Temperature-induced phase transition in h-MoO3: Stability loss mechanism uncovered by Raman spectroscopy and DFT calculations

This paper reports the study of the metastable hexagonal molybdenum oxide (h-MoO3) rods by looking at the vibrational, structural and morphological properties. The MoO3 as-synthesized rods were prepared by the precipitation method and characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy, revealing a hexagonal phase and submicrometric size of the MoO3. The vibrational modes of the h-MoO3 were calculated by density-functional perturbation theory (DFPT) and used by first time to do the signature of the experimentally observed Raman modes, filling a gap in this field. Experimental temperature-dependent Raman spectroscopy study was carried out on h-MoO3 rods and pointed out to a phase transition in the 675-690 K temperature range. This phase transition was confirmed by scanning electron microscopy that was used to analyze the morphological changes in the MoO3 samples during the heating cycle. Temperature-dependent Raman data analysis combined with DFT calculations allowed us to confirm the mechanism that underlies the stability loss of the hexagonal phase at the critical temperature and to correlate the wavenumber difference of two specific Raman bands with the real temperature of the sample.