Features of elastic phase interaction in the crystal Gd2(MoO4)3

The elastic interaction of paraelastic-paraelectric (¯42m) and ferroelastic-improper ferroelectric (mm2) phases coexisting in a Gd₂ (MoO₄)₃ crystal is investigated for a phase transition in the neighborhood of T_c=433 K. The energetic favorability of the monodomain state of the ferroelastic phase being formed is shown and the reasons for the impossibility of reaching the plane of zero mean strains on the phase interface boundary are analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 68–72, March, 1990.The authors are grateful to I. M. Vitebskii for useful discussion and valuable remarks.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.     

Gd2(MoO4)3:Er3+ Nanophosphors for an Enhancement of Silicon Solar-Cell Near-Infrared Response

In an attempt to take full advantage of near-infrared part of the solar spectrum, Gd₂(MoO₄)₃:Er³⁺ nanophosphors have been proposed as potential luminescent materials to enhance the response of the silicon solar-cell. Upon excitation with low-energy near-infrared photons, intense upconverted emissions at 545, 665, 800, and 980 nm, for which energies higher than the bandgap of silicon solar-cell, have been achieved with conversion efficiencies of 0.12%, 0.05%, 0.83%, and 1.35%, respectively. Development of nanophosphors for photovoltaic purposes could open up an approach in achieving high-efficiency silicon-based solar-cell by means of the up-conversion of the sub-bandgap near-infrared part of the solar spectrum (E < 1.12 eV) to visible/near-infrared photons.     

Eu3+掺杂Gd2W2O9和Gd2（WO4）3纳米荧光粉发光性质研究

采用共沉淀法制备了不同Eu3+掺杂浓度的Gd2W2O9和Gd2(WO4)3纳米发光材料.通过对纳米材料样品的X射线衍射谱(XRD)和场发射扫描电镜(FE-SEM)照片的观察和分析,对样品的结构和形貌进行了表征.测量了各样品的发射光谱、激发光谱,计算了各样品的部分J-O参数和Eu3+5D0能级量子效率,绘制了不同基质中Eu3+发光的浓度猝灭曲线,对Eu3+掺杂的Gd2W2O9和Gd2(WO4)3纳米发光材料的光致发光性质进行了研究.实验结果证明,与较常见的Gd2(WO4)3:Eu一样,Gd2W2O9:Eu中Eu3+5D0→7F2跃迁的红色发光也能被395nm和465nm激发光有效激发,具有近紫外(蓝光)相对激发效率高,猝灭浓度大的优点,有潜力成为高效的近紫外(蓝光)激发白光LED用红色荧光粉材料.     

Spectral properties of Pr3+ ions in Bi2(MoO4)3 crystal

A Bi₂(MoO₄)₃ single crystal doped with Pr³⁺ ions has been grown by the Czochralski technique. The polarized absorption and fluorescence spectra as well as the fluorescence decay curve of Pr³⁺ ions in the crystal were measured at room temperature. The spectroscopic parameters, including the Judd–Ofelt intensity parameters Ω_t (t=2, 4, and 6), spontaneous emission probabilities, fluorescence branching ratios, radiative lifetimes, stimulated emission cross sections, and fluorescence quantum efficiencies, were calculated. The spectral properties related to laser performance of this crystal were analyzed. The ¹ D ₂ multiplet of the crystal may be a good upper level for a solid-state laser.     

Electrooptical and photoelastic phenomena in ferroelectric crystals (survey)

Relations between optical and ferroelectric properties are discussed as a basis for analyzing and generalizing this author's and other authors' research data pertaining to the electrooptical, the photoelastic, and other effects in ferroelectric crystals.     

