Electron diffraction and high resolution microscopy have been performed on Mg1-xAlxB2 with x~0.5. This composition displays a superstructure with a repeat period of exactly 2c along the c axis and about 10 nm in the a-b plane. The superstructure results in ring-shaped superreflections in the diffraction pattern. Irradiation by a strong electron beam results in a loss of the superstructure and a decrease of about 1% in the c lattice parameter. In-situ heating and cooling on the other hand showed that the superstructure is stable from 100 K to 700 K. Possible origins for the superstructure are proposed.
@article{arxiv.cond-mat/0109301,
title = {The complex superstructure in Mg1-xAlxB2 at x~0.5},
author = {H. W. Zandbergen and M. Y. Wu and H. Jiang and M. A. Hayward and M. K. Haas and R. J. Cava},
journal= {arXiv preprint arXiv:cond-mat/0109301},
year = {2009}
}