Biaxial Hard Compression, Anisotropic Elastic Property, and Pressure-Induced Isosymmetric Phase Transition in Ammonium Bicarbonate

Abstract

The elastic property, exceptional anisotropic compression, and pressure-induced isosymmetric phase transition in ammonium bicarbonate (AB) are studied by in situ synchrotron X-ray diffraction, Raman spectroscopy, and computational methods. The exceptional anisotropic compression as a result of biaxial hard compression is induced by stiff hydrogen-bonding “double-wine-rack” geometric motifs. The large values of elastic anisotropy such as Young’s moduli, Shear moduli, and Poisson’s ratio verify the anisotropic nature in AB. The biaxial hard compression further induces an isosymmetric phase transition at 2 GPa through the generation of new N–H···O hydrogen bonds. Our work first demonstrates the stiff mechanical study of “double-wine-rack” geometric hydrogen-bonding networks and its influence on the phase transition, which can be used as a model to investigate the robust nature of hydrogen bonds and the structural origin of isosymmetric phase transition. Moreover, the hydrogen-bonding “double-wine-rack” geometric mechanism also provides an inspiration to construct a covalent-bonding “double-wine-rack” geometric model favoring anomalous biaxial mechanical/thermal responses. The determination of the high-pressure phase of AB provides new information to understand the composition of the ternary system H2O–CO2–NH3.