Morphological determination of modulated-cell parameters of Rb2ZnBr4

The morphology of Rb2ZnBr4 single crystals as observed in growth sphere experiments is analyzed on the basis of geometrical morphological laws extended to take into account the incommensurate modulated nature of these crystals. Especially in the past, identification of crystals faces (hkl) on the basis of these classical morphological laws has been a powerful tool in the determination of 3-dimensional cell parameters. In analogy, the relative magnitude of the modulation wave vector (the fourth lattice constant in the language of the superspace approach) can be deduced now from the identification of satellite faces described by four low rational indices (hklm).