Soft manifold dynamics behind negative thermal expansion

Minimal models are developed to examine the origin of large negative thermal expansion in underconstrained systems. The dynamics of these models reveals how underconstraint can organize a thermodynamically extensive manifold of low-energy modes which not only drives negative thermal expansion but extends across the Brillioun zone. Mixing of twist and translation in the eigenvectors of these modes, for which in ZrW2O8 there is evidence from infrared and neutron scattering measurements, emerges naturally in our model as a signature of the dynamics of underconstraint.     

Frustrated soft modes and negative thermal expansion in ZrW2O8

Negative thermal expansion (NTE) in cubic ZrW2O8 has generated much interest due to its large, isotropic, and temperature independent behavior. Here, x-ray absorption fine structure data are presented for various atom pairs, providing evidence that the low-energy modes causing NTE correspond to the correlated vibrations of a WO4 tetrahedron and its three nearest ZrO6 octahedra. This involves translations of the WO4 as a rigid unit along each of the four <111> axes. The interconnectivity of these modes prevents an anisotropic soft mode from developing, a new geometrical phenomenon that we call the "frustrated soft mode."     

High pressure behavior of ZrW2O8: Gruneisen parameter and thermal properties

High pressure Raman spectroscopic studies are carried out on negative thermal expansion material ZrW2O8. The system exhibits amorphization at 2.2+/-0.3 GPa via an intermediate orthorhombic phase. In the cubic phase most modes below 50 meV are found to have negative Gruneisen parameter. Using the reported phonon density of states thermal properties are calculated and compared with the reported results. In contrast to the earlier belief, the present results show that modes of energies much higher than 10 meV also contribute substantially to the negative thermal expansion.     

新型负热膨胀氧化物材料的研究

综述了这年来国际上兴起的氧化物ZrW2O8,Y2W3O12等新型负热膨胀材料的研究及其进展。对研究此类负热膨胀氧化物的重要性,其优异的负热膨胀性能以及它的结构特点、机制及可能的应用作了论述。建议我国加大此项课题的研究力度。     

Origin of negative thermal expansion in cubic ZrW2O8 revealed by high pressure inelastic neutron scattering

Isotropic negative thermal expansion has been reported in cubic ZrW2O8 over a wide range of temperatures (0-1050 K). Here we report the direct experimental determination of the Grüneisen parameters of phonon modes as a function of their energy, averaged over the whole Brillouin zone, by means of high pressure inelastic neutron scattering measurements. We observe a pronounced softening of the phonon spectrum at P = 1.7 kbar compared to that at ambient pressure by about 0.1-0.2 meV for phonons of energy below 8 meV. This unusual phonon softening on compression, corresponding to large negative Grüneisen parameters, is able to account for the observed large negative thermal expansion.     

快速烧结合成Zr1-xMxW2O8(M=Hf,Sn,Y)固溶体的Raman光谱研究

首次采用快速烧结合成技术制备了Zr1-xMxW2O8 (M=Hf,Sn,Y)系列固溶体.制备的合适条件为:温度1523～1553 K,时间30 min～1 h,同传统固相反应烧结相比,该方法合成时间和能耗显著降低.XRD和Raman光谱分析表明,Zr1-xMxW2O8 (M=Hf,Sn,Y)固溶体具有α-ZrW2O8的...     

红外辐射材料的制备与光谱研究

本文开展了红外辐射材料ＺｒＯ２、Ａｌ２Ｏ３及其复合氧化物ＺｒＯ２－Ａｌ２Ｏ３的纳米材料制备方法研究,对其开展了ＦＴＩＲ,ＮＩＲ ＦＴ－Ｒａｍａｎ、ＸＲＤ和红外发射光谱研究,研究表明复合氧化物具有较好的红外发射性能。在构效关系的研究基础上,探讨其实际应用价值。     

Structural description of pressure-induced amorphization in ZrW2O8

ZrW2O8 undergoes a high-pressure amorphization transition above 1.5 GPa to a phase which is recoverable to ambient conditions. Reverse Monte Carlo modeling of neutron and x-ray total scattering data from ZrW2O8 recovered from approximately 4 GPa shows that the large increase in density on pressurizing ZrW2O8 is accommodated within the structure by increased bonding between the WO4 tetrahedra. This increases the tungsten coordination; changes to the ZrO6 octahedral environment are not required. This densified crystal-based model, which contains significant local disorder within a distorted periodic structure, is also in reasonable agreement with x-ray and neutron total scattering data measured in situ at high pressure.     

