Mechanisms of ionization of <Emphasis Type="Italic">F</Emphasis><Subscript>2</Subscript>-centers in laser media on the basis of LiF crystals

F₂ color centers with a superhigh concentration (5000-cm^–1 absorption coefficient at 450 nm) were formed by high-density electron beams in a layer of LiF crystals of micrometer thickness. The F₂-centers excited by high-power nanosecond wide-band optical pulses (the “soft” pumping regime) efficiently amplified the laser radiation and showed high stability under these conditions. A low stability of F₂-centers to laser radiation (the “hard” excitation regime) is explained by the dissociation of (F ₂ ⁺ , F^–) pairs induced by two-step ionization of F₂-centers: (2hν > 4.5 eV) → F₂ → (F₂)* → F ₂ ⁺ + e; F + e → F^–; F ₂ ⁺ + F^– → 3F.     

EPR-Determined Anisotropy of the <Emphasis Type="Italic">g</Emphasis>-Factor and Magnetostriction of a Cu<Subscript>2</Subscript>MnBO<Subscript>5</Subscript> Single Crystal with a Ludwigite Structure

Electron paramagnetic resonance (EPR) and magnetostriction of the Cu₂MnBO₅ single crystal have been studied. The EPR spectrum consists of a single Lorentzian line due to the exchange-coupled system of spins of Cu²⁺ and Mn³⁺ ions. It has been established experimentally that the g-factor in the paramagnetic region is strongly anisotropic and anomalously small, which is not typical of the exchange-coupled system of spins of Cu²⁺ and Mn³⁺ ions. At a temperature of 150 K, the g-factors along the crystallographic a, b, and c axes are 2.04, 1.96, and 1.87, respectively. Such small effective g-factor values can be due to the effect of the anisotropic Dzyaloshinskii–Moriya exchange interaction between the spins of Cu²⁺ and Mn³⁺ ions directed along the a axis. The presence of two Cu²⁺ and Mn³⁺ Jahn–Teller ions occupying four nonequivalent positions in the crystal is responsible for the absence of the inversion center. It is found that the behavior of the magnetostriction of Cu₂MnBO₅ is not typical of transition-metal crystals but is closer to the behavior of crystals containing rare-earth ions.     

Magnetic circular dichroism and optical absorption in TmAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The polarized spectra of absorption and magnetic circular dichroism in a TmAl₃(BO₃)₄ single crystal are studied in the region of ³ H ₆ → ³ F ₄, ³ H ₆ → ³ F ₃, and ³ H ₆ → ³ F ₂ electronic transitions in the Tm³⁺ ion. The structure of the spectra is interpreted qualitatively. It is shown that the magnetic circular dichroism of the ³ H ₆ → ³ F ₄ transition is determined by the contribution from the splitting of the ground state, whereas the magnetic circular dichroism of the ³ H ₆ → ³ F ₃ transition is governed by the contribution from the splitting of an excited state in a trigonal crystal field.     

High-temperature complex impedance and modulus spectroscopic studies of doped Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ferroelectric ceramics

Lead-free Na_0.5Bi_0.5TiO₃ (NBT) and (1 ? x)Na_0.5Bi_0.5TiO₃ + xBaTiO₃ with x = 0.1 and 0.2 (where x = 0.1 and 0.2 are named as NBT1 and NBT2, respectively), (1 ? y)Na_0.5Bi_0.5TiO₃ + yBa_0.925Nd_0.05TiO₃ with y = 0.1 and 0.2 (where y = 0.1 and 0.2 are named as NBT3 and NBT4, respectively)-based relaxor ferroelectric ceramics were prepared using the sol-gel method. The crystal structure was investigated by X-ray diffraction (XRD) at room temperature (RT). The XRD patterns confirmed the presence of the rhombohedral phase in all the samples. The electrical properties of the present NBT-based samples were investigated by complex impedance and the modulus spectroscopy technique in the temperature range of RT–600 °C. The AC conductivity was found to increase with the substitution of Ba²⁺ ions to the NBT sample whereas it significantly decreased with the addition of Nd³⁺ ions. The more anion vacancies in Ba-added samples and the lower anion vacancies in Nd-added samples were found to be responsible for higher and lower conductivities, 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.     

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.     

