A parallel plate capacitor of capacitance 12.5 pF is charged by a battery connected between its plates to potential difference of 12.0 V . The battery is now disconnected and a dielectric slab ( $\epsilon_{\mathrm{t}}=6$ ) is inserted between the plates. The change in its potential energy after inserting the dielectric slab is $\_\_\_\_$ $\times 10^{-12} \mathrm{~J}$.