Luminescence and recombination processes in bulky Li6Gd x Y1 − x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li₆Gd _x Y_{1 − x} (BO₃)₃:Eu have been investigated. The steady-state X-ray luminescence (XRL) spectra, the temperature dependences of the XRL intensity, and the thermally stimulated luminescence (TSL) spectra have been measured for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃:Eu, and Li₆Gd(BO₃)₃:Eu compounds in the temperature range of 90–500 K. It is established that the band at 312 nm, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ ions and the group of lines at 580–700 nm, which are due to the ⁵ D ₀ → ⁷ F _J (J = 0 … 4) transitions in Eu³⁺ ions dominate in the XRL spectra. The XRL intensity in these bands increases several times with a change in temperature from 100 to 400 K for undoped crystals. The likely mechanisms of the observed temperature dependence of the XRL intensity and their relationship with the features of electronic-excitation energy transfer in these crystals are discussed. The spectra of all crystals under study exhibit a dominant composite TSL peak with a maximum at 110–160 K and a series of weaker peaks, the composition and structure of which depend on the crystal type. The nature of the shallow traps, which are responsible for the TSL at temperatures below room, and their relationship with the defects of lithium cation sublattice are discussed.     

Temperature dependence of the luminescence of Li6Gd x Y1 ? x (BO3)3:Eu crystals

The luminescence and recombination processes in crystals of lithium borates Li₆Gd _x Y_{1 ? x} (BO₃)₃:Eu (LGYBO:Eu) have been studied. The photoluminescence (PL) spectra upon selective photoexcitation to lower excited states of Gd³⁺ and Eu³⁺ ions and the temperature dependences of the photoluminescence intensity at different energies of excitation photons have been measured in a wide temperature range: 10?C500 K. We observed a photoluminescence band at 3.97 eV, which is due to the ⁶ P _J ?? ⁸ S _7/2 transitions in Gd³⁺ ions, and a characteristic line spectrum in the range of 1.6?C2.2 eV, which is related to the radiative f-f transitions in impurity Eu³⁺ ions, occurring mainly from the lower excited ⁵ D ₀ level to the ⁷ F _J states (J = 0, 1, ??, 6). The influence of the O-Eu charge transfer states and the vibrational relaxation between the ⁶ I _J and ⁶ P _J levels of the Gd³⁺ ion on the temperature dependences of the intrinsic and impurity luminescence intensities is discussed.     

Specific heat of Eu x Sr1−x Te

In this paper, we report on measurements of the specific heatC of single-crystalline Eu _x Sr_1–x Te at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the Néel temperatureT _N=9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent =–0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close toT _N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay ofC is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: becomes negative and decreases continuously towards –1 atxx _c. This concentration dependence of was previously observed in the diluted ferromagnetic system Eu _x Sr_1–x S. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentrationx lower than the critical concentrationx _c reveal spin-glass behavior in the specific heat. In addition, the dependence ofT _N on magnetic fields is discussed. The data yield a normalized magnetic phase boundaryB _c(T)/B_c(T=0) vs.T _N(B)/T_N(B=0) which is independent of concentration.     

Magnetic hyperfine fields in Y2Fe17−x Si x compounds

Y₂Fe_17–x Si _x compounds withx=0, 0.5, 1.0, 1.5, 2.0, 2.5 and3.0 were investigated by magnetic measurement andNMR. It is found that with increasing Si content the Curie temperatureT _C increases while the average Fe magnetic moment _Fe decreases. NMR study indicates that Si preferentially substitute the Fe atoms at 4f sites, which is responsible for the increase ofT _C.     

Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_{1 ? x} Ca _x B₆ (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) ? ρ(H))/ρ(H) ≈ 7 × 10⁵ detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_0.74Ca_0.26B₆ solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200?350 cm²/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_0.74Ca_0.26B₆ solid solution are comparable with the charge carrier mobility μH = 400?600 cm²/(V s) for the undoped EuB₆ compound. The anomalous behavior of the transport and magnetic parameters of the Eu_{1 ? x} Ca _x B₆ solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.     