Weak ferromagnetism in BiFeO<Subscript>3</Subscript>-based multiferroics

The magnetic properties of the Bi_{1 ? x} Ln _x FeO₃ (Ln is a rare-earth ion), Bi_{1 ? x} A _x FeO_{3 ? x/2} (A is an alkali earth ion), and BiFe_{1 ? x} Ti _x O_{3 + δ} solid solutions in magnetic fields up to 14 T have been studied. The concentration ranges of the existence of the ferroelectric phase described by the space group R3c have been determined. It is shown that the substitution of the rare-earth ions for the Bi³⁺ ions leads to a sharp decrease in the critical fields inducing the metamagnetic transition from a modulated antiferromagnetic state to a weakly ferromagnetic one; however, the modulated structure in the concentration range of the R3c phase is mainly retained. The substitution of the alkali earth ions (x ～ 0.1) for the bismuth ions leads to the total destruction of the modulated structure and to the implementation of the weakly ferromagnetic state within the R3c phase. A homogeneous weakly ferromagnetic state has been revealed when the Ti⁴⁺ ions (x = 0.1) are substituted for the Fe³⁺ ions in the ferroelectric R3c phase.     

Low-temperature thermal-expansion anomaly in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6</Subscript>

Single-crystal samples of the Bi_{2 + x}Sr_{2 ? x ? y}Cu_{1 + y}O_{6 + δ} system revealed anomalous (negative) thermal expansion in the temperature range 10–20 K. Magnetic fields of 1–3 T were found to strongly affect the position and width of the anomaly region. A thermal-expansion singularity was detected at temperatures T≈30–50 K, which may be related to the formation of a pseudogap.     

Secondary ion emission from a GaAs single crystal upon bombardment with Bi<Stack> <Subscript> <Emphasis Type="Italic">m</Emphasis> </Subscript> <Superscript>+</Superscript> </Stack> cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bi_m⁺(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga⁺ and Ga₂⁺ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga⁺ ions upon bombardment with Bi₂⁺–Bi₅⁺–ions is 35–75%.     

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.     

Effect of heterovalent substitution of rare-earth elements on the magnetic and transport properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>

The Y_{(1 ? x} Ce _x Ba₂Cu₃O₇ system with low cerium concentrations has been synthesized. The cerium solubility limit measured using x-ray powder diffraction analysis is about 2.4 at. %. The temperature dependences of the magnetization M(T) are measured for samples cooled in a magnetic field (FC) and in a zero field (ZFC). The difference between the magnetizations M _ZFC-M _FC at 77.4 K, which is proportional to the pinning potential, passes through a maximum at x = 0.0156. This concentration corresponds to the average distance (equal to eight lattice constants) between the impurity ions in the plane of the rare-earth elements, which is comparable to the diameter of Abrikosov vortices in YBa₂Cu₃O₇.     

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.     

Specific features of optical absorption in rare-earth orthoaluminate DyAlO<Subscript>3</Subscript>

Polarization spectra of optical absorption of the 4f-4f transition ⁶ H _15/2 → ⁶ F _3/2 in the rare-earth orthoaluminate DyAlO₃ are theoretically and experimentally studied at the temperature T=78 K. It is shown that the nontrivial character of the anisotropy of the polarization absorption spectra at low temperatures can be explained by the J-J mixing of excited multiplets of the 4f ⁹ configuration of Dy³⁺ ions in a low-symmetry crystal field of the orthoaluminate structure. The energy and wave functions of the Stark sublevels within the excited ⁶ F _5/2 multiplet in the 4f ⁹ configuration of the Dy³⁺ rare-earth ion in the crystal field of C _s symmetry are numerically calculated.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Effect of the chemical state of silver on the luminescence properties of GeO<Subscript>2</Subscript>-Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ag films

A sol-gel method is used to prepare GeO₂-Eu₂O₃-Ag films in which the luminescence efficiency of Eu³⁺ ions during UV excitation is comparable to that in films activated by organic europium complexes. The luminescence spectra of these films are recorded, and the films are also studied using EPR and x-ray diffraction. The main origin of this effect is found to be complex Eu-Ag centers with a high quantum yield of the intracenter transfer of excitations to the rare-earth activator from silver ions and Ag _m ⁿ⁺ oligomer clusters located on the surface of silver nanoparticles.     

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.     

