Structures of the high temperature phases of rubidium nitrate

Abstract

Applying Rietveld's profile refinement method to the neutron powder diffraction patterns, the structures and orientations of the nitrate group in RbNO3 are determined. Phase III (437 to 492 K) has cubic CsCl‐structure, Pm3m, with one molecule per unit cell. The structural analysis gives a slight preference to the twelve‐orientations model for rigid NO 3−. Phase II (492 to 557 K) has rhombohedral unit cell with space group R3m, phase I (557 to 583 K) cubic NaCl‐type unit cell, Fm3m, with four molecules per unit cell. In both of these phases the aragonite type of disorder for the rigid nitrate group is proposed.

Polymorphs of rubidium nitrate and their crystallographic relationships

“…Its crystal structure was originally suggested to have the Rb atoms and NO3 groups forming a NaC1-type cation-anion arrangement. Single crystal X-ray (21) and neutron powder (14) diffraction studies also concluded a similar structure with an aragonite-type configuration of the orientationally$isordered NO3 groups. The structure (14), with a = 7.32(2) A, Z = 4 and space group Fm3m, is shown in Fig.…”

“…The Rb atoms were at cell comers, while the NO3 group was orientationally disordered over mainly aragonite-type (each NO3 group plane normal to a body diagonal of the surrounding Rb atom cube) and partly calcite-type (each NO3 group plane bisects a cell edge of the surrounding Rb atom cube) configurations. Recently, powder (14) and single crystal (15) neutron diffraction studies concluded that a 12-fold orientationally disordered model for the NO3 group (each NO3 group plane being parallel to a face of the surrounding Rb cube) is preferable to an 8-fold disordered model (pure aragonite-type configuration). This model is compatible with the observed single crystal X-ray diffuse scattering intensity patterns that occur perpendicular to the reciprocal lattice axes (I?, 18).…”

“…X-ray powder diffraction data of I1 have been indexed on the basis of three different crystal systems, viz trigonal (3,6,14), tetragonal (2), and cubic (16). Single crystal diffraction methods to determine the crystal system have not been successful so far, as various reports (2,19,20,21) indicate that single crystals (grown in IV) fragment during the 111 + I1 transition, and are unsuitable for single crystal diffraction studies in 11.…”

“…The unit cells of the proposed trigonal, tetragonal, or cubic crystal systems are geometrically closely related and this makes difficult discrimination (22) between the fit to the observed X-ray diffraction powder pattern, although as 111 has been established to have a cubic crystal system (14,15), a lowering of crystal symmetry to trigonal or tetragonal with higher temperature phase appears unlikely. Antiferroelectricity has been reported for I1 (23)(24)(25), which requires arrangements of antiparallel dipoles within the unit cell.…”

“…lattice with a = 7.32 A and Z = 4 (2,6,14,20,21). Its crystal structure was originally suggested to have the Rb atoms and NO3 groups forming a NaC1-type cation-anion arrangement.…”