The phase transition between dimerized-antiferromagnetic and   uniform-antiferromagnetic phases in impurity-doped spin-Peierls cuprate   CuGeO3
  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. 
T. Masuda, A. Fujioka, Y. Uchiyama, I. Tsukada, and K. Uchinokura (The   University of Tokyo)