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 Pa3̄. 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: Pa3̄: Ia3̄.