In its gas phase, RbCl is diatomic with a bond length estimated at 2.7868 Å. This distance increases to 3.285 Å for cubic RbCl, reflecting the higher coordination number of the ions in the solid phase. Depending on conditions, solid RbCl exists in one of three arrangements or polymorphs as determined with holographic imaging:
The sodium chloride polymorph is most common. A cubic close-packed arrangement of chloride anions with rubidium cations filling the octahedral holes describes this polymorph. Both ions are six-coordinate in this arrangement. This polymorph's lattice energy is only 3.2 kJ/mol less than the following structure's.
At high temperature and pressure, RbCl adopts the caesium chloride structure. Here, the chloride ions form a simple cubic arrangement with chloride anions occupying the vertices of a cube surrounding a central Rb+. This is RbCl's densest packing motif. Because a cube has eight vertices, both ions' coordination numbers equal eight. This is RbCl's highest possible coordination number. Therefore, according to the radius ratio rule, cations in this polymorph will reach their largest apparent radius because the anion-cation distances are greatest.
Sphalerite (tetrahedral 4:4)
The sphalerite polymorph of rubidium chloride has not been observed experimentally. This is consistent with the theory; the lattice energy is predicted to be nearly 40.0 kJ/mol smaller in magnitude than those of the preceding structures.
Synthesis
The most common preparation of pure rubidium chloride involves the reaction of its hydroxide with hydrochloric acid, followed by recrystallization: Because RbCl is hygroscopic, it must be protected from atmospheric moisture, e.g. using a desiccator. RbCl is primarily used in laboratories. Therefore, numerous suppliers produce it in smaller quantities as needed. It is offered in a variety of forms for chemical and biomedical research.
Rubidium chloride is an excellent non-invasive biomarker. The compound dissolves well in water and can readily be taken up by organisms. Once broken in the body, Rb+ replaces K+ in tissues because they are from the same chemical group. An example of this is the use of a radioactive isotope to evaluate perfusion of heart muscle.
