Negative volume thermal expansion via orbital and magnetic orders in Ca₂Ru₁-(x)Cr(x)O₄(0 < x < 0.13)

Ca?RuO? undergoes a metal-insulator transition at T(MI)=357 K, followed by a well-separated transition to antiferromagnetic order at T(N)=110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at T(MI) and induces ferromagnetic behavior at T(C). The lattice volume V of Ca?Ru?-(x)Cr(x)O? (0 < x < 0.13) abruptly expands with cooling at both T(MI) and T(C), giving rise to a total volume expansion ΔV/V ≈ 1%, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between T(C) and T(MI) represents an Invar effect. The two phase transitions, which surprisingly mimic the classic freezing transition of water, suggest an exotic ground state driven by an extraordinary coupling between spin, orbit, and lattice degrees of freedom.     

The study of thermal expansion properties in solid solutions Ln2−xCrxMo3O12 (Ln=Ho and Lu) by high temperature X-ray diffraction

A systematic study of the formation, crystal structures and thermal expansion properties of solid solutions Ln_2?xCr_xMo₃O₁₂ (Ln=Ho and Lu) has been performed. Rietveld refinement results indicate that compounds Ho_2?xCr_xMo₃O₁₂ with 0≤x≤0.2 and Lu_2?xCr_xMo₃O₁₂ with 0≤x≤0.5 have orthorhombic structures and show negative thermal expansion in the temperature range of 200–800 °C. Compounds Ln_2?xCr_xMo₃O₁₂ with 1.7≤x≤2.0 have monoclinic structures and show strong positive thermal expansion in the temperature range of 25–300 °C. While compounds Ln_2?xCr_xMo₃O₁₂ with 1.7≤x≤2.0 adopt orthorhombic structures and show very low positive thermal expansion from 500 to 800 °C. Though the crystal structure plays the key rule in determining the thermal expansion properties, chemical composition and temperature also show important effect on the thermal expansion properties of these compounds.     

Y(Ba_(1-x)Gd_x)_2Cu_3O_(7-δ)的临界电流密度特征

多晶样品Y(Ba1-xGdx)2Cu3O7-δ(YBGCO)(x=0.0,0.02,0.03,0.05,0.08,0.10)是由标准固相反应法制备获得。XRD的Rietveld精修显示,Gd成功地部分替代YBa2Cu3O7-δ(YBCO)晶胞中的Ba位。实验结果表明,对于某一固定磁场Y(Ba1-xGdx)2Cu3O7-δ的临界电流密度(Jc)在x=0.05有最大值。此外,尽管YBGCO的超导温度(Tc)明显地单调下降,但微量的Gd掺杂导致磁场中YBGCO的Jc的提高。Jc随着Gd掺杂量不同而变化的特征行为可以认为,是由Gd掺杂引起的两种不同的效果竞争共同作用造成的,即纳米尺度范围内空间分布不均匀性和超导性的变化。     

Oxygen vacancy induced magnetism in BaTiO_(3-δ) and Nb:BaTiO_(3-δ) thin films

The unusual magnetic properties are observed in oxygen deficient BaTiO3-δ(BTO) and Nb:BaTiO3-δ(BNTO) thin films fabricated on SrTiO3 substrates by laser molecular beam epitaxy.The distinct magnetic hysteresis loops are observed in the oxygen deficient BTO and BNTO thin films in a temperature range of 5 to 300 K,whereas the diamagnetism is observed in both BTO and BNTO annealed at 1 atm of oxygen.The dopant Nb only enhances the magnetization in BNTO thin films,but has little effect on the coercivity.The magnetism of BTO and BNTO films is proposed to be the oxygen vacancies by origin.     

