Acentricity and phase transitions for some AM2X6 compounds

Abstract Phases of the type AM 2 X 6 with a pyrochlore related structure were studied by x-ray diffraction, DSC, and second harmonic techniques and in one case by Tl 205 NMR. Although these phases are ideally centric and cubic, several were found to be acentric. Two, TlNbWO 6 and RbNbWO 6 were found to be noncubic, but transitions to higher symmetry were observed for both phases above room temperature.