Novel compound powder

Abstract

This invention provides a novel compound powder that can be used as a negative thermal expansion material exhibiting negative thermal expansion characteristics in a lower temperature range and with a higher thermal shrinkage rate. The compound powder of this invention contains at least one element A selected from a specific element group such as P, and is a composite comprising: substrate particles formed from a compound containing element A and O (oxygen), and a coating layer formed from a metal compound containing Ca and element A covering the surface of the substrate particles, and the compound powder exhibits negative thermal expansion characteristics. The compound powder can be prepared by: placing a sample of the compound powder and pure water in a pressure-resistant container at room temperature and sealing it; subjecting it to heating and pressurization under specified conditions to obtain a treated solution; performing solid-liquid separation on the treated solution to remove insoluble components and obtaining a filtrate; and using the filtrate as a test solution for an ion dissolution test, wherein the amount of at least one ion of element A dissolved in the test solution is less than 3000 ppm by mass per 1 g of the sample, and the compound powder exhibits negative thermal expansion characteristics.