Compositional dependence of negative thermal expansion in the Prussian Blue analogues M(II)Pt(IV)(CN)6 (M = Mn, Fe, Co, Ni, Cu, Zn, Cd)

The effect of M(II) substitution on the magnitude of the negative thermal expansion (NTE) behavior within a series of Prussian Blue analogues, M(II)Pt(IV)(CN)(6) for M(II) = Mn, Fe, Co, Ni, Cu, Zn, Cd, has been investigated using variable-temperature powder X-ray diffraction (100-400 K). The NTE behavior varies widely with M(II) substitution, from near zero thermal expansion in NiPt(CN)(6) (alpha = dl/l dT = -1.02(11) x 10(-)(6) K(-)(1)) up to a maximum in CdPt(CN)(6) (alpha = -10.02(11) x 10(-)(6) K(-)(1)). The trend in the magnitude of the NTE behavior, with increasing atomic number (Z) of the M(II) ion, follows the order Mn(II) > Fe(II) > Co(II) > Ni(II) < Cu(II) < Zn(II) < Cd(II), which correlates with the trends for M(II) cation size, the lattice parameter, and structural flexibility as indicated by the temperature-dependent structural refinements and Raman spectroscopy. Analysis of the temperature dependence of the average structures suggests that the differences in the thermal expansion are due principally to the different strengths of the metal-cyanide binding interaction and, accordingly, the different energies of transverse vibration of the cyanide bridge, with enhanced NTE behavior for more flexible lattices.     

Zn(NCN)单轴的负热膨胀性及机理研究

负热膨胀(NTE)是一种反常的物理现象, 已在合金和框架结构化合物等材料中被观察到, 但NTE材料的种类仍然有限. 本文合成了一种单轴NTE材料Zn(NCN), 该材料在c轴方向及在100~475 K下的热膨胀系数为-3.35×10^?6 K^?1, 而a轴和b轴方向则呈低热膨胀性, 体积具有低的热膨胀系数[6.13×10^?6 K^?1(100~475 K)]. 通过同步辐射X射线衍射、 扩展X射线吸收精细结构和拉曼光谱等方法, 研究了Zn(NCN)的NTE机理. 结果表明, Zn—N键具有明显的横向振动, 一些低频振动模Grüneisen参数为负值. 直接的实验证据表明, N=C=N的横向振动以及准刚性ZnN₄四面体的耦合旋转和扭摆导致了c轴方向的NTE.     

Pronounced negative thermal expansion from a simple structure: cubic ScF(3)

Scandium trifluoride maintains a cubic ReO(3) type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at ～300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, α(l) ≈ -14 ppm K(-1)). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW(2)O(8) and positive thermal expansion above ～1100 K. While the cubic ReO(3) structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF(3) is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.     

(Fe1-xNix)ZrF6固溶体晶体结构与可调控的热膨胀性质

绝大部分物质具有热胀冷缩的基本性质,然而近年来的研究发现一些化合物具有反常的负热膨胀性质,其为有效调节物质热膨胀系数(CTE)提供了可行性,尤其调控各向同性化合物热膨胀性质是一个重要的研究方向。本文以双ReO₃结构的固溶体(Fe_1-xNi_x)ZrF₆为研究对象,对(Fe_1-xNi_x)ZrF₆固溶体的制备、晶体结构以及热膨胀调控开展了深入研究。(Fe_1-xNi_x)ZrF₆固溶体呈现全程固溶特性,通过Ni²⁺对Fe²⁺进行化学替代的方法实现了(Fe_1-xNi_x)ZrF₆热膨胀系数在大范围内的有效调控(−3.24×10⁻⁶– +18.23 × 10⁻⁶ K⁻¹,300–675 K),尤其,在(Fe_0.5Ni_0.5)ZrF₆化合物中得到了零膨胀性能。作为一种典型的框架结构化合物,晶胞中F原子横向热振动的差异是导致各自不同热膨胀差异的本质原因。该研究给我们提供了一个基于开放式框架结构化合物的热膨胀调控方法。     

Unusual non-bulk properties in nanoscale materials: thermal metal-metal bond contraction of gamma-alumina-supported Pt catalysts

Variable-temperature X-ray absorption spectroscopy measurements of sub-nanometer Pt nanoparticles on a high-surface-area gamma-alumina support reveal that the Pt-Pt bonds exhibit contraction upon heating [i.e., negative thermal expansion (NTE)]. Bare clusters under a He environment show an average linear expansion coefficient of -2.5 x 10-5 K-1 over the temperature range studied. Adsorption of hydrogen results in an overall bond relaxation and less dramatic Pt-Pt thermal bond-length contraction. From the temperature effect on bond length, disorder parameters, and the X-ray absorption near edge structure (XANES) spectra, temperature-dependent charge transfer between the support and the Pt clusters was concluded to be responsible for the observed behavior.     

