Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.     

Magnetic properties, magnetoresistance, and magnetic-field-induced phase transitions in Tb0.95Bi0.05MnO3 and Eu0.8Ce0.2Mn2O5 multiferroics

This paper reports on a study of the magnetic properties, magnetoresistance, and phase transitions in the semiconducting manganite multiferroics Tb_0.95Bi_0.05MnO₃ and Eu_0.8Ce_0.2Mn₂O₅ whose dielectric properties have been a subject of an earlier study. An analysis of these properties has led us to the conclusion that the above crystals at temperatures T ≥ 180 K undergo phase separation with the formation of a dynamic periodic alternation of quasi-2D layers of manganese ions in different valence states, i.e., charge-induced ferroelectricity. This state exhibits a giant permittivity and ferromagnetism in the layers containing Mn³⁺ and Mn⁴⁺ ions. At low temperatures (T < 100 K), the phase volume is occupied primarily by the dielectric phase. Studies of the magnetic properties and the effect of the magnetic field on the dielectric properties of crystals substantiate the scenario of the formation of a state with giant permittivity put forward by us. At low temperatures, Tb_0.95Bi_0.05MnO₃ exhibits features at the points of phase transitions in pure TbMnO₃. A ferromagnetic moment is observed to exist at all the temperatures covered. At the boundaries of the quasi-2D layers, magnetic-field-induced jumps of the electrical resistivity caused by metamagnetic transitions in the layers with Mn³⁺ and Mn⁴⁺ ions are observed. At temperatures T ≥ 180 K, the electrical resistivity undergoes a periodic variation in a magnetic field which is a manifestation of carrier self-organization. A high magnetic field is capable of shifting the phase transition from 180 K to higher temperatures and inducing additional phase transitions.     

The effect of codopants on spectral-kinetic characteristics of luminescence of scintillation glasses doped with terbium ions

The effect of Ce³⁺ and Pr³⁺ ions on spectral-kinetic characteristics of luminescence of lithium–phosphate–borate glasses is studied. It is shown that terbium ion luminescence caused by transitions from ⁵D₃ and ⁵D₄ multiplets to the ground 7FJ term is detected in samples containing Tb³⁺/Ce³⁺ and Tb³⁺/Pr³⁺. It has been found that an increase in the concentration of cerium ions from 0.2 to 1 wt % leads to an increase in the intensity of main luminescence bands of terbium ions. In Tb³⁺/Pr³⁺ glasses, a decrease in the relative light yield is observed with an increase in the concentration of Pr³⁺ ions. Processes of energy transfer between Tb³⁺/Ce³⁺ and Tb³⁺/Pr³⁺ ions are discussed.     

Optical anisotropy of Tb3+ ions in a garnet structure

Low-temperature measurements of the absorption spectra of linearly polarized light are performed by Tb³⁺ ions of paramagnetic Tb₃Ga₅O₁₂ and magnetically ordered Tb₃Fe₅O₁₂ crystals in the optical transition domains⁷F₆–⁷F₀,⁷F₆–⁷F₁ in a transverse magnetization geometry. It is shown that at T = 100 K an abrupt anisotropy of the Tb³⁺ ion absorption spectra is observed in a terbium ferrite-garnet depending on the orientation of the magnetization vector I relative to the crystallgraphic axes, where the strongest changes in energetic level structure are noted at 18–25° angles between I and the [110] axis. Reduction of the temperature to 30 K results in isotropization of the spectra, which is associated with a high local anisotropy of the magnetic moment of the terbium ion in the ferrite-garnet structure at low temperatures. Dispersion dependences are obtained for the absorption index and the contribution to the refractive index n' due to the Tb³⁺ optical transitions being examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 102–107, March, 1988.In conclusion, the authors are grateful to G. S. Krinchik for stimulation of this research and useful discussions and to A. I. Popov for useful discussion.     

The influence of gallium on magnetocaloric effect in Gd60Tb40 alloys

The influences of gallium substitution for terbium in Gd₆₀Tb₄₀ on the phase formation, Curie temperature and magnetic entropy change have been investigated. A series of Gd₆₀Tb_40−xGa_x with x=0, 1, 3 and 5 alloys were prepared by arc-melting method. The X-ray diffraction (XRD) analysis reveals that a small amount of Ga substitution for terbium in Gd₆₀Tb₄₀ can form the solid solution (Gd, Tb). The Curie temperature (T_c) increases from 270 K for Gd₆₀Tb₄₀ to 297 K for Gd₆₀Tb₃₇Ga₃, while the maximum magnetic entropy changes ΔS_{M, max} decreases from 5.15 J/K kg for Gd₆₀Tb₄₀ to 3.32 J/K kg for Gd₆₀Tb₃₇Ga₃ with increasing the Ga content.     

