Synthesis and Characterization of Co5Cr2(P2O7)4 – New Pyrophosphate as a Colouring substance

Abstract

A new pyrophosphate(V) of the formula Co5Cr2(P2O7)4 was obtained in the system Co2P2O7-Cr4(P2O7)3 as a result of solid-state reactions taking place between different reactants. The new compound crystallizes in the orthorhombic system and belongs to the family of pyrophosphates of the general formula M5(II+)M2(III+)(P2O7)4 and is probably isostructural with Fe5(II+)Fe2(III+)(P2O7)4. Powder diffraction pattern, infrared spectrum and SEM image of the new compound were presented. As a new potential inorganic pigment, Co5Cr2(P2O7)4 was tested for its thermal stability, particle size distribution and colour properties, which were studied both for powder and after introduction into organic matrix and leadless ceramic glaze. The colour of Co5Cr2(P2O7)4 powder was defined as deep grey with the colour coordinates L*/a*/b* = 60.63/-1.42/-3.41 and according to the hue angle (h degrees = 247.39°) it belongs to the blue region. Co5Cr2(P2O7)4, with its relatively high thermal stability (tm = 1230 +/- 10 °C) and appropriate colour properties, is a good candidate to be used as inorganic pigment for colouring of acrylic paints. In the case of leadless glaze, the obtained compound acts as a dye.