Temperature-Dependent Synthetic Routes to and Thermochemical Ranking of α- and β-SrNCN

White powdery beta-SrNCN was obtained by the solid-state metathesis between Srl(2) and ZnNCN at 843 K, by the reaction of Sl(1)2, CsCN, and CsN3 in tantalum cylinders at the same temperature, and from the direct reaction between elemental Sr and H2NCN dissolved in liquid ammonia. The solid-state reactions carried out at a higher 973 K yield white alpha-SrNCN. Both experimental data (X-ray diffraction (XRD), infrared spectroscopy, differential scanning calorimetry (DSC)) as well as GGA density-functional phonon calculations show that the beta-phase is thermochemically more stable, by a minute 2 kJ/mol (electronic-structure theory) and about 6 kJ/mol (DSC), whereas the alpha-phase is slightly more dense. In addition, both XRD and DSC measurements reveal two distinct (endothermic) steps for the beta-to-alpha phase transition, that is, first around 920 +/- 20 K, then at 985 +/- 15 K based on the X-ray data. Thermochemically, the upper heat effect is larger by a factor of 20.