Static and dynamic structures of CD3ND3GeCl3 studied by TOF high resolution neutron powder diffraction and solid state NMR

Abstract

CD₃ND₃GeCl₃ has been studied by high-resolution neutron powder diffraction and ²H NMR investigation over a wide temperature range to understand the static and dynamic structures and successive phase transitions. A monoclinic crystal of CD₃ND₃GeCl₃ (Phase IV, space group P2₁/n) undergoes a phase transition to an orthorhombic system (Phase III, Pnma) at 203 K where the symmetry of the anionic sublattice and the geometry of the N–DCl hydrogen bonds change. Furthermore, Phase III undergoes a phase transition to a trigonal system (Phase II, R3m) at 349 K where an orientational disorder of the cation is excited. Our ³⁵Cl NQR measurement also suggested an onset of the reorientation of the pyramidal GeCl₃ ⁻ anion just below the phase transition to Phase II with an activation energy E_a = 60.5 kJ mol⁻¹. However, a transition from Phase II to a cubic phase (Phase I, Pm3m), which was reported to be 396 K on the CH₃NH₃GeCl₃, was not complete by 475 K. The Rietveld analysis at 475 K suggested that only 7% of the sample transformed to a cubic perovskite structure in which not only orientational disorder of the cation but also positional disorder of the chloride ion were confirmed, similar to CH₃NH₃GeCl₃.