Growth and single-crystal refinement of phase-III potassium nitrate, KNO₃

Oriented single crystals of the high-temperature phase of KNO3 (phase III), a ferroelectric compound that may also occur as an atmospheric aerosol particle, were grown at room temperature and pressure by atomizing a solution of KNO3 in water and allowing droplets to dry on a glass substrate. The crystals are up to 1 mm across and are stable unless mechanically disturbed. There is no evidence of the spontaneous transformation of phase III to the room-temperature stable phase (phase II), even after several months. Single-crystal structure determinations of phase III were obtained at 295 and 123 K. The unit cell regained its room-temperature dimensions after warming from 123 K. The phase-III KNO3 structure can be viewed as the stacking parallel to the c axis of alternating K atoms and planar NO₃ groups. The NO₃ groups connect the planes of K atoms, where each O is fourfold coordinated to one N and three K. Each K atom has nine O nearest neighbors, with three bonds at 2.813 and six at 2.9092 Å. The interatomic K - N - K distance alternates from 5.051 to 3.941 along the c axis. The N - O distances increase from 1.245 (2) Å at 295 K to 1.2533 (15) Å at 123 K. The nitrate group has a slight non-planarity, with the N atoms 0.011 Å above the O plane and directed toward the more distant K of the K - N - K chain.