On the Crystal Structure of the Trigonal (and Cubic) Phase of Potassium Nitrite.

Ionic Dynamics in Plastic Crystal KNO2 Studied by 39K and 15N NMR

“…In Phase I (cubic, Fm3m, a = 6.66 Ä), the presence of isotropically disordered orientations of N 0 2 ions was predicted from structural and thermal studies [2,9]. In accordance with these results, a narrowed single 15N spectrum with a width of ca.…”

“…In this phase, forming a rhombohedral lattice (R3m), the NO J ion is on the C3-axis which is perpen- dicular to the molecular plane [2]. From these crystal data, in-plane disorder as for the anionic orientation is expected.…”

“…The plastic phase I forms an NaCl-type cubic lattice (Fm3m) [2] between 314.7 K (Ttri) and the melting temperature (Tm) of 710 K. Phase II, having a rhombohedral struc ture (R3m) [2,12], is stable above 264.1 K(TtrI1). De tailed structure is unknown for Phase III, which was reported to form a monoclinic lattice below ^trll [2,13]. In the present study, we intend to reveal the ionic dynamics in the plastic phase I and the disor dered structure expected in Phase III of K N 0 2 by measuring 39K and 15N N M R relaxation times and 15N NMR spectra.…”

“…In K N 0 2, three solid phases I, II, and III from the hightemperature side have been reported [9]. The plastic phase I forms an NaCl-type cubic lattice (Fm3m) [2] between 314.7 K (Ttri) and the melting temperature (Tm) of 710 K. Phase II, having a rhombohedral struc ture (R3m) [2,12], is stable above 264.1 K(TtrI1). De tailed structure is unknown for Phase III, which was reported to form a monoclinic lattice below ^trll [2,13].…”

“…400 K above room temperature [1], In this phase of NaCl-or CsCl-type cubic structure [2][3][4][5], highly disordered N 0 2 orientations and rapid ionic motion of large amplitude are expected. The heat ca pacity curve at the transition to the plastic phase in each salt [6][7][8] (at the low-temperature transition for K N 0 2 [9]) is accompanied by a long tail on the lowtemperature side extending over 100 K, suggesting some disordered structure in the low-temperature phase.…”