X-ray diffraction studies on nitric acid dihydrate

Amorphous nitric acid hydrates in the concentration range 25 to 50 mol% HNO3 were prepared by low-temperature quenching of the respective HNO3/H2O aerosols. Subsequently, a properly selected thermal treatment enabled the phase distribution of two thermodynamically stable (NAM, β-NAT) and three metastable phases (α-NAD, β-NAD, α-NAT) to be determined between 180 and 232 K by X-ray powder diffractometry. The intriguing phase distribution of the two nitric acid dihydrates was interpreted on the basis of the classical nucleation theory and Ostwald's step rule using known crystallographic data on α- and β-NAD.