Phase transition of KD3(SeO3)2 studied by neutron diffraction

Crystal structure of low temperature phase of KD3(SeO3)2 was studied by neutron diffraction method. Space group is Pbcn at the high temperature phase and changes to P21/b below the transition point. At the low temperature phase, striking twin patterns due to spontaneous shear strain were observed. Suitable corrections were applied to the observed data and the result of structure analysis revealed that a hydrogen atom which is disordered at the high temperature phase is distinctly bonded to one of the oxygens of SeO3. This induces slightly different orientational changes and deformations of two SeO3 tetrahedra connected by the hydrogen. Discussion on the phase transition mechanism is given in parallel to that of KDP. (author)