High‐pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd₂(MoO₄)₃] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure‐transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure‐transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO₄ stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short‐range and long‐range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure‐transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A₁ symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm⁻¹, soften continuously through the first three phases when increasing pressure. The strong A₂ mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm⁻¹ at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the Mo O stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

The IR and Raman spectra of Tb0.2Dy0.9Gd0.9(MoO4)3

The IR and Raman spectra of the ternary molybdate Tb_0.2Dy_0.9Gd_0.9(MoO₄)₃ have been recorded and analysed on the basis of the vibrations of MoO²⁻₄ ions. The splitting of the non-degenerate Mo—O symmetric stretching mode due to the vibrational interaction between neighbouring ions is observed. The large splitting observed for the v₃(F₂) mode is probably due to the large number of MoO²⁻₄ ions and strong interlayer coupling present in the unit cell.     

X-ray and dielectric studies of Sm2(MoO4)3

The polycrystalline samples of Sm₂(MoO₄)₃ have been prepared by heating appropriate quantities of oxides. The X-ray powder diffraction technique has been used to provide information about lattice parameter, space group and basic structure. The dielectric constant has been measured as a function of frequency (560 Hz-13 MHz) and temperature (30–220°C) to know the ferroelectric transition temperature.     

Acoustooptical and elastic properties of laminated KY(MoO4)2 crystals

Data on acoustic (absorption and velocity of sound), optical (refractive index and optical absorption coefficient), and photoelastic (coefficients of acoustooptical quality and photoelastic constants) properties of KY(MoO₄)₂ crystals are obtained. It is shown that, not only does the anisotropy of binding forces lead to a significant anisotropy of acoustic and photoelastic properties, but it also determines anomalously high elastic nonlinearity in the direction of the Y-axis perpendicular to cleavage planes.     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

6Li and 7Li MAS NMR spectra of complex lyonsite-type Li2Zn2(MoO4)3 and LiRb3Hf2(MoO4)6 lithium molybdates

⁶Li and ⁷Li MAS NMR spectra of complex Li₂Zn₂(MoO₄)₃ and LiRb₃Hf₂(MoO₄)₆ molybdates of lyonsite-type structure are studied at temperatures ranging from 25 to 100°C. NMR signals are attributed to their sources. It is shown that the presence of a transition metal such as Hf in the +4 oxidation state in the structure of lyonsite contributes to the increased mobility of lithium ions along the channels of the crystal structure.     

基质发光材料Na5Eu(MoO4)4和NaEu(MoO4)2的喇曼光谱

本文研究了基质发光材料Na5Eu(MoO4)4和NaEu(MoO4)2的喇曼光谱.用群论方法分析了它们的内、外振动模式,对实验振动模进行了指认.本文还将上述喇曼光谱与Na5Eu(WO4)4、CaWO4、SrWO4、CaMoO4和SrMoO4的喇曼光谱进行了比较,Na5Eu(MoO4)4和NaEu(MoO4)2的喇曼光谱结果表明(MoO4)2-离子团的高能量拉伸内振动是产生Eu3+的5D2→5D1,5D1→5D0-多声子无辐射弛豫的原因,这导致了Eu3+在这类材料中主要产生5DO→7Fj的发光跃迁.     

Quantum efficiency of Nd-doped lasers measured by pump-induced crystal heating: application to the Nd3+:Gd2(MoO4)3 crystal

In this paper we show how the quantum efficiency of Nd³⁺-doped laser crystals can be measured by means of a very simple method. This method is based on a multiwavelength study of pump-induced crystal heating, and its major advantage is the simplicity of the required experimental set-up. It has been used to determine the quantum efficiency of the main infrared laser channel (⁴ F _3/2?⁴ I _11/2) of the Nd³⁺:Gd₂(MoO₄)₃ non-linear laser crystal. The value obtained for the quantum efficiency (φ=0.97) is in good agreement with that obtained from the Judd–Ofelt formalism (φ=0.95). Received: 25 October 2000 / Revised version: 2 January 2001 / Published online: 27 April 2001     

Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat C _total of crystalline and amorphous Eu₂(MoO₄)₃ is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of C _total for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 623–627 (25 October 1998)     

