Effects of Nd substitution on the polarization properties and electronic structures of bismuth titanate single crystals

Single crystals of Nd-substituted Bi₄Ti₃O₁₂ ferroelectrics were grown by a self-flux method, and the effects of Nd substitution on the polarization properties and optical transmission spectra of the crystals have been investigated. Bi_3.52Nd_0.48Ti₃O₁₂ crystals showed a smaller remanent polarization along the a axis of 29 μC/cm² than Bi₄Ti₃O₁₂ crystals (46 μC/cm²), and this result is in good qualitative agreement with the calculation of the spontaneous polarization based on Rietveld analysis of neutron diffraction. Electronic band structure calculations using the structural data suggest that the orbital hybridization between the Nd 5d and O 2p states stabilizes oxide ions in the perovskite layers, which is consistent with the much smaller leakage current observed for Nd-substituted crystals.     

镥基闪烁晶体的研究进展

由于高能物理实验、核医学成像、安全检查和地质探矿等领域的迫切需要,具有高密度、快衰减、高光输出和低成本等优良特性的闪烁晶体成为关注的焦点,特别是Ce~(3+)激活的镥(Lu)基化合物,其开发、研究和应用方兴未艾。简要综述了硅酸镥、氧化镥和铝酸镥等闪烁晶体的生长技术、闪烁性能和应用,并展望了镥基闪烁晶体的发展趋势。     

Spectral Absorptivity and Thermal Conductivity of BGO and BSO Melts and Single Crystals

The present work describes the results of spectral absorptivity, α, and thermal conductivity, λ, studies for compound oxides Bi₄Ge₃O_12, Bi₁₂GeO_20, Bi₄Si₃O_12, and Bi₁₂SiO₂₀ in molten and monocrystalline states. The data for the spectral absorptivity were obtained by placing the sample onto a mirror and using the transmission method. To obtain the data on the thermal conductivity of crystals, the stationary method of two identical samples was used. The data for the thermal conductivity of melts were obtained by a new stationary relative method in which the thermal conductivity of the crystal is used as a reference. Special attention is focused on numerical and experimental error analysis at high temperature. The studies have shown that α in the range of a transmission band strongly depends on crystal purity. It varies from 0.0005 cm⁻¹ to 0.03 cm⁻¹ for Bi₄Ge₃O₁₂ and reaches 0.15 cm⁻¹ for Bi₄Si₃O₁₂. It was found that α is significantly greater for melts than for crystals, reaching (150 to 200) cm⁻¹ for the Bi₄Ge₃O₁₂ melt. The thermal conductivity of the melts under investigation was found to be much smaller than that of the corresponding crystals.     

Growth,structure and infra-red spectra of CsRP4O12 crystals

CsRP₄O₁₂ (where R is a rare-earth ion) crystals have been grown from highly concentrated phosphoric acid solutions. X-ray and infra-red spectral studies were performed on these crystals. Structurally, the CsRP₄O₁₂ crystals can be divided into two types: ringtype and chain-type. The crystal chemistry of CsRP₄O₁₂ crystals has been briefly discussed and the absorption bands for representatives from each structural modification have been determined.     

Flux growth of LiNdP4O12 single crystals

LiNdP₄O₁₂ single crystals were grown by slow-cooling a Li₂OP₂O₅ flux. Crystal morphology and macroscopic crystal quality depend on the solution composition in the ternary system Li₂ONd₂O₃P₂O₅. Under the favorable growth conditions, platy crystals of 15 × 10 × 1 mm³ were obtained.     

Relaxation of optical excitations in crystals of the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system with radiation defects

The photoluminescence and short-time persistence (afterglow) kinetics in pure and doped Y₂O₃-Al₂O₃ crystals excited with UV laser pulses (12 ns, 337 nm) were studied using samples irradiated with gamma quanta from a ⁶⁰Co source to a dose from 10⁴ to 10⁷ Gy. The relaxation time of the samples studied increases, with decreasing symmetry of the crystal lattice, in the following order: garnet—orthoaluminate—ruby—yttria. The afterglow duration and intensity significantly decrease in gamma-irradiated crystals, which is explained by the predominant recombination of close electron-hole pairs. Garnet-neodymium crystals are characterized by high radiation stability and fast relaxation kinetics.     

Gamma luminescence of Pr<Superscript>3+</Superscript>- and Ce<Superscript>3+</Superscript>-doped yttrium aluminum garnet crystals

We have studied the gamma luminescence of undoped and Pr³⁺- or Ce³⁺-doped Y₃Al₅O₁₂ crystals gamma-irradiated at 77 and 300 K. The results demonstrate that, depending on temperature, three excitonassisted activator luminescence excitation mechanisms are possible in YAG crystals.     

