Giant extrinsic negative thermal expansion in vanadium pentoxide nanocrystalline films

Vanadium pentoxide gels, V2O5 center dot 1.6H(2)O, 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 degrees 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 x 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.