Melting Behavior of Alkaline-Earth Metal Carbodiimides and Their Thermochemistry from First-Principles

We present a combined experimental and theoretical investigation targeted at the thermochemical properties of a series of alkaline-earth metal carbodiimides. Their Gibbs energies and decomposition temperatures were calculated on the basis of phonons derived from density functional theory. The theoretical decomposition temperatures arrive at 1270, 1224, and 1185 K for CaNCN, α-SrNCN, and tetragonal BaNCN, respectively. Only the melt of tetragonal BaNCN is maintained at ∼1173 K, which is slightly below its calculated decomposition temperature. Experimentally, the melt of BaNCN did not decompose below 1273 K. On the contrary, both CaNCN and α-SrNCN partially decompose by forming a mixture of their carbides, metals, and nitrogen. The calculated Gibbs energies also show that the tetragonal phase of BaNCN is more stable than the rhombohedral one. We conclude that the melt of BaNCN is useful in the crystal growth of oxynitride perovskites such as BaTaO₂N.