Enhancement of mechanical hardness in SnO_xN_y with a dense high-pressure cubic phase of SnO₂

Controlling crystalline phases in polymorphic materials is critical not only for the fundamental understanding of the physics of phase formation but also for the technological application of forbidden, but potentially useful physical properties of the nominally unstable phases. Here, using tin oxide (SnO₂) as a model system, we demonstrate a new way to enhance the mechanical hardness of an oxide by stabilizing a high-pressure dense phase through nitrogen integration in the oxide. Pristine SnO₂ has a tetragonal structure at the ambient pressure, and undergoes phase transitions to orthorhombic and cubic phases with increasing pressure. Leveraging the enhanced reactivity of nitrogen in plasma, we are able to synthesize tin oxynitride (SnON) thin films with a cubic phase same as the high-pressure phase of SnO₂. Such nitrogen-stabilized cubic SnON films exhibit a mechanical hardness of ∼23 ± 4 GPa, significantly higher than even the nitride counterpart (Sn₃N₄) as the result of the shortened atomic distance of the denser, high-pressure cubic phase. Moreover, SnON has a heavily doped, n-type semiconducting property with a controllable optical bandgap. Our work will provide new opportunities to search for and to utilize beneficial, but hidden physical properties that exist in a particular phase stable only at extreme conditions.