Thermal expansion properties of Lu2-xFexMo3O12

The structures and thermal expansion properties of Lu2-xFexMo3O12 have been investigated by X-ray diffraction(XRD).XRD patterns at room temperature indicate that compounds Lu2-xFexMo3O12 with x≤1.3 exhibit an orthorhombic structure with space group Pnca;compounds with x=1.5 and 1.7 have a monoclinic structure with space group P21/a.Studies on thermal expansion properties show that the linear thermal expansion coefficients of orthorhombic phase vary from negative to positive with increasing Fe content.Attempts to make zero thermal expansion materials indicate that zero thermal expansion can be observed in Lu1.3Fe0.7Mo3O12 in the temperature range of 200-400℃.     

Entropy increase in the amorphous-to-crystalline phase transition in zirconium tungstate

It has been shown that zirconium tungstate (ZrW₂O₈) exhibits isotropic negative thermal expansion and undergoes pressure-induced amorphization above 1.5 GPa, at room temperature. Now, we have found that amorphous ZrW₂O₈ undergoes endothermic recrystallization, and thus have an overall entropy lower than that of the crystalline phase. This counterintuitive behavior can be rationalized owing to the same low-energy modes already shown to be responsible for the isotropic negative thermal expansion and the anomalously high heat capacity at low temperatures exhibited by ZrW₂O₈. Our findings point to an entire class of materials that should behave similarly to ZrW₂O₈ and constitute direct experimental evidence for an overall entropy increase in an amorphous-to-crystalline transition.     

一种新颖的稀土-2-羟基烟酸的配合物的结构和光谱性质研究

通过水热反应合成了一种新颖的稀土2-羟基烟酸的配合物{[LaL(HL)(H2O)3]1/3(SO4)2/3(H3O)2H2O}(H2L=2-hydroxynicotinic acid)。X-ray单晶衍射分析可知,2-羟基烟酸通过羧基O和羟基O原子桥连La离子,形成二维层状结构,层与层之间通过弱作用力拓展为三维结构。二维相关光谱分析表明N—H面外弯曲振动和C—H面外弯曲振动对磁微扰比较敏感,这可能是吡啶环上π电子云在磁微扰下变形引起C—H和N—H面外弯曲振动的响应,SO24-的ν4振动和羧基不对称伸缩振动对磁微扰也比较敏感;热微扰下N—H伸缩振动比较敏感。此外还对化合物进行了紫外和热重分析。     

Phase transformation of natural titanite: An infrared,Raman spectroscopic,optical birefringence and X-ray diffraction study

The thermal behaviour of natural titanite has been studied using infrared and Raman spectroscopy, linear birefringence and synchrotron X-ray diffraction. Natural single crystals from Rauris, Austria (Fe 1.8%. Al 3.8%) were investigated and two additional samples from the Smithonian Institute Washington (Fe 2.8%, Al 0.5% and Fe 8.8%, Al 13.9%) served for IR and birefringence measurements. The coupled substitution of Fe and Al for Ti and OH and F for O leads to the formation of long ranging defect fields which are assumed to account for an increased density of antiphase boundaries. Infrared absorption spectra show increased bandwidths with increasing defect concentration. The phase transition near 500 K is clearly seen in the IR spectra of samples with low defect concentrations but not for those with high defect densities. X-ray diffraction shows strong anisotropic diffuse scattering in the Rauris sample leading to the average symmetry A2/a at room-temperature. A second anomaly at ca. 850 K was found by X-ray diffraction techniques using synchrotron radiation and by a break in the temperature evolution of the IR spectra.     

氢化非晶硅薄膜的热导率研究

采用铂电极为加热电阻,研究了厚度为300—370nm等离子体化学气相沉积(PECVD)工艺制备的氢化非晶硅(a-Si：H)薄膜的热导率随衬底温度的变化规律.用光谱式椭偏仪拟合测量薄膜的厚度,得到了沉积速率随衬底温度变化规律,傅里叶红外(FTIR)表征了在KBr晶片衬底上制备的a-Si：H薄膜的红外光谱特性,SiH原子团键合模的震动对热量的吸收降低了薄膜热导率.从动力学角度分析了薄膜热导率随平均温度升高而增大的原因,并比较了声子传播和自由电子移动在a-Si：H薄膜热导率变化上的作用差异. 关键词： 非晶硅 热导率 薄膜 热能     