Energy transfer mechanisms in the KCaLa1−x−yCexTby(PO4)2 phases

The various mechanisms involved in the green emission of KCaLa_1?x?yCe_xTb_y(PO₄)₂ under UV excitation are analyzed for a weak terbium concentration (y = 0.05). Ce³⁺ → Tb³⁺ transfer can be described by the Dexter model, but only for a weak cerium concentration. For higher cerium contents the cerium lifetime temperature dependence can be fitted by using a model involving energy migration between Ce³⁺ ions before Ce³⁺ → Tb³⁺ transfer. The investigation of the variation of the terbium emission vs temperature involves the presence of energy-trapping defects in the material.     

Luminescence of Tb3+ ions in silicate glasses

The optical absorption and photoluminescence emission spectra of terbium doped sodium and lithium aluminium silicate glasses have been measured as a function of terbium concentration. Optical absorption has been measured over the wavelength range from 250 nm to 40 μm and the absorption bands attributed to Tb³⁺ ions have been identified. Luminescence emission occurs in two groups of bands in the blue and in the green. The green ⁵D₄ → ⁷F_J emission is more intense than the blue ⁵D₃ → ⁷F_J. The green luminescence is enhanced at the expense of the blue when the Tb³⁺ ion concentration reaches 0.5 molar%, which corresponds to an ion separation of 20 Å. The green emission is quenched when the Tb³⁺ ion concentration exceeds 5 molar%, corresponding to an ion separation of 9.5 Å. It is concluded that energy transfer from ⁵D₃ to ⁵D₄ levels begins at Tb³⁺ ion separations of 20 Å, and that the process is multipolar. Exchange dipole processes set in at 9.5 Å and quench the green emission. The ion separations at which the two processes occur in silicate glasses are much larger than those at which similar processes set in crystalline material. This enhancement of energy transfer processes in silicate glass is attributed to inhomogeneous broadening of the absorption and emission bands. The detailed structure of the emission bands, particularly that of the ⁵D₄ → ⁷F_6,5,4 doublets, is used to suggest that the Tb³⁺ ions occupy two different sites with rhombohedral and cubic symmetries.     

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Syntheses of optically efficient (La1−x−yCexTby)F3 nanocrystals via a hydrothermal method

Optically efficient cerium and terbium doped lanthanide fluoride (La_1−x−yCe_xTb_y)F₃ nanocrystals with different doping concentrations have been synthesized by a hydrothermal route in the presence of ethylenediamine tetraacetic acid disodium salt (EDTA). The results showed that the formation of nanocrystals with different morphologies depends on terbium ion Tb³⁺ doping concentration, but independent of cerium ion Ce³⁺ doping concentration. With increase in Tb³⁺ doping concentration, the morphologies of nanocrystals evolved from a spherical shape to a plated-like one. In addition, both the photoluminescence quantum yield (PL QY) and the fluorescence lifetime of nanocrystals increased with the increase in Ce³⁺ doping concentration in cerium and terbium co-doped system. The PL QY reached up to 55%, and the lifetime up to 7.3 ms. Transmission electron microscopy (TEM), X-ray diffraction (XRD), selected area electron diffraction (SAED), X-ray fluorescence (XRF), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV-vis) absorption, photoluminescence (PL) and infrared (IR) spectroscopies were employed to characterize the properties of nanocrystals. The growth mechanism of nanocrystals with different morphologies and optical properties of nanocrystals with different doping concentrations were investigated.     

The properties of the (H2O)6 water cluster that depend on its density during temperature transitions

Water clusters (H₂O)₆ are simulated by the Monte Carlo method with the Metropolis function at various temperatures (T ₁ = 273 K, T ₂ = 298 K, and T′₁= 373 K) and densities (ρ₁ = 0.9998 g/cm³, ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system. It is established that the number of retained most probable configuration types at ρ₁ = 0.9998 g/cm³ during temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K is smaller than at ρ₃ = 0.00059 g/cm³. This result was acquired on the background of the following invariable parameters of the system with the same temperature transitions for each of three values of density: (i) the average number of retained most probable configuration types, (ii) the average fraction of weight coefficients of the most probable configuration types, and (iii) the average potential energy. The configuration type that was retained among the most probable configuration types of the system for all values of density (ρ₁ = 0.9998 g/cm², ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system for temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K was also revealed.     

