The phase transition, hygroscopicity, and thermal expansion properties of Yb2-xAlxMo3O12

Materials with the formula Yb2-xAlxMo3O12 （x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8） were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffraction, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3012 solid solutions crystallize in a single monoclinic phase for 1.7 〈 x 〈 2.0 and in a single orthorhombic phase for 0.0 〈 x 〈 0,4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhonlbic phase transition temperature of A12Mo3012 can be reduced by partial substitution of A13＋ by Yb3＋, and the Yb2-zAlxMo3012 （0.0 〈 x 〈 2.0） materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of A13＋ for Yb3＋ in Yb2Mo3012 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be -9.1 x 10-6/K, -5.5 x 10-6/K, 5.74 x 10-6/K, and 9.5 x 10 6/K for Ybl.sAlo.2（MoO4）3, Yb1.6Alo.4（MoO4）3, Ybo.4All.6（Mo04）3, and Ybo.2Al1.8（MoO4）3, respectively.     

Thermal properties of high-k Hf_1-xSi_xO₂

Classical atomistic simulations based on the lattice dynamics theory and the Born core-shell model are performed to systematically study the crystal structure and thermal properties of high-k Hf1-xSixO2 . The coefficients of thermal expansion, specific heat, Grneisen parameters, phonon densities of states and Debye temperatures are calculated at different temperatures and for different Si-doping concentrations. With the increase of the Si-doping concentration, the lattice constant decreases. At the same time, both the coefficient of thermal expansion and the specific heat at a constant volume of Hf1-xSixO2 also decreases. The Grneisen parameter is about 0.95 at temperatures less than 100 K. Compared with Si-doped HfO2 , pure HfO2 has a higher Debye temperature when the temperature is less than 25 K, while it has lower Debye temperature when the temperature is higher than 50 K. Some simulation results fit well with the experimental data. We expect that our results will be helpful for understanding the local lattice structure and thermal properties of Hf1-xSixO2 .     

Phase transition and thermal expansion property of Cr_(2-x)Zr_(0.5x)Mg_(0.5x)Mo_3O_(12) solid solution

Compounds with the formula Cr2-xZr0.5xMg0.5xMo3O12（x = 0.0, 0.3, 0.5, 0.9, 1.3, 1.5, 1.7, 1.9） are synthesized, and the effects of Zr4＋ and Mg2＋ co-incorporation on the phase transition, thermal expansion, and Raman mode are investigated. It is found that Cr2-xZr0.5xMg0.5xMo3O12 crystallize into monoclinic structures for x 〈 1.3 and orthorhombic structures for x _〉 1.5 at room temperature. The phase transition temperature from a monoclinic to an orthorhombic structure of Cr2Mo3O12 can be reduced by the partial substitution of （ZrMg）6＋ for Cr3＋. The overall linear thermal expansion coefficient decreases with the increase of the （ZrMg）6＋ content in an orthorhombic structure sample. The co-incorporation of Zr4＋ and Mg2＋ in the lattice results in the occurrence of new Raman modes and the hardening of the symmetric vibrational modes, which are attributed to the MoO4 tetrahedra sharing comers with ZrO6/MgO6 octahedra and to the strengthening of Mo-O bonds due to less electronegativities of Zr4＋ and Mg2＋ than Cr3＋, respectively.     

Fe2-xYxMo3O12系列材料的相变及负膨胀性研究

本文对Fe2-xYx(MoO4)3(x=0.0,0.2,0.4,0.5,0.6,0.8,1.0,1.2,1.4,1.6,1.8,2.0)系列材料的相变及负膨胀性能进行了研究.通过对Fe2-xYx( MoO4)3系列材料的XRD和拉曼谱的分析发现,当x≤0.4时Fe2-xYxMo3O12在常温下是单斜结构;当x≥0.5时...     