热红外发射率光谱在盐渍化土壤含盐量估算中的应用研究

利用傅里叶变换红外光谱仪对绿洲盐渍化土壤进行野外测量,采用光谱平滑迭代法对温度和发射率进行分离,得到了盐渍化土壤的热红外发射率数据。通过对盐渍化土壤发射率光谱的特征分析,得出8～13 μm土壤发射率随盐分含量的增加而减小,发射率光谱对盐分因子的响应在8～9.5 μm较敏感。分析了原始发射率光谱、一阶导数、二阶导数和标准化比值与含盐量之间的相关性,表明土壤发射率与含盐量呈负相关关系,发射率一阶导数与含盐量的相关性最高,相关系数最大为0.724 2,对应波段为8.370 745～8.390 880 μm。建立了土壤发射率一阶导数与盐分含量的二次函数回归模型,模型拟合的决定系数为0.741 4,验证结果的均方根误差为0.235 5,说明利用热红外发射率光谱反演土壤盐分含量的方法可行。     

Ab Initio Study on Structures and Vibrational Spectra of M+(H2O)Ar2 (M = Cu,Au)

Previous investigations have shown that it is difficult to acquire the infrared (IR) spectra of M⁺(H₂O) (M?=?Cu, Au) using a single IR photon by attaching an Ar atom to M⁺(H₂O). To explore whether the IR spectra can be obtained using the two Ar atoms tagging method, the geometrical structures, IR spectra and interaction energies are investigated in detail by ab initio electronic structure calculations for M⁺(H₂O)Ar₂ (M?=?Cu, Au) complexes. Two conceivable isomeric structures are found, which result from different binding sites for two Ar atoms. CCSD(T) calculations predict that two Ar atoms are most likely to attach to Cu⁺ for the Cu⁺(H₂O)Ar₂ complex, while the Au⁺(H₂O)Ar₂ complex prefers the isomer in which one Ar atom attaches to an H atom of the H₂O molecule and the other one is bound to Au⁺. Moreover, the calculated binding energies of the second Ar atom are smaller than the IR photon energy, and so it is possible to obtain the IR spectra for both Cu and Au species. The changes in the spectra caused by the attachment of Ar atoms to M⁺(H₂O) are discussed.     

锂锰尖晶石红外光谱的研究

本文对锂锰尖晶石的红外光谱进行了研究。由于锂锰尖晶石的晶体结构属于Fd3m空间群,锂离子占据四面体空隙(8a位置),锰离子占据八面体空隙(16d位置)。根据群论的知识,对锂锰尖晶石晶体中离子的振动方式与红外活性之间的内在关系进行了讨论。并列出了锂锰尖晶石的红外光谱实验数据。通过理论分析,我们推断:位于618.6和501.5cm~(-1)的红外吸收带分别来源于Mn(Ⅳ)-O和Mn(Ⅲ)-O键在晶体中的不对称伸缩振动(单元为Mn(Ⅳ)O_6和Mn(Ⅲ)O_6八面体),位于1124cm~(-1)的弱红外吸收带来源于Li-O键的不对称伸缩(单元为LiO_4四面体)。还有一些低于400cm~(-1)的可能吸收带在400～4000cm~(-1)范围内未能检测到。这一结论的可靠性通过锂锰尖晶石和掺杂的锂锰尖晶石的红外光谱实验数据得到证实。     

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...     

Raman scattering from CuBr and CuI

Detailed temperature-dependent Raman spectra of CuI and CuBr are reported for the first time. Spectra have been recorded between room temperature and 6K. Peaks arising from scattering by the zone-center optic phonon modes have been identified and their frequencies compared with those determined by neutron scattering and infrared measurements. The LO phonon energy in CuBr is found to reflect the negative thermal expansion at low temperatures while the TO phonon frequency does not follow this behavior. Second-order features of the CuI spectrum are identified using available phonon dispersion curves.     

Infrared emission from surfaces under low-energy electron impact

The infrared photon emission from metal surfaces stimulated by the impact of low-energy electrons of kinetic energies between 0–10 eV has been measured. The results are presented as isochromat spectra from clean Ag and Na surfaces under different temperatures. Some IR emission features have been associated tentatively with inverse photoemission processes.     

无水五硼酸钾晶体高温性质振动光谱的研究

利用自行设计的反应器研究了高温条件下水蒸汽对无水五硼酸钾晶体结构的影响,借助于X射线衍射、红外光谱、Raman光谱等手段分析了无水五硼酸钾晶体高温性质的振动光谱。在水蒸汽的作用下,五硼酸钾的B₂O₃/K₂O(摩尔比)变小,X射线衍射分析表明750 ℃时晶体中有K₅B₁₉O_<sub>31存在,而红外光谱与Raman光谱分析表明由于水蒸汽的作用使得晶体中的三角形B₍₃₎—O结构单元向四面体B₍₄₎—O结构单元转变,四面体的B₍₄₎—O含量增加并且硼氧网络结构被进一步打断。