A New Three-Dimensional Incommensurately Modulated Cubic Phase (in ZrP2O7) and Its Symmetry Characterization via Temperature-Dependent Electron Diffraction

A detailed electron diffraction study has been made of the ZrP2O7 member of the AM2O7 family of framework structures and of its temperature-dependent structural phase transformations. The room-temperature phase corresponds to a primitive 3×3×3 superstructure phase of probable space group symmetry Pa . An observed glide extinction condition when viewed down <001> axes could, however, also be compatible with resultant Pbca space group symmetry. The high-temperature phase corresponds to the normal unmodulated parent structure. An intervening intermediate phase that exists over a relatively narrow temperature window (~5–10°C) around 294°C is shown to be a three-dimensional incommensurately modulated cubic phase characterized by incommensurate primary modulation wave vectors in the close vicinity of 0.31 <100>* and probable superspace group symmetry P: Pa : Ia .