Interpenetration as a Mechanism for Negative Thermal Expansion in the Metal–Organic Framework Cu3(btb)2 (MOF‐14)

Metal–organic framework materials (MOFs) have recently been shown in some cases to exhibit strong negative thermal expansion (NTE) behavior, while framework interpenetration has been found to reduce NTE in many materials. Using powder and single‐crystal diffraction methods we investigate the thermal expansion behavior of interpenetrated Cu₃(btb)₂ (MOF‐14) and find that it exhibits an anomalously large NTE effect. Temperature‐dependent structural analysis shows that, contrary to other interpenetrated materials, in MOF‐14 the large positive thermal expansion of weak interactions that hold the interpenetrating networks together results in a low‐energy contractive distortion of the overall framework structure, demonstrating a new mechanism for NTE.     

Trends in negative thermal expansion behavior for AMO2 (A=Cu or Ag; M=Al, Sc, In, or La) compounds with the delafossite structure

Powder neutron diffraction data were obtained from 30 to 600 K for CuAlO₂, CuInO₂, CuLaO₂, 2H CuScO₂, 3R CuScO₂, and AgInO₂. Rietveld refinements of these data showed negative thermal expansion (NTE) of the O-Cu-O linkage in all cases. This behavior was especially strong for CuLaO₂ and CuScO₂, where it persisted up to our maximum measuring temperature of 600 K. This NTE in turn caused NTE of the c cell edge, which was moderated by the positive thermal expansion of the M-O bonds. The NTE behavior increases in the CuMO₂ series as the size of M increases. No NTE behavior was found for the O-Ag-O linkage in AgInO₂; nonetheless, this compound did exhibit NTE for the c cell edge at low temperatures. For CuLaO₂ there is NTE for both the a and c cell edges at low temperatures. Structural trends for compounds with the delafossite structure are discussed with respect to both composition and temperature.     

Valence instabilities, phase transitions, and abrupt lattice expansion at 5 K in the YbGaGe system

The powder synchrotron X-ray diffraction technique was used to study the thermal expansion behavior of the mixed valence layered compound, YbGa1.05Ge0.95 in the temperature range 3-1123 K. A surprising abrupt isosymmetric phase transition, accompanied by a dramatic volume increase (negative thermal expansion), was found at 5 K induced by a sudden Yb valence transition from +(2 + epsilon) toward +2. At high temperatures, the material undergoes a transformation to a highly disordered structure until it eventually collapses at 1123 K to a structure with isovalent Yb ions and flat Ga/Ge planes (AlB2 type).     

Influence of conformational change of chain unit on the intrinsic negative thermal expansion of polymers

Negative thermal expansion (NTE) behavior has roused wide interest for the control of thermomechanical properties of functional materials. Although NTE behaviors have been found in kinds of compounds, it remains challenging for polymers to achieve intrinsic NTE property. In this work, we systematically studied the conformational change of dibenzocyclooctadiene (DBCOD) derivatives between chair (C) and twist-boat (TB) forms based on density-functional theory (DFT) calculations, and found clear evidence of the relationship between the structure of DBCOD units and the thermal contraction behavior of the related polymers. In order to obtain the polymer with NTE property, two conditions should be met for the thermal contracting DBCOD related units as follows: (i) the TB conformation can turn into C conformation as the temperature increases, and (ii) the volume of C conformation is smaller than that of TB conformation. This rule should offer a guidance to exploration of the new intrinsic NTE polymers in the future.     

普鲁士蓝类化合物负热膨胀及机理

负热膨胀化合物可调控材料的热膨胀系数,在复合材料、精密仪器等方面具有重要的应用前景,成为近年来化学、物理和材料工程领域的研究热点之一.因其晶体结构主要由—M—CN—M—含有双原子氰根(CN)链组成,许多普鲁士蓝类化合物呈现反常的热膨胀性质.本文综述了普鲁士蓝类负热膨胀化合物结构、热膨胀机制与系数调控等方面的研究进展.以氰根配体数量为分类主线,将具有反常热膨胀性的氰根配体化合物分为氰化物、六氰基和八氰基普鲁士蓝类化合物等进行介绍,从局域结构和平均结构角度分析了N和C原子的横向振动对负热膨胀贡献的角度解释了机理,从客体离子或分子嵌入的方法分析了热膨胀调控原理,并对新型普鲁士蓝类负热膨胀化合物的设计及应用进行了展望.     

