New Insights into Planar Defects in Layered α-MoO₃ Crystals

The observation of regular ( h0 l) planar defects in α-MoO₃ crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies because of thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of α-MoO₃. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to H _xMoO₃ precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into α-MoO₃ belt crystals by heating in liquid sulfur at 300 °C. These defects are undetectable by both atomic force microscopy and scanning electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of α-MoO₃ at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges and the argon-plasma-induced Raman spectral evolutions of the as-grown α-MoO₃ belt crystals provide new insights into the planar defects with regard to their formation and characteristics.