Near-zero thermal expansion of In_(2(1-x))(HfMg)_xMo_3O_(12) with tailored phase transition

Solid solutions of In_(2(1-x)(HfMg)_xMo_3O_(12) are synthesized by solid state reaction with the aim to reduce the phase transition temperature of In_2Mo_3O_(12) and improve its thermal expansion property.The effects of(HfMg)~(6+) incorporation on the phase transition and thermal expansion are investigated.It is shown that the monoclinic-to-orthorhombic phase transition temperature obviously decreases and the coefficient of thermal expansion(CTE) of the orthorhombic becomes less negative and approaches to zero with increasing the content of(HfMg)~(6+).A near zero thermal expansion covering the case at room temperature(RT) is achieved for the solid solutions with x ≥ 0.85,implying potential applications of this material in many fields.     

ZrV2-xPxO7固溶体的相变与热膨胀性质的研究

本文采用固相烧结法制备了ZrV2-xPxO7@=0,0．2,0．4,0．6,0．8,1）系列材料．粉末X射线衍射（XRD）分析表明,所制备材料的结构为单一立方相．应用变温拉曼光谱研究该材料相变,变温拉曼光谱研究结果表明,材料起始相变温度随着P5＋替代V5＋量的增加逐渐降低,x=0,0．4,0．8,1对应的相变温度分别为383K,363K,273K,213K．热膨胀测试结果表明：随着P替代量的增加正一负膨胀转变温度先降低后增加,x=0,0．2,0．4,0．6,0．8,1对应的正一负膨胀转变温度分别为429K,403K,372K,390K,398K和435K．本文指出了该系列材料存在两个相变过程,为设计和制备ZrV2O7基室温附近的负热膨胀材料奠定了良好的基础．     

The effect of intragranular microstress in Al₂O₃-SiC nanocomposites

A theoretical model is established to investigate the intragranular particle residual stress in Al2O3-SiC nanocomposites.Using this model,we calculate the average compressive stress on the Al2O3 grain boundary(GB) and the average tensile stress within Al2O3 grains caused by SiC nanoparticles.The normal compressive stress strengthens the GB,and the average tensile stress weakens the grains.The model gives a reasonable interpretation of the strength changes of Al2O3-SiC nanocomposites with the number of SiC particles.     

Photonic generation of stable microwave signals from a dual-wavelength Al₂O₃:Yb³⁺ distributed-feedback waveguide laser

We report the fabrication and characterization of a dual-wavelength distributed-feedback channel waveguide laser in ytterbium-doped aluminum oxide. Operation of the device is based on the optical resonances that are induced by two local phase shifts in the distributed-feedback structure. A stable microwave signal at ～15 GHz with a -3?dB width of 9 kHz was subsequently created via the heterodyne photodetection of the two laser wavelengths. The long-term frequency stability of the microwave signal produced by the free-running laser is better than ±2.5 MHz, while the power of the microwave signal is stable within ±0.35 dB.     

Influence of the Jahn-Teller distortion on magnetic ordering in TbMn_1-xFe_xO₃

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3(x = 0,0.02,0.05,0.10,and 0.20) single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring(NN) FM interaction dominant over the next nearest neighbor(NNN) AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

The interplay between the lattice and magnetism in La(Fe_11.4Al_1.6)C_0.02 studied by powder neutron diffraction

The crystallographic structure and magnetic properties of La(Fe 11.4 Al 1.6 )C 0.02 are studied by magnetic measure- ment and powder neutron diffraction with temperature and applied magnetic field. Rietveld refinement shows that La(Fe 11.4 Al 1.6 )C 0.02 crystallizes into the cubic NaZn 13 -type with two different Fe sites: Fe I (8b) and Fe II (96i), and that Al atoms preferentially occupy the Fe II site. A ferromagnetic state can be induced at a medial temperature of 39 K–139 K by an external magnetic field of 0.7 T, and a large lattice is correspondingly found at 100 K and 0.7 T. In all other conditions, La(Fe 11.4 Al 1.6 )C 0.02 has no net magnetization in the paramagnetic (T > T N = 182 K) or antifer- romagnetic states, and thus keeps its small lattice. Analysis of the Fe–Fe bond length indicates that the ferromagnetic state prefers longer Fe–Fe distances.     

