Structural transformation and thermochromic behavior of Co2+-doped Zn3(PO4)2•4H2O hopeites

The thermal history of Co2+-doped hopeite Zn-3(PO4)(2)center dot 4H(2)O was investigated combining thermogravimetric analyses, X-ray diffraction and UV-Visible spectroscopy. For the two dehydration steps, Zn-3(PO4)(2)center dot 4H(2)O -> Zn-3(PO4)(2)center dot 2H(2)O -> alpha + delta-Zn-3(PO4)(2), the temperatures increases linearly with the Co2+ doping rate (130-340 degrees C). The coexistence of alpha and delta phases formed after dehydration was explained considering a complex and unusual intergrowth phenomenon. Finally these phases transform into the high temperature stable phase, gamma-Zn-3(PO4)(2), at a temperature also controlled via the Co2+ doping rate (350-900 degrees 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 Zn-3(PO4)(2)center dot 2H(2)O or alpha + delta-Zn-3(PO4)(2) and finally to distorted octahedral/trigonal bipyramid sites in gamma-Zn-3(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.