Ni-free, black ceramic pigments: a reappraisal of crystal structure, optical properties and technological behaviour of Co-Cr-Fe-Mn spinels.

A reappraisal of crystal structure, optical properties and technological behaviour of black spinels in the Co--Cr--Fe--Mn system was carried out to define the best compositions and disclose the role of crystal chemistry in colouring performance and pigment-glaze interactions. Twenty ternary and quaternary spinel formulations were designed using crystallochemical criteria and prepared by simulating the industrial synthesis. Powder samples were characterized through X-ray diffraction (Rietveld), optical, Raman and Mössbauer spectroscopies, and technological testing in several glazes and glassy coatings. Black spinels fall in a relatively narrow field of unit cell and inversion parameters in between "chromites" and "magnetites". Their cation distribution is governed by Co2+ occupancy of tetrahedrally-coordinated site T and Cr3+ occupancy of octahedrally-coordinated site M, with Mn3+ hosted at M, while Mn2+ and Fe3+ are distributed over both sites. Nevertheless, Raman and Mössbauer spectra indicate a growing disorder in cation partitioning going towards the iron-rich terms. The pigment technological behaviour depends to a large extent on crystal chemistry of spinels, with no effect by grain or crystallite size. The best colouring performance corresponds to recommended compositions able to withstand corrosion and change in crystal chemistry in contact with melted glaze. Three different pathways can give rise to excellent black pigments: (a) strongly disordered spinels having an inversion parameter i~0.2; (b) moderately disordered spinels, free of Mn3+, with i<0.1; (c) apparently ordered and normal spinels halfway from cobalt chromite and cobalt ferrite. Poor technological performances are due to different conditions leaving room to colour bleaching (low Cr amount), glaze bloating (excess of Mn3+) and limited resistance to corrosion (oversaturation of site M by Cr+Fe with formation of kenotetrahedral spinels or eskolaite--hematite solid solutions).