We have performed detailed magnetic susceptibility measurements as well as synchrotron x-ray diffraction studies to determine the temperature vs concentration (T - x) phase diagram of Cu1−xMgxGeO3. We observe clear double peaks in the magnetic susceptibility implying two antiferromagnetic (AF) transition temperatures in samples with Mg concentrations in the range 0.0237 ≤x≤ 0.0271. We also observe a drastic change in the inverse correlation length in this concentration range by x-ray diffraction. The drastic change of the AF transition temperature as well as the disappearance of the spin-Peierls (SP) phase have been clarified; these results are consistent with a first-order phase transition between dimerized AF (D-AF) and uniform AF (U-AF) phases as reported by T. Masuda {\it et al.} \lbrack Phys. Rev. Lett. {\bf 80}, 4566 (1998)\rbrack. The T - x phase diagram of Cu1−xZnxGeO3 is similar to that of Cu1−xMgxGeO3, which suggests that the present phase transition is universal for Cu1−xMxGeO3.
@article{arxiv.cond-mat/9906130,
title = {The first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in Cu_(1-x)M_xGeO_3},
author = {T. Masuda and I. Tsukada and K. Uchinokura and Y. J. Wang and V. Kiryukhin and R. J. Birgeneau},
journal= {arXiv preprint arXiv:cond-mat/9906130},
year = {2009}
}