High-temperature synthesis and unknown properties of M3Cr4(PO4)6, where M = Zn or Mg and a new solid solution Zn1.5Mg1.5Cr4(PO4)6

Abstract

For the first time, double phosphates(V) Zn3Cr4(PO4)6 and Mg3Cr4(PO4)6 were synthesized by non-waste solid-state reaction, performed in the temperature range 350–1150 °C. The change in the phase composition of samples during the synthesis, depending on the heating temperature, was examined using the XRD method. The results of the DTA measurements enabled both the correct selection of the heating temperatures of the samples and the determination of the thermal stability of the phases obtained. Zn3Cr4(PO4)6 and Mg3Cr4(PO4)6 have relatively high thermal stability in air, i.e. they melt incongruently at 1340 °C and about 1450 °C, respectively. Crystallites of the obtained double phosphates, observed by SEM method, are similar in shape, when their sizes are of the order of 0.5–6 µm. IR spectra of both double phosphates are presented. A new solid solution of the formula Zn1.5Mg1.5Cr4(PO4)6 was also obtained. It is stable in air up to 1412 °C. The energy gaps of Zn3Cr4(PO4)6, Mg3Cr4(PO4)6 and Zn1.5Mg1.5Cr4(PO4)6 are 4.00, 3.60 and 3.75 eV, respectively, as estimated from their UV–Vis DR spectra. The colour of all phases expressed in the coordinates of CIE L*C*h* colour space is very similar, and according to the RAL system, it can be described as boulder brown. All the obtained phases are good candidates to be as inorganic pigments.