Dielectric Response and Heat Capacity Measurement of Ammonia Borane (NH$_{3}$BH$_{3})$

Ammonia Borane (NH$_{3}$BH$_{3})$, henceforth AB, has attracted a lot of interest recently because of its potential as a hydrogen storage material. It is known to exhibit a solid-solid phase transition at T$_{P} \quad \sim $225 K but the underlying mechanism is unclear. AB being a classical example of donor-acceptor complex, a high dipole moment is expected in the solid, with a probable ferroelectric or antiferroelectric behavior$^{1}$. We thus carried out dielectric measurements on AB using an ac impedance bridge technique over a temperature range of 200--250 K, and found a dielectric transition at 225 K. The dielectric constant was measured at 100 Hz, 1 kHz and 10 kHz, and, was calibrated against standard materials$^{2}$. It is found to decrease with increase in frequency. We also observed the hysteresis at 10 kHz frequency using a ramping rate of 0.25 K/min. The hysteresis behavior is consistent with an antiferroelectric transition at 225K. The transition mechanism was studied also by specific heat measurements, clearly indicating a first-order transition, with a half width at half height of 0.3 K, in contrast to literature values of 10-K. Details of sample preparation, experimental procedure and data analysis will be explained in the presentation. [1] Weaver et al, \textit{J. Chem. Phys.} 1958, 29, 1-2. [2] Bull. Acad. Sci. USSR, \textit{Phys. Ser.} 1960, 24, 1327.