Low-Temperature Transitions of Some Ammonium Salts

Abstract

AbstractX-ray diffraction studies were performed on ammonium fluoroborate [NH4BF4], ammonium perchlorate [NH4ClO4], ammonium chloride [NH4CI] and ammonium phosphate [(NH4)H2PO4] within a temperature range of ambient to −190°C. For these experiments a specimen holder was designed to effect rapid cooling of the sample. Results indicate that the fluorob orate and perchlorate salts undergo a polymorphic transition at −190°C. The decreasing intensities of the (011) and (112) reflections of the orthorhombic modification indicates a firstorder reaction. No transition was observed when these salts were cooled to −40°C. The ammonium chloride shows a transition at about −30°C which confirms the results obtained by G. H. Goldschmidt and D. G. Hurst. The ammonium phosphate (NH4)H2PO4, which is tetragonal at ambient temperature also undergoes a transition if cooled to −190°C. The kinetics of this reaction are discussed.