The high-pressure behavior of ammonium iodide (NH4I) has been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 40 GPa. The first-order phase transition from phase IV to V is confirmed by XRD measurements for the first time. Fitting the measured volumetric compression data to the third order Birch–Murnaghan equation of state reveals bulk moduli of B0 = 14.3 ± 1.3 and 28.5 ± 2.6 GPa (with fixed B′0 = 4) for phases IV and V, respectively. Still, by analyzing the red shift of the N–H symmetric and asymmetric modes and the intramolecular distances, it is concluded that hydrogen bonding is the dominant effect upon compression over the whole pressure range.
