Rubidium acetate (RbOAc) has two polymorphic crystal structures. A phase transition occurs at approximately 500 K, with the high-temperature phase being orthorhombic (Pmmn) and the low-temperature phase monoclinic (P2₁/m). Six oxygen atoms surround the Rb⁺ ion (one in a bidentate coordination mode and four in monodentate bridging modes), forming a polymeric bilayer structure. Both acid salts are obtained simultaneously during synthesis.
Comparison of sodium, potassium, and rubidium acetates: Sodium, potassium, and rubidium acetates all exhibit layered structures (β-NaOAc represents the sodium salt). As the cation radius increases, the metal–oxygen distance increases, the interlayer spacing decreases, and hygroscopicity increases (NaOAc is the least hygroscopic, whereas RbOAc deliquesces immediately). The phase-transition temperature increases with increasing cation radius (Na: 259 K; K: 423 K; Rb: 498 K), whereas the order of the melting points is reversed.
