B. Doudin

Study of the modulated phase of (C3H7NH3)2CdCl4 by single-crystal X-ray diffraction

Bis(n-propylammonium) tetrachlorocadmate shows a modulated phase stable between two orthorhombic phases on the temperature scale. The superspace group could be determined unambiguously on the basis of the selection rules. In addition to Bragg main reflections, 1137 first-order and 107 second-order satellites with an intensity larger than 3a were measured. The refinement revealed a displacive modulation of harmonic type. The CdCI 6 octahedra show modulation amplitudes larger than 0.2 A. To a first approximation, the dynamics of the octahedra can be analyzed in terms of a rotation about the a axis and a translation along the c axis. The low-temperature phase can be considered as a lock-in of the modulated one with a change of q from 0.42b* to 1 b*. In this phase the rotation of the octahedra about a is frozen and a unique hydrogen-bonding scheme is adopted between the alkylammonium ion and the C1 atoms. For the modulated phase, Mr= 374.5, superspace group PAbs-T~(O,fl, O), a = 7.585 (5), b = 7.359 (1), c = 25.053 (6) A, V= 1398 (1) A 3, Z = 2 (primitive cell), MoKa, 2=0.71069A, ~=22.5cm -~, wR(F) =0.072 for 1791 independent reflections [wR(F)= 0.088 for first-order satellites].

Limits of the Patterson method for the determination of one-dimensionally modulated structures

The Patterson function can reveal the modulation of a basic structure if its summation terms include only satellite reflections. Examples of Patterson summations for various types of modulated structures illustrate the difficulties of interpretation and the bias occurring in the determination of the amplitudes and phases of the modulation. The opportunity to use the Patterson function to determine modulated structure is discussed.

The study of incommensurate modulated phases by diffraction methods

Abstract Incommensurate modulated crystals are part of a more general group of materials known as aperiodic crystals. They can be interpreted as a periodic deformation of a conventional three-dimensional structure, the deformation of which is independent of the periodicity. Incommensurate modulated phases are mainly detected by diffraction owing to the presence of the so-called satellites, generally weak reflections surrounding Bragg peaks or systematically absent reflections.

X-ray study of the reentrant modulated phase of Bis-Propylammonium Tetrachlorocuprate(II) at high temperature

Abstract The modulated phase of (C3H7NH3)CuCl4 stable between 386 and 412 K has been analyzed by single-crystal X-ray diffraction. The structure was solved in the frame of the superspace group approach. The modulation is characterized by a wavelike displacement of the CuCl4 layer.