CONTRIBUTIONS ON THE THERMAL-BEHAVIOR OF ANHYDROUS PHOSPHATES .4. CHEMICAL-TRANSPORT REACTIONS OF PYROPHOSPHATES M2P2O7 (M = MG, CR, MN, FE, CO, NI, CU, ZN, CD)-THE 1ST TIME PREPARATION OF CHROMIUM(II) - PYROPHOSPHATE

Pyrophosphates M2P2O7 (M = Mg, Cr, Mn, Fe, Co, Ni, Zn, Cd) can be crystallized by chemical transport reactions. Chlorine (TBAR almost-equal-to 1 000-degrees-C) and NH4Cl (TBAR almost-equal-to 850-degrees-C) were used as transport agents for the pyrophosphates of Mg, Mn, Co, Ni, Cu, Zn, and Cd which can be obtained easily by thermal decomposition of the metal ammonium phosphates MNH4PO4.H2O or MNH4PO4.6H2O precipitated from aqueous solutions. Fe2P2O7 as well as the pyrophosphate of divalent chromium, which was prepared for the first time, could be synthesized in situ from metal monophosphide and metal(III) orthophosphate. The reduction of CrPO4 by CrP followed by a transport reaction (condensed equilibrium phases: Cr2P2O7,s + CrPs; transport agent: iodine; 1050 --> 950-degrees-C) led to light blue well shaped crystals of Cr2P2O7 which adopt the Thortveitit structure. An interpretation of the experimental results using estimated heats of formation of Fe2P2O7 and Co2P2O7 is given. Including these data an evaluation of the influence of various heterogenous and homogenous equilibria on the behaviour of pyrophosphate in the presence of transport agents was possible.