Alloying barbituric and thiobarbituric acids: from solid solutions to a highly stable keto co-crystal form

Alloying isomorphous barbituric and thiobarbituric acids yields solid solutions of formula BA x TBA 1−x ( x < 0.8); for x = 0.5 the isomorphous, stable keto co-crystal BA 0.5 TBA 0.5 is observed, which melts at 265 °C, i.e. ca. 10 and 20 °C higher than the melting points of BA and TBA.