Ionic conductivity in the system Li9−4xZrx〈PO4〉3

Lithium ion conductors of composition Li_9−4xZr_x〈PO₄〉₃ (0.0 < x < 2.0) have been prepared by the conventional solid state reaction and also by a wet chemical method. X-ray diffraction patterns reveal identical crystallographic morphology for the compounds prepared by the two methods. However, they can be prepared and sintered at much lower temperatures by the wet chemical method. Their conductivities are also higher. Compared to pure Li₃PO₄, one of the end member of the series, the conductivity increases sharply with substitution of lithium by zirconium. The highest conductivity has been obtained in the composition Li_1.8Zr_1.8〈PO₄〉₃ (x = 1.8), prepared by the wet chemical method. The conductivity value of 10⁻³ ω⁻¹ cm⁻¹ is obtained at around 225°C with an activation energy of about 0.5 eV.     

The NASICON solid solution Li1−x La x /3Zr2(PO4)3: optimization of the sintering process and ionic conductivity measurements

The Li_1−x La _{x /3}Zr₂(PO₄)₃ NASICON-type compounds (0 ≤ x ≤ 1) have been synthesized in powder form by a sol-gel method and sintered for ionic conductivity measurements. In order to improve the compactness of the ceramic without decomposition of the compound, several sintering processes have been tested for one member of the solid solution (x = 0.6): the use of sintering aids (ZnO, B₂O₃, TiO₂ and LiNO₃), a ball-milling of the synthesized powder, a flash heating, high isostatic pressure, and spark plasma sintering. Finally, a satisfactory compactness of 85% is obtained compared to the referenced value (63%) obtained by uniaxial and isostatic pressing. The ionic conductivity study was performed by impedance spectroscopy. It shows that, despite the formation of vacancies, the substitution Li⁺→ 1/3 La³⁺ + 2/3 □ has unfortunately no influence on the conduction for 0 ≤ x ≤ 0.7 since the ionic conductivity remains identical to the LiZr₂(PO₄)₃ one. For higher x values, the ionic conductivity strongly decreases.     

Luminescence of Eu2+–vc− dipoles and their associates in CsI:Eu crystals

Spectral-kinetics properties of photo-scintillation excited with single light pulses of a nitrogen laser (λ=337.1 nm, t_1/2=5 ns, Q=1 mJ) have been studied in CsI:Eu crystals at temperature within 80–300 K. It is found that the exponential decay of 463 nm emission band has a time constant which grows from 0.85 μs at 78 K to 1.6 μs at 380 K. Such an anomalous temperature behavior of 463 nm emission decay kinetics is discussed in terms of the crystal thermal expansion. It has been proposed that 463 nm emission is caused by a cluster center consisting of three dipoles Eu²⁺v_c^? bounded with each other in a hexagon. Owing to the exchange resonance in the cluster, the energy passes from an excited dipole to a non-excited one and the distance between them gets longer due to thermal expansion of the crystal.     

Thermoelectric properties of the Nd2−x Ce x CuO4−y system

The temperature dependence of the Seebeck coefficient (in region 300–900 K) and the lattice constants of Nd_2–x Ce_xCuO_4–y (x=0, 0.05, 0.10, 0.15) with different oxygen content were measured. The value of S is always negative and it decreases with both Ce content and oxygen non-stoichiometry. At a certain concentration of Ce and oxygen in the material, the Seebeck coefficient becomes temperature-independent.The authors would like to thank Professor J. Horák of University of Chemical Technology, Pardubice for helpful discussions.     

Specific features of the electronic structure and spectral properties of NdNi5 − x Cu x compounds

The spectral properties of the intermetallic compounds NdNi_{5 ? x} Cu _x (x = 0, 1, 2) have been studied using optical ellipsometry in the wavelength range 0.22–16 μm. It has been established that substitution of copper atoms for nickel leads to noticeable changes in the optical absorption spectra, plasma frequencies, and relaxation frequencies of conduction electrons. Spin-polarized calculations of the electronic structure of these compounds have been performed in the local spin density approximation allowing for strong electron correlations (LSDA + U method) in the 4f shell of the rare-earth ion. The calculated electron densities of states have been used to interpret the experimental dispersion curves of optical conductivity in the interband light absorption region.     

