  We discovered a first-order phase transition between
dimerized-antiferromagnetic and uniform-antiferromagnetic phases in
impurity-doped spin-Peierls (SP) cuprate Cu{1-x}MgxGeO3. As impurity
concentration increases, linear reduction of SP transition temperature (T{SP})
and linear increase of Neel temperature (TN) are observed up to x = 0.023. At
this critical concentration (xc) SP transition suddenly disappears and TN jumps
discontinuously. The peak of the susceptibility at xc around TN is not so sharp
as those at other concentrations, which indicates the phase separation of low
and high concentration phases. These results indicate the existence of the
first-order phase transition between dimerized-antiferromagnetic and
uniform-antiferromagnetic long-range orders.
