Preparation of the negative thermal expansion material cubic ZrMo₂O₈

The negative thermal expansion material cubic ZrMo₂O₈ can be prepared by the carefully controlled dehydration of ZrMo₂O₇(OH)₂·2H₂O. The quality of the end product is dependent upon the route used to prepare the ZrMo₂O₇(OH)₂·2H₂O. The influence of both the Zr:Mo ratio in the solution used to prepare the hydrate and the type of acid used in its preparation (HCl, H₂SO₄, CH₃CO₂H, HNO₃ and HClO₄) are examined. Not all acids provide media suitable for the preparation of ZrMo₂O₇(OH)₂·2H₂O. Cubic ZrMo₂O₈ could only be obtained from precipitates containing the hydrate. Starting solutions with a Zr:Mo ratio in excess of 1:2 were necessary to avoid coprecipitation of amorphous MoO₃. The nature of the acid used in the hydrate synthesis affects the morphology of both the hydrate precursor and the cubic ZrMo₂O₈ particles. Many of the syntheses examined led to cubic ZrMo₂O₈ contaminated with amorphous impurities. Highly crystalline pure material could be obtained by using a perchloric acid medium for the synthesis of the hydrate precursor.