Mechanism Of Negative Thermal Expansion Research Articles

Neutron diffraction studies of structure and increasing splitting of LO-TO phonons in Pb1−xCdxTiO3

Pb 1 − x Cd x Ti O 3 (0.0⩽x⩽0.08) compounds in the tetragonal phase were prepared by the solid state reaction. Neutron powder diffraction study revealed that Pb1−xCdxTiO3 exhibits an unusual structural property. Although the tetragonality (c∕a) is enhanced by Cd substitution for Pb, the cation displacement of spontaneous polarization unexpectedly decreases, which is associated with the reduced Curie temperature. The increasing of the LO-TO splitting in Pb1−xCdxTiO3 shows an anomalous behavior in comparison with other previously studied PbTiO3-based systems. It was interestingly found that a similarity exists in the behavior of the solubility dependence of the LO-TO splitting and the negative thermal expansion (NTE) in the PbTiO3-based systems, which suggests that Pb–O hybridization plays an important role in NTE mechanism of PbTiO3.

Giant extrinsic negative thermal expansion in vanadium pentoxide nanocrystalline films

AbstractVanadium pentoxide gels, V2O5·1.6H2O, give rise to xerogel layers that exhibit a preferred orientation. X‐ray diffraction of this xerogel displays the 00l peaks typical of a turbostratic stacking of the V2O5 ribbons along a direction parallel to the substrate. The distance along the c‐axis is observed from the interlayer spacing to decrease continuously with increasing temperature up to 180 °C, as observed by high‐temperature X‐ray diffraction. This contraction may be described by an extrinsic mechanism of negative thermal expansion (NTE). The coefficient of NTE as large as –1.5 × 10–3 K–1 was observed. Full recovery of the interlayer spacing is obtained after cooling the sample to room temperature in open air, where water molecules are reabsorbed, indicating that the process is reversible and the heating process can be repeated without losing NTE. The structure of the xerogel was explored further using differential scanning calorimetry as well as infrared spectroscopy. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

EXAFS and XRD study of local dynamics in Cu2O and Ag2O

Copper and silver oxide share the rather unusual cuprite structure, formed by two interpenetrating networks of corner-sharing M4O tetrahedra (M = Cu, Ag). Negative thermal expansion of the cell parameter has been observed from 5 to about 200 K for Cu2O and up to 470 K for Ag2O by powder diffraction. In the case of Ag2O, it has been shown by EXAFS that the lattice contraction is accompanied by an expansion of the Ag-O distance and that the deformation of the Ag4O tetrahedral is not negligible. In this work we present a comparison between EXAFS and XRD results for both compounds. The comparison between local and lattice thermal expansions, measured by EXAFS and XRD, respectively, and the differences found between the two compounds, allow to gain original insight on the mechanism of negative thermal expansion in framework structures.

Negative Thermal Expansion in (HfMg)(WO4)3

A single‐phase material (HfMg)(WO4)3 with an orthorhombic structure, A2 (WO4)3‐type tungstate, has been successfully prepared for the first time by the calcination of HfO2, MgO, and WO3, substituting Hf4+ and Mg2+ for A3+ cations in A2(WO4)3. The new material shows a negative thermal expansion coefficient of approximately −2 ppm/°C from room temperature to 800°C. The mechanism of negative thermal expansion is assumed to be the same as that of Sc2(WO4)3.

