Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001     

Level-Crossing-Untersuching der Stark-Aufspaltung der tiefsten1 P 1-Terme von Ca,Sr und Ba

Using the level-crossing technique, magnitude and sign of the tensor polarizability α_ten of the lowest¹ P ₁-levels of Ca, Sr, and Ba have been measured by investigating the splitting into Zeeman-sublevels in superimposed homogeneous electric and magnetic fields. The experimental results are: α_ten/g _J =?13.6(3) kHz/(kV/cm)² for Ca α_ten/g _J =?14.32(15) kHz/(kV/cm)² for Sr α_ten/g _J =?10.72(10) kHz/(kV/cm)² for Ba, where theg _J -values are approximatelyg _J≈1. With greater accuracy the ratios of these constants have been determined by eliminating the electric-field strength measurement: (α_ten(Sr)/g _J (Sr))/(α_ten(Ca)/g _J (Ca))=1.050(8) (α_ten(Sr)/g _J (Sr))/(α_ten(Ba)/g _J (Ba))=1.336(2). The polarizability constants are discussed with regard to the state vectors of the lowestsp ¹ P ₁- and³ P ₁-levels. The tensor polarizability of the 6s 6p¹ P ₁-level of Ba I is considered in relation to oscillator strengths of electric dipole transitions to neighbouring levels.     

Effect of Ba substitution for Sr in Bi2Sr2CaCu2O y superconducting films prepared by rf-sputtering method

In order to investigate the effect of the distance between the CuO₂ layer and the SrO layer, d _Cu-Sr, on the superconductivity in the Bi₂Sr₂CaCu₂O _y system, we have substituted Ba for Sr. Bi-(Sr, Ba)-Ca-Cu-O superconducting thin films were prepared by the rf-magnetron sputtering method. Rietveld analysis was employed to estimate d _Cu-Sr. Samples with various d _Cu-Sr were obtained by changing the post-annealing temperature as well as the Ba concentration. T_c was found to increase with increasing d _Cu-Sr.     

New parametric scattering in photorefractive Sr0.61Ba0.39Nb2O6:Cr

A new class of light-induced parametric scattering, not included in the conventional model, has been discovered in photorefractive Sr(0.61)Ba(0.39)Nb(2)O(6):Cr illuminated by two coherent beams. A novel model of multiwave mixing of coherent noise and transmitted light is developed to explain the new scattering phenomena. The model includes all known types and predicts a multitude of new types of parametric scattering. Generalized phase-matching conditions for parametric scattering in photorefractive crystals are proposed.     

Chemical diffusion of water in the double perovskites Ba4Ca2Nb2O11 and Sr6Ta2O11

The mechanism and kinetics of water incorporation in the double perovskites Ва₄Ca₂Nb₂O₁₁ and Sr₆Ta₂O₁₁ has been investigated (T = 300÷500 °C and aH₂O = 1 · 10^− 3÷2.2 · 10^− 2). The formation of hydration products Ba₄Ca₂Nb₂O₁₁·xH₂O and Sr₆Ta₂O₁₁·xH₂O (0.2 < x < 0.50) was limited by the diffusion of H₂O. It has been found that the concentration dependences of D˜_H2O are the same for both samples: small increasing of D˜_H2O with increasing x. The temperature dependences of the chemical diffusion coefficients of water for compositions of Ba₄Ca₂Nb₂O₁₁·0.35H₂O and Sr₆Ta₂O₁₁·0.35H₂O could be described with close activation energies of E_a = 0.38 ± 0.03 eV and E_a = 0.49 ± 0.03 eV, respectively. The chemical diffusion coefficients of water are nearly one order of magnitude smaller for tantalate Sr₆Ta₂O₁₁. This result correlates with lower oxygen and proton conductivities in Sr₆Ta₂O₁₁ as the consequence of lower mobilities.     

Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 doubly doped with Ce and Cr: PHoto- and thermoluminescence investigations

Congruent Sr x Ba 1 m x Nb 2 O 6 (SBN, x=0.61) doped with Ce or Cr ions exhibits enhanced photorefractive properties and new spectral features like increased red sensitivity. Here special emphasis is placed on the luminescence features of doubly doped Ce+Cr SBN crystals. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers und their recombination. The well separated thermo-luminescence peaks detected and their spectral line shape in emission point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb 4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb 4+ polarons and of VIS-centers are estimated to be 0.18 - 0.02 v eV and 0.30 - 0.05 v eV respectively. Possible excitation and recombination mechanisms in SBN:Ce+Cr are discussed.     

