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 ${\mathrm{Cu}}_{1-x}{\mathrm{Mg}}_x{\mathrm{GeO}}_3.$ We observe clear double peaks in the magnetic susceptibility implying two antiferromagnetic (AF) transition temperatures in samples with Mg concentrations in the range $0.0237<~x<~\mathrm{0.0271.}$ 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 phase have been clarified; these results are consistent with a first-order phase transition between dimerized AF and uniform AF phases as reported by T. Masuda et al. [Phys. Rev. Lett. 80, 4566 (1998)]. The $T-x$ phase diagram of ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{GeO}}_3$ is similar to that of ${\mathrm{Cu}}_{1-x}{\mathrm{Mg}}_x{\mathrm{GeO}}_3,$ which suggests that the present phase transition is universal for ${\mathrm{Cu}}_{1-x}{M}_x{\mathrm{GeO}}_3.$

• Received 8 June 1999

DOI:https://doi.org/10.1103/PhysRevB.61.4103