Negative thermal expansion in hydrated HZSM-5 orthorhombic zeolite

Orthorhombic HZSM-5 zeolite, in hydrated form and with the silica/alumina molar ratio of 30, exhibits a complex thermal expansion behavior over the temperature range 40–840 °C. Strong negative average overall linear coefficient of thermal expansion ( α=−9.0×10 −6 K −1) was found in this region. Thermal expansion behavior of HZSM-5 is anisotropic, with the a and c axes contracting more than the b axis. Five different thermal expansion regions could be established over the overall studied temperature range. The first region, between 40 and 80 °C, demonstrated positive average thermal expansion with α=6.19×10 −6 K −1. The second one, identified between 80 and 440 °C, displays negative average thermal expansion with α=−7.38×10 −6 K −1. The next region between 440 and 520 °C shows a small positive average thermal expansion with α=3.89×10 −6 K −1. The fourth region has a strong negative average thermal expansion with α=−6.22×10 −6 K −1, while the last one exhibits a very strong negative average thermal expansion with α=−25.75×10 −6 K −1. It could be concluded that the release of H 2O molecules does not provoke negative average thermal expansion of HZSM-5. One can ascribe the negative thermal expansion behavior of orthorhombic HZSM-5 from 80 to 440 °C mainly to the action of H 3O + species. The thermal expansion behavior in the range between 440 and 520 °C could be correlated to the processes of dehydroxylation. Above 520 °C the strong negative thermal expansion seems to be related to the intrinsic mechanism of negative thermal expansion, i.e., transverse vibrations of the bridging oxygen atoms between two rigid polyhedrons.

Pair Distribution Function as a Probe for Zeolite Structures

ABSTRACTIn this article we describe how we have used the pair distribution function of zeolites to solve complex structural problems. A first study was dedicated to refining the structure of disordered zeolite beta, a zeolite that is used in the alkylation of benzene. A second study is a work in progress geared toward the determination of the mechanism of negative thermal expansion of zeolite chabazite, which has been found to be one of the most contracting materials known. Preliminary results suggest that the individual Si-Ox distances and Ox-Si-Ox angles in the tetrahedral are deformed with temperature while the average tetrahedra dimensions stay constant.

Strong Negative Thermal Expansion along the O−Cu−O Linkage in CuScO2

The structure of 2H CuScO2 has been refined from single-crystal X-ray diffraction data obtained at room temperature and from neutron powder diffraction data obtained from 11 to 1206 K. Strong negative thermal expansion (−4.0 × 10-6/K) has been found for the apparent Cu−O bond length below 300 K. This is the first example of strong negative thermal expansion in a metal oxide based on an O−M−O linkage instead of a M−O−M linkage. It is now the transverse thermal motion of Cu, instead of the transverse thermal motion of oxygen, that causes the negative thermal expansion. As expected for this mechanism for negative thermal expansion, the thermal displacement parameter for Cu is very pronounced perpendicular to the linear O−Cu−O linkage.

Variable-temperature microcrystal X-ray diffraction studies of negative thermal expansion in the pure silica zeolite IFR.

Variable-temperature single-crystal X-ray diffraction using a synchrotron X-ray source has allowed the mechanism of negative thermal expansion in the pure silica zeolite IFR to be studied in greater detail than was previously possible over the temperature range 30-557 K. The results have allowed the changes in average atomic position with temperature to be measured and the structural features that are important in negative thermal expansion to be identified. The structure of zeolite IFR can be split into two regions: columns of fused rings that expand with temperature and the intercolumn regions, which tend to contract on heating. These competing changes combine to produce a material that contracts parallel to the crystallographic a and b axes and expands in the c-direction. Correlations between zeolite structure and thermal expansivity are also reported.

Negative thermal expansion materials

The recent discovery of negative thermal expansion over an unprecedented temperature range in ZrW 2O 8 (which contracts continuously on warming from below 2 K to above 1000 K) has stimulated considerable interest in this unusual phenomenon. Negative and low thermal expansion materials have a number of important potential uses in ceramic, optical and electronic applications. We have now found negative thermal expansion in a large new family of materials with the general formula A 2(MO 4) 3. Chemical substitution dramatically influences the thermal expansion properties of these materials allowing the production of ceramics with negative, positive or zero coefficients of thermal expansion, with the potential to control other important materials properties such as refractive index and dielectric constant. The mechanism of negative thermal expansion and the phase transitions exhibited by this important new class of low-expansion materials will be discussed.