First-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in Cu1-xMxGeO3

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 dear double peaks in the magnetic susceptibility implying two antiferromagnetic (AF) transition temperatures in samples with Mg concentrations in the range 0.0237 less than or equal to x less than or equal to 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 er al. [Phys. Rev. Lett. 80, 4566 (1998)]. 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.