Structural study of the low‐temperature phase transition in Cs3D(SeO4)2

The X-ray structure of tricaesium deuteriumbiselenate, Cs3D(SeO4)2, M, = 686.7, has been studied at 25, 190 and 297 K with particular attention being paid to the low-temperature phase transition at 7",.3 = 180 K. The structure of Cs3H(SeO4)2 has also been studied at 297 K. The data were refined in the monoclinic space group C2/m, Z = 2, at 297 and 190 K, and in P2j/m, Z = 2, at 25 K (Mo Ka radiation, a = 0.71073A). For Cs3D(SeO4)2, R(F2)=0.0365 for 1801 unique reflections at 297 K, R(F 2) = 0.0389 for 1787 unique reflections at 190 K and R(F 2) = 0.0405 for 3514 unique reflections at 25 K. For Cs3H(SeO4)2, R(F 2) = 0.0637 for 1265 unique reflections at 297 K. Temperature effects on the structure are mainly noticeable in the Cs--O bond distances of the oxygen coordination polyhedra of Cs (0.016/~ per 100 K on average). Accompanying the 1",.3 transition, the space group changes from C2/m to P21/m, and the two SeO4 groups in the dimer become nonequivalent with one adopting HSeO£ character and the other SeO 2- character. As a result, the dimers have a net dipole moment and are arranged in an antipolar way, similar to K3D(SO4)2. An examination of the room-temperature structure of Cs3D(SeO4) 2 and other M3H(XO4)2-type crystals reveals that the non-H atoms lie in approximately the same position in both cases and that the only major difference is that half of the hydrogens in other M3H(XOa)2-type crystals are involved in hydrogen-bonded dimers which are formed with two different adjacent selenate groups. The successive transitions in Cs3D(SeO4)2 are characterized as an order-disorder transition of the donor-oxygen atom at T,.~, reorganization of the hydrogen bonds at To2 and an order~tisorder transition of the proton at 7",.3.