A humidity-controlled precipitation technique enabling discovery of Rb3(H1.5PO4)2

Abstract

The previously unknown compound Rb3(H1.5PO4)2 is successfully synthesized here using a newly developed variant of aqueous precipitation crystal growth. The approach exploits the phenomenon of boiling point elevation in concentrated solutions. Crystals of the title compound were obtained upon heating a stoichiometric solution from 110 to 150 ​°C under a high steam partial pressure of 0.83 ​atm. Single crystal X-ray diffraction studies revealed Rb3(H1.5PO4)2 crystallizes in space group C2/m and is isostructural to Cs3(H1.5PO4)2. As evidenced by thermal analysis, Rb3(H1.5PO4)2 does not undergo a phase transition to a trigonal superprotonic phase upon heating. Even under a steam partial pressure of 0.82 ​atm, under which dehydration is suppressed to a temperature of 263 ​°C, no polymorphic transition is detected. The behavior parallels that of Cs3(H1.5PO4)2 and contrasts that of several structurally and chemically similar selenate compounds. The crystal growth approach developed here may prove particularly useful for obtaining water soluble compounds which are thermodynamically or kinetically disfavored at temperatures close to ambient.