Comparison of emission and absorption core profiles of Sr and Ba resonance lines in an air-acetylene flame

Under optically thin conditions, the core of emission-line profiles of the resonance lines (¹P₁-¹S₀ transition) of Sr and Ba have been measured with a pressure-scanned Fabry-Pe´rot interferometer in a laminar, pre-mixed, shielded air-acetylene flame at 1 atm (T = 2275 K). Under identical experimental conditions, where Doppler and collisional broadening are almost equally important, these profiles are compared with Zeeman-scanned absorption profiles of the same resonance lines. The values for the full width at one-half maximum (FWHM) and the shift of the emission and absorption core profiles for Sr and Ba proved to be identical within an experimental scatter of 5 and 20%, respectively. Obviously the radiation emitted by the Sr and Ba atoms in the flame is thermal, which means that the spectral shape of the line is consistent with Maxwell-Boltzmann equilibrium.     

Raman spectroscopy of BaTiO3/(Ba,Sr)TiO3 superlattices

A series of BaTiO₃/Ba_{1 ? x} Sr _x TiO₃ (BT/BST) superlattices were prepared by pulsed laser deposition on MgO substrates with a constant period of 80 Å (40 Å BT and 40 Å BST) and varying compositions of the BST layer so that the Ba/Sr concentration ratios were 0/100, 30/70, 40/60, 50/50, 60/40, 70/30, 100/0. The soft mode E(1TO) of the polarized Raman spectra transformed depending on the Ba/Sr ratio in the BST layer. As the Sr concentration in the BST layers increased from 0 to 100%, the E(1TO) soft mode half-width varied from 171 to 103 cm^?1 and its frequency increased from 31 to 109 cm^?1 due to the interaction between the epitaxial layers forming the superlattices.     

Effect of gamma irradiation on the dielectric response of a Sr0.75Ba0.25Nb2O6 relaxor single crystal

The effect of gamma irradiation from a ⁶⁰Co source at a dose D = 4.75 × 10⁴ R on the dielectric properties of a Sr_0.75Ba_0.25Nb₂O₆ (SBN-75) single crystal with a smeared phase transition is investigated in the low-and infralow-frequency ranges. The behavior of the dielectric response is compared with the behavior of the depolarization currents induced in unirradiated and irradiated samples of the Sr_0.75Ba_0.25Nb₂O₆ compound. The observed changes in the electrical properties of the Sr_0.75Ba_0.25Nb₂O₆ single crystal in the temperature range of the smeared phase transition are associated with the stabilization of the ferroelectric phase of the material in an internal bias field generated by radiation-induced defects.     

Dielectric relaxation and ac conductivity study of Ba(Sr1/3Nb2/3)O3

Single phase perovskite Ba(Sr_1/3Nb_2/3)O₃ ceramics was prepared using the columbite precursor method. X-ray diffraction (XRD) technique is used to verify the single phase formation. It stabilizes in hexagonal phase with lattice constants a=12.1243 Å and c=15.3747 Å. Impedance analysis shows distributed relaxation time. Single semicircular arc observed in complex impedance plot confirms that only semiconducting grains are contributing to the polarization. The scaling behavior of both Z″ and M″ infers that the dynamical processes are temperature independent. Comparison of the impedance and electrical modulus data shows that the bulk response has contributions from both localized, i.e. defect relaxation, and non-localized conduction, i.e. ionic or electronic conductivity. The ac conductivity of the ceramics at higher temperatures indicates NTCR (negative temperature coefficient of resistance) behavior like semiconductors. The ac conduction activation energies are estimated from Arrhenius plots and conduction is largely due to hopping process.     

Synthesis,Structure, and Dielectric Characteristics of Sr0.61Ba0.39Nb2O6 Single Crystals and Thin Films

Physics of the Solid State - The structure, lattice dynamics, and dielectric characteristics of Sr0.61Ba0.39Nb2O6 (SBN-61) single crystals and SBN-61/(001)MgO thin films have been studied. The...     

The Peierls instability and superconductivity in quasi-2-d La2-x(Ba,Sr)xCuO4 compounds

The Peierls instability in the quasi-2-d La_2-x(Ba,Sr)_xCuO₄ is reexamined by taking the weak interlayer coupling into account. A two-step Peierls transition theory is developed, the general misunderstandings on the Peierls instability are therefore clarified, and a number of experimental anomalies observed in La₂CuO₄ are well explained. By suggesting an additional change of space group symmetries at the lower transition point, the calculated band structures are also intepreted. The possibility of CDW coexistence with the high-T_c superconductivity, the possible evidence for the 3-d superconductivity of the high-T_c materials are discussed with this theory.     

Preparation of porous (Ba,Sr)TiO3 by adding corn-starch

A new method of preparing porous (Ba,Sr)TiO₃ ceramics has been introduced, using an ordinary ceramics processing technique. The effect of corn-starch on the positive temperature coefficient of resistivity characteristics and microstructure of the porous (Ba,Sr)TiO₃ ceramics has been investigated. When the corn-starch addition was 1-20 wt%, the PTCR jump was over 10⁶ and 1-2 orders higher than that of samples without corn-starch. Also, it was found that the (Ba,Sr)TiO₃ ceramics had porous microstructure by the addition of corn-starch. The porosity of the ceramics with 20 wt% corn-starch was 44%. The electrical properties of the (Ba,Sr)TiO₃ ceramics have been discussed, based on the microstructure, resistivity of grain boundaries, donor concentration of grains and the electrical potential barrier of grain boundaries.     

