Exploring the Structural and Elastic Properties of CH 3 NH 3 GeI 3 under High Pressure Based on First Principle Calculations

Methylamine iodide based chalcogenide (CH 3 NH 3 PbI 3 ) perovskite is a new material of wide interest in the photovoltaic field because of its high conversion efficiency. However, it contains Pb, a heavy metal element, and its structure is not very stable, therefore researchers continue to explore its replacement. It has been found that CH 3 NH 3 GeI 3 has similar excellent photoelectric properties to CH 3 NH 3 PbI 3 , but there are few literatures on its mechanical properties. In this paper, the mechanical properties of CH 3 NH 3 GeI 3 are studied by first-principles to provide theoretical guidance for its experimental research and practical application. The results show that under high pressure, the T-phase transforms into C- and O- phases at 0.05 GPa and 0.10 GPa, respectively, and in addition, the Young's modulus E of the O-phase keeps decreasing with increasing pressure, and its Platts ductility index (G/B) is always greater than 0.57, while the T-phase always stays below 0.57, which proves that the T-phase is brittle and the O-phase is ductile. Mechanical stability conditions are met for all three phases as the pressure increases.