Synthesis of condensed cobalt (II)-zinc phosphate with the concrete anionic composition
Authors: Kochkodan Olha , Antraptseva Nadiya , Kozachuk Tetiana 
Published: August 2019

Сondensed cobalt(II)-zinc phosphates with the concrete anionic composition (with а linear structure of anion with = 2–8 of the general formula (Co1-хZnх)(n+2)2PnO3n+1, 0<х<1.00, and a cyclic with = 4 – (Со1-xZnx)2P4O12, 0<x<1.0) were synthesized by heat treatment in the isothermal conditions of crystallohydrates  of  composition  Со1-хZnх(H2PO4)2·2H2O (0<x<1.00).

The heat treatment of Со1-хZnх(H2PO4)2·2H2O (0<x<1.00) was carried out in the air in the range of 100–350 °C (± 5 °). The sample was maintained at a predetermined temperature for 0.5, 1.5, 3.0, 5.0 and 7.0 hours. Heat treatment products were identified using a set of analytical methods: chemical, X-ray, IR spectroscopy, quantitative chromatography on paper.

It has been determined that the formation of condensed phosphates in products of heat treatment Со1-хZnх(H2PO4)2·2H2O (0<x<1.00) at 100 °C for 0.5–3.0 h does not occur. The processes of anionic condensation begin under the heat treatment for 5.0–7.0 h at 100 °C and deepen for further temperature rise.

With the increase in the duration of heat treatment at 150 °C to 3.0–7.0 h, the formation of condensed phosphates with a linear structure of anion with a degree of polycondensation = 2–5 of the general formula (Со1-хZnх)(n+2)2PnO3n+1. is recorded. The degree of conversion of monophosphate anion to polyphosphate is 61-73%, respectively. Similar changes in the composition of heat treatment products are realized with the destruction of the structure and complete amorphization of the solid phase. The formation of a new crystalline lattice is recorded at 225 °C.

The sample, which lasts 0.5 h, is a crystalline phase identified as Со1-хZnхH2P2О7 with an admixture of Со1-хZnхP2О7. The maximum amount of diphosphate (52.9 % of the total content of P2O5) is formed during the firing of Со1-хZnх(H2PO4)2·2H2O for 1.5 h at 225 °C. The amount of diphosphate is reduced by almost 2 times during heat treatment for 7.0 hours. Similar changes in the composition of linear condensed phosphates are observed at 275 °C: with the increase in the duration of heat treatment the number of low-molecular phosphates with = 2–4 decreases, the high-molecular with = 5–8 of the general formula (Со1-хZnх)(n+2)2PnO3n+1 increases . Fosted condensed phosphate with a cyclic structure of the anion with = 4 – cyclotetraphosphate of the composition (Со1-хZnх)2Р4O12 . It increases the temperature to 350 °C and becomes the only heat treatment product.

Quantitative dependences of the content of condensed phosphates with different anion structure and phosphate acids, which released as intermediate products, on the temperature regime and roasting duration were established.

The influence of cation nature on the conditions of synthesis and quantitative composition of the condensed phosphates of cobalt(II)-zinc with concrete anionic composition  (with а linear structure of anion with = 2–8 of the general formula (Co1-хZnх)(n+2)2PnO3n+1, 0<х<1.00, and a cyclic with = 4 – (Со1-xZnx)2P4O12, 0<x<1.0) is shown.