Phase Transition and Negative Thermal Expansion Property of ZrMnMo_3O_(12)

A novel material of ZrMnMo_3O_(12) with negative thermal expansion is presented.The phase transition temperature and coefficient of thermal expansion(CTE) are investigated by temperature-dependent x-ray diffraction and Raman spectra,It is shown that ZrMnMo_3O_(12) adopts monoclinic structure with space group P21/a(No.14)from 298 to 358 K and transforms to orthorhombic with space group Pnma(No.62) above 363 K.The linear CTE obtained from the results of XRD refinement is-2.80×10~(-6) K~(-1) from 363 to 873 K.The CTE of the bulk cylinder ceramic measured by a thermal dilatometer is-4.7×10~(-6) K~(-1) from 373 to 773 K approximatively.     

FFPI-FBG hybrid sensor to measure the thermal expansion and thermo-optical coefficient of a silica-based fiber at cryogenic temperatures

In this Letter, a sensor consisting of a fiber Bragg grating(FBG) and a fiber Fabry–Perot interferometer(FFPI)sensor is developed to measure the coefficient of thermal expansion(CTE) and the thermo-optical coefficient(TOC) of a silica-based optical fiber at cryogenic temperatures. The FFPI is fabricated by welding together two acid-etched fibers. The temperature performance of the FFPI-FBG hybrid sensor is studied in the temperature range of 30–273 K. The CTE and TOC of the optical fiber at cryogenic temperatures are derived analytically and verified by experiments.     

Thermal Expansion of Ni_3A1 Intermetallic Compound:Experiment and Simulation

The thermal expansion of Ni_3Al intermetallic compound is determined by a thermal dilatometer and simulated by the molecular dynamics method.The results of the linear thermal expansion coefficients are presented from200 K up to the maximum temperature of 1600 K.The single phase of Ni_3Al intermetallic compound is confirmed by x-ray diffraction together with DSC melting and solidification peaks,from which the solidus and the liquidus temperatures are obtained to be 1660 and 1695 K,respectively.The measured linear thermal expansion coefficient increases from 1.5×10~(-5) to 2.7×10~(-5) K~(-1) in the experimental temperature range,in good agreement with the data obtained by the molecular dynamics simulation,just a slight difference from the temperature dependence coefficient.Furthermore,the atomic structure and position are presented to reveal the atom distribution change during thermal expansion of Ni_3Al compound.     

Low thermal expansion and broad band photoluminescence of Zr0.1Al1.9Mo2.9V0.1O12

A new material of Zr0.1Al1.9Mo2.9V0.1O12 is synthesized by the traditional solid state synthesis method.The phase transition,coefficient of thermal expansion,and luminescence properties of Zr0.1Al1.9Mo2.9V0.1O12 are explored with Raman spectrometer,dilatometer,and x-ray diffraction(XRD)diffractometer.The results show that the Zr0.1Al1.9Mo2.9V0.1O12 possesses the strong broad-band luminescence characteristics almost in the whole visible region.The sample is crystallized in a monoclinic structure group of P21/a(No.14)crystallized at room temperature(RT).The crystal is changed from monoclinic to orthorhombic structure when the temperature increases to 463 K.The material has very low thermal expansion performance in a wide temperature range.Its excellent low thermal expansion and strong pale green light properties in a wide temperature range suggest its potential applications in light-emitting diode(LED)and other optoelectronic devices.     

Thermal Expansion Anomaly and Spontaneous Magnetostriction of Y2Fe14Al3 Compound

The structure and magnetic properties of Y2Fe14Al3 compound are investigated by means of x-ray diffraction and magnetization measurements. The Y2Fe14Al3 compound has a hexagonal Th2Ni17-type structure. Negative thermal expansion is found in Y2Fe14Al3 compound in the temperature range from 403 to 491K by x-ray dilatometry. The coefficient of the average thermal expansion is α^- = -2.54Х 10^-5 K^-1. The spontaneous magnetostrictive deformations from 283 to 470 K are caJculated by means of the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation ωs decreases from 5.74 × 10^-3 to nearly zero with temperature increasing from 283 to 470K, the spontaneous linear magnetostrictive deformation λc along the c-axis is larger than the spontaneous linear magnetostrictive deformation λa in basal-plane in the same temperature below 350 K.     