Luminescence of Tl<Superscript>+</Superscript> ions in a KZnF<Subscript>3</Subscript> crystal

The luminescence spectra of a KZnF₃: Tl⁺ crystal are investigated in the energy range from 4.75 to 5.9 eV at temperatures of 10–300 K upon excitation into the A absorption band (5.7–6.3 eV). At T=300 K, the luminescence spectra exhibit an intense band with a maximum at 5.45 eV, which is attributed to single Tl⁺ ions substituted for K⁺ ions. The 5.723-eV intense narrow band observed at T<20 K is assigned to the ³Γ_1u-¹Γ_1g zero-phonon transition, which is weakly allowed by the hyperfine interaction. The luminescence decay is studied as a function of temperature. The main characteristics of the luminescence spectra are adequately described in terms of the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect for an excited sp configuration of the Tl⁺ ion with the use of the parameters obtained earlier from analyzing the absorption spectra of the system under investigation.     

Optical characteristics of radiation from N<Stack><Subscript>2</Subscript><Superscript>*</Superscript></Stack> dimers in a barrier discharge

The spectral and power characteristics of radiation of the second positive system of nitrogen (C ³Π _u → B ³Π _g ) in Ar-N₂ and Ar-N₂-Cl₂ mixtures excited by barrier discharge have been studied experimentally. Addition of argon to N₂ increased the radiation power by sixfold. In the triple mixture Ar-N₂-Cl₂ = 210/0.5/0.005, minor chlorine additions increased the intensity of the C ³Π _u → _B ³Π _g transition by 26% compared to Ar-N₂ mixtures. Radiation power density of 2.7 mW/cm² has been achieved. In both binary and triple mixtures, the second positive system of nitrogen was the major contributor to radiation, while the contributions of the fourth positive system of N ₂ ^* (D ³Σ _u ⁺ → B ³Π _g ), the Vegard-Kaplan transition of N ₂ ^* (A ³Σ _u ⁺ → X ¹Σ _g ⁺ ), and the D′ → A′ band of Cl ₂ ^* were negligibly small.     

Magnon satellite bands in the optical spectrum of antiferromagnetic Rb<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The evolution of optical absorption in a two-dimensional antiferromagnet is investigated in the range of the transition ⁶A_1g → ⁴A_1g, ⁴E _g (⁴G) observed in manganese ions in an external magnetic field inducing noncollinearity of the magnetic structure. It is revealed that hot and cold satellites of the exciton-magnon bands appear in the optical absorption spectrum and then increase in intensity. The shapes of the magnon satellite bands corresponding to a two-dimensional magnetic structure are calculated. It is demonstrated that magnons at the inner points of the Brillouin zone appreciably contribute to the absorption. The zero-point magnetic oscillations play a decisive role in the absorption associated with the magnon decay at low temperatures.     

Comparative studies of microwave absorption in the singlet paramagnets HoVO<Subscript>4</Subscript> and HoBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> in strong pulsed magnetic fields

Microwave absorption in the tetragonal singlet paramagnets HoVO₄ (zircon structure) and HoBa₂Cu₃O _x (x ≈ 6, layered perovskite structure) is studied and compared in pulsed magnetic fields up to 40 T at low temperatures. These paramagnets are characterized by a singlet-doublet scheme of the low-lying levels of the Ho³⁺ ion in a crystal field. In a magnetic field directed along the tetragonal axis, HoVO₄ exhibits resonance absorption lines at wavelengths of 871, 406, and 305 μm, which correspond to electron transitions between the low-lying levels of the Ho³⁺ ion in the crystal field. The positions and intensities of these absorption lines in HoVO₄ are well described in terms of the crystal-field formalism with the well-known interaction parameters. The absorption spectra of HoBa₂Cu₃O _x at a wavelength of 871 μm exhibit broad resonance absorption lines against the background of strong nonresonance absorption. The effects of low-symmetry (orthorhombic, monoclinic) crystal-field components, the deviation of a magnetic field from a symmetry axis, and various pair interactions on the absorption spectra of the HoVO₄ and HoBa₂Cu₃O _x crystals are discussed. Original Russian Text ? Z.A. Kazeĭ, V.V. Snegirev, M. Goaran, L.P. Kozeeva, M.Yu. Kameneva, 2008, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2008, Vol. 133, No. 3, pp. 632–645.