Isotropic Negative Thermal Expansion Research Articles

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.

Lattice dynamical calculation of isotropic negative thermal expansion inZrW2O8over 0–1050 K

We report detailed lattice dynamical calculations of the large isotropic negative thermal expansion (NTE) over 0--1050 K in ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ in its cubic phase. The results bring out the unusually dominant quantitative contributions to NTE of the transverse acoustic, and librational and translational optic phonons below 8 meV. The contributions of these phonons are reduced in the high pressure orthorhombic phase which therefore shows NTE only below 270 K. The calculations show a fair agreement with the published thermal expansion data in both phases.

Synthesis and Properties of the Negative Thermal Expansion Material Cubic ZrMo2O8

A new phase, cubic ZrMo2O8, has been prepared by the low-temperature dehydration of ZrMo2O7(OH)2·2H2O. This material displays isotropic negative linear thermal expansion (α = −5.0 × 10-6 K-1) over a large temperature range. Unlike the previously reported cubic ZrW2O8, it does not undergo any phase transformations on heating at atmospheric pressure, and it does not display any pressure-induced phase transformations below 0.6 GPa at room temperature.

Pressure-induced amorphization and negative thermal expansion in ZrW2O8

It has recently been shown that zirconium tungstate (ZrW2O8) exhibits isotropic negative thermal expansion over its entire temperature range of stability. This rather unusual behavior makes this compound particularly suitable for testing model predictions of a connection between negative thermal expansion and pressure-induced amorphization. High-pressure x-ray diffraction and Raman scattering experiments showed that ZrW2O8 becomes progressively amorphous from 1.5 to 3.5 gigapascals. The amorphous phase was retained after pressure release, but the original crystalline phase returned after annealing at 923 kelvin. The results indicate a general trend between negative thermal expansion and pressure-induced amorphization in highly flexible framework structures.

Strong negative thermal expansion in siliceous faujasite

Strong isotropic negative thermal expansion (α = –4.2 × 10–6 K–1) found for siliceous faujasite over a temperature range of 25 to 573 K is attributed to transverse vibrations of the bridging oxygen atoms, a model supported by structural refinements of X-ray diffraction data as a function of temperature.