YBa_2(Cu_(1-x)Fe_x)_3O_(7-δ)超导体结构相变及其超导电性的研究

我们对YBa_2(Cu_(1-x)Fe_x)_3O_(7-δ)(0≤x≤0.07)超导体的结构相变与超导电性进行了系统研究。x射线衍射分析表明样品在x≈0.04时存在一个从正交晶到四方晶的结构相变过程,原胞体积随铁的组分增加而变大。R-T曲线测量得到零电阻温度随铁组分的增加迅速下降到液氮温度以下,零电阻温度T_c(O)随x增大而减小且在结构相变点没有突变。微结构分析表明从正交晶到四方晶的相变过程伴随着孪晶现象的消失。     

YBa_2(Cu_(1-x)Fe_x)_3O_(7-δ)超导体结构相变及其超导电性的研究

我们对YBa_2(Cu_(1-x)Fe_x)_3O_(7-δ)(0≤x≤0.07)超导体的结构相变与超导电性进行了系统研究。x射线衍射分析表明样品在x≈0.04时存在一个从正交晶到四方晶的结构相变过程,原胞体积随铁的组分增加而变大。R-T曲线测量得到零电阻温度随铁组分的增加迅速下降到液氮温度以下,零电阻温度T_c(O)随x增大而减小且在结构相变点没有突变。微结构分析表明从正交晶到四方晶的相变过程伴随着孪晶现象的消失。     

LaBaCu_2O_(5-δ)氧化物超导体的压缩特性

本文对La-Ba-Cu-O超导体的压缩特性进行了实验测量及理论计算,并利用所获实验数据写出了其Bridgman状态方程。     

A-site ordered perovskite CaCu_3Cu_2Ir_2O_(12-δ) with square-planar and octahedral coordinated Cu ions

A novel CaCu_3Cu_2Ir_2O_(12-δ) polycrystalline sample was synthesized at 8 GPa and 1373 K.Rietveld structural analysis shows that this compound crystallizes in an AA'_3B_4O_(12)-type A-site ordered perovskite structure with space group Im-3.Xray absorption spectra reveal a +2-charge state for both the square-planar and octahedral coordinated Cu ions,and the valence state of Ir is found to be about +5.Although the A-site Ca and the A'-site Cu~(2+) are 1:3 ordered at fixed atomic positions,the distribution of B-site Cu~(2+) and Ir~(5+) is disorderly.As a result,no long-range magnetic ordering is observed at temperatures down to 2 K.Electrical transport and heat capacity measurements demonstrate itinerant electronic behavior.The crystal structure is stable with pressure up to 35.7 GPa at room temperature.     

X-ray diffraction,electrical and magnetic studies of solid solution La0.6Sr0.4MnO3-δ□δ (0.0 ≤ δ ≤ 0.2)

Nonstoichiometric substituted lanthanum manganite powder has been synthesized by solid state reaction method. X-ray diffraction shows that the material exists in single phase having a perovskite structure. A structural phase transition separating a rhombohedral phase and an orthorhombic phase was observed at the rate δ ≈ 0.127 corresponding to a Mn³⁺ concentration of about 85 %. The material is ferromagnetic for 0 ≤ δ ≤ 0.127 and antiferromagnetic for 0.15 ≤ δ ≤ 0.20 The Curie temperature is very sensitive to the presence of vacancies. The investigation of electrical properties shows metallic behavior at low temperature for δ ≤ 0.10 and as insulator for 0.127 ≤ δ ≤ 0.20. The variation of activation energies with the rate of vacancies has been investigated.     

YBa_2(Cu_(0.95)M_(0.05))_3O_(7-δ)的代换效应及其中子衍射研究

X射线衍射实验表明YBa_2(Cu_(0.95)M_(0.05))_3O_(7-δ)(M=Ti,V,Cr,Mn,Fe,Co,Ni,Cu和Zn)均为单相结构。Fe,Co,Ni和Zn对Cu的替代使超导临界温度T_c显著下降,而同样含量的Ti,V,Cr,Mn对Cu的替代并未对超导性能产生显著影响。并利用中子衍射分析了Ti,Mn,Fe和Co对Cu原子的取代,发现代换原子对Cu的两个晶位各自存在不同的择优占据,从而为分析不同晶位Cu在超导机制中的作用提供了依据。同时,本文研究了YBa_2·(Cu_(0.95)M_(0.05))_3O_(7-δ)的磁性,讨论了用3d金属离子代换Cu离子时对磁性的影响,以及磁性与超导性能之间的联系。     

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     

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.     

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.     

