Ca3N2 and Mg3N2: Unpredicted High-Pressure Behavior of Binary Nitrides

High-pressure synthesis allows both fundamental and materials science research to gain unprecedented insight into the inner nature of materials properties at extreme environment conditions. Here, we report on the high-pressure synthesis and characterization of γ-Ca3N2 and the high-pressure behavior of Mg3N2. Investigation of M3N2 (M=Ca, Mg) at high-pressure has been quite challenging due to the high reactivity of these compounds. Ex situ experiments have been performed using a multianvil press at pressures from 8 to 18 GPa (1000-1200 °C). Additional in situ experiments from 0 to 6 GPa (at RT) at the multianvil press MAX 80 (HASYLAB, Beamline F.2.1, Hamburg) have been carried out. The new cubic high-pressure phase γ-Ca3N2 with anti-Th3P4 defect structure exhibits a significant increase in coordination numbers compared to R-Ca3N2. Contrary, Mg3N2 shows decomposition starting at surprisingly low pressures, thereby acting as a precursor for Mg nanoparticle formation with bcc structure. Soft X-ray spectroscopy in conjunction with first principles DFT calculations have been used to explore the electronic structure and show that γ-Ca3N2 is a semiconductor with inherent nitrogen vacancies