Structural transitions in NaNH2 via recrystallization under high pressure**Project supported by the National Key R&D Program of China (Grant Nos. 2018YFA0305900 and 2016YFB0201204), the National Natural Science Foundation of China (Grant Nos. 51632002, 51572108, 11504127, 91745203, and 11634004), Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT_15R23), and National Fund for Fostering Talents of Basic

Abstract

Multiple phase transitions are detected in sodium amide (NaNH2), an important hydrogen storage material, upon compression in diamond anvil cells (DAC) by using Raman spectroscopy and x-ray diffraction (XRD) measurements. Additional Bragg reflections appear on lower and higher angle sides of the original ones at ∼ 1.07 GPa and 1.84 GPa, accompanied by obvious changes in Raman spectroscopy, respectively. It reveals that NaNH2 undergoes the high-pressure phase sequence (α–β–γ) up to 20 GPa at room temperature. Spectral analysis indicates an orthorhombic structure with PBAN space group for the γ phase. We also experimentally observe high pressure induced recrystallization in alkaline amide compounds for the first time.