Critical neutron scattering study of the compositional phase transition   in Mg-doped CuGeO3
  Cu1-xMgxGeO3 undergoes a first-order phase transition at a critical concentration xc between an antiferromagnetic (AF) state on dimerized lattice (D-AF) and an AF Neel state on undistorted uniform lattice (U-AF). Previous magnetic susceptibility measurements showed xc = 0.023 while a recent neutron scattering study reported xc = 0.027 +- 0.001. The present critical scattering due to antiferromagnetic fluctuations near the superlattice reflection (0,1,1/2) unambiguously determines xc = 0.028 +- 0.001 at TN = 3.4 ~ 4 K. Also at T = 1.3 K, the phase boundary was determined as xc = 0.028 +- 0.001 by observation of a jump of an effective magnetic moment across xc. 
M. Nishi, H. Nakao, Y. Fujii, T. Masuda, K. Uchinokura, and G. Shirane