Structural transformation and thermochromic behavior of Co2+-doped Zn3(PO4)2.4H2O hopeites
Résumé
The thermal history of Co2+-doped hopeite Zn3(PO4)2*4H2O was investigated combining thermogravimetric analyses, X-ray diffraction and UV-Visible spectroscopy. For the two dehydration steps, Zn3(PO4)2*4H2O → Zn3(PO4)2*2H2O → α + δ-Zn3(PO4)2, the temperatures increases linearly with the Co2+ doping rate (130-340 °C). The coexistence of α and δ phases formed after dehydration was explained considering a complex and unusual intergrowth phenomenon. Finally these phases transform into the high temperature stable phase, γ-Zn3(PO4)2, at a temperature also controlled via the Co2+ doping rate (350-900 °C). Both phase transformations are associated with variations of Zn2+/Co2+ local environments, from a distorted octahedral site in hopeite to a distorted tetrahedral site in Zn3(PO4)2*2H2O or α + δ-Zn3(PO4)2 and finally to distorted octahedral/trigonal bipyramid sites in γ-Zn3(PO4)2. The successive changes of the Co2+ coordination induce colour variation from pink (octahedral coordination) to blue (tetrahedral coordination), then violet (5 and 6-fold coordination). Co2+-doped hopeites can so be used as efficient over-heat temperature indicators.