Influence of europium doping on the structural phase-transition temperature of β− and α−CoMoO4 polymorphs

This work presents a new possibility to increase the phase transition temperature of CoMoO₄ polymorphs (β and α) by europium doping. The material Co_0.98Eu_0.02MoO₄ was produced by the co-precipitation method and its thermochromic property was investigated. An accurate characterization of the material was performed using X-ray diffraction, Raman spectroscopy, diffuse reflectance spectroscopy, thermal analysis, and morphological analysis. The results showed that an irreversible first order phase transition between the two allotropic forms occurred at around 950 °C, and it is associated with a drastic color change from purple (beta) to green (alpha). This color change confirmed the promising potential of the material for use as a temperature indicator that could be observed by the naked eye. Photoluminescence spectroscopy helped to elucidate the effect of the presence of europium added to the cobalt molybdate, which also enabled the material to be used as a light emitter under UV excitation.