Optic characteristics of Pr3+ doped NaY(MoO4)2 crystal

The polarized absorption and emission spectra of Pr³⁺ ions in NaY(MoO₄)₂ single crystal were investigated. The standard and modified Judd–Ofelt theory have been applied to the measured optical absorption intensities to determine the spectroscopic parameters, including the Judd–Oflet intensity parameters Ω_t (t=2,4,6), the radiative transition rates, branching ratios, and emission cross-sections for various excited levels of Pr³⁺ ions. In samples with Pr³⁺ ions concentration of 2.00×10²⁰ cm^-3, the excitation of the ¹ D ₂ manifold decays non-radiatively by the electric dipole–dipole transfer between Pr³⁺ neighbors. The good spectroscopic properties show the possible application of the Pr³⁺ doped NaY(MoO₄)₂ crystal as a solid-state laser. PACS 42.70.Hj; 78.20.-e     

基质发光材料Na5Eu(MoO4)4和NaEu(MoO4)2的制备和发光

本文研究了化学计量的基质发光材料Na5Eu(MoO4)4和NaEu(MoO4)2.它们都属于四方晶系,空间群为I41/α .NaEu(Mo04)2属于白钨矿(scheelite)结构,一个晶胞中有2个分子式NaEu(MoO4)2(z=2);而Na5Eu(Mo4O)4属于类自钨矿(scheelite-like)结构,一个晶胞中有4个分子式Na5Eu(MoO4)4(z=4).X射线衍射确定的晶胞常数,对Na5Eu(MoO4)4,α=1.1439nm和c=1.1486nm(密度d计算=4.01g/cm3,z=4);对NaEu(MoO4)2,α=0.5236nm和c=1.1439nm(密度d计算=5.24g/cm3,z=2).由差热法和X射线衍射法确定了Na2MoO4-Eu2(MoO4)3的相图.Na5Eu(MoO4)4分解为NaEu(MoO4)2的温度是700℃.本文确定了制备纯 Na5Eu(MoO4)和NaEu(MoO4)2的方法,并研究了上述两种材料的激发光谱和发光光谱.确定了用X射线粉末衍射谱和发光光谱鉴别Na5Eu(MoO4)4和NaEu(MoO4)2的方法.在365nm紫外光激发下,Na5Eu(MoO4)4的发光比Na5Eu(WO4)4的强得多,在室温下,Na5Eu(MoO4)4,NaEu(MoO4)2和Na5Eu(WO4)4的相对发光积分亮度比为3.8:3.2:1.0.     

水热法制备的Li4-3xEux(MoO4)2微晶及其发光性能

采用水热法制备了Li_4-3xEu_{x(MoO4)2系列红色荧光粉.通过X射线衍射(XRD)、扫描电镜(SEM )和荧光分析(FL)对产物的微结构、形貌和发光性能进行表征分析.XRD分析表明,制备的Li4-3xEux(MoO4)2微晶均为白钨矿四方结构.SEM结果显示:随着x的增大,Li4-3xEux(MoO4)2微晶的晶粒尺寸相应减小,在0.2~0.5 μm之间变化.荧光分析结果表明:源于Eu³⁺的⁵D0→⁷F2和⁵D0→⁷F1电荷转移的592 nm和614 nm的特征发射峰显现明显,后者的发射强度远远大于前者.随着x的增大,样品中Eu³⁺的两个特征发射峰的强度先增大后减小,在x=1.0时达到最大.}     

Er3+掺杂NaAl(MoO4)2的荧光光谱与上转换发光研究

用液相沉淀法制备得到掺稀土离子Er3+和Ce3+的NaAl( MoO4)2,X射线衍射图样显示NaAl(MoO4)2是单斜晶体结构.掺稀土离子Er3+和Ce3+的NaAl(MoO4)2的拉曼光谱研究表明,掺Er3+离子的NaAl(MoO,)2比掺Ce3+离子具有更强荧光性能,掺杂Er3+离子的NaAl( MoO4)2晶...