Specific heat of quasi-one-dimensional spin 1/2 system CuNb2O6

The specific heat C of CuNb2O6 with quasi-one-dimensional zig-zag spin S = 1/2 chains has been measured in the temperature (T) range between 1.6K and 40K. For the orthorhombic phase, a sharp anomaly of C is observed at the antiferromagnetic transition temperature T-N similar to 7.3 K. The T-dependence of C of the monoclinic phase, whose ground state has a spin gap, can he well described by the exchange parameters determined by fitting the magnetic susceptibility chi calculated for a model of a ferromagnetic-antiferromagnetic alternating bond to the experimentally obtained chi. The gap-value estimated from the low temperature behavior of C is about 21 K, which is also consistent with the ones obtained by other measurements. Results of the measurements of the magnetic susceptibilities chi and the specific heats C of M-Cu1-xZnxNb2O6 (0 less than or equal to x less than or equal to 0.5) and their structural parameters are also presented.