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

We measured the magnetic susceptibilities of Cu$_6$Ge$_6$O$_{18}$-xH$_2$O ($x = 0 \sim 6$). Susceptibility above the antiferromagnetic (AF) transition temperature ($T_{\rm N}$) 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 $T_{\rm N}/T_{\rm max}$, where $T_{\rm max}$ 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.