Stabilization and Metallization of Sodium Halide Perovskites

Halide perovskites with chemical formula ABX3 have been pointed out as viable candidates for next-generation photovoltaic materials due to their squander conversion efficiency that can reach 25.5% under controlled conditions. To avoid stability issues, devices are usually made of alloys at the A-site, combining atoms such as Cs and Rb, and molecules such as MA and FA. Sodium should also be a good candidate for this task; however, it is believed that it would not form a stable compound owing to its small ionic radius. Here we show that application of hydrostatic pressures at the 6.0-7.1 GPa range can stabilize NaPbX3, X = Cl, Br, I orthorhombic perovskites (γ) with respect to the ilmenite phase, that is the most stable phase for NaPbX3 in the uncompressed case. We will also discuss the general stability of these compounds in comparison to a wide variety of different crystal phases, and analyze the stability of Na impurities in pure inorganic compounds such as CsPbX3, showing that Na prefers the Pb substitutional position. We also find that application of pressures ≥ 50GPa is capable of producing semiconductor-to-metal transitions in γ-NaPbX3 perovskites.