Monoclinic RbD2PO4: Room temperature synthesis, chemical and structural stability upon heating

Monoclinic RbD2PO4 polycrystals (DRDP) were synthesized via the room temperature crystallization of RbH2PO4 (RDP) dissolved in D2O. Powder X-ray diffraction (XRD) data collected at T = 25 °C indicate that this deuterated compound crystallizes in spacegroup P21/m with unit cell parameters a = 7.688 Å, b = 6.192 Å, c = 4.781 Å and β = 109.02°, and is isomorphic with the intermediate-temperature phase of its hydrogenated counterpart RDP. We found no evidence of a previously reported [Phase Transitions80, 17 (2007)] polymorphic phase transition in DRDP upon heating from room temperature to 210 °C. 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 °C and 210 °C, which indicates the absence of significant deuterium-hydrogen isotope exchange. Further heating to 240 °C leads to the thermal decomposition of the title compound via dehydration. - Highlights: • P21/m monoclinic RbD2PO4 is structurally and chemically stable up to 210 °C. • Unit cell thermal expansion of monoclinic RbD2PO4 is anisotropic. • Volume of monoclinic RbD2PO4 is about 1% greater than that of hydrogenated RDP. • Room temperature RbD2PO4 is isomorphic with intermediate-temperature RbH2PO4. • RbD2PO4 decomposes chemically via dehydration at 240 °C