[en] The previously determined structure of CsCuBr₃ is rationalized and can be understood as a combination of a Jahn-Teller effect and magnetic exchange interactions. Its hc structure is a consequence of a spin-Peierls effect. The unusual occurrence of this effect arises because of alternating stretched and bent Cu-Br-Cu superexchange paths. (author). 2 figs., 20 refs

[en] The connection between the C-M₂O₃ compounds (example Y₂O₃) and a CaF₂-type structure is studied. The space group (SG) of Y₂O₃ is T_h⁷, that of CaF₂ is O_h⁵, the first being a subgroup of the latter (index 32). The distortion mode is single and belongs to a point on the Σ-line in the first BZ of O_h⁵. This same mode occurs in yellow InCl (SG T⁴, heavily distorted NaCl-structure). Although the space groups of the parent structures are the same, these differ in the cation-anion ratio, so that the resemblance is restricted to cations on a threefold axis. We observed previously in InCl an invariant connected to this mode. This same invariant, also occurs in Y₂O₃. It is a ferri-distortive scalar of symmetry A_2g (with respect to O_h) and invariant for the point group of the crystal (T_h). There appear two types of Y³⁺-ions. Y₁, with site-symmetry S₆, and Y₂ (C₂). In the surroundings of the latter, screws and twists are observed. These lead to antiferro-ordered expressions of A_1u and A_2u-type.

[en] We derive the crystal-structures of FeS₂ (pyrite), N₂ and CO₂ and similar species, where the double-ions or molecules can be regarded as linear entities, placed in a face-centred cubic (FCC) lattice. The interaction between two of these elongation directions is determined by three constants. A minimum is reached by orienting the elongations of neighbouring ions or molecules in a perpendicular way, which points to electrostatic quadrupole-quadrupole interactions and which has been noted before in ortho-hydrogen and para-deuterium. For several points in the Brillouin zone the interaction is calculated. Especially the X-points are of interest and determine the space group Pa3^¯, which is a subgroup of Fm3^¯m, the original group of the fcc lattice. We derive also an alternative setting for Pa3^¯. The conclusion is made that for all these compounds with a Pa3^¯-structure an electrostatic quadrupole-quadrupole interaction is present.