Volborthite is a good candidate material to study strongly geometrical frustrated spin systems, since this compound possesses 2D Kagomé structure of Cu(II) ions. Although the Kagomé structure of this compound is not perfect, absence of typical ordering was reported by magnetic measurements and our previous heat capacity measurements. We have observed remarkable sample dependences around 1 – 2 K even in the samples obtained in the same batches. We have performed TG and heat capacity measurements in order to investigate the effect of heat treatment. As a result, we found no distinct changes for the sample with heat treatment up to 100 °C. It indicates that degradation of sample quality due to the release of water molecules does not occur in this temperature. Partial defect of water and hydrogen bonding might cause slight change of low-temperature heat capacity. Removal of inter-layer water produced by heating the sample up to 600 °C significantly affects the physical properties of volborthite while surface water does not affect spin properties.
Fig. 1. Temperature dependence of heat capacity of volborthite at low temperature around 1 K is shown in a plot of CpT −1 vs T. Serious sample dependence was observed in low temperature magnetic behavior. The sample A and the samples B and Bf are obtained as different batches.
Fig. 2. Temperature dependence of heat capacity of volborthite at low temperature around 1 K after heat treatment up to 100 °C and 600 °C. Temperature dependences of heat capacity before and after TG measurement up to 100 °C show almost the same trend. The orange square data shows heat capacity after TG measurement up to 600 °C.