Magnetic characterization of Ca substituted Ba and Sr hexaferrites

A magnetic characterization has been worked out for the solid solution from Ba and Sr hexaferrites (BaFe_{1 2}O_{1 9}, SrFe_{1 2}O_{1 9}) towards CaO-xFe₂O₃ (2 ? x ?5.5). Measurements of Curie temperature, saturation magnetization, magnetic anisotropy, together with Mössbauer characterization indicate that the intrinsic properties of the studied compounds do not change appreciably with increasing Ca content. These results, together with the X-ray data, are consistent with the formation of an undistorted M-type cell with a low content of iron and oxygen vacancies.     

Magneto-electrogyration in cubic Sr(NO3)2, Ba(NO3)2, and Pb(NO3)2

The simultaneous application of electric and magnetic fields on single crystals of Sr-, Ba-and Pb-dinitrate yields an additional term to the pure electrogyration and Faraday effect. The symmetric part of the fourth-rank tensor of this magneto-electrogyration has been completely determined with the aid of a high-resolution computer-aided polarimetric device. In Pb(NO₃)₂ a maximum magneto-electrogyration of about 10% of the magnitude of electrogyration is observed when a magnetic field of 1000 kA/m (1.2 Tesla) is applied along [111]. The effects in Sr(NO₃)₂ and Ba(NO₃)₂ are much smaller. Index of refraction, electrogyration, Faraday effect, and the new magneto-electrogyration obey the same sequence: Pb(NO₃)₂ > Ba(NO₃)₂ > Sr(NO₃)₂.     

Oxide ion conduction in Nd9.33(SiO4)6O2 and Sr2Nd8(SiO4)6O2 single crystals grown by floating zone method

Single crystals of Nd_9.33(SiO₄)₆O₂ and Sr₂Nd₈(SiO₄)₆O₂ oxide ion conductors with the oxyapatite structure were grown without any macroscopic defect by the floating zone method. The oxide ion conductivity of the Sr₂Nd₈(SiO₄)₆O₂ single crystal varied with the distance from its seed crystal along the growth direction, because of its changing deficiency in Sr content. Its stoichiometric portion had a lower electrical conductivity by about five orders of magnitude at 600 °C than the value for Nd_9.33(SiO₄)₆O₂ single crystal. Structural refinement of neutron diffraction data for powdered Nd_9.33(SiO₄)₆O₂ single crystal showed that the previous structure analysis was misleading. Cation vacancies were present only at 4f site and its channel oxygen site was fully occupied in space group of P6₃/m. The oxygen had an anisotropic displacement along the channel in the refined oxyapatite structure.     

Structural Characteristics of Sr0.61Ba0.39Nb2O6/MgO(001) Thin Films Grown by RF-Cathode Sputtering

Physics of the Solid State - Thin films of congruent composition of the system of solid solutions of barium-strontium niobates Sr0.61Ba0.39Nb2O6 (SBN:61) with thicknesses from 30 to 630 nm on a...     

Physical properties of double perovskite compounds ALaVMoO6 (A=Ca, Sr, Ba)—a possible half-metallic antiferromagnetic system

Double perovskite compounds ALaVMoO₆ (A=Ca, Sr, Ba) have been synthesized and their electrical and magnetic properties have been investigated. Magnetization measurements have indicated the possible antiferromagnetic transitions at 120 and 130 K for A=Ca and Sr samples, respectively. Electrical resistivity ρ for this system shows metallic temperature dependence from 300 to 20 K, though the sample with A=Ca shows weak semiconducting behavior in the low temperature region (<70 K). Considering the magnetic and electrical properties and assuming the V³⁺S=1 and Mo⁴⁺S=1 valence and spin states, the samples with A=Ca and Sr can be promising candidates for half-metallic antiferromagnets.     

Eu activated M6AlP5O20 (M=Sr/Ba/Mg) novel red phosphors

Eu³⁺ activated M₆AlP₅O₂₀ (where M=Sr/Ba/Mg) phosphors prepared by combustion synthesis and the completion of the synthesis was confirm by XRD (X-ray diffraction) patterns. The surface morphology studied by scanning electron microscopy (SEM) and photoluminescence (PL) properties has been reported in this paper. The Eu³⁺ PL emission spectrum was observed in M₆AlP₅O₂₀ phosphors (where M=Sr/Ba/Mg) at 592 (orange) and 618 nm (red) region, the spectrum due to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions at mercury free excitation, respectively. Its considerable emission intensity under 350 nm excitations makes it possible candidate materials as red component of tricolor luminescence materials and for near ultra violet light emitting diode (n-UVLED) phosphors.