First-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases

We have performed detailed magnetic susceptibility measurements as well as synchrotron x-ray-diffraction studies to determine the temperature vs concentration (Formula presented) phase diagram of (Formula presented) We observe clear double peaks in the magnetic susceptibility implying two antiferromagnetic (AF) transition temperatures in samples with Mg concentrations in the range (Formula presented) 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 (Formula presented) phase diagram of (Formula presented) is similar to that of (Formula presented) which suggests that the present phase transition is universal for (Formula presented).