热收缩化合物——负热膨胀性及成因

文章综述了负热膨胀化合物、负热膨胀机理与应用等方面的进展.负热膨胀是最近十多年来新兴的研究领域,目前已经发现较多化合物具有负热膨胀性能,它们广泛分布在类似ZrW2O8开放式框架结构化合物、磁性合金、反钙钛矿结构的Mn3AX、PbTiO3基铁电化合物、纳米颗粒等领域.在负热膨胀机理研究方面,原子热振动机理研究相对充分,成功地解释了一部分框架式结构化合物负热膨胀机理;然而,较多负热膨胀起源与非振动机理相关,如：物质磁性、铁电性、电子作用、纳米尺寸效应等.文章最后从实际应用角度出发对未来负热膨胀材料研究进行了展望.     

Negative thermal expansion in framework compounds

We have studied negative thermal expansion (NTE) compounds with chemical compositions of NX₂O₈ and NX₂O₇ (N=Zr, Hf and X=W, Mo, V) and M₂O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.      

Negative thermal expansion: Mechanisms and materials

Negative thermal expansion (NTE) of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications. Progresses in this field develop markedly and update continuously our knowledge on the NTE behavior of materials. In this article, we review the most recent understandings on the underlying mechanisms (anharmonic phonon vibration, magnetovolume effect, ferroelectrorestriction and charge transfer) of thermal shrinkage and the development of NTE materials under each mechanism from both the theoretical and experimental aspects. Besides the low frequency optical phonons which are usually accepted as the origins of NTE in framework structures, NTE driven by acoustic phonons and the interplay between anisotropic elasticity and phonons are stressed. Based on the data documented, some problems affecting applications of NTE materials are discussed and strategies for discovering and design novel framework structured NET materials are also presented.     

Effects of Al particles and thin layer on thermal expansion and conductivity of Al-Y_2Mo_3O_(12) cermets

Low thermal expansion composites are difficult to obtain by using Al with larger positive thermal expansion coefficient(TEC) and the materials with smaller negative TECs. In this investigation, Y_2Mo_3O_(12) with larger negative TEC is used to combine with Al to obtain a low thermal expansion composite with high conductivity. The TEC of Al is reduced by 19%for a ratio Al:Y_2Mo_3O_(12) of 0.3118. When the mass ratio of Al:Y_2Mo_3O_(12) increases to 2.0000, the conductivity of the composite increases so much that a transformation from capacitance to pure resistance appears. The results suggest that Y_2Mo_3O_(12) plays a dominant role in the composite for low content of Al(presenting isolate particles), while the content of Al increases enough to contact each other, the composite presents mainly the property of Al. For the effect of high content Al, it is considered that Al is squeezed out of the cermets during the uniaxial pressure process to form a thin layer on the surface.     

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...     

Y_(2-x)(LiMg)_xMo_3O_(12)固溶体吸水性与热膨胀性能研究

Y2-x(LiMg)xMo3O12固溶体(x=0、0.1、0.3、0.5、0.7、0.9、1.1、1.3、1.5、1.7、1.9和2.0)采用固相法制备。当x=0时,由于严重吸水,拉曼峰宽化。随着(LiMg)掺入量的增加,拉曼峰趋于尖锐并发生劈裂,说明固溶体的吸水性得到明显的降低,结晶性能提高,变温拉曼光谱显示失水对应的温度也在降低。在900cm-1和370cm-1附近出现的新峰,与Li-O和Mg-O的振动有关。随着(LiMg)量的增加,Li-O和Mg-O的振动加强,并导致MoO4与YO6振动减弱,也可能与热膨胀系数的变化有关。     

Negative thermal expansion in NbF_3 and NbOF_2:A comparative theoretical study

Thermal expansion control is always an obstructive factor and challenging in high precision engineering field. Here,the negative thermal expansion of Nb F_3 and Nb OF_2 was predicted by first-principles calculation with density functional theory and the quasi-harmonic approximation(QHA). We studied the total charge density, thermal vibration, and lattice dynamic to investigate the thermal expansion mechanism. We found that the presence of O induced the relatively strong covalent bond in Nb OF_2, thus weakening the transverse vibration of F and O in Nb OF_2, compared with the case of Nb F_3.In this study, we proposed a way to tailor negative thermal expansion of metal fluorides by introducing the oxygen atoms.The present work not only predicts two NTE compounds, but also provides an insight on thermal expansion control by designing chemical bond type.