Pressure-induced structural transition and thermodynamic properties of RhN₂ and the effect of metallic bonding on its hardness

The elastic constant,structural phase transition,and effect of metallic bonding on the hardness of RhN 2 under high pressure are investigated through the first-principles calculation by means of the pseudopotential plane-wave method.Three structures are chosen to investigate for RhN 2,namely,simple hexagonal P6/mmm(denoted as SH),orthorhombic Pnnm(marcasite),and simple tetragonal P4/mbm(denoted as ST).Our calculations show that the SH phase is energetically more stable than the other two phases at zero pressure.On the basis of the third-order Birch-Murnaghan equation of states,we find that the phase transition pressures from an SH to a marcasite structure and from a marcasite to an ST structure are 1.09 GPa and 354.57 GPa,respectively.Elastic constants,formation enthalpies,shear modulus,Young’s modulus,and Debye temperature of RhN 2 are derived.The calculated values are,generally speaking,in good agreement with the previous theoretical results.Meanwhile,it is found that the pressure has an important influence on physical properties.Moreover,the effect of metallic bonding on the hardness of RhN 2 is investigated.This is a quantitative investigation on the structural properties of RhN 2,and it still awaits experimental confirmation.     

The influence of interfacial barrier engineering on the resistance switching of In₂O₃:SnO₂/TiO₂/In₂O₃:SnO₂ device

The I–V characteristics of In2O3:SnO2/TiO2/In2O3:SnO2 junctions with different interfacial barriers are inves- tigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interfacial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interfacial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.     

Sc2－xGaxW3O12体系负热膨胀性能研究

通过固相反应法,在1100 ℃下成功制备出了系列Ga掺杂Sc_2-xGa_xW₃O₁₂（x=0, 0.05, 0.1, 0.2, 0.3, 0.5, 0.8）固溶体.X射线粉末衍射结构精修表明,Ga以替代Sc的形式成功进入Sc_2-xGa_xW₃O₁₂晶格,但不能获得端元组分Ga₂W₃关键词： 负热膨胀 热膨胀系数 Rietveld结构精修     

The magnetic properties of diluted CoFe₂O₄ nanomaterials

The magnetic properties of (Cox Fe1-x )A(Zn1-xFe1+x) BO4 are studied using mean-field theory and the probability distribution law to obtain the saturation magnetization, the coercive field, the critical temperature, and the exchange interactions with different values of D (nm) and x. High-temperature series expansions (HTSEs) combined with the Pad′e approximant are used to calculate the critical temperature of (Cox Fe1-x) A (Zn1-x Fe1+x) BO4 , and the critical exponent associated with magnetic susceptibility is obtained.     

Zero and controllable thermal expansion in HfMgMo_(3-x)W_xO_(12)

HfMgMo_(3-x)W_xO_(12) with x = 0.5, 1.0, 1.5, 2.0, and 2.5 are developed with a simple solid state method. With increasing the content of W, solid solutions of Hf Mg Mo3-xWx O12 crystallize in an orthorhombic structure for x≤2.0 and a monoclinic structure for x2.0. A near-zero thermal expansion(ZTE) is realized for HfMgMo_(2.5)W_(0.5)O_(12) and negative coefficients of thermal expansion(NCTE) are achieved for other compositions with different values. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO_4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and x-ray diffraction.     