Flux growth of single magnetic sublattice antiferromagnetic garnets Ca3 Fe2 Ge3 O12 and Ca3 Mn2 Ge3 O12

Magnetic single crystals garnets Ca₃ X₂ Ge₃ O₁₂ with X = Mn³⁺ or Fe³⁺ containing Ca²⁺ and Ge⁴⁺ are of great interest due to the rise of an antifer-romagnetic order in one definite octahedral site. The optimal conditions for obtaining single magnetic-sublattice garnets of large size (1 cm in diameter) have been analyzed. Two groups of solvents have been tested: Bi₂ O₃ or PbO based flux. The best results were obtained with PbO flux and starting composition : % Moles 44 PbO 1b 35 GeO₂ 1b 15 CaO 1b 6 X₂ O₃.     

Wavelength dependence of Verdet constant of Pr doped terbium gallium garnet crystal

Samples of the magneto-active material – Pr³⁺: Tb₃Ga₅O₁₂ crystals with Pr³⁺ ion concentration of 1%, 2%, 3% and 5%. The Verdet constant measurement has been carried out at room temperature in the 400–1500 nm range for all crystal samples and was compared with a pure Tb₃Ga₅O₁₂ material. A high value of the Verdet constant for 5% Pr³⁺: Tb₃Ga₅O₁₂ crystal was obtained at room temperature – namely, 324.5, 200.1 and 68.7 rad/(T·m) for 532, 632.8 and 1064 nm, respectively. The Verdet constant of Pr doped TGG crystal at 1064 nm is much more higher than that of TGG. The superior performance of the materials indicates that Pr³⁺: Tb₃Ga₅O₁₂ crystals have great potential to meet the increasing demand for magneto-optical devices in the VIS-NIR wavelength.     

Growth and characterization of Yb doped garnet crystals for scintillator application

Several concentration of Yb-doped Lu₃Al₅O₁₂ (Yb:LuAG) and Lu₃Ga₅O₁₂ (Yb:LGG) single crystals were grown by the micro-pulling-down method. The crystals were seeded-grown in the 1 1 1 direction and transparent and crack free crystals were obtained. Photoluminescence spectra and decay kinetics of these crystals were studied. Charge transfer luminescence of Yb³⁺ was observed in both crystals. Mean decay time of about 25 ns at 90 K and strong thermal quenching at room temperature was measured for Yb 5%:LuAG. Radioluminescence intensity was compared to the standard BGO sample.     

High resolution X-ray diffraction studies on unirradiated and irradiated strontium hexaferrite crystals

High-resolution X-ray diffraction technique, employing a three-crystal monochromator?Ccollimator combination is used to study the irradiation induced defects in flux grown Sr-hexaferrite crystals irradiated with 50?MeV Li³⁺ ion beams at room temperature with a fluence value of 1 × 10¹⁴ ions/cm². The diffraction curves of the irradiated crystals suggest the possibility of creation of low angle grain boundaries and other point/clusters of defects causing amorphization in the irradiated crystals. The perfection of the irradiated and unirradiated (0001) cleaved surfaces of the crystals is studied using the bulk method of X-ray topography. The topographs supplement the findings suggestive of modifications in the crystalline quality of SrFe₁₂O₁₉ on irradiation with SHI of Li^3?+?. Etching of the (0001) cleaved surfaces in H₃PO₄ at 120°C suggests that the dissolution characteristics of the surfaces get affected on irradiation with SHI of Li^3?+?, besides supporting the findings of HRXRD and X-ray topography regarding modifications in the perfection of SrFe₁₂O₁₉ on irradiation.     

High temperature Raman spectroscopy of silver tetratungstate,Ag8W4O16

Polarized Raman spectra have been obtained on oriented single crystals of Ag₈W₄O₁₆ from 20°C to the melting point (620°C) and also of the melt, both in air and in an oxygen atmosphere. The temperature dependences of the low-frequency Ag⁺-ion vibrations show no anomalies, in contrast to those in ionic conductors such as α-AgI. Polarization versus temperature measurements have been made on the crystals under the same conditions as the Raman measurements. The results indicate that there is decomposition of the crystals in air which produces irreversible conducting phases. The oxygen atmosphere impedes this decomposition, and the results in this case confirm an earlier prediction that there is no crystallographic transformation of the crystals up to the melting point and that the (W₄O₁₆)^8? ions continue to be noncentrosymmetric in the melt.     

