Preparation of zirconium tungstate (ZrW2O8) by the amorphous citrate process

Citrate gels were prepared from aqueous solutions of zirconium dinitrate oxide, ammonium metatungstate and citric acid in the range of pH 1.1–10. The citric acid to total metal molar ratio was varied from 0.66 to 1.33. The thermal decomposition of these precursor gels and the oxide phase evolution were studied by thermal analysis (DTA, TG, EGA-MS) and X-ray powder diffraction (XRD), respectively. Complex stability appeared to be largely insensitive to pH variations, whereas consistency and thermal behaviour of the gels were found to be influenced by pH and molar ratio of the starting solutions. Under optimized thermolysis conditions, phase-pure trigonal ZrW2O8 was obtained which was converted to cubic ZrW2O8 by up-quenching to 1125 °C for a very short time (<1 min). The cubic ZrW2O8 has particle sizes in the lower micron to submicron range and shows a negative linear coefficient of thermal expansion CTE = −10.2 × 10−6 K−1 (30–120 °C). The polymorphism of ZrW2O8 is discussed with respect to its preparation by the amorphous citrate process and other soft chemistry routes.