Negative thermal expansion in single-component systems with isotropic interactions

We have devised an isotropic interaction potential that gives rise to negative thermal expansion (NTE) behavior in equilibrium many-particle systems in both two and three dimensions over a wide temperature and pressure range (including zero pressure). An optimization procedure is used in order to find a potential that yields a strong NTE effect. A key feature of the potential that gives rise to this behavior is the softened interior of its basin of attraction. Although such anomalous behavior is well-known in material systems with directional interactions (e.g., zirconium tungstate), to our knowledge, this is the first time that NTE behavior has been established to occur in single-component many-particle systems for isotropic interactions. Using constant-pressure Monte Carlo simulations, we show that as the temperature is increased, the system exhibits negative, zero, and then positive thermal expansion before melting (for both two- and three-dimensional systems). The behavior is explicitly compared to that of a Lennard-Jones system, which exhibits typical expansion upon heating for all temperatures and pressures.     

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.     

The role of spontaneous polarization in the negative thermal expansion of tetragonal PbTiO3-based compounds

PbTiO(3)-based compounds are well-known ferroelectrics that exhibit a negative thermal expansion more or less in the tetragonal phase. The mechanism of negative thermal expansion has been studied by high-temperature neutron powder diffraction performed on two representative compounds, 0.7PbTiO(3)-0.3BiFeO(3) and 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3), whose negative thermal expansion is contrarily enhanced and weakened, respectively. With increasing temperature up to the Curie temperature, the spontaneous polarization displacement of Pb/Bi (δz(Pb/Bi)) is weakened in 0.7PbTiO(3)-0.3BiFeO(3) but well-maintained in 0.7PbTiO(3)-0.3Bi(Zn(1/2)Ti(1/2))O(3). There is an apparent correlation between tetragonality (c/a) and spontaneous polarization. Direct experimental evidence indicates that the spontaneous polarization originating from Pb/Bi-O hybridization is strongly associated with the negative thermal expansion. This mechanism can be used as a guide for the future design of negative thermal expansion of phase-transforming oxides.     

Molecular dynamics study of tryptophylglycine: a dipeptide nanotube with confined water

To investigate the mechanism of structural changes of a peptide nanotube and water confined inside the channel, the helical peptide tryptophylglycine monohydrate (WG.H2O) was studied by molecular dynamics (MD) simulations using the three-dimension parallel MD program ddgmq (software package) and a consistent force field. Simulations were performed on both the water-containing system and a model system without water molecules. The details of the structural behavior with temperature are investigated for the entire simulated temperature range. Phase transitions were obtained at 115, 245, 270, 310, and 385 K, due to the contributions of both the peptide and the confined water subsystems. The crystalline, amorphous, liquidlike, liquid, and superheated phases of water were observed in the temperature ranges 40-115, 115-245, 245-310, 310-385, and >385 K, respectively. At 300 K, the diffusion constant of the confined water is 0.46 x 10-5 cm2 s-1, a value comparable to that of other peptide nanotubes. The empty peptide system melts at 440 K. Mechanisms of the negative thermal expansion (NTE) along the tube axis were investigated for different temperature ranges. The contraction of the crystalline water (or amorphous water) draws also the tube walls in and leads to NTE below 245 K. The other NTEs appear to be connected to the collapse of the ice network or the solid peptide network between 245 K and room temperature or from 310 to 440 K, respectively.     

EXAFS studies on adsorption irreversibility of Zn(II) on TiO2: temperature dependence

