We have studied the phase behavior at normal pressure of NH4HSO4 (AHS)
over the 100-420 K temperature region using ac calorimetry on single
crystal samples. From the changes in the ac-calorimetric curves record
ed on successive thermal cycles of the samples, it is inferred that me
tastable phases can be induced in the previously reported region of it
s intermediate ferroelectric phase (154 K < T < 220 K) at 1 atm. An ad
ditional thermal anomaly was found to occur at 220 K that seems to ind
icate the coexistence of the low and intermediate phase in the 154 K <
T < 220 K temperature range. No thermal anomalies were observed in th
e paraelectric region 270 K < T < 420 K, where it has been claimed tha
t some proton transport occurs in AHS.