Structure of a high temperature phase of potassium dideuteriophosphate (DKDP)

We previously determined the crystal structures of the high temperature phases of KH2PO4 [J.A. Subramony, S. Lovell, B. Kahr, Chem. Mater. 10 (1998) 2053. [6]]. These triclinic and monoclinic phases were obtained by heating the room temperature tetragonal form until new crystal phases were identified by polarization microscopy. These samples were subsequently cooled to room temperature thereby preserving the metastable high temperature phases for analysis. KD2PO4 is distinct from KH2PO4 in that it crystallizes at room temperature in a monoclinic phase unknown for its isotopomer, therefore the question as to whether it would support the corresponding high temperature phases remained open until our report here of the transformation of monoclinic KD2PO4 to an isomorphous triclinic high temperature phase: space group P 1 ¯ with a=7.475(1) Å, b=7.440(1) Å, c=7.184(1) Å, α=88.53(1) Å, β=86.81(1)°, γ=88.09(1)°, V=398.58 Å3. This is the first known high temperature phase of DKDP, a material coveted at room temperature for its optical properties.