Compositional variation of photoluminescence from Mn doped MgAl2O4 spinel

Spinel (MgAl₂O₄) crystals doped with 1.0% Mn have been grown by floating zone (FZ) technique with various Mg compositions, x = MgO/Al₂O₃, from 0.2 to 1.0. Compositional variations of photoluminescence are evaluated for a fluorescence thermometer application using crystals grown. Strong photoluminescence (PL) peak is observed at λ from 512 to 520 nm from the crystals grown from compositions, x, from 0.3 to 1.0. Peak wavelength of PL increases linearly from 512 to 520 nm with x. Weak PL peaking at λ = 750 nm is also observed from the specimens. Compositional variations of PL are considered to be due to the variation of crystal field surrounding the Mn²⁺ ions. The variation of crystal field strength agrees with the compositional variation of lattice constant.     

Bridgman growth and defects of Nd<Superscript>3+</Superscript> : Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> laser crystals

Nd³⁺ : Sr₃Ga₂Ge₄O₁₄ crystals have been grown by the modified Bridgman method. The growth defects, such as striations, scattering particles and dislocations were investigated. Some featherlike striations were observed in as-grown crystals. EPMA analysis suggested that these inclusions were caused by the segregation of Nd₂O₃ from the melt. Chemical etching results showed that the dislocation density was in the range of 10³ ∼ 10⁵/cm².     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

Facetting and optical perfection in Czochralski grown garnets and ruby

Studies of ruby (Al₂O₃/Cr³⁺) and rare-earth aluminium garnet single crystals, in particular the mixed garnets formed between Y₃Al₅O₁₂ (YAG) and Dy₃Al₅O₁₂ (DyAG), have shown that the formation of facets on the solid/liquid interface, which give rise to a strained central core within the crystals, is dependent upon the shape of the solid/liquid interface. Development of the strained and facetted core can be prevented by modifying the growth conditions to produce a flat solid/liquid interface and as a result the optical perfection of the crystals is greatly improved. Certain crystals, e.g. DyAG, grow naturally with aflat interface, and in the present work this has been shown to be due to the optical absorption characteristics of this material. In other materials, e.g. YAG and ruby, the interface shape can be controlled by the rate at which the growing crystal is rotated. The changes in temperature gradient produced in a YAG melt by changes of crystal rotation rate have been measured, and their effect upon crystal perfection is described.     

Conductivity pre-exponential factors for some new superionic conductors

The pre-exponential factors obtained from the ionic conductivity studies on Na₂(La, Al)ZrP₃O₁₂, Na₂(La, Al)TiP₃O₁₂, NH₄Zr₂V₃O₁₂ and AlPO₄:Li⁺ have been analysed. The compensation law has been found to be valid for these materials indicating that the entropy is directly related to the activation energy. The 1/α vsβ plots show straight lines for most of the superionic materials except for a few and this variation has been discussed.     

The perfection of bridgman-grown Bi4Ge3O12 crystals

Single crystals of Bi₄Ge₃O₁₂ were grown by the vertical Bridgman technique. As shown by X-ray diffraction line profiles these crystals exhibit a very high degree of perfection. Bi₄Ge₃O₁₂ is used as detector material for high energy radiation. Very perfect crystals are preferred because defects play a prominent role in radiation damage.     

Surface crystallization and second-order optical non-linearity in Gd2O3–Bi2O3–B2O3 glasses

Some ternary Gd₂O₃–Bi₂O₃–B₂O₃ glasses are prepared, and crystallization behavior and second harmonic intensity are examined to develop new non-linear optical crystallized glasses. The glasses with Gd₂O₃ contents of 8–14 mol% have large densities of over 6 g/cm³ and large refractive indices of ～ 1.9. Transparent surface crystallized glasses consisting of two kinds of crystalline phases with different morphologies, i.e. plate shape and needle shape crystals, are fabricated by heat-treatment at temperatures between glass transition and crystallization temperatures. From second harmonic generation microscope observations, micro-Raman scattering spectra and XRD analyses, plate shape crystals are determined to be non-linear optical Gd_xBi_1KxBO₃ and needle shape crystals are Bi₃B₅O₁₂ having no second-order optical non-linearity. Since crystallized glasses consisting of Gd_xBi_1KxBO₃ crystals exhibit relatively strong SHGs, they have a high potential for application to light control devices.     

Synthesis and properties of RbRP4O12 crystals

We have investigated the system Rb₂O-R₂O₃-P₂O₅-H₂O within the temperature range 300 to 800° C. A composition diagram showing fields of crystallization of different phases is given. The crystal chemistry of these crystals are discussed briefly. The crystals were characterized by various methods: IR spectroscopy, differential thermal analysis, laser spectroscopy and others. RbNdP₄O₁₂ crystals have a greater number of polymorphic modifications than any other tetraphosphates.