Neutron diffraction study on hydrogen bond structure in K3H(SeO4)(2) and K3D(SeO4)(2) crystals

Neutron diffraction experiments on the room- and low-temperature phases of K3H(SeO4)(2) (T-trs = 21 K) and K3D(SeO4)(2) (T-trs = 105 K) have been perf ormed using the time-of-flight high-resolution powder diffractometer HRPD a t the ISIS pulsed neutron source. Data were collected at 2 and 30 K for K3H (SeO4)(2) and 2 and 130 K for K3D(SeO4)(2) over the d-spacing range 0.7-2.4 Angstrom. The room-temperature phases of the crystals have monoclinic stru cture belonging to the space group A2/a. Rietveld refinement combined with the published dielectric and calorimetric studies reveals that H/D atoms ar e disordered over two symmetry-equivalent positions. The hydrogen bonds are non-linear with an O-H/D O angle of 175(1)degrees. The hydrogen bond lengt h is 2.495(1) Angstrom, in K3H(SeO4)(2) and 2.533(2) Angstrom in K3D(SeO4)( 2). The separation between the half occupancy hydrogen positions is 0.32(1) Angstrom for K3H(SeO4)(2) and 0.550(4) Angstrom for K3D(SeO4)(2). The rela tion between these structural parameters and the isotope effect in the tran sition temperature is discussed. Diffraction patterns of the low-temperatur e phases indicated that the structure change accompanying the transition is very small.