Magnetism of Cu₆ Ge₆ O₁₈-x H₂ O (x = 0 6), a compound of the one-dimensional Heisenberg S = 1/2 model with competing antiferromagnetic interactions

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We measured the magnetic susceptibilities of Cu₆Ge₆O₁₈-xH₂O (x = 0 6). Susceptibility above the antiferromagnetic (AF) transition temperature (T N) agrees with susceptibility of the one-dimensional Heisenberg S = 1/2 model with competing AF interactions. From the estimated ratio between nearest-neighbor and next-nearest-neighbor AF exchange interactions, the spin system is probably located near a boundary between spin systems with gapless and gapped magnetic excitation. The value of T N/T max, where T max is temperature at which the susceptibility is maximum, shows a unique dependence on x. The x dependence can be explained qualitatively by taking phase transition as a function of x and the effect of disorder into account.