Effect of pressure on the ferroelectric properties of telluric acid ammonium phosphate

Telluric acid ammonium phosphate (TAAP) is a new class-m ferroelectric with a transition temperature of $45^0C$. Recent Raman spectroscopy studies of this crystal suggest that the phase transition takes place predominantly by the order-disorder nature of the hydrogen bonds of the type N-H-O. The hydrogen-bonded ferroelectrics are known to be very sensitive to external pressure and deuteration. In view of this, the effect of hydrostatic pressure on ferroelectric properties of TAAP has been studied up to 6 kbar. It was found that $T_c$ shifts to lower temperatures under the influence of external pressure. At low pressures, $T_c$ decreases more rapidly than at high pressures. The pressure derivative evaluated from the linear portion of the curve is $-2.69^0C kbar^{-1}$. The effect of pressure on the spontaneous polarisation is to reduce its value although the continuous nature of the transition does not undergo any change up to the maximum pressure employed. These results and the shift of $T_c$ to higher temperature upon deuteration support the order-disorder nature of the hydrogen bonds during the phase transition