Crystal structures and electronic properties of Sn3N4 polymorphs synthesized via high-pressure nitridation of tin

Abstract

The crystal structures, compression behaviors, and electronic properties of Sn₃N₄ polymorphs were investigated by means of high-pressure in situ measurements, transmission electron microscopy, and DFT ab initio calculations. The direct nitridation of tin above a pressure of 29.5 GPa by using a laser-heated diamond anvil cell resulted in the synthesis of highly crystalline monoclinic Sn₃N₄ which so far is consistent with the reported one. On the other hand, it was newly discovered that the monoclinic Sn₃N₄ undergoes phase transition twice along with decompression at ambient pressure. Advanced synchrotron X-ray powder diffraction measurements and TEM analyses on the recovered sample demonstrate that the newly recovered Sn₃N₄ crystallizes with orthorhombic symmetry and consists of irregularly shaped SnN₅ and SnN₆. DFT ab initio calculations reveal that this newly recovered orthorhombic Sn₃N₄ is an indirect band gap semiconductor having a narrower electronic energy gap than that of cubic spinel-type Sn₃N₄. The present results demonstrate that high-pressure nitridation is a powerful method to synthesize highly crystalline Sn₃N₄ polymorphs and reveal the new crystal chemistry of group IVA nitrides with respect to the variety of crystal structures and electronic properties.