Robust Nonmetallicity of NH₃BH₃ under Extreme Deviatoric Stresses

Ammonia borane (NH3BH3) has been widely used as an effective hydrogen source for synthesizing prominent high-temperature superconducting superhydrides. Since unreacted NH3BH3 may coexist with synthesized superhydrides, it is imperative to evaluate whether NH3BH3 would turn metallic and thus potentially interfere with transport measurements on superconducting superhydrides. Previous theoretical work indicated that NH3BH3 is nonmetallic for up to 300 GPa under hydrostatic conditions, but studies showed that deviatoric stresses can drastically alter the electronic structure of materials, turning insulators into metals. Here, we show the calculated bandgap evolution of the high-pressure phase of NH3BH3 at 300 GPa under deviatoric compressive and shear stresses prevalent in high-pressure experiments. Our results reveal that NH3BH3 maintains nonmetallicity under extreme deviatoric stresses up to the highest strains limited by dynamic or elastic instability. This finding clarifies a long-standing question about the robustness of the nonmetallic nature of NH3BH3 and establishes pertinent benchmarks of its properties at ultrahigh pressure.