that the most probable ground state of Yb³⁺ ions in the YbRh₂Si₂ crystal is the Kramers doublet Γ _t6 ^? .     

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na<Subscript>0.4</Subscript>Y<Subscript>0.6</Subscript>F<Subscript>2.2</Subscript>:Er<Superscript>3+</Superscript>: II. Luminescence self-quenching and microparameters and macrorates of energy transfer

Na_0.4Y_0.6F_2.2:Er³⁺ (NYF:Er³⁺) crystals with an Er concentration up to 15% were grown by the Bridgman-Stockbarger method. The luminescence kinetics was investigated for a series of NYF:Er³⁺ crystals (0.5–15% Er), as well as the concentration and temperature quenching of the luminescence from radiative Er levels upon selective laser excitation. It is shown that the luminescence from the ⁴S_3/2 level is quenched significantly with increasing temperature and concentration. The luminescence from the ⁴G_11/2, ²G(H)_9/2, ⁴F_9/2, and ⁴I_9/2 levels is quenched mainly due to nonradiative multiphonon transitions. The concentration quenching of the luminescence from the ⁴I_11/2 and ⁴I_13/2 levels was not observed. Possible schemes of the self-quenching of excited levels of erbium are considered and the microparameters and macrorates of self-quenching are estimated by model quantum-mechanical calculation. Based on the comparison of the calculated and experimental self-quenching rates, the most probable mechanisms and schemes of self-quenching are determined. The self-quenching of the ⁴S_3/2 level of erbium was investigated experimentally and theoretically. Good agreement is obtained between the experimental and the calculated kinetic curves and the dependences of the self-quenching rates on Er concentration. It is concluded that NYF:Er³⁺ crystals are promising as active media for tunable lasers with laser diode pumping.     

Spectral-luminescent properties of Eu<Superscript>3+</Superscript> ions in inorganic D<Subscript>2</Subscript>O and POCl<Subscript>3</Subscript>-SnCl<Subscript>4</Subscript> solvents

Absorption and luminescence spectra of Eu³⁺ ions in D₂O and POCl₃-SnCl₄ inorganic solvents were measured. Oscillator strengths and spontaneous radiation transition probabilities for the most intense electron transitions in Eu³⁺ were calculated. Judd-Ofelt parameters, the radiative lifetime of the ⁵ D ₀ metastable level of Eu³⁺ in D₂O-Eu³⁺ and POCl₃-SnCl₄-Eu³⁺ solutions, and matrix elements for radiative transitions from the ⁵ D ₀ level were determined. The luminescence lifetime of the ⁵ D ₀ level of Eu³⁺ in these solutions was measured. The photoluminescence quantum yield for transitions from the ⁵ D ₀ level of Eu³⁺ in ⁵ D ₀ and POCl₃-SnCl₄-Eu³⁺ solutions was found to be 0.32 and 0.88, respectively.     

Influence of the ground state of the Pr<Superscript>3+</Superscript> ion on magnetic and magnetoelectric properties of the PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe₃(BO₃)₄ single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T _N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial (l ∥ c), while, in a strong magnetic field H ∥ c (H _cr ～ 43 kOe at T = 4.2 K), a Fe³⁺ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostriction, and electric polarization. The threshold field values determined in the temperature interval 2–32 K are used to plot an H-T phase diagram. The contribution of the Pr³⁺ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.     

Spectral properties of neodymium in POCl<Subscript>3</Subscript>-SbCl<Subscript>5</Subscript>-Nd<Superscript>3+</Superscript>

The absorption and luminescence spectra of neodymium in a binary inorganic solvent—phosphorus oxychloride-antimony pentachloride—are measured. The spectra are analyzed in terms of the Judd-Ofelt theory. The Judd-Ofelt parameters Ω_λ, the oscillator strengths of the main absorption bands, the spontaneous emission probabilities, the radiative lifetime, the luminescence quantum yields, and the stimulated emission cross sections for the ⁴ F _3/2 → ⁴ I _11/2 laser transition are calculated.