Neutron diffraction study on hydrogen bond structure inK3H(SeO4)2 and K3D(SeO4)2 crystals

Abstract

Neutron diffraction experiments on the room- and low-temperature phases ofK3H(SeO4)2 (Ttrs = 21 K) and K3D(SeO4)2 (Ttrs = 105 K)have been performed using the time-of-flight high-resolution powderdiffractometer HRPD at the ISIS pulsed neutron source. Data were collected at 2and 30 K for K3H(SeO4)2 and 2 and 130 K for K3D(SeO4)2 overthe d-spacing range 0.7-2.4 Å. The room-temperature phases of the crystalshave monoclinic structure belonging to the space group A2/a. Rietveldrefinement combined with the published dielectric and calorimetric studiesreveals that H/D atoms are disordered over two symmetry-equivalentpositions. The hydrogen bonds are non-linear with an O-H/D···O angleof 175(1)°. The hydrogen bond length is 2.495(1) Å inK3H(SeO4)2 and 2.533(2) Å in K3D(SeO4)2. The separationbetween the half occupancy hydrogen positions is 0.32(1) Å forK3H(SeO4)2 and 0.550(4) Å for K3D(SeO4)2. The relationbetween these structural parameters and the isotope effect in the transitiontemperature is discussed. Diffraction patterns of the low-temperature phasesindicated that the structure change accompanying the transition is very small.