The heat capacity of the layer compound tetrachlorobis (methylammonium) manganese—II (CH3NH3)2MnCl4, from 10 to 300k

The heat capacity of the layer compound, tetrachlorobis (methylammonium) manganese II, (CH₃NH₃)₂MnCl₄, has been measured over the range 10K <T<300K. In this region, two structural phase transitions have been observed previously by other techniques: one transition is from a monoclinic low temperature (MLT) phase to a tetragonal low temperature (TLT) phase, and the other is from TLT to an orthorhombic room temperature (ORT) phase. The present experiments have shown that the lower transition (MLT→TLT) occurs at T = 94.37±0.05K with ΔH_t = 727±5 J mol^?1 and ΔS_t = 7.76±0.05 J K^?1 mol^?1, and the upper transition (TLT→ORT) takes place at T = 257.02±0.07K with ΔH_t = 116±1J mol^?1 and ΔS_t = 0.451±0.004 J K^?1mol^?1. These results are discussed in the light of recent measurements on (CH₃NH₃)₂CdCl₄, and also with regard to a recent theoretical model of the structural phase transitions in compounds of this type.In addition to the structural phase transitions, (CH₃NH₃)₂MnCl₄ also undergoes magnetic ordering at T < 150K. The magnetic component to the heat capacity, as deduced from a corresponding states comparison of the heat capacity of the present compound with that of the Cd compound, is shown to be consistent with the behaviour expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Spectral and luminescence properties of gadolinium gallium garnet epitaxial films doped with terbium

Gadolinium gallium garnet single-crystal films containing terbium are grown through liquid-phase epitaxy from a supercooled solution melt in the PbO-B₂O₃ system. The optical absorption spectra in the wavelength range 0.2–10.0 μm and the luminescence spectra excited by synchrotron radiation with energies in the range 3.5–30.0 eV are investigated at temperatures of 10 and 300 K. It is revealed that the optical absorption spectra contain an absorption band with the maximum at a wavelength λ ≈0.260 μm, which corresponds to the spin-allowed electric dipole transition between the electronic configurations 4f ⁸(⁷ F ₆) → 4f ⁷(⁸ S)5d of the Tb³⁺ ions. The narrow low-intensity absorption bands attributed to the 4f → 4f transitions from the ⁷ F ₆ ground level to the ⁷ F _0–5 multiplet levels of the Tb³⁺ ions are observed in the wavelength range 1.7–10.0 μm. In the luminescence spectra measured at a temperature of 10 K, the highest intensity is observed for a band with the maximum at a wavelength λ ≈ 0.544 μm, which is associated with the ⁵ D ₄ → ⁷ F ₅ radiative transition in the Tb³⁺ ion.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Optical spectra and emission characteristics of terbium-doped potassium–lead double chloride crystals (KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript>:Tb<Superscript>3+</Superscript>)

Optical transitions in KPb₂Cl₅:Tb³⁺ crystals are studied experimentally and theoretically. The absorption cross-section spectra are plotted and the oscillator strengths of transitions from the ground terbium state to excited multiplets are determined. Intensity parameters Ω_t for KPC:Tb³⁺ are determined by the Judd–Ofelt method to be Ω₂ = 2.70 × 10^–20 cm², Ω₄ = 7.0 × 10^–20 cm², and Ω₆ = 0.72 × 10^–20 cm². These values were used to calculate such characteristics of spontaneous radiative transitions as oscillator strengths, probabilities of radiative transitions, and radiative lifetimes. The emission spectra of KPb₂Cl₅:Tb³⁺ crystals upon UV excitation and the decay kinetics of luminescence from the excited ⁵ D ₃ and ⁵ D ₄ levels are studied experimentally, the lifetimes of these levels are determined, and the dependences of the rates of nonradiative relaxation from the excited ⁷ F _j (j = 0–5), ⁵ D ₄, and ⁵ D ₃ levels to lower-lying terbium levels are calculated. It is shown that the population of the ⁵ D ₄ level in KPC:Tb³⁺ crystals occurs according to a cascade scheme, which leads to quenching of the ⁵ D ₃ level. The calculated data agree well with the known experimental rates of multiphonon nonradiative transitions for Dy:KPC, Nd:KPC, Er:KPC, Tb:KPB, and Nd:KPB crystals. It is shown that transitions in the near-IR (3–6 μm) region in double halide crystals (MPb₂Hal₅) are almost unquenched and the rates of nonradiative relaxation of excited levels spaced by energy gaps ΔE _ji > 1000 cm^–1 are W _ji ^NR < 10³s^–1. This circumstance suggests that it is possible to obtain stimulated emission in KPb₂Cl₅:RE³⁺ crystals in the IR spectral region up to 6 μm.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Excitation and emission processes of Tb in porous anodic alumina