Effects of W~(6+) occupying Sc~(3+) on the structure,vibration, and thermal expansion properties of scandium tungstate

We experimentally investigate effects of W~(6+)occupying the sites of Sc~(3+)in the unit cell of Sc_2 W_3 O_(12)(Sc_8 W_(12) O_(48))on the structure, vibration and thermal expansion. The composition and structure of the doped sample(Sc_6 W_2)W_(12) O_(48±δ)(with two W~(6+)occupying two sites of Sc~(3+)in the unit cell of Sc_8 W_(12) O_(48)) are analyzed and identified by combining the x-ray photoelectron spectroscopy and the synchronous x-ray diffraction with first-principles calculations based on density functional theory. Results show that the crystal with even W~(6+)occupying even Sc~(3+)in the unit cell is stable and maintains the orthorhombic structure at room temperature. The structure of the doped sample is similar to that of Sc_2 W_3 O_(12), and with even W occupying even positions of Sc in the unit cell and constituting the WO_6 octahedra. Raman analyses show that the doped sample possesses stronger W–O bonds and wider Raman linewidths than those of Sc_2 W_3 O_(12). The sample doped with W also exhibits intrinsic negative thermal expansion in the measured range of 150 K–650 K.     

First-order character of the displacive structural transition in BaWO₄

Nearly all displacive transitions have been considered to be continuous or second order,and the rigid unit mode(RUM) provides a natural candidate for the soft mode.However,in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in BaWO 4:a 1.6% volume collapse,coexistence of phases,and hysteresis on release of pressure.Such first-order signatures are found to be the same as the soft modes in BaWO 4,which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism.By the refinement of atomic displacement parameters,we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaO 8 polyhedrons to the small displacive distortion of rigid WO 4 tetrahedrons.Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds.     

A2( MoO4)3(A=Al,Cr,Fe)结构与相变的Raman光谱研究

用XRD、Raman光谱和DSC研究了Al2 (MoO4)3、Cr2( MoO4)3和Fe2 (MoO4)3的结构与相变.Al2(MoO4)3、Cr2 (MoO4)3和Fe2 (MoO4)3在室温下为单斜相,分别在483 K、673 K和783 K附近转变为正交相.发现MO4四面体的对称和反对称伸缩振动模的频率和相对强...     

Spectroscopic characterization of Yb:Sc₂O₃ transparent ceramics

Yb:Sc2O3 transparent ceramics are fabricated by a conventional ceramic process and sintering in H2 atmosphere. The room-temperature spectroscopic properties are investigated, and the Raman spectrum shows an obvious vibration characteristic band centred at 415 cm 1 . There are three broad absorption bands around 891, 937, and 971 nm, respectively. The strongest emission peak is centred at 1.04 μm with a broad bandwidth (11 nm) and an emission cross-section of 1.8×10 20 cm 2 . The gain coefficient implies a possible laser ability in a range from 990 nm to 1425 nm. The energy-level structure shows that Yb:Sc 2 O 3 ceramics have large Stark splitting at the ground state level due to their strong crystal field. All the results show that Yb:Sc2O3 transparent ceramics are a promising material for short pulse lasers.     

Genesis,structural, and transport properties of La<Subscript>2</Subscript>Mo<Subscript>2-x</Subscript>W<Subscript>x</Subscript>O<Subscript>9</Subscript> prepared via mechanochemical activation

Fast oxide-ion conductors La₂Mo_2-xW_xO₉ (x = 0–1) have been prepared using mechanochemical activation (MA) of starting oxides in a high-power planetary ball mill. Studies of La₂Mo_2-xW_xO₉ genesis and structural properties using thermal analysis, XRD, SEM, IR, and Raman spectroscopy have revealed that MA results in the formation of an amorphous precursor, while the cubic β-phase is formed after calcination at 700–900 °C. Due to a high dispersion of powders, high-density pellets of W-LAMOX ceramics have been obtained already after sintering at 950 °C. Their electrical conductivity measured by the impedance spectroscopy depends on the W concentration being sufficiently high (up to 5.6?10^?3 S/cm at 630 °C) at temperatures below 650 °C.