On heating KNOa at atmospheric pressure, a solid-state phase transformation occurs from e-KNO3 to fl-KNO3 at ,-, 128°C. On cooling from ~200°C, fl-KNO3 transforms at ~ 124°C to y-KNO3, which reverts to e-KNO3 at ,-, 100 °C. The precise temperature range in which the various phases occur depends on the previous treatment of the sample. X-ray powder diffractometer data have been used to determine the structures of y-and fl-KNO3, and the hitherto ambiguous situation concerning the NO3 orientation and disorder has been resolved. For y-KNO3 the space group is R3m with hexagonal axes a= 5.487 (1), c=9.156 (3)A (91°C), Z=3, while for fl-KNO3 the corresponding data are R3m, a=5.425 (1), c= 9-836 (4) ,~ (151 °C), Z= 3. In ~'-KNO3, there are two equivalent types of NOa positions, but within each cell the NO3 groups all belong to one type. With fl-KNO3, each NO3 is in disorder between the two types of position. Relationships between the structures of the e-, B-and y-phases are given.
IntroductionAt atmospheric pressure, KNO3 can exist in several different phases depending on the temperature. When heated from room temperature through the transition at ~ 128 °C, KNO3 transforms from an orthorhombic structure (~-KNO3) to a trigonal structure (fl-KNO3). On cooling the fl-phase from ~200°C, KNO3 passes through another trigonal phase (y-KNO3) between ,-~ 124°C and ~ 100°C before reverting to the ~-phase. y-KNO3 exhibits ferroelectric properties (Sawada, Nomura & Fujii, 1958).The structure of ~-KNOa has been studied by Edwards (1931) and more recently by Nimmo & Lucas (1972, the latter using single-crystal neutron diffraction data; the Nimmo & Lucas structure has been confirmed by Holden & Dickinson (1975) with single-crystal X-ray data. The unit cell is orthorhombic with a=5.414 (2), b=9.166 (9), c=6.431 (9) A (25°C), Z=4; the space group is Pmcn and the structure is of the aragonite type. fl-KNOa has been investigated by Tahvonen (1947), Shinnaka (1962) and Stromme (1969), and y-KNO3 by Barth (1939) and Stromme (1969). In these phases, the NO3 conformation and orientation is uncertain. Ordered models have been considered for y-KNOa, static and dynamic disordered models for/3-and y-KNO3, and free rotation models for fl-KNO3. The 7-KNO3 space group has been taken as R3m, but both R3m and R3c have been suggested for fl-KNO3. All fl-and y-KNO3 structural studies have used the X-ray powder diffraction photographic data of Tahvonen (1947) and Barth (1939), respectively. The latter data contained only ten observed reflexions. Shinnaka (1962) also made X-ray diffuse scattering photographic studies.The aim of the present study was to determine the fl-and y-KNO3 structures, and to relate the ~-, fl-and ),-KNO3 structures. Experimental fl-and ~,-KNO3 data were collected on an X-ray powder diffractometer incorporating a small electrical sample furnace. The briquet sample of analytical grade KNO3 was continuously rotated about the diffraction vector. 09--20 scans were made with 3(20)=0.05 ° steps and a step-time of 100 s. Filtered Cu K~ rad...
