Monoclinic RbD$_{2}$PO$_{4}$: room temperature synthesis, chemical and structural stability upon heating

Monoclinic RbD$_{2}$PO$_{4}$ polycrystals (DRDP) were synthesized via the room tempearture crystallization of RbH$_{2}$PO$_{4}$ (RDP) dissolved in D$_{2}$O. Powder x-ray diffraction (XRD) data collected at T$=$25$^{\circ}$C indicate that this deuterated compound crystallizes in spacegroup P2$_{1}$/m with unit cell parameters a$=$7.688{\AA}, b$=$6.192{\AA}, c$=$4.781{\AA} and $\beta =$109.02, and is \textit{isomorphic} with the intermediate-temperature phase of its hydrogenated counterpart RDP. We found no evidence of previously reported [\textit{Phase Transitions} \textbf{80}, 17 (2007)] polymorphic phase transition in DRDP upon heating from room temperature to 210$^{\circ}$C. All lattice parameters vary smoothly within this temperature range, demonstrating that the P2$_{1}$/m phase persists upon heating. In addition, the unit cell volume of monoclinic DRDP is $\sim$ 1{\%} greater than that of its RDP polymorph at all temperatures between 150$^{\circ}$C and 210$^{\circ}$C, which indicates the absence of significant deuterium-hydrogen isotope exchange. Further heating to 240$^{\circ}$C leads to the thermal decomposition of the title compound via dehydration.