The group 14 elements form well-known nitrides and oxynitrides. Spinel type nitrides, namely Si₃N₄, Ge₃N₄ and Sn₃N₄, offer a new class of compounds with potentially useful mechanical and optoelectronic properties. Silicon oxynitride (Si₂N₂O) ‘Sinoite’ and the isostructural germanium oxynitride (Ge₂N₂O) are known for decades. Sinoite, first discovered in a meteorite in 1962, is a noted refractory ceramic. An oxynitride of the group 14 element ‘Sn’, however, is still unknown.

          Here, we report on the high-pressure high-temperature (HP-HT) synthesis of the first tin oxynitride Sn₂N₂O (SNO). SNO was synthesized at p = 20 GPa and T = 1200 -1500 °C using a large volume press (LVP) installed at the beamline P61B at DESY, Hamburg. SNO is found to exhibit a novel Rh₂S₃-type crystal structure where all Sn atoms are in sixfold coordination, in contrast to the fourfold coordination of Sinoite. Materials are characterized by elemental analysis, angle-dispersive powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) in combination with automated diffraction tomography (ADT). An isothermal bulk modulus of K_o = 193(5) GPa was determined by in-situ synchrotron X-ray diffraction in a diamond anvil cell. Along with the reaction enthalpy as function of pressure (Figure 1), the bulk modulus and the band structure, the structure model is also supported by DFT calculations.

Figure 1. Reaction enthalpy (PBE results) of the reaction SnO₂ + Sn₃N₄ -> 2 Sn₂N₂O as a function of pressure. The labels indicate (1) transformation spinel-type Sn₂N₂O to Pbcn- Sn₂N₂O (at 4.6 GPa), (2) ∆H = 0 (12.0 GPa), (3) transformation rutile-type SnO₂ to α-PbO₂-type SnO₂ (12.5 GPa), and (4) transformation α-PbO₂-type SnO₂ to pyrite-type SnO₂ (17.2 GPa).