Adsorption irreversibility of Zn(II) on TiO2 at various temperatures was studied using a combination of classical macroscopic methods and extended X-ray absorption fine structure (EXAFS) spectroscopy. When the temperature was increased from 5 to 40 degrees C, the Zn(II) adsorption capacity increased by 130%, and adsorbed Zn(II) became more reversible. The standard Gibbs free energy change (DeltaG 0) of the adsorption reaction at 5, 20, and 40 degrees C was determined to be -19.58+/-0.30, -22.28+/-0.10, and -25.14+/-0.21 kJ mol(-1), respectively. And the standard enthalpy (DeltaH 0) and entropy (DeltaS 0) were 24.55+/-2.91 kJ mol(-1) and 159.13+/-0.53 J mol(-1)K(-1), respectively. EXAFS spectra results showed that the hydrated Zn(II) was adsorbed through fourfold coordination with an average ZnO bond distance of 1.98+/-0.01 A. Two ZnTi atomic distances of 3.25+/-0.02 and 3.69+/-0.03 A were observed, which corresponded to an edge-sharing linkage mode (strong adsorption) and a corner-sharing linkage mode (weak adsorption), respectively. As the temperature increased from 5 to 40 degrees C, the number of strong adsorption sites (N1) remained relatively constant while the number for the weak adsorption sites (N2) increased by 31%. These results indicate that the net gain in adsorption capacity and the decreased adsorption irreversibility at elevated temperatures were due to the increase in available weak adsorption sites (N2) or the decrease in the ratio of N1/N2. Both the macroscopic sorption/desorption equilibrium data and the molecular level evidence of this study suggest that in a given environmental system (e.g., soils or natural waters) zinc and other similar heavy metals are likely more mobile at higher temperatures.     

Colossal Negative Thermal Expansion in Electron‐Doped PbVO3 Perovskites

Colossal negative thermal expansion (NTE) with a volume contraction of about 8 %, the largest value reported so far for NTE materials, was observed in an electron‐doped giant tetragonal perovskite compound Pb_1?xBi_xVO₃ (x=0.2 and 0.3). A polar tetragonal (P4mm) to non‐polar cubic structural transition took place upon heating. The coefficient of thermal expansion (CTE) and the working temperature could be tuned by changing the Bi content, and La substitution decreased the transition temperature to room temperature. Pb_0.76La_0.04Bi_0.20VO₃ exhibited a unit cell volume contraction of 6.7 % from 200 K to 420 K. Interestingly, further gigantic NTE of about 8.5 % was observed in a dilametric measurement of a Pb_0.76La_0.04Bi_0.20VO₃ polycrystalline sample. The pronounced NTE in the sintered body should be attributed to an anisotropic lattice parameter change.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

Hydrogen in N-methylacetamide: positions and dynamics of the hydrogen atoms using neutron scattering

This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O...H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior of the phonon vibrations together with the observation of NTE, could be justified by the "vibrational" polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally, this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.     

Effects of doping Fe cations on crystal structure and thermal expansion property of Yb2Mo3O12

Crystal structures and thermal expansion properties of Yb_(2-x)Fe_xMo_3O_(12)(x=0.0,0.6,1.0,1.1,1.4) solid solutions have been studied by X-ray powder diffraction(XRPD) at different temperatures.Rietveld analysis of the XRPD data shows that Yb_(2-x)Fe_xMo_3O_(12) solid solutions adopt orthorhombic structure and have variable thermal expansion coefficients controlled by the ratio of Yb~(3+) to Fe~(3+).Yb_2Mo_3O_(12) shows anisotropic negative thermal expansion property,induced by the reductions in average Yb-O-Mo angle and average apparent Mo2-O bond length with increasing temperatures.As more Yb~(3+) substituted by Fe~(3+),the linear thermal expansion coefficients of Yb_(2-x)Fe_xMo_3O_(12) increase from negative to positive.A near-zero thermal expansion coefficient of 0.55×10~(-6)K~(-1) for Yb_(0.6)Fe_(1.4)Mo_3O_(12) is observed in the temperature range of 573-873 K     

Low thermal expansion behavior and electrical conductivity of Mn3(Cu0.5SixGe0.5−x)N at low temperatures

Anti-perovskite manganese nitrides with the general formula Mn₃(Cu_0.5Si_xGe_0.5?x)N (x = 0.05, 0.1, 0.15, 0.2) were fabricated by mechanical ball milling followed by solid state sintering. The temperature dependence of thermal expansions, magnetic properties and electrical conductivities were investigated in the temperature range of 77–300 K. The results show that the operation-temperature window of negative thermal expansion (NTE) shifts to lower temperature and the magnitude of NTE becomes smaller with increasing Si content. Very low average coefficients of thermal expansion of 1.3 × 10^?6 K^?1 and 1.65 × 10^?6 K^?1 were observed in Mn₃(Cu_0.5Si_0.1Ge_0.4)N and Mn₃(Cu_0.5Si_0.15Ge_0.35)N within the temperature range of 77–300 K, respectively. In addition, the electrical conductivities of all the samples are in the range of 2.5–3.5 × 10⁵ (ohm m)^?1.