The relationship between crystal structures and thermochromism in CoMoO4.

Abstract

The visible thermochromism of crystalline CoMoO4 was investigated in a temperature range from 300 to 1000 °C. The microcrystals were obtained by the coprecipitation method followed by calcination. Through spectroscopic techniques, it was possible to monitor the phase transition from samples in the β-phase to α-phase, which is associated with the color change of the material from purple (β) to green (α). The phase transition was irreversible and proceeded approximately at 550 °C. Both polymorphs belong to C2/m (#12) space group and crystallize in the monoclinic arrangement. The colorimetric parameters allowed to follow the color diferences obtained in the material according to the applied temperature. The relationship between crystal structure, colorimetric parameters, and spectroscopic properties was described in detail and help the understood of the thermochromic behavior in cobalt molybdate. The increase in the temperature allowed the band-gap tuning from 2.8 to 1.5 eV due to the change of color from purple to green, confrming the promising potential of these materials for use as temperature indicators that can be observed by the naked eye.