Avoiding the invasion of HO into YMoO by coating with CN to improve negative thermal expansion properties.

The hygroscopicity of YMoO has serious influences on its mechanic and negative thermal expansion (NTE) properties. The reported partial ion substitution for Y in YMoO could reduce the hygroscopicity, however, the expected NTE properties disappear disappointedly. In this investigation, it is found that avoiding the invasion of crystal water and extending the NTE temperature range of YMoO to room temperature could be realized together by heating with CO(NH). The X-ray diffraction patterns, infrared absorption spectra, scanning electron microscopy images and X-ray photoelectron spectra suggest the formation of small hydrophobic molecules (CN, C, etc.) coated on the surface, which could clog the microchannels in YMoO to avoid the invasion of crystal water. The investigation paves a way to improve the NTE properties by neglecting the influences of water molecules on the stretching vibrations of MoO tetrahedra and the transverse vibrations of bridge oxygen atoms.