LOW-TEMPERATURE STRUCTURE OF MG2NIH4 - EVIDENCE FOR MICROTWINNING

X-ray powder diffraction patterns of the monoclinic low-temperature phase of Mg//2NiH//4 were analyzed in terms of a structure model which allowed for microtwinning. In contrast to previous studies, the intensity profiles of reflections that were broadened due to twinning and of reflections that had been attributed to a second, orthorhombic phase could be reproduced. A structure refinement based on this model gave a better fit (R//p equals 0. 12, R//F equals 0. 04) and more accurate metal atom parameters than those reported previously. In samples which were quenched from temperatures above 300 degree C the average distance between twin planes was found to be a few lattice translation periods.