Phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in the impurity-doped spin-peierls cuprate CuGeO3

We report a first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic phases in impurity-doped spin-Peierls (SP) cuprate Cu1-xMgxGeO3. As Mg concentration increases, linear reduction of the SP transition temperature (T-SP) and linear increase of the Neel temperature (T-N) are observed for x up to x(c) similar or equal to 0.023. At x(c) the SP transition suddenly disappears and T-N jumps discontinuously. The peak of the susceptibility at x(c) around T-N is not as sharp as those at other concentrations, which indicates the separation of low and high concentration phases. These results indicate the existence of a first-order phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic long-range orders.