On the magnetic susceptibility of Pd1−x Ag x in the range 0.5≦x≦1

The weak variation of the magnetic bulk susceptibility of Pd_1–x Ag _x with temperature T and silver mole fractionx within 0.5x1 has been investigated in the range 5KT400K. Experimental evidence can be given for an intersection point of the susceptibility isotherms (T=const,x) atx=0.55. The observed dependence of on T andx is interpreted by means of a semiphenomenological alloy susceptibility function (T,x).     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Spontaneous reorientation of the electric polarization in Eu1 − x Ho x MnO3 multiferroics

The magnetic, dielectric, and ferroelectric properties of Eu_{1 ? x} Ho _x MnO₃ single crystals (0 < x ≤ 0.5), where magnetic ordering can be varied from the canted antiferromagnetic phase to modulated spin structures, have been studied. It has been found that a ferroelectric state appears at x ≥ 0.2 and low temperatures. As the temperature decreases and the holmium content increases, the electric polarization in this state is reoriented from the a axis to the c axis. It has been shown that the polarization is reoriented owing to a change in the spin rotation plane in the cycloidal phase from the ab to cb plane because of the stabilization of the latter upon an increase in the rare-earth contribution to the anisotropy energy. The T-x phase diagram of magnetic and ferroelectric states has been constructed.     

Thermal conductivity of single crystals of the Ca1 − x Y x F2 + x solid solution

The thermal conductivity of single crystals of the solid solution of yttrium fluoride in calcium fluoride Ca_{1 ? x} Y _x F_{2 + x} with the fluorite structure (x ≤ 0.20) and the Ca_0.27Y_0.73F_2.73 phase with the tisonite structure has been studied by the absolute steady-state longitudinal heat flow method in the temperature range 50–300 K. It has been established that the thermal conductivity drops sharply with increasing yttrium trifluoride concentration, especially in the low-temperature region.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Mössbauer studies of the charge transfer process in the system (Li0.5Fe0.5)1−xFexFe2O4

Conclusion The formation of heterovalent pairs seems to be proved for the compounds rich in Fe²⁺ ions (x0.5) and the electron-phonon coupling seems to change with decreasing concentration of Fe²⁺ ions.Nevertheless the pairs model is not necessarily an unique interpretation of the data for x0.5 /1/ and the broadening of the Fe^m+ ions spectrum could be worked considering electronic and atomic disorders.     

Investigations of the Spin-Hamiltonian Parameters for the Rhombic Mo5+ Centers in Ca1−x Y x MoO4 Crystal

The spin-Hamiltonian parameters (g factors g _i and hyperfine structure constants A _i , where i = x, y, z) of the rhombic Mo⁵⁺ center in Ca_1?x Y _x MoO₄ crystal are calculated from the high-order perturbation formulas based on the two-mechanism model for the rhombic d¹ tetrahedral clusters with the ground state |d _z 2〉. In these formulas, besides the contributions due to the widely applied crystal-field (CF) mechanism concerning CF excited states, those due to the charge-transfer (CT) mechanism (which is omitted in CF theory) concerning CT excited states are considered. The calculated results are in reasonable agreement with the experimental values. The calculations show that because of the great relative importance of CT mechanism for the components of spin-Hamiltonian parameter along x and y axes, the accurate and complete calculations of spin-Hamiltonian parameters for Mo⁵⁺ and other high valence state dⁿ ions in crystals should take account of both the CF and CT mechanisms. The defect model of the rhombic Mo⁵⁺ center is also confirmed from the calculations.     

Ionic conductivity of Li4−2xMgxSiO4

Solid solutions Li_4−2xMg_xSiO₄ in the system Li₄SiO₄Mg₂SiO₄ were prepared at 1000°C. The single phase region extended up to x=0.5. The conductivity is much higher than that of the end member Li₄SiO₄, and passes through a maximum at Li_3.2Mg_0.4SiO₄ with values of 2.3 × 10⁻⁵ S cm⁻¹ at 200°C, rising to 1.5 × 10⁻² S cm⁻¹ at 400°C. The solid solutions are easy to prepare, are stable in air, and retain their conductivity over a long period of time.     

Dielectric nonlinearity of crystalline Li2−x NaxGe4O9 (x≈0.23)

The dielectric nonlinearity of ferroelectric Li_2−x Na_xGe₄O₉ (x≈0.23) crystals is measured in the neighborhood of the phase transition temperatures. The magnitude of the nonlinear coefficient β is estimated from the shift in T _c and the reduction in ɛ _max under the influence of E ₌, from the dielectric nonlinearity in the paraphase, and from the temperature dependence of P _s in crystalline Li_2−x Na_xGe₄O₉ (x≈0.23). The resulting values of β are 1.87, 1.26, 2.17, and 1.17×10⁻⁹ (CGSE cm²)⁻², respectively. The mechanism for the phase transition in crystalline Li_2−x Na_xGe₄O₉ (x≈0.23) is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1070–1072 (June 1999)