The strong photoluminescence (PL) of porous anodic alumina (PAA) with terbium deposition is reported. PAA, which has a regular pore morphology, is considered an effective template for fabricating luminescent composites. Tb was deposited onto PAA films by immersion in alcoholic solution with terbium chloride followed by heat treatment. The PL spectra demonstrate typical bands of Tb³⁺ corresponding to ⁵D₄ → ⁷F_j (j = 3, 4, 5, 6,) electron transition, with the maximum at 18,360 cm⁻¹ (⁵D₄ → ⁷F₅). The PL mechanism of Tb³⁺ was systematically studied with annealing temperature. The non-radiative relaxation channel is provided by OH hydroxyls at the surface of porous anodic alumina and, after annealing at 900 °C, the PL yield is highly improved. The PL intensity of Tb³⁺ increases with laser power and a saturation phenomenon, associated with the ratio of Tb³⁺ to Tb⁴⁺ ions, is observed at approximately 90 W/cm². Based on a theoretical model, the optical cross-section σ of terbium in PAA is estimated, with a value close to that of other porous materials doped with the rare-earth elements.     

D3→FJ emission of Tb doped sol–gel silica

Amorphous silica samples doped with 0.1 and 1 mol% of terbium (Tb) were synthesized by the sol–gel method. In addition to the green light associated with ⁵D₄→⁷F_J transitions of Tb³⁺, the sample containing 0.1 mol% also emitted blue light as a result of ⁵D₃→⁷F_J transitions during photoluminescence (PL) measurements. As a result of concentration quenching this blue emission was not observed for the samples doped with the higher concentration (1 mol%). However the blue ⁵D₃ →⁷F_J emission was observed in the 1 mol% doped samples during cathodoluminescence (CL) measurements. Since a rough calculation indicated that the excitation rate in the CL system where the blue emission is observed may be similar to a laser PL system under conditions where the blue emission is not observed, the difference is attributed to the nature of the excitation sources. It is suggested that during the CL excitation incident electrons can reduce non-luminescent Tb⁴⁺ ions in the silica, substituting for Si⁴⁺ ions, to the excited (Tb³⁺)? state and that these are responsible for the blue emission, which does not occur during PL excitation.     

Cathodoluminescence properties of Tb3+-doped Na3YSi2O7 phosphors

Tb³⁺-doped Na₃YSi₂O₇ phosphors were prepared by the sol–gel method and then characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, and cathodoluminescence spectroscopy. The XRD results reveal that the Tb³⁺ ions have been introduced as dopants into the Na₃YSi₂O₇ host lattice. Under low-voltage electron beam excitation, the phosphors exhibit the characteristic emissions of Tb³⁺ (⁵ D _3,4→⁷ F _J , J=3–6 transitions). The luminescence color of the phosphors can be tuned from greenish-blue to bluish-green and to green by controlling the Tb³⁺ concentration within the 0.0005–0.15 (x value). The optimum Tb³⁺ doping concentration is 10 mol%, and the “dead voltage” is approximately 1.35 kV. All results indicate that the sample is a phosphor candidate for field-emission displays.     

The effects of added foreign ions in Gd2O2S : Tb; crystal field calculations, lifetimes, photo-luminescence and absorption spectra

A spectroscopic study is carried out in which the effects of added Ca²⁺ and Ru⁴⁺ transition metal ions on some characteristics of the emission of Gd₂O₂S : Tb³⁺ phosphors (energy levels, intensities, lifetimes) are examined and compared. In order to distinguish the Tb³⁺ emissions from impurity ones, the electronic energy levels of trivalent terbium are determined and the energy level scheme is completed by a crystal field analysis. The optical spectra reveal no terbium doped impurity phase; however, other rare earth ions present as impurities in the starting materials are detected. They are identified, and the influence of the added Ca²⁺ and Ru⁴⁺ on their emission lines is also examined.