Crystal growth, transport behavior and X-ray detection of non-perovskite-phase NH4Pb2Br5

Abstract

“125” phase APb₂X₅ (A = Cs, NH₄; X= Cl, Br, I) materials usually exist as an impurity phase during the process of preparing halide perovkites, and is normally regarded as having negative effects. However, there has been little research about them, and their transport properties seem mysterious. Herein, we report the growth of a centimetre-sized non-perovskite-phase NH₄Pb₂Br₅ crystal achieved using the solvent-volatilization crystallization method. This crystal belonged to the I4/mcm (No. 140) space group with lattice parameters of a = b = 8.419 Å, c = 14.468 Å, α = β = γ = 90°. The as-grown NH₄Pb₂Br₅ crystal exhibited high and anisotropic resistivities, specifically values of 1.29 × 10¹² Ω cm and 4.38 × 10¹¹ Ω cm for the ab plane and c direction, respectively. The anisotropic mobilities of an Au/NH₄Pb₂Br₅/Au device were determined, based on the time-of-flight (TOF) technique, to be 0.27 ± 0.03 cm² V⁻¹ s⁻¹ (0.24 ± 0.04 cm² V⁻¹ s⁻¹) and 0.27 ± 0.03 cm² V⁻¹ s⁻¹ (0.24 ± 0.04 cm² V⁻¹ s⁻¹) for electrons (holes) along the ab plane and c direction, respectively. Finally, the Au/NH₄Pb₂Br₅/Au device exhibited a good anisotropic performance for X-ray detection, with relatively high X-ray sensitivity levels of 31.0 μC Gy⁻¹ cm⁻² and 43.6 μC Gy⁻¹ cm⁻² under an external bias of 5 V and low detection limit of 72.5 nGy s⁻¹ along the ab plane and c direction.