Measurement of thermal expansion coefficient of nonuniform temperature specimen

A new technique is developed to measure the longitudinal thermal expansion coefficient of C/C composite material at high temperature.The measuring principle and components of the apparatus are described in detail.The calculation method is derived from the temperature dependence of the therinal expansion coefficient.The apparatus mainly consists of a high temperature environmental chamber,a power circuit of heating,two high-speed pyrometers,and a laser scanning system.A long solid specimen is resistively heated to a steady high-temperature state by a steady electrical current.The temperature profile of the specimen surface is not uniform because of the thermal conduction and radiation.The temperature profile and the total expansion are measured with a high-speed scanning pyrometer and a laser slit scanning measuring system,respectively.The thermal expansion coefficient in a wide temperature range(1000-3800 K)of the specimen can therefore be obtained.The perfect consistency between the present and previous results justifies the validity of this technique.     

Negative Thermal Expansion and Spontaneous Magnetostriction of Nd2Fe16.5Cr0.5 Compound

The structural,thermal expansion,and magnetic properties of the Nc/2^16.5^0.5 compound are investigated by means of x-ray diffraction and magnetization measurements.The Nd2^616.560.5 compound has a rhombohedral Th2Znn-type structure.There exists a small negative thermal expansion resulting from a spontaneous magnetostriction in the magnetic state of the^2^16.5 Cr0.5 compound.The average thermal expansion coefficient is-1.06×10^6/K in a temperature range 299-394 K.The spontaneous magnetostrictive deformation and the Curie temperature are discussed.     

High Solubility of Hetero-Valence Ion （Cu^2＋） for Reducing Phase Transition and Thermal Expansion of ZrV1.6P0.4O7

Large thermal expansion at room temperature and high phase transition temperature of ZrV2O7 limit its practical applications and are reduced by the high solubility of hetero-valence ion （Cu^2＋） on the basis of an equal-valence substitution of P^5＋ for V^5＋. The temperature-dependent Raman spectra show that Zr0.9CU0.1V1.6P0.4O6.9 maintains a normal parent cubic structure till 173 K and transforms to a 3 × 3 × 3 cubic superstructure below 173K. Temperature dependent x-ray diffraction patterns of Zr0.9CU0.1V1.6P0.4O6.9 show near zero and negative thermal expansion. High solubility of lower valence Cu^2＋ relates to an equal-valence substitution of smaller pH for P^5＋, which extends the bond angle of V（P）-O-V in ZrV1.6P0.4O7 close to 180°. The change of microstructure is considered to be responsible for reduced phase transition temperature and thermal expansion.     

First-principles calculations on the elastic and thermodynamic properties of NbN

<正>The elastic and thermodynamic properties of NbN at high pressures and high temperatures are investigated by the plane-wave pseudopotential density functional theory（DFT）.The generalized gradient approximation（GGA） with the Perdew-Burke-Ernzerhof（PBE） method is used to describe the exchange-correlation energy in the present work.The calculated equilibrium lattice constant a₀,bulk modulus B₀,and the pressure derivative of bulk modulus B₀’ of NbN with rocksalt structure are in good agreement with numerous experimental and theoretical data.The elastic properties over a range of pressures from 0 to 80.4 GPa are obtained.Isotropic wave velocities and anisotropic elasticity of NbN are studied in detail.It is indicated that NbN is highly anisotropic in both longitudinal and shear-wave velocities. According to the quasi-harmonic Debye model,in which the phononic effect is considered,the relations of（V-V₀）/V₀ to the temperature and the pressure,and the relations of the heat capacity C_V and the thermal expansion coefficientαto temperature are discussed in a pressure range from 0 to 80.4 GPa and a temperature range from 0 to 2500 K.At low temperature,C_V is proportional to T³ and tends to the Dulong-Petit limit at higher temperature.We predict that the thermal expansion coefficientαof NbN is about 4.20×10^-6/K at 300 K and 0 GPa.     

Conductive property of Zr_(0.1)Fe_(0.9)V_(1.1)Mo_(0.9)O_7 with low thermal expansion

Low thermal expansion materials are mostly ceramics with low conductive property, which limits their applications in electronic devices. The poor conductive property of ceramic ZrV_2 O_7 could be improved by bi-substitution of Fe and Mo for Zr and V, accompanied with low thermal expansion. Zr_(0.1) Fe_(0.9) V_(1.1 )Mo_(0.9 )O_7 has electrical conductivity of 8.2× 10~(-5) S/cm and 9.41× 10~(-4) S/cm at 291 K and 623 K, respectively. From 291 K to 413 K, thermal excitation leads to the increase of carrier concentration, which causes the rapid decrease of resistance. At 413–533 K, the conductivity is unchanged due to high scattering probability and a slowing increase of carrier concentration. The conductivity rapidly increases again from533 K to 623 K due to the intrinsic thermal excitation. The thermal expansion coefficient of Zr_(0.1) Fe_(0.9) V_(1.1 )Mo_(0.9 )O_7 is as low as 0.72× 10~(-6 )K~(-1) at 140–700 K from the dilatometer measurement. These properties suggest that Zr_(0.1) Fe_(0.9) V_(1.1 )Mo_(0.9 )O_7 has attractive application in electronic components.     

