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

Alloying isomorphous crystals of barbituric acid (BA) and thiobarbituric acid (TBA) yields solid solutions of general formula BATBA (x < 0.8); for x = 0.5 the stable co-crystal BATBA, isomorphous with the parent keto forms, is observed, which melts at 265 °C, i.e. ca. 10 and 20 °C higher than the melting points of BA and TBA, respectively. While the BATBA solid solutions with x > 0.5 are stable, those with x < 0.5 convert, with time or temperature, to the BATBA co-crystal.