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Double-ReO3-type structure compound NaSbF6 undergoes a low-temperature rhombohedral to high-temperature cubic phase between 303 and 323 K, as revealed by temperature-dependent X-ray diffractions. Although many double-ReO3-type fluorides exhibit either low thermal expansion or negative thermal expansion (NTE), NaSbF6 exhibits positive thermal expansion (PTE) with a large volumetric coefficient of thermal expansion, αv = 62 ppm/K, in its cubic phase. Raman spectroscopy reveals that the low-frequency transverse vibration of fluorine atoms is stiffened in NaSbF6, compared with the typical NTE compound CaZrF6 with the same structure. The related weak contraction associated with the polyhedral rocking would be overcome by the notable elongation of the Na–F bond length on heating, thus leading to the large volumetric PTE. Unlike ScF3 and CaZrF6 which are insulators with a wide band gap, a relative small band gap of 3.76 eV was observed in NaSbF6. The small band gap can be attributed to the hybridization between ...

As the Mn doping level ( x ) increases in cubic (Ga 0.7 Cu 0.3 ) 1- x Mn x NMn 3 ( x ≤ 0.4) compounds, the temperature window of negative thermal expansion (NTE) is shifted to lower temperatures and broadened due to the strengthening of ferromagnetism. Large coefficient of linear thermal expansion ( α ~ − 22.8 ppm/K) is observed for x = 0.25 and 0.3 at cryogenic temperatures ( x = 0.3 powders, the thermal expansion of epoxy resin is effectively reduced. Nearly zero thermal expansion ( α ~ 1 ppm/K) and remarkably improved thermal conductivity are achieved at cryogenic temperatures in the composite with 50 vol.% addition of the NTE filler.

Ni–Cu alloy nanowire array, with uniform length and diameter, was well fabricated by using alternating current electrodeposition. Both X-ray diffraction and in situ heating extended X-ray absorption fine structure technique were employed to characterize its structural change with temperature. As expected, a weak concentration fluctuation of Ni and Cu atoms was found in the as-prepared nanowires. However, this concentration fluctuation can be easily eliminated by heating the as-prepared sample at 200 °C, which resulting in an approximately homogeneous Ni–Cu solid solution. From the nanostructural point of view, the serviceability temperature of Ni–Cu alloy nanowire array in magnetism could not exceed 200 °C. The Ni–Ni, Ni–Cu and Cu–Cu bonds were found to have different thermal expansion behaviors, causing the distortion of the crystal cell. An average thermal expansion coefficient (1.1 × 10 −5 K −1 ) was obtained. The structural change of Ni–Cu alloy nanowires with temperature was discussed.

Core−shell nanoparticles were widely studied because of their potential application. However, there are few reports about the interfacial structure and its stability. In this paper, Au@SiO2 nanoparticles have been synthesized by using the traditional sol−gel method. In situ extended X-ray absorption fine structure technique was used to characterize the structural change with temperature. It is found that the interfacial bonding between the Au core and the silica shell has changed from a Au−N bond to a Au−Si bond when the temperature is above 600 °C. The Au−silica interface with Au−Si bonds between the Au core and silica shell can be stably existed in the temperature range of 600 to 1000 °C. The mechanism of bond change is described. The thermal expansion coefficient of Au nanoparticles in the Au@SiO2 core−shell system is obtained to be 1.4 × 10−5 K−1, which is almost the same with Au bulk. The details of the structural changes and thermal expansion behavior are discussed in this paper.

Scandium trifluoride (ScF3), adopting a cubic ReO3-type structure at ambient pressure, undergoes a pronounced negative thermal expansion (NTE) over a wide range of temperatures (10 K–1100 K). Here, we report the size effects on the NTE properties of ScF3. The magnitude of NTE is reduced with diminishing the crystal size. As revealed by the specific heat measurement, the low-energy phonon vibrations which account for the NTE behavior are stiffened as the crystal size decreases. With decreasing the crystal size, the peaks in high-energy X-ray pair distribution function (PDF) become broad, which cannot be illuminated by local symmetry breaking. Instead, the broadened PDF peaks are strongly indicative of enhanced atomic displacements which are suggested to be responsible for the stiffening of NTE-related lattice vibrations. The present study suggests that the NTE properties of ReO3-type and other open-framework materials can be effectively adjusted by controlling the crystal size.

In situ x-ray diffraction and x-ray absorption fine structure techniques were used to study the structural change of ordered Co nanowire array with temperature. The results show that the Co nanowires are polycrystalline with hexagonal close packed structure without phase change up until 700 °C. A nonlinear thermal expansion behavior has been found and can be well described by a quadratic equation with the first-order thermal expansion coefficient of 4.3×10−6/°C and the second-order thermal expansion coefficient of 5.9×10−9/°C. The mechanism of this nonlinear thermal expansion behavior is discussed.