Electrical and structural properties of (Pb0.5Fe0.5)Sr2(RE0.5Ca0.5)Cu2Oz (RE: rare-earth element)

(Pb_0.5Fe_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_z is known to have so-called “1-2-1-2” crystallographic structure which is one of typical structures of high-temperature superconducting cuprates (HTSC’s). While it possesses structural characteristics of HTSC’s, superconductivity has not been reported so far. In this study, phase formation, electrical properties and structural characteristics of this compound are investigated. It is shown that this phase can be synthesized using several kinds of rare-earth element and that post-annealing in O₂ is effective for decreasing electrical resistivity. The latter suggests that this “1-2-1-2” can be made superconducting by appropriate carrier-doping into the [Cu–O₂] plane. Preliminary Rietveld structural refinement using powder X-ray diffraction data show that the initial structural model based on the nominal (Pb,Fe)Sr₂(Y,Ca)Cu₂O_z is basically correct except for true site-distribution of Fe and Cu.     

自发有序Ga_（0．5）In_（0．5）P合金的拉曼光谱研究

自发有序Ｇａ＿（０．５）Ｉｎ＿（０．５）Ｐ合金的拉曼光谱研究李国华，刘振先，韩和相，汪兆平，董建荣（中国科学院半导体研究所，半导体扭晶格国家重点实验室北京１０００８３）（中国科学院半导体研究所，半导体材料科学实验室北京１０００８３））ＲａｍａｎＳｃａ...     

Dielectric loss of half-doped manganite La_(0.5)Ca_(0.5)MnO_3

The dielectric loss tanδ of half-doped manganite La 0.5 Ca 0.5 MnO 3 is investigated using Green's function technique. The La 0.5 Ca 0.5 MnO 3 is described by the Kondo-lattice model in the double exchange limit, taking into account the Jahn-Teller distortion and the super-exchange interaction between the localized electrons. It is found that the intensity of tanδ decreases with increasing | ε JT |, V, and U. It is also observed that the transition temperature T P rises as | ε JT | and U increase. It is worth noting that T P remains unchanged and the strength of tanδ increases with increasing g. The calculated dielectric loss results are explained theoretically, and these behaviors are in qualitative agreement with the experimental results.     

MOCVD生长Ga0.5In0.5P外延层光学性质的研究

本文用光致发光(PL)研究了MOCVD(金属氧化物化学气相沉积)生长的有序Ga_0.5In_0.5P外延层的光学性质.发现有序程度较强的Ga_0.5In_0.5样品的PL谱中一峰的能量随温度升高,先增大而后又减小.根据已有的报道和本文的实验结果,提出了一个有序Ga_0.5In_0.5P的模型,模型中将有序Ga_0.5In_0.5P看作阶宽随机分布的Ⅱ型多量子阱结构,能带边之下存在带尾态,并用该模型对实验结果进行了较好的解释.     

纳米晶稀土复合氧化物Dy_(0.5)Sr_(0.5)CoO_(3-Y)的光谱特性研究

研究了纳米晶Dy0 5 Sr0 5 CoO3 Y 的光谱特性 ,结果表明 :当晶粒尺寸减小至纳米量级时 ,发射光谱的激子峰波长蓝移 ;XRD ,FTIR谱表明 ,与体材料相比 ,纳米晶的衍射峰宽化 ,纳米晶的红外谱带大劈裂和宽化 ,表现为纳米尺寸量子化效应。     

