System CoO–P2O5–Cr2O3: new compound, homogeneity range of the phase with howardevansite-type structure and phase equilibria in subsolidus area

Abstract The ternary oxide system CoO–P 2 O 5 –Cr 2 O 3 is an area of ​​search, among others, for new inorganic pigments or dyes. As a result of the research conducted in this work, concerning phase equilibria in the subsolidus region of this system, a new double phosphate of the formula Co 13 Cr 4 P 10 O 44 was discovered. This compound was obtained by multi-stage heating of a stoichiometric mixture of CoCO 3 , Cr 2 O 3 and (NH 4 ) 2 HPO 4 in the temperature range of 350–1050 °C. Both the calculated crystallographic data and the infrared spectrum indicate that Co 13 Cr 4 P 10 O 44 is not isostructural with Cu 13 Fe 4 V 10 O 44 , known from the literature. The new phosphate, in a powder form, has an ash-violet colour. It melts incongruently at 1160 °C. The second finding, resulting from our studies, is that the previously known phase, assigned to a precisely defined formula Co 3 Cr 4 (PO 4 ) 6 , has in fact a range of homogeneity, which can be described as Co 3+1.5x Cr 4-x (PO 4 ) 6 (0 ≤ x < 0.323). The paper also presents the division of the subsolidus region of the CoO–P 2 O 5 –Cr 2 O 3 system into subsidiary subsystems. Investigations by the differential thermal analysis method have shown that almost all of the mixtures of phases coexisting in the solid state in equilibrium have relatively high melting points, i.e. above 1100 °C. Therefore, not only the new phosphate Co 13 Cr 4 P 10 O 44 discovered in this work, but also these mixtures can be potentially used for colouring of commercially available middle-temperature ceramic glazes with a glazing temperature of 1050 °C.