RbD2PO4: Room temperature synthesis, chemical and structural stablity upon heating

Monoclinic RbD2PO4 polycrystals were synthesized via the room temperature crystallization of RbH2PO4 dissolved in D2O. Powder x-ray diffraction (XRD) data collected at T=25 ºC indicate that this deuterated compound crystallizes in space group P21/m with unit cell parameters a=7.688Å, b=6.192Å, c=4.781Å and β=109.02°, and is isomorphic with the intermediate-temperature phase of its hydrogenated counterpart rubidium dihydrogen phosphate (RDP). We found no evidence of previously reported [ Phase Transitions 80, 17 (2007)] polymorphic phase transition in rubidium dideuterium phosphate (DRDP) upon heating from room temperature to 210 ºC. All lattice parameters vary smoothly within this temperature range, demonstrating that the P21/m phase persists upon heating. In addition, the unit cell volume of monoclinic DRDP is ~1% greater than that of its RDP polymorph at all temperatures between 150 ºC and 210 ºC, which indicates the absence of significant deuterium-hydrogen isotope exchange. Further heating to 240 ºC leads to the thermal decomposition of the title compound via dehydration. Finally, we demonstrate that, unlike its monoclinic polymorph, tetragonal DRDP synthesized at room temperature has a thermal behavior similar to that of its hydrogenated counterpart (RDP) both from the structural and from the chemical point of view