Negative thermal expansion materials

Abstract

A method for making tungstate compounds according to the formula XW.sub.2 O.sub.8, wherein X is selected from the group consisting of zirconium, hafnium, and combinations thereof, is described. Also described are compositions that include the tungstate compounds, and a method for reducing the positive thermal expansion of a material using such tungstate compounds. Substantially single phase compounds can be made by practicing the method, which method also reduces the synthesis time relative to known methods, and expands the reactants that can be used for the synthesis over the methods reported previously. The tungstate compounds generally can be made by forming a solution, particularly an aqueous solution, comprising a W.sup.6+ compound and a second metal compound wherein the metal is selected from the group consisting of Zr.sup.4+, Hf.sup.4+ and combinations thereof. The solvent is removed from the solution by any known method to leave the solutes as a solid product. The solid product is then heated to a temperature of greater than about 600.degree. C., preferably at least about 1165.degree. C., more preferably from about 1165.degree. C. to about 1250.degree. C., and even more preferably to about 1200.degree. C. The heated product is then cooled to ambient temperature, generally relatively rapidly, to produce substantially single phase tungstate compounds.