PHASE-TRANSITION IN RUBIDIUM NITRATE AT 346-K AND STRUCTURE AT 296-K, 372-K, 413-K AND 437-K

Rubidium nitrate, RbNO3, M(r) = 147.474, trigonal, space group P3(1) for both structures: below the irreversible first-order phase transition at 346 K and above. Another crystal exhibited space group P3(2). Crystal growth from aqueous solution at 290 K. At 296 K: a = 10.474 (1), c = 7.443 (1) angstrom, V = 707.2 (2) angstrom 3, D(x) = 3.116 (1) g cm-3, refractive indices n(omega) = 1.5250 (1) and n(epsilon) = 1.5267 (3). After heating (to 372 K) in a quartz capillary: a = 10.506 (3), c = 7.469 (3) angstrom, V = 714.0 (5) angstrom 3, D(x) = 3.087 (3) g cm-3. F(000) = 612, lambda(Ag K-alpha) = 0.56087 angstrom, mu = 82.7 cm-1, Z = 9. Least-squares refinement with anisotropic temperature factors and an isotropic extinction parameter confirmed two different structures (before and after heating). Final R = 0.0219 (0.0301) for 970 (727) unique reflections with I > 3-sigma(I). Both structures differ from that reported in the literature. The Rb atoms form a pseudocubic sublattice with nine pseudocubes per unit cell. Each NO3 group is essentially parallel to one of the faces of its surrounding Rb-atom pseudocube. The phase transition is associated with a 90-degrees flipping of one third of the NO3 groups. On heating the crystal in a quartz capillary, a phase transition occurs between 296 and 372 K. The same crystal was used for measurements at 296, 372, 413, 437 and 296 K, respectively. Once formed, the new phase proved to be stable at all these temperatures. Differential thermal analysis revealed a transition temperature of 346 K.