Structure and colour of diphosphate pigments with square pyramid environment around chromophore ions (Co2+, Ni2+, Cu2+)

A series of diphosphate compounds of SrZn1−xMxP2O7 composition (M = Co, Ni, Cu) has been prepared by a conventional ceramic route. In this low cost and easily prepared series, the zinc site is five-coordinated with square pyramid geometry. The members of the whole solid solutions exhibit bright coloration due to the pyramidal crystal field around the M2+ chromophore. The systematic investigation for these chromophores with d7, d8 and d9 electronic configuration allows obtaining various site distortions (characterized from X-ray diffraction) and original colorations like the orange nickel-doped compounds, the bright cyan copper-doped compounds or the grey cobalt-doped compounds. In these latter, while a part of strontium is substituted for barium of calcium, the grey coloration can be shift on the green to red colorimetric parameter axis, recalling somewhat the “alexandrite effect”.