Singlet ground state determined by isolated ${\mathrm{Cu}}^{2+}$ chain topology in microporous ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$

The copper silicate ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ and its dehydrated form ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$ exhibit a singlet ground state at low temperature, which is conditioned by the topology of the isolated edge-sharing ${\mathrm{CuO}}_6$ (or ${\mathrm{CuO}}_5$) chains. The structural analysis of the hydrated and dehydrated forms indicates a strong alternation of the exchange interaction parameter within these chains. The magnetic susceptibility and specific heat measurements confirm spin gaps $\Delta =96\mathrm{K}$ for ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ and $\Delta =85\mathrm{K}$ for ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$. The reversible dehydration of the water molecules from the microporous framework in the structure affords unique opportunity for tuning the magnetic interaction parameters in these compounds.

Figure 1

(Color online) Fragment of crystal structure of (a) ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ and (b) ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$ comprised of ${\mathrm{CuO}}_6$ octahedra, ${\mathrm{SiO}}_4$ tetrahedra, and ${\mathrm{H}}_2\mathrm{O}$ molecules in ball-and-stick representation.

Figure 2

Temperature dependences of magnetic susceptibility $(\chi =M∕H)$ in ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ (엯) and ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$ (◻), taken at 1 T. Full lines are fits as described in text.

Figure 3

Temperature dependences of the specific heat $C$ of ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}\bullet 2{\mathrm{H}}_2\mathrm{O}$ (엯) and ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{11}$ (∎). The inset at the top zooms on the low-temperature data. The inset at the bottom presents calculations [Eq. (4)] of the magnetic specific heat for both systems (dashed, hydrated; line, dehydrated).