Polymorphs of rubidium nitrate and their crystallographic relationships

The crystal structure relationships between the four thermal polymorphs (IV – 437 K – III – 492 K – II – 564 K – I) of RbNO3are discussed. Trigonal IV, a = 10.55(2), c = 7.47(2) Å at 298 K, Z = 9, and space group P31(or its enantiomorph P32) has Rb atoms arranged on a pseudocubic sublattice defined by and related segments. The ordered NO3groups are enclosed within respective pseudocubes and form close to ideal 8-fold anion–cation coordination with the surrounding Rb atoms. At IV → III, each of the appropriate (1/3) <10.1> and (1/3) <11.1> segments of IV (forming the Rb atom sublattice) transform to a cell edge of an ideal s.c. lattice of Rb atoms, a = 4.39(1) Å, Z = 1, and space group Pm3m. The orientationally disordered NO3groups in III form another close to ideal 8-fold anion–cation coordination with the Rb atoms. At III → II, the s.c. lattice of III changes to form a larger b.c.c. lattice, a = 8.84(3) Å, Z = 8, in II. A parallel-axes unit cell relationship, i.e. <100>III//<100>IIand 2(a)III = (a)II, exists between III and II. The NO3group in II is postulated to form a close to ideal 8-fold anion–cation coordination with the surrounding Rb atoms. At II → I, II transforms to a f.c.c. lattice, a = 7.32 Å, Z = 4, and space group Fm3m. The NO3group in I forms a close to ideal 6-fold anion–cation coordination with the surrounding Rb atoms. It is suggested that and related segments in II, which form a trigonal Rb atom sublattice, a = 7.65(3) Å and α = 109.47°, after II → I, transform to orthogonal cell edges of the cubic lattice of I.