Structural properties of ammonium iodide under high pressure

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 B-0 = 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.