Na$_{2}$Cu$_{2}$Si$_{4}$O$_{11}\cdot $H$_{2}$O: A new S=1/2 chain material with microporous structure

We report the first S=1/2 chain microporous silicate, Na$_{2}$Cu$_{2}$Si$_{4}$O$_{11}\cdot $H$_{2}$O. This compound was synthesized hydrothermally and its structure determined through single-crystal X-ray diffraction. It consists of pyroxene-type chains of distorted edge sharing CuO$_{6}$ octahedra separated by SiO$_{4}$ tetrahedra and running parallel to Na$^{+}$ filled channels. Susceptibility data was fitted using the S=1/2 alternating exchange parameters and J$_{1}$/K$_{b}$ and J$_{2}$/K$_{b}$ were determined to be 102 K and 16 K respectively. The spin gap $\Delta $ was extracted from the low temperature data and decreases with increasing magnetic field, also 2$\Delta $/K$_{b}$Tsp= 3.9 close to 3.53, the expected value for a Spin-Peierls transition. These observations seem to indicate that this system is a two step Spin-Peierls system where a structural dimerization, already present at room temperature, is followed by a magnetic transition at lower temperature.