Comparison of band structure and superconductivity in FeSe_(0.5)Te_(0.5) and FeS

The isovalent iron chalcogenides,FeSe_(0.5)Te_(0.5) and FeS,share similar lattice structures but behave very differently in superconducting properties.We study the underlying mechanism theoretically.By first principle calculations and tightbinding fitting,we find the spectral weight of the d_(X~2-Y~2) orbital changes remarkably in these compounds.While there are both electron and hole pockets in FeSe_(0.5)Te_(0.5) and Fe S,a small hole pocket with a mainly d_(X~2-Y~2) character is absent in FeS.We find the spectral weights of d_(X~2-Y~2) orbital change remarkably,which contribute to electron and hole pockets in FeSe_(0.5)Te_(0.5) but only to electron pockets in FeS.We then perform random-phase-approximation and unbiased singular-mode functional renormalization group calculations to investigate possible superconducting instabilities that may be triggered by electron-electron interactions on top of such bare band structures.For FeSe_(0.5)Te_(0.5) ,we find a fully gapped s~±-wave pairing that can be associated with spin fluctuations connecting electron and hole pockets.For Fe S,however,a nodal dxy(or d_(x~2-y~2) in an unfolded Broullin zone)is favorable and can be related to spin fluctuations connecting the electron pockets around the corner of the Brillouin zone.Apart from the difference in chacogenide elements,we propose the main source of the difference is from the d_(X~2-Y~2) orbital,which tunes the Fermi surface nesting vector and then influences the dominant pairing symmetry.     

Crystal structure and spin state of Fe0.5Co0.5(NCS)2(bpa)2

Fe(NCS)₂(bpa)₂, Co(NCS)₂(bpa)₂, and Fe_0.5Co_0.5(NCS)₂(bpa)₂ were synthesized. X-ray structural analysis revealed that the structure was the same among the three complexes. Although the metal-ligand distance in the mixed crystals was shorter than the distance in Fe(NCS)₂(bpa)₂ and was similar to that in Co(NCS)₂(bpa)₂, the spin state remained temperature-independent Fe^II high-spin state in ⁵⁷Fe M?ssbauer spectra.     

First-Principles Investigations of Pb_(0.5)Ba_(0.5)TiO_3 Alloys Based on Structure Predictions

Crystal structure predictions of Pb_(0.5)Ba_(0.5)TiO_3 alloys under different pressures are performed based on the particle swarming optimization algorithm.The predicted stable ground-state and high-pressure phases are tetragonal ferroelectric(I4mm) and cubic para-electric(Fm3m),respectively,whose structural details have not been reported.The pressure-induced colossal enhancements in piezoelectric response are associated with the mechanical and dynamical instabilities instead of polarization rotation.The band gap of the tetragonal phase is indirect and that of the cubic phase is always direct.As pressure increases,the alloy displays the similar band-gap behaviors to PbTiO_3,while different from BaTiO_3,which is attributed to the different orbital contributions to the valence bands.Our calculated results are in good agreement with the available data.     

LaB0.5Mn0.5O3(B=Co,Fe)的磁性及电输运特性的研究

采用固相烧结工艺制备了钙钛矿锰氧化物LaCo0.5Mn0.5 O3和LaFe0.5 Mn0.5O3,系统研究了样品的磁性、电输运特性和磁电阻效应.室温下LaCo0.5M0.5O3和LaFe0.5Mn0.5O3均呈现顺磁行为.电子顺磁共振得到LaFe0.5Mn0.5O3样品的朗德因子g=1.9661,与Fe3+离子(g=...     

Formation of unusual Cr~(5+) charge state in CaCr_(0.5)Fe_(0.5)O_3 perovskite

A new oxide CaCr_(0.5)Fe_(0.5)O_3 was prepared under high pressure and temperature conditions. It crystallizes in a B-site disordered Pbnm perovskite structure. The charge combination is determined to be Cr~(5+)/Fe~(3+) with the presence of unusual Cr~(5+) state in octahedral coordination, although Cr~(4+) and Fe~(4+) occur in the related perovskites CaCrO_3 and CaFeO_3.The randomly distributed Cr~(5+) and Fe~(3+) spins lead to short-range ferromagnetic coupling, whereas an antiferromagnetic phase transition takes place near 50 K due to the Fe~(3+)–O–Fe~(3+) interaction. In spite of the B-site Cr~(5+)/Fe~(3+) disorder,the compound exhibits electrical insulating behavior. First-principles calculations further demonstrate the formation of CaCr_(0.5)~(5+)Fe_(0.5)~(3+)O_3 charge combination, and the electron correlation effect of Fe~(3+) plays an important role for the insulting ground state. CaCr_(0.5)Fe_(0.5)O_3 provides the first Cr~(5+) perovskite system with octahedral coordination, opening a new avenue to explore novel transition-metal oxides with exotic charge states.