Magnetic properties of Cu6Ge6O18-xH2O (x=0-6):: A compound of S=1/2 Heisenberg competing antiferromagnetic chains coupled by interchain interaction -: art. no. 214421

We studied magnetic properties of powder samples of Cu6Ge6O18-xH(2)O (x=0-6). Susceptibility above the antiferromagnetic (AF) transition temperature (T-N) agrees with susceptibility obtained from the one-dimensional Heisenberg S=1/2 model with competing AF interactions. Since the estimated ratio (0.27-0.31) between nearest-neighbor and next-nearest-neighbor AF exchange interactions is close to a critical value (0.24-0.30) which determines whether a spin gap exists or not, the spin system in Cu6Ge6O18-xH(2)O 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 the temperature at which the susceptibility is maximum, shows a unique dependence on x. We speculate that a spin gap due to competing AF interactions exists at x<1.54 and that magnetic excitation is gapless at x>1.54, explaining qualitatively the dependence of T-N/T-max on x.