Modulated phase ofK2MoxW1−xO4mixed compounds using orthorhombic symmetry: A powder di

Abstract

4 as exceptions among the members of AЈAЉBX 4 family because their thermotropic phase transitions do not follow the commonly observed sequence and because their modulation wave vector, unusually, lies on the pseudohexagonal plane. Until now, the modulated phases of these compounds have been interpreted using hexagonal symmetry. The present work indicates, however, that a model consisting of threefold twinned orthorhombic modulated domains better describes the satellites observed in the temperature range of 620-740 K. The Ccmm space group is proposed for the average structure of the whole family K 2 Mo x W 1−x O 4 , whereas the Ccmm͑␣00͒000 with the wave vector q = ͑0.5+ ␦͒a ‫ء‬ is the appropriate superspace group for the description of the modulated structures.The coexistence of three orthorhombic domains, each of which is rotated by 120°from each other, indeed reproduces a pseudohexagonal basis, which is a common feature in the AЈAЉBX 4 family. The usage of the twinning model allows a much simpler interpretation of the diffraction pattern and provides a more consistent way to describe the phase-transition sequence shown by the mixed